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Minimum Design Loads for Buildings and Other Structures provides requirements for general structural design and the means for determining dead, live, soil, flood, wind, snow, rain, atmospheric ice, and earthquake loads, as well as their combinations, which are suitable for inclusion in building codes and other documents. This Standard, a complete revision of ASCE/SEI 7-02, includes revised and significantly reorganized provisions for seismic design of structures, as well as revisions in the provisions for determining live, flood, wind, snow, and atmospheric ice loads. Supplement No. 1, which is included with the Standard, ensures full and complete coordination between ASCE/SEI 7-05 and the 2006 International Building Code. The updates which comprise Supplement No. 1 are seamlessly integrated into this volume and are not available anywhere else. ASCE/SEI 7-05 is an integral part of building codes in the United States. The earthquake load provisions in ASCE 7-05 are substantially adopted by reference in the 2006 International Building Code and the 2006 NFPA 5000 Building Construction and Safety Code. Many other provisions, including calculations for wind and snow loads, are also adopted by reference by both IBC and NFPA model building codes. Structural engineers, architects, and those engaged in preparing and administering local building codes will find this Standard an essential reference in their practice.

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... In earlier research and practices, the wind effect was simplified as an air pressure, which was statically applied to surfaces of structures [10]. The method was easy to apply for engineers and leads to conservative results in most cases, and thus adopted in standards [11,12]. However, since the basic assumption of air pressure is inconsistent with the dynamic nature of wind, the method does not provide accurate results. ...

... The average wind speed and standard deviation were 8.6 m/s and 5.2 m/s, respectively. After future wind speed data are obtained from the proposed prediction models for the 10min averaging data, instantaneous wind speed can be calculated using the predicted average wind speed, according to code [11]. The instantaneous wind speed can be used to assess more severe wind conditions with higher wind speeds. ...

Operation safety and efficiency of high-speed trains are prone to strong winds. This study proposes a wind hazard warning system to predict wind speed and provide warning for safe and efficient operation of trains, and shows the implementation to the Lanzhou-Xinjiang High-Speed Railway. The system is composed of monitoring, remote and central processing, and execution modules. The system uses field monitoring data to predict wind speed and analyze responses of trains. Wind speed is predicted through integrating three models based on artificial neural networks and the predictions are incorporated into operation control for high speed trains under wind load. The results show that the proposed predictive model is capable of predicting wind speed, and the system is capable of providing reasonable warning for high-speed trains. It is anticipated that the system will improve the operation and management of high-speed trains for high safety and efficiency.

... There are few available structured texts that treat the topic of structural loads in sufficient detail. The major reference used in the course is the ASCE 7-02 publication 13 . The one or two textbook available are not concise and contain a lot of extraneous information, and are therefore not very suitable for this 2-credit hour undergraduate course. ...

... The response quantities can be estimated from the median value only if at least 7 pairs of ground motions are used. The newer version of the ASCE Standard 7-16 [17], however, increased the minimum number of ground motions to 11. This value was not based on a detailed study, but was judgmentally selected. ...

The impact of the type of target response spectrum and the number of ground motions on the response of buildings is investigated. The parametric study involves the selection of ground motions based on the Eurocode 8 spectrum, a conditional spectrum using the official seismotectonic model of Slovenia, a uniform hazard spectrum based on the seismotectonic model of the SHARE project, and the corresponding conditional spectrum. In addition to the variation of the target response spectrum, the number of selected ground motions was varied from 7 to 60. Selected sets of ground motions were used to investigate the seismic response of eight reinforced concrete buildings with fundamental vibration periods from 0.15 s to 1.76 s. The aim of the study was to analyse the variation of target displacements obtained by a pushover-based method, i.e. the median displacement corresponding to a seismic intensity with a return period of 475 or 2475 years, and the median spectral acceleration causing collapse, which was estimated by incremental dynamic analysis. It was found that the target spectrum and the number of ground motions have a limited impact on the target displacement, especially if it corresponds to seismic intensities with a return period of 475 years. Additionally, the impact of the number of ground motions on the median spectral acceleration causing collapse is much lower than the impact of the target response spectrum. When the conditional spectrum was used as the target spectrum for ground motion selection instead of the design response spectrum prescribed by Eurocode 8, the resulting median spectral acceleration causing collapse increased by a factor of between 1.2 and 2.3.

... So far, many methods have been investigated for the strengthening of structural members [8,19] and many models were presented by researchers for predicting the behavior of strengthened members [17,21,12]. Different guidelines like ASCE 7-05 are available for this purpose [2]. The design guidelines address some specifications and parameters of steel frames by the engineering judgment and non-linear analyzes. ...

Due to several reasons as the low resistance of constructed concrete and also change in codes or application of structures, some concrete frames need to be retrofitted. By adding the steel brace to the reinforced concrete, many seismic parameters such as resistance, ductility, stiffness and the resistance reduction coefficient change. This study experimentally investigates the impact of adding the prop, the convergent steel brace and also the convergent steel brace with a ductile ring on resistance, stiffness, ductility and energy dissipation of RC frames. Four samples of RC frames with one span and one story with the same characteristics were constructed and retrofitted by different methods. All frames were subjected to the cyclic loading, and the hysteresis and pushover-displacement graphs of them were plotted. The novelty of the work was using such braces in RC frames. The results obtained from the tests showed that although the frame retrofitted by the X-brace showed a better performance in terms of resistance and stiffness, but the retrofitted frame with a ring also showed a better behavior in terms of resistance and stiffness compared to the RC frame and the sample with the jacket as well as compared to the sample with the X-brace showed more ductility and energy dissipation (with a slight reduction in resistance).

... The first step is to confirm the isolated building period T e that is the key to confirm the effective radius of curvature of the spherical surface of VFPB. It is recommended that the target isolated building period T e , be selected such that [55], ...

• Shang Jiying
• Ping Tan
• Yafei Zhang
• Peng Mi

Herein, analytical and experimental investigations are conducted to characterize the performance of a novel variable friction pendulum bearing (VFPB) capable of progressively exhibiting different hysteretic properties for different displacement amplitudes. An analytic scheme comprising existing nonlinear elements is proposed, which can be easily implemented using currently available analysis software. The analytic-scheme accuracy is verified through comparison with experimental data. Thereafter, a design method is developed for seismically isolated buildings employing VFPB. Elastic-plastic time-history analyses are performed to study the seismic performance of the base-isolated building using VFPB, by comparing its performance with that of a friction pendulum bearing (FPB). Both the buildings isolated employing VFPB and FPB exhibit excellent aseismic performance. The VFPB is more effective than FPB in reducing the response of superstructure without increasing isolator displacement for the rare earthquake scenario, especially for very rare earthquake scenario. The stiffness and damping of VFPB change to predictable values at calculable and controllable displacement. Therefore, it can be designed to achieve multiple performance objectives corresponding to different levels of ground shaking.

In this paper, the time-varying autoregressive (TVAR) model is integrated with K-means – clustering technique to detect damage in the steel moment resisting frame. The damage is detected in the frame using non-stationary acceleration response of the structure excited using ambient white noise. The proposed technique identifies and quantifies the damage in the beam-to-column connection and column-to-column splice plate connection caused due to loosening of the connecting bolts. The algorithm models the non-stationary acceleration time history and evaluates the TVAR coefficients (TVARC) for pristine and damage states. These coefficients are represented as a cluster in the TVARC subspace and segregated and classified using K means – segmentation technique. The K-means – approach is adapted to simultaneously perform partition clustering and remove outliers. The topological and statistical parameters of the TVARC clusters are used to quantify the magnitude of the damage. The damage in quantified using Mahalanobis distance (MD) and Itakura distance (ID) serving as statistical distance between the healthy and damage TVARC clusters. MD calculates a multidimensional statistical distance between two clusters using the covariance between the state vectors. Whereas, ID measures the dissimilarity of the AR parameter between reference state and unknown states. These statistical distances are used as DSF to detect and quantify the initiation and progression of the damage in the structure under ambient vibrations. The outcome of both the damage sensitive features (DSF) corroborate with experimental investigation, thereby improving the robustness of the algorithm by avoiding false damage alarms.

This paper presents an effective method to design semi-active fluid viscous damper (SAFVD) system for mitigating the seismic responses of the nonlinear frames considering multiple safety and convenience criteria. This method is based on defining a multi-objective optimization problem with discrete objective functions of minimization of inter-story drift as safety criterion as well as absolute acceleration as convenience criterion where an improved version of the non-dominated sorting genetic algorithm (NSGA-II) has been used to solve it and find out the Pareto-optimal solutions. For this purpose, the maximum damping coefficient of the semi-active damper and parameters of the control algorithm have been selected as design variables. The effect of the uncertainty of the input seismic excitation has been considered using the mean responses under multiple real earthquakes recommended for the site that the structure is located in. To assess the effectiveness of the proposed methodology, a numerical example has been conducted on an eight-story nonlinear shear building frame with hysteretic bilinear elasto-plastic behavior. The results show that the designed optimal SAFVD system has the capability of a significant reduction in seismic responses and improving both the safety and convenience of the entire building. Also comparing the seismic performance of SAFVD system with that of active control system shows that SAFVD could perform as effective as active control system in structural response mitigation.

This study presents an exposure model for the residential building stock in the Middle East, developed for the purposes of multi-hazard risk assessment. The exposure model provides the number of buildings, number of dwellings and population in 12 countries with their corresponding physical characteristics, geographical location and economic value. The main sources of data used to develop this model were housing and population census surveys, existing literature and the judgment of local experts. The study also includes an overview of the most common building types in different parts of the region. A simplified multi-hazard exposure taxonomy is introduced to identify relevant building features according to the type of natural hazard. The exposure model was disaggregated at a fine spatial resolution, using a combination of various remote sensing datasets, and overlapped with hazard maps to identify population and buildings exposed to floods and earthquakes. The results from this study represent a significant step towards a better understanding of risk due to natural hazards in Jordan, Syria, Palestine, Saudi Arabia, Lebanon, United Arab Emirates, Yemen, Oman, Kuwait, Qatar, Bahrain and Iraq.

An alternative design methodology which can be conveniently adopted by practicing engineers is proposed to enhance the seismic performance of existing reinforced concrete moment resisting frame (RC-MRF) buildings by employing viscous dampers. Here, the response indicators of the unretrofitted building are calculated using nonlinear response history analysis (NLRHA); viscous damper characteristics are calculated by estimating the peak viscous damper forces in terms of supplemental viscous damping and peak story shear forces of the unretrofitted building; and currently available expressions are used for the displacement profile of the retrofitted building and the equivalent single degree of freedom (SDOF) system displacements, to calculate the supplemental viscous damping. The effectiveness of the distribution of the viscous damper constants proportional to story parameters is also investigated.

This paper presents a particle swarm optimizer for production of endurance time excitation functions. These excitations are intensifying acceleration time histories that are used as input motions in endurance time method. The accuracy of the endurance time methods heavily depends on the accuracy of endurance time excitations. Unconstrained nonlinear optimization is employed to simulate these excitations. Particle swarm optimization method as an evolutionary algorithm is examined in this paper to achieve a more accurate endurance time excitation function, where optimal parameters of the particle swarm optimization are first determined using a parametric study on the involved variables. The proposed method is verified and compared with the trust-region reflective method as a classical optimization method and imperialist competitive algorithm as a recently developed evolutionary method. Results show that the proposed method leads to more accurate endurance time excitations.

Local geology of site and its properties can influence the input seismic wave specifications dramatically such as frequency content and duration while passing through soil stratums, making it inevitable to perform site-specific ground response analysis. For such analysis, soil's dynamic properties including shear wave velocity (Vs) and shear modulus (G) are required, which are usually obtained using laboratory or field tests, commonly too expensive and time consuming for regions having deep sedimentary deposits. Electrical resistance of ground can be put into practice as a cost-effective alternative in assessing soils dynamic properties in order to be utilized in site response analysis, since the subsurface materials such as soil and rock have unique electrical properties. In this study, the feasibility of site response analysis using electrical logging has been evaluated using field data measurements of electrical resistivity and borehole logging through Guilan province, which is located on the southern shoreline of the Caspian Sea and in a region with high seismicity having deep sedimentary deposits. Nonlinear site response analysis as well as equivalent linear one is carried out in order to obtain the spectral response of the region, separately for the eight data points of the study. The acceleration time histories were synthesized using spectral matching technique integrating with probabilistic seismic hazard analysis. According to the results, the NL and EQL response and design spectrums, which are obtained in this study, show decent agreement with Iran's 2800 seismic design spectrum, indicating acceptable performance of site response analysis using electrical resistivity logging.

Reinforced concrete (RC) frames infilled with unreinforced masonry are quite common all across the globe since many decades. In this study the special moment resisting RC framed buildings with vertical mass and stiffness irregularities have been analysed with modal analysis using ETABS 2016. Regular model having regular distribution of mass and stiffness in elevation were analysed and designed as per IS 1893:2002 by equivalent static method. Comparison has been done among the fundamental periods of 6th, 9th and 12th storeys among regular, irregular and bare framed buildings. Results show that there is significant contribution of infill in fundamental periods and there is significant effect of location and magnitude of irregularity in fundamental period. The fundamental periods of the irregular buildings were found longer than regular and shorter than bare frame buildings. Furthermore, it has been found that there are positive correlation between elevation of mass irregularity & fundamental time period and negative correlation between elevation of stiffness irregularity and fundamental period. Fundamental period was longest with mass irregularities in top storey and stiffness irregularities in second storey. There was not significant effect in fundamental period when mass irregularity was in bottom portion of building and stiffness irregularity was in top portion of the building.

• Mohammad Alembagheri
• Pejman Sharafi
• Zhong Tao
• Kamyar Kildashti

In current practice, a relatively wide range of different interconnections and assembly systems is being used for the design of modular buildings made of volumetric prefabricated frames. The integration strategies and design criteria for providing structural robustness or prevention of disproportionate collapse for such buildings are completely different with those with conventional systems. This is mainly because of their fundamentally different configuration of joints and the redundancy they provide for the structure. This paper studies the robustness of multistory modular structures, made of prefabricated volumetric steel frames, against progressive collapse, which can be triggered by various column or module loss scenarios. In this study, the flexibility of modules and propagation of the damage inside the individual modules are taken into account, and the buckling of columns, as well as the effects of material nonlinearity on the responses and capacity of the entire system, are also considered. Load redistribution patterns and possible gravity-induced collapse mechanisms are inspected in various collapse scenarios for a typical mid-rise modular steel frame. In the numerical model, each individual prefabricated volumetric module is composed of discrete beam and column frame elements, connected through conventional rigid beam–column connections. The volumetric modular steel frames are connected via corner vertical and horizontal intermodule joints; each one is modeled utilizing one axial and two shear springs with predefined nonlinear force–displacement behavior in a 3D finite element analysis. The local damage scenarios are assumed to be due to instantaneous loss of columns or entire modules, for which the interactions between the modular units, overall structural robustness, load redistribution, possible collapse mechanisms, and progressive collapse response of the frames are investigated. The results indicate that the additional redundancy may help the modular steel frames survive from some corner column or single module removals but will not be able to maintain their robustness under combined module removal scenarios.

Seismic behavior of Steel Moment Resisting Frames (SMRFs) with Drilled Flange (DF) connections as well as Reduced Beam Section (RBS) and Welded Unreinforced Flange-Bolted web (WUF-B) as a Pre-Northridge connection have been compared analytically considering Far-field earthquake-induced. The backbone curves of RBS and WUF connections are extracted from available studies and the backbone curve of DF connection is presented in this study in order to simulating the buildings models. DF, RBS, WUF connections and Panel Zone (PZ) are numerically modeled based on the proposed models provided by the prior researches and these models are applied to analyze low-and high-rise buildings designed in accordance with the relevant standards. Incremental Dynamic Analysis (IDA) process is utilized to evaluate the effects of DF connection on structural seismic response of SMRFs. Afterwards, the structures' performance in different response levels is probabilistically assessed by means of IDA and fragility curves. The results show that the seismic demand of SMRFs with DF and RBS connections are so similar, specifically for low-rise buildings. Likewise, SMRFs with DF connection provide up to 43% higher seismic demand in high-rise buildings compared to RBS connection. Eventually, DF connection can be used as an authentic option in SMRFs.

• Samyak D. Parekar
• Debarati Datta

Generally, the buildings located in earthquake-prone regions are subjected to a sequence of earthquakes. The sequence of earthquakes is termed as mainshock–aftershock (MS–AS). Most of the time the rehabilitation of structure before the occurrence of aftershock cannot be carried out due to a short interval of time between mainshock and aftershock results in additional damage to the structure. The irregular distribution of mass, stiffness and strength along the height of the building may exhibit the poor seismic performance of the structures. The present study examined the influence of stiffness irregularity on seismic demands of 3-, 6- and 9-storey steel moment-resisting frames by comparing the mean seismic demands on reference regular frames under mainshocks and MS–AS seismic sequences. Stiffness irregular frames are obtained by modifying the stiffness at three different locations (bottom, middle and top storey) along with the height of reference regular frame. MS–AS seismic sequences are generated by using a randomized approach. The comparison between reference and stiffness irregular frames shows that the effect of stiffness irregularity on the height-wise variation of interstorey drift ratio is significant. Results of the effect of MS–AS seismic sequence show that aftershock increases the structural responses of both reference regular and stiffness irregular frames.

The Pacific Earthquake Engineering Research (PEER) Center has expanded its Tall Building Initiative project to include the seismic performance of existing tall buildings. A candidate 35-story steel building with representative details from that era was analyzed. Pushover analysis and nonlinear dynamic response history results for the as-built building are discussed in a companion paper. In this paper, a potential retrofit approach using viscous dampers is presented. The performance criteria for the retrofit focused on reducing story drifts and accelerations to enhance safety in large events, and promote continued occupancy in more moderate ones. Retrofit study starts by selecting damper locations within architectural constraints. The overall effective damping ratio is calculated in order to satisfy a target roof drift limit. Three conventional damper placement methods are proposed, and the structural responses including story drift ratio, floor acceleration, and damper responses are compared. Possible improved schemes are later presented taking into account of performance-related and cost-related factors. A refined damper design scheme is proposed based on an overall cost-benefit analysis. Other design issues for FVD such as nonlinearity, bracing stiffness, and damper configurations are discussed briefly at the end.

This paper presents a seismic response evaluation of self-centering prestressed concrete frames with infill walls (SCPC-IW frames). Three types of infill walls with different load capacity were selected to be installed in a bare self-centering prestressed concrete frame (SCPC-Bare frame). Beam web friction devices are included in the beam-column connections to provide energy dissipation capacity. Numerical models of the SCPC frame, and the three SCPC-IW frames were established and analyzed in a finite element software, OpenSees. Firstly, the nonlinear static analyses and nonlinear dynamic analyses under 44 ground motions were performed. Secondly, the incremental dynamic analyses (IDAs), seismic fragility analyses, and collapse resistance capacity evaluation were conducted. The results showed that the initial stiffness, energy dissipating capacity of SCPC-IW frames are increased in comparison to SCPC frames. After adding infill walls, the lateral displacement of the SCPC-IW frames decreases. However, the residual deformation and axial compression ratio of the SCPC-IW frame increases. As the infill walls strength increases, the increased amplitude of residual displacement becomes much larger than the decreased amplitude of the drift. It is concluded that when the load capacity of infill walls ranges from 37% to 79% of the web friction force, the SCPC-IW frames can achieve a good balance in terms of stiffness, energy dissipation, and self-centering capability as well as collapse resistance capability.

• Mohammad Javanmard
• Aliakbar Yahyaabadi

Given the inherent uncertainty in seismic response, seismic performance assessment of structures should be conducted within a probabilistic framework. One of the most efficient probabilistic approaches is the IM-based probabilistic seismic demand analysis (PSDA). In this method, an intermediate parameter, which is known as the intensity measure (IM), is used to decouple the seismological and structural uncertainties. Two intensity measures of \(\mathrm{IM}_\mathrm{oc}\) and \((S_\mathrm{a})_\mathrm{rms}\) were introduced for near-fault pulse-like records in previous research. These IMs are defined based on the optimal combination of spectral displacements and root-mean-square of spectral accelerations at effective periods, respectively. In this research, to consider the efficiency of these IMs under a set of 90 records that contains both near-fault and ordinary ground motion records, we conducted the PSDA for five moment-resisting frames with the number of stories ranges from 3 to 15. Results show that \(\mathrm{IM}_\mathrm{oc}\) and the advanced intensity measure of \( \mathrm{IM}_{{1}\mathrm{I} \& 2\mathrm{E}}\) exhibit the highest correlation with the expected damage for the most frames, especially moderate and relatively long-period ones. \( \mathrm{IM}_{{1}\mathrm{I} \& 2\mathrm{E}}\) is defined based on the inelastic spectral displacement with the higher-mode modification. In addition, comparison of the drift hazard curve of different frames shows that by increasing the structural height, the amount of drift hazard will decrease. However, comparing to other cases, the reduction rate of drift hazard along with increasing the number of stories from three to six is significant.

A generalized fiber beam element formulation is proposed to accurately capture the formation of multiple plastic regions with a coarse mesh, which usually occurs in the process of structural collapses. The strong gradient of displacement near plastic regions in a fiber beam element can be accurately described using a special plastic enrichment function. The two types of the plastic enrichment functions are suggested for the cases where the plastic region is located fully inside an element and spread over a node, respectively. In this approach, the optimal shape of the plastic enrichment function can be updated by reflecting plastic deformation at the previous loading step. Furthermore, if plastic regions appear in multiple locations in an element, the corresponding plastic enrichment function can be adaptively reconstructed on the basis of plastic region distribution without introducing additional degrees of freedom. The effectiveness of the proposed method is investigated in terms of accuracy and computational cost through several numerical experiments.

Historical earthquakes in developing countries, particularly in South America, have caused devastating effects in essential buildings, such as hospitals. As a result, some structural retrofits and new designs of essential facilities in South American countries have concentrated on increasing the seismic design forces beyond what is normally required by building codes. This increased-strength approach can lead to quasi-elastic structural response during earthquakes with much larger induced floor accelerations than expected from code-designed structures. In critical structures, sub-par performance of critical non-structural elements, such as medical equipment in a hospital, due to high-induced floor accelerations can have consequences as devastating as structural failures. Passive seismic protection systems, such as seismic isolation, provide a practical means of controlling seismic demand and are now common in developed countries. However, the applications of protection systems have been far fewer in developing countries due to economic considerations and lack of technical expertise. This paper presents the experimental and numerical studies supporting the development of a novel low-cost sustainable protection system incorporating recycled automobile tires for isolating designated floors or rooms in essential buildings in developing countries. The main innovation of the proposed floor isolation system is the geometric arrangement of the rubber tires that allows isolation from both, horizontal and vertical floor accelerations. Horizontal and vertical cyclic testing conducted on tire specimens are first described followed by the development of a numerical model able to reproduce the experimental results. Finally, an existing case-study hospital in Ecuador is considered for evaluating numerically the seismic performance of the proposed floor isolation system.

The Indian subcontinent has experienced several earthquakes, and the major ones have caused significant damage to the buildings and lifeline structures. Researchers have realized the importance of developing a robust design method which can reduce the probability of occurrence of such damages. This paper discusses the currently followed seismic analysis techniques, their applicability and limitations, as well as the newly emerging displacement-based design methods. There is a need for a paradigm shift from the conventional force-based approach to the more rational displacement-based design methods to limit damages and to get uniform risk structures. Researchers are now focusing on the refinement of these new design methods to make them applicable to all types of buildings, bridges, and other structures.

• Anil Kumar
• Vasant Matsagar

Fragility functions are determined for braced steel moment frames (SMFs) with plans such as square-, T-, L-, U-, trapezoidal-, and semicircular-shaped, subjected to blast. The frames are designed for gravity and seismic loads, but not necessarily for the blast loads. The blast load is computed for a wide range of scenarios involving different parameters, viz. charge weight, standoff distance, and blast location relative to plan of the structure followed by nonlinear dynamic analysis of the frames. The members failing in rotation lead to partial collapse due to plastic mechanism formation. The probabilities of partial collapse of the SMFs, with and without bracing system, due to the blast loading are computed to plot fragility curves. The charge weight and standoff distance are taken as Gaussian random input variables. The extent of propagation of the uncertainties in the input parameters onto the response quantities and fragility of the SMFs is assessed by computing Sobol sensitivity indices. The probabilistic analysis is conducted using Monte Carlo simulations. The frames have least failure probability for blasts occurring in front of their corners or convex face. Further, the unbraced frames are observed to have higher fragility as compared to counterpart braced frames for far-off detonations.

In the present paper the effects of aerodynamic damping and earthquake loads on the dynamic response of flexible-based wind turbines are studied. A numerical analysis framework (NAF) is developed and applied. NAF is based on a user-compiled module that is developed for the purposes of the present paper and is fully coupled with an open source tool. The accuracy of the developed NAF is validated through comparisons with predictions that are calculated with the use of different numerical analysis methods and tools. The results indicate that the presence of the aerodynamic loads due to the reduction of the maximum displacement of the tower attributed to the dissipation of earthquake excitation energy in fore-aft direction. Emergency shutdown triggered by strong earthquakes results to a rapid change of aerodynamic damping, resulting to short-term instability of the wind turbine. After shutdown of the wind turbine, enhanced dynamic response is observed. For the case where the wind turbine is parked, the maxima displacement and acceleration of tower-top increase linearly with the peak ground acceleration. With the use of the least-square method a dimensionless slope of tower-top displacements is presented representing the seismic response coefficient of tower that can be used to estimate the tower-top acceleration demand. Moreover, on the basis of the seismic response coefficient, an improved model for the evaluation of load design demand is proposed. This model can provide accurate predictions. KEYWORDS aerodynamic damping, dynamic behavior, earthquake intensity, seismic demand, wind turbines 1 | INTRODUCTION Wind energy technology has a leading role in the renewable energy sector. 23.4 GW of new installed capacity of wind turbines that was added in China 1 for 2016; this value corresponds to the 42.8% of the overall added capacity globally for the same year. New installed wind farms are located along the northwest and southeast coasts of China. These wind farms are located in earthquake prone areas, close to the Eurasian and Pacific seismic belts. Wind farms that are located in the aforementioned areas are susceptible to damage on the event of an earthquake. Over the past decades it has been well accepted that earthquakes have a significant effect on the structural dynamic response of wind turbines. 2 Response spectrum method and finite element method (FEM) have been commonly used for the estimation of the seismic load demand of wind turbines. 3 With the use of the response spectrum method the rotor and nacelle are modeled as point lumped masses at tower-top. The seismic load demand of wind turbines can be calculated on the basis of the natural periods, mode shapes, and mass distribution of the tower. 4,5 Usually the aerodynamic damping and higher-order modes are neglected because of the overestimation of the seismic load demand. On the basis of experiments, 6 it is concluded that the seismic excitation increases significantly the lateral displacement of tower-top. On the other hand FEM is applied for the analysis of wind turbines in time domain. Very commonly, the aerodynamic loads are ignored 7-9 or simplified as thrust loads that are applied at rotor. 10 On the basis of this simplification, Zhao and Maisser 11 and Sapountzakis et al 12 used a multibody dynamic method for assessing the behavior of wind turbines under seismic excitation more effectively. However, aerodynamic loads play an important role on the dynamic behavior of towers of wind turbines especially for the case of large diameter wind turbines. 13,14 The interaction between aerodynamic and earthquake loads should be accounted for the estimation of the response of wind turbines under an earthquake event.

A double-layer grid space structure is a conventional long span structure used where large column-free areas are required. Due to its' large indeterminacy and the redundancy of its structural configuration, it is normally considered in design practice, that progressive collapse will not be triggered when the loss of an individual member occurs. However, research and several prior accidents have shown that progressive collapse could occur following the loss of some critical members when the structures are subject to abnormal loading such as heavy snow. To investigate the structural behavior of this type of structure, a 3D finite element model of a double-layer space structure grid was built by the authors, several collapse scenarios have been investigated using an implicit method which follows the alternative path method defined in GSA. In addition, case studies have been made using the explicit method which is to simulate the whole process of the structural collapse. In the analysis, different members failure or support collapses were studied. The response of the structure was investigated and the correspondent potential of progressive collapse was discussed in detail. Methods to mitigate the progressive collapse of this type of space structure have also been recommended.

This study aimed at proposing a new ground motion scaling method for precise spectral matching between the response accelerations and the standard design spectrum regarding the normalization procedure. In this method, the dispersion in acceleration and displacement responses were decreased significantly. Moreover, 11 parameters were used for normalizing response accelerations, including peak ground acceleration, peak ground velocity, peak ground displacement, Arias intensity, Housner intensity, cumulative absolute velocity, maximum incremental velocity, energy index, acceleration spectrum intensity, velocity spectrum intensity, and specific energy density. Three sets of non-pulse-like, near-field pulse-like and non-pulse-like long-duration ground motions were taken into account to investigate the effect of earthquake characteristics on normalizing parameters. Hence, by performing sensitivity analysis, suitable parameters were determined for normalizing response accelerations. Statistical results illustrated that normalizing response accelerations to acceleration spectrum intensity, Housner intensity, and peak ground displacement led to minimum dispersion at acceleration-sensitive, velocity-sensitive, and displacement-sensitive regions, respectively. Moreover, the results showed that suitable normalizing parameters could be determined considering the period of vibration. It was concluded that normalizing response accelerations and scaling of them could lead to appropriate spectral matching and low dispersion. Finally, drift dispersion for four, eight, and twelve-story steel special concentrically braced frame structures under nonlinear fiber-element time-history analysis was evaluated carefully. The results indicated that the inter-story drift dispersion decreased acceptably in all structures.

Engineering structures may inevitably be subjected to multiple natural hazards (such as earthquakes and winds) during their life cycles. This paper presents an efficient multihazard fragility methodology based on the structural demand models. The approach is applied to two steel‐concrete composite frame structures (SCCFSs), with and without buckling‐restrained braces (BRBs), aiming to evaluate the effect of BRBs on controlling the structural responses and fragilities under the combined earthquake and wind loads. In total, 120 earthquake records are selected, and 120 sets of wind drag force time histories are simulated by considering the spatial variation along the height of the exemplar building. The combined "earthquake–wind" events are stochastically assembled, in which the intensities of these two hazards are modeled using the Monte Carlo simulation. The OpenSees platform is employed to calculate the dynamic responses of the SCCFSs with and without BRBs under simultaneous earthquake and wind loads. The goodness of fits of the first‐, second‐, and third‐order polynomial in predicting the structural demand are evaluated, and the optimal polynomial is employed to generate the multihazard fragility surfaces at different damage states. The numerical results indicate that the structural responses and fragilities under the combined earthquake and wind are higher than those under an individual hazard, while the influencing extent varies with the relative intensities of these two hazards. The impact of multiple hazards and the control effect of BRBs on the structural responses and fragilities are systematically quantified and discussed in details.

In this paper, post-earthquake fire modeling was studied in steel structures with different levels of ground motion intensity. Three-story structure modeling was implemented under post-earthquake fire by using the OpenSees software. The performance of the structure was investigated under different levels of ground motion intensity to the level of life safety. The structure was subjected to seismic and thermal analysis by applying standard fire load up to 880 s of fire. The 9-point thermal gradient for beam and column profiles under heat was analyzed by heat transfer analysis in the software. By comparing obtained results from the seismic analysis and post-earthquake fire analysis, it can be seen that for different levels of ground motion intensity, the behavior of the structure is different when it is exposed to post-earthquake fire than being exposed to the earthquake alone, which can affect the performance-based design of the structure. Therefore, in the design of structures, the effect of post-earthquake fire should be taken into account, considering the seismic zone of the structure and the time required to extinguish the fire.

In this study, the use of high damping rubber bearing (HDRB) with various design properties in mitigating the seismic effects for steel buildings was investigated. For this, a generalized demand on the analytical model of HDRB was introduced and eighteen different models of HDRB were examined comparatively. These models were created by considering three significant isolation parameters of HDRB such as isolation period T (2, 2.5, and 3 s), effective damping ratio β (0.05, 0.10, 0.15), and post-yield stiffness ratio λ (3 and 6). The benchmark low (3-storey), mid (6-storey), and high-rise (9-storey) steel buildings were equipped with different isolation systems of HDRB and then subjected to a set of earthquake ground motions through nonlinear time history analyses in order to evaluate the actual nonlinear behaviour of the bearings in the base-isolated steel buildings in service. The base-isolated frames were assessed by the variation of the selected structural response parameters such as isolator displacement, relative displacement, inter-storey drift ratio, absolute acceleration, base shear, hysteretic curve, and dissipated energy. The effectiveness of the isolation parameters on the nonlinear response of the steel buildings with HDRB under earthquakes was comparatively evaluated to generate alternatively innovative isolation system. It was shown that the seismic performance of the base-isolated structure was remarkably influenced by the isolation parameters. The most favourable base isolation model was obtained when the higher value of the isolation period and effective damping ratio combined with the low post-yield stiffness ratio.

Compared with conventional buildings, the design of interconnections in multistory modular buildings requires special attention , due to different load distribution mechanisms, level of redundancy, integration strategies, and the stability requirements, in particular under accidental load conditions. In this paper, the robustness of corner-supported modular steel buildings, subject to various sudden loss scenarios, is studied through investigating the collapse-resisting capacities and the probable gravity-induced progressive collapse mechanisms. The aim is to evaluate the minimum requirements and special considerations for the design of interconnections for robustness. To that end, archetypal precast room-sized volumetric modules with different heights are analyzed using the finite-element macromodelling method and the alternate load path approach. The load redistribution mechanisms are inspected, nonlinear dynamic responses are determined , and probable collapse mechanisms are identified for buildings with different heights and configurations. The dynamic increase factors are computed and compared with the values suggested by the design codes. Given that the individual modules are mainly made of inherently robust structures, the focus of this study is on the performance of interconnections and interactions between an assemblage of units. The results indicate that due to high redundancy in these systems, there is great reserve strength against gravity-induced progressive collapse scenarios, triggered by instantaneous removal of components.

Blast loads create massive damage and reduce the structure's strength and stability, leading to internal and external damage. Due to this damage under extreme loads, efforts have been made to improve methods of structural analysis and design to resist blast loads. The research mainly focuses on the effect of surface blast on low-rise and high-rise buildings. In this study, five buildings with the same plan configuration are considered and designed according to Indian Standards. These buildings are modelled in SAP2000 software. The displacement responses are represented in terms of nonlinear time histories for different charge weights and standoff distances. Strengthening measures can be done to minimize the dynamic effect of the building due to blast loading. The responses significantly change at lower standoff distances for high-rise buildings and the same for standoff distances greater than 10 m. No significant effect is observed on the response of building between single and double bracings.

• Hamid Kazemi
• Abbasali Sadeghi

The Impact induced by vehicles collision to external buildings' columns is one of the research scenarios of collision. Therefore, in this study, the reliability analysis of steel moment-resisting frame structure with 2-story has been conducted under the impact of light vehicle collision considering uncertainty in material and applied loads using simulation-‌based methods. The mentioned structure is modeled in OpenSees software two-dimensionally and the sensitivity analysis of the studied random variables is performed using Monte Carlo simulation-based method in Matlab software. Then, the limit state functions are proposed based on the maximum permitted beam rotation of damaged bay. Finally, the failure probability and reliability index of the mentioned frame is investigated and compared according to performance levels under the impact of light vehicle collision with speeds 20, 40, 60 and 80 km‌/‌h. The results showed that the random variables such as mass and velocity of vehicle and yield strength of material were the most influential variables in the failure probability and the control variates-based subset simulation method compared to Monte Carlo method estimated the failure probability with permissible error rate, less sample number and short running duration.

• Richard Clarke

There have been damaging hurricanes and earthquakes throughout the history of the Caribbean. However, the intensity and frequency of these natural events have increased. The Caribbean is comprised of territories with dependencies on developed countries, as well as territories that are sovereign states. In September 2017, Hurricane Maria impacted the island of Dominica resulting in damage equivalent to 226% of the Gross Domestic Product. There were 65 fatalities, 1.37 billion US$ in damage, and a recovery period of approximately 5 years. In August 2018, a significant earthquake occurred near Trinidad, the larger of the twin-island state of Trinidad and Tobago, which provides critical support to the Caribbean Community. There were no fatalities or significant injuries, and approximately 12 million US$ in damage, which with some exceptions, was not sufficient for major disruption. In this paper, the results of rapid visual assessment of the buildings after these events are reported with an emphasis on damage in terms of actual versus expected performance. The following are considered: (1) causes of excessive damage; (2) existing approaches that may be used for adequately improving performance, and (3) possible research ideas for the cases where cost-effective approaches to improving performance do not exist at this time. It was concluded that: (i) the main cause of the damage due to the hurricane is an underestimation of the expected wind speed and that adequate performance is possible if building codes are enforced and (ii) the earthquake damage was excessive relative to the extent of ground shaking. Given the general similarity of construction between Trinidad and the rest of the Caribbean, it was also concluded that the Caribbean as a whole has inadequate buildings as regards earthquake resistance and an extensive retrofit effort is required.

This paper presents an experimental and numerical investigation into the buckling behaviour of axially loaded cold-formed steel (CFS) zed and hat sections. In total, 12 experimental tests are reported, of those 6 tests are on CFS zed sections and the remaining 6 are on CFS hat sections. The failure modes, ultimate loads, and load-displacement curves of the specimens were reported and analysed. The experiments revealed the deformation mechanism of both the zed and hat sections failing in distortional mode. Nonlinear finite element (FE) models are then described for both the zed and hat sections, which include material non-linearity and geometric imperfections. The validated finite element models were then used for the purpose of parametric studies comprising 140 models, which include 70 models each for CFS zed and hat sections. Ten different cross-sections were analysed in the parametric study for both the zed and hat sections. The axial strengths obtained from the experimental tests and FE analysis were used to assess the performance of the current design guidelines as per the Direct Strength Method (DSM) for both the zed and hat sections. From the comparison, it was found that the design strengths are un-conservative by 7% and 10% on average for CFS zed and hat sections, respectively. An improved design equation was therefore proposed for those CFS zed section columns, which failed by either distortional buckling or through a combination of distortional and global interactive buckling. The proposed equation gave a close comparison against FE results, being conservative to the FE results by only 3%. Reliability analysis was also performed to confirm the reliability of the proposed design equation.

Tornadoes pose a significant threat to residential communities, causing enormous physical damage and losses to their social fabric. The dominant type of single-family residential buildings in the USA is light-frame wood construction, which is especially susceptible to tornado effects. Previous studies considering resilience of light-frame wood buildings have focused primarily on assessing damage, developing damage functions, and exploring different repair methods. Studies related to sustainability have focused mainly on environmental impacts or carbon usage. Practically all of these studies have been geared to assessment of individual buildings. In this study, we couple resilience and sustainability to evaluate their tradeoffs or alignments at the community level from a life-cycle stance. The life-cycle cost and carbon footprint are reflected in the construction and repair of damages due to the tornado hazard, as well as regular repair and maintenance that occurs during the life of the residence. Uncertainties in the randomness in tornado occurrence, size of the tornado footprint, and variation in wind speed intensities within the tornado footprint, and capacities of the building structure and envelope play a significant role in building performance and are considered. We explore a number of repair strategies that might be adopted at the community level in decision-making and policy formulation for homeowners, home builders and community planners. These strategies provide a framework for integrating minimum cost and carbon footprint objectives in risk-informed decision-making, a topic that appears to be lacking in the literature.

• Jin Wang
• Shuyang Cao

This paper aims to study the characteristics of tornado-induced wind loads over smooth and rough ground and subsequently examine the tornado wind load provisions in ASCE 7–16. The pressure measurement on a cubic building subjected to stationary tornado vortices was conducted in the tornado simulator at Tongji University, China. The overall uplift and base shear for MWFRS and wind loads for Components & Cladding are analyzed to assess the newly proposed tornado wind load provisions. The results indicate that the simulated tornado vortices with swirl ratio of 0.72 can represent the EF3 Spencer tornado vortices. In addition, the external building surfaces are significantly affected by the terrain when they are immersed below 3 to 5 times the roughness element height. Tornado would induce more intense aerodynamic loads on roof, leeward wall and side wall than those from conventional boundary-layer winds. The Extended method by amplifying the gust effect factor to 0.9 and the Simplified method by introducing a tornado factor in ASCE 7–16 are able to conservatively evaluate the overall uplift and base shear on the building without dominant openings. However, these two approaches would significantly underestimate the overall uplift on building with a dominant opening as much as 21% and 22%, respectively. The underestimation of peak negative force on the edge zone of wall can reach 39% ~ 48% using Extended method.

"You never change things by fighting the existing reality. To change something, build a new model that makes the existing model obsolete." — Buckminster Fuller. • The Neo-Deterministic Seismic Hazard Assessment (NDSHA) method, proposed some twenty years ago, is found to reliably and realistically simulate the wide suite of earthquake ground motions that may impact civil populations as well as their heritage buildings. • The scenario-based NDSHA modeling technique is developed from comprehensive physical knowledge of: (i) the seismic source process; (ii) the propagation of earthquake waves; and (iii) their combined interactions with site conditions. • Thus, NDSHA effectively accounts for the tensor nature of earthquake ground motions. • Observations from recent destructive earthquakes in Italy: (i) Mw 5.9 Emilia 2012; (ii) Central Italy Mw 6.3 L'Aquila 2009; and 2016–2017 Seismic Crisis - Mw 6.1 Amatrice; Mw 5.9 Visso; Mw 6.5 Norcia; Mw 5.7 L'Aquila; and (iii) Mw 7.8 Nepal 2015 - have all confirmed the validity of NDSHA's approach and application. • Although damaging earthquakes cannot yet be predicted with ultimate precision, intermediate-term (several months) and middle-range (few 100 s km scale) predictions of main shocks above a pre-assigned threshold (based on seismicity "alarms" generated by interpretive algorithms like CN and M8) may be properly used for the implementation of low-key preventive safety actions.

• Massimiliano Ferraioli

The current generation of seismic design codes is based on a linear elastic force-based approach that includes the nonlinear response of the structure implicitly through a response modification factor (named reduction factor R in American codes or behaviour factor q in European codes). However, the use of a prescribed behaviour factor that is constant for a given structural system may fail in providing structures with the same risk level. In this paper, the behaviour factor of reinforced concrete frame structures is estimated by means of nonlinear static (pushover) and nonlinear incremental dynamic analyses. For this purpose, regular reinforced concrete frames of three, five, seven, and nine storeys designed for high ductility class according to the European and Italian seismic codes are investigated, and realistic input ground motions are selected based on the design spectra. Verified analysis tools and refined structural models are used for nonlinear analysis. Overstrength, redundancy, and ductility response modification factors are estimated, and the effects of some parameters influencing the behaviour factor, including the number of bays and the number of storeys, are evaluated. The results are finally compared with those obtained from a previous paper for steel moment-resisting frames with the same geometry. According to the analysis results, the behaviour factors in the case of pushover analysis are significantly higher than those obtained in the case of nonlinear response history analysis. Thus, according to the pushover analysis, the behaviour factor provided by European and Italian standards seems highly conservative. On the contrary, the more refined nonlinear dynamic analysis shows that the code-prescribed value may be slightly nonconservative for middle-high-rise frame structures due to unfavourable premature collapse mechanisms based on column plastic hinging at the first storey. Thus, some modifications are desirable in local ductility criteria and/or structural detailing of high ductility columns to implicitly ensure that the recommended value of the behaviour factor is conservative. 1. Introduction The structures may behave inelastically under earthquake strong ground motions. Thus, the nonlinear dynamic analysis (NDA) would be essential in predicting the seismic response, since it accounts for the redistribution of forces in the inelastic range. However, the nonlinear dynamic analysis implies more effort than linear static (LSA) or linear dynamic (LDA) analysis, in both performing the analysis since it requires additional data (i.e., hysteretic behaviour of materials and spectrum-compatible input records) and interpreting the results (i.e., failure mechanisms and acceptance criteria). Therefore, the NDA is perceived as too complicated and time-consuming in the structural design process. On the other side, the nonlinear static analysis (NSA) is generally less accurate than NDA, and the results should be validated especially for structures with important higher-mode and torsional effects [1–6]. Consequently, the current practice is generally founded on more simple and easy methods based on the linearly elastic analysis. Despite the recent development of deformation-based design (DBD) methods, the Force-Based Design (FBD) approach remains the standard method to design structures for seismic loads. According to the design linear elastic method, the design force level is defined by scaling the seismic forces with a single reduction factor (named response modification factor R in UBC97 [7], IBC [8], and NEHRP [9] or behaviour factor q in Eurocode 8 [10]) that aims to account for overstrength, energy dissipation, and plastic redistribution capacity. This factor that implicitly considers the inelastic behaviour is defined as the ratio (q = Fe/Fd) of the maximum seismic force (Fe) on the structure that responds elastically to the design seismic force (Fd). Many studies in the literature were focused on formulas to evaluate the response modification factor and its components [11–14] that are primarily related to the period of the structure, ductility, damping, hysteretic behaviour, soil conditions, and distance to the epicentre of the earthquake. The first studies in the literature investigated the influence of the most important parameters (i.e., period, site effects, and ductility) on the response of structures using single-degree-of-freedom (SDOF) models [15, 16]. Some of them were implemented in design standards. For example, both UBC94 [7] and NEHRP 94 [17] were based on the studies of Wu et al. [12]. The outcomes of Newmark and Hall [11] were used in various US seismic codes [7–9, 18]. The R-μ-T relations proposed by Vidic et al. [19] inspired both Eurocode 8 [10] and Italian seismic code [20, 21]. More recent studies were dedicated to real complex buildings. The estimation of the components of the behaviour factor requires analysing realistic code-designed buildings, since material safety factors, design criteria, detailing provisions, and practical construction aspects may significantly affect the response reduction factors. Mwafy and Elnashi [22] presented a comprehensive study to calibrate the R-factors using 12 RC buildings designed with Eurocode 8 [10]. Zafar [23] conducted a parametric study involving RC framed buildings to evaluate the effect of dimensional and material properties on R-factor. AlHamaydeh et al. [24] studied the R-factors of three RC framed buildings of 4, 16, and 32 stories for two levels of seismicity in Dubai. Massumi et al. [25] examined the structural overstrength in 25 reinforced concrete framed structures and showed that the overstrength factor remains approximately constant (with average values of 2.5 and 1.7 for buildings designed using, respectively, the Iranian Code and North American Codes) for buildings with a number of floors between 4 and 10. Al-Ahmar and Al-Samara [26] investigated the effects of the number of stories and bays and the bay span on the seismic response modification factors of 25 special moment-resisting frames (SMRFs) buildings. The main weakness of the force-based design approach is that a predefined behaviour factor is used, which is constant regardless of the structure's geometric configuration. The value proposed could be unrealistic, excessively conservative, or not representative of the actual nonlinear behaviour of the building. Most of the past studies in the literature have focused on evaluating the ductility component of the response reduction factor for single-degree-of-freedom (SDOF) systems. However, the overstrength is also very important in calibrating the force reduction factor and may vary widely depending on many factors such as the structural system, the ductility class, and the period of the structure. This paper aims to evaluate and clarify the above. For this purpose, the behaviour factors of moment-resisting RC-frame structures of high ductility class were evaluated performing inelastic pushover and dynamic analyses of RC frames of different heights and configurations. Three-, five-, seven-, and nine-storey reinforced concrete frame structures were designed according to the Italian Code [20]. Nonlinear static pushover analyses for uniform and modal load patterns were carried out to identify the load-displacement relationship and estimate the ductility, overstrength, and redundancy reduction factors. The incremental dynamic analysis using a set of time-history earthquake records was carried out to estimate the behaviour factors, and the results were compared with the values obtained by the pushover analysis. The effects of some parameters influencing the reduction factor, including the number of spans and the number of storeys, were investigated. The values calculated were related to the value of q-factor provided by the Italian Code [20]. The results were finally compared with those obtained from a previous study on steel moment-resisting frames having the same geometry. 2. Evolution of Seismic Codes and Behaviour Factor First introduced in the ATC-3-06 report [27] in the late 1970s, the R-factor was then used in seismic codes to obtain economical designs based on simple elastic analysis but allowing plastic deformation under earthquake ground motion. Since the 1980s, in many studies, seismic codes, and documents, the R-factor has been decomposed into different components. For example, in ATC-19 [28] and ATC-34 [29], the R-factor was calculated as the product of three factors accounting for overstrength, redundancy, and ductility. Nowadays, most of the current seismic design codes (ASCE SEI/ASCE 7-05 [30], Eurocode 8 [10], BIS IS 1893 [31], NZS1170 [32], MDRAP [33], NTC-2018 [20], and IBC [8]) are based on a reduction factor that implicitly accounts for the nonlinear response of the structure under the design earthquake ground motion. On the other side, many procedures for performance-based seismic assessment are included in codes and guidelines (ATC 19 [28], ATC 34, [29], ATC 40 [18], FEMA 440 [34], Annex B EC8 [10], FEMA 356 [35], FEMA 450 [36], FEMA 451 [37], and FEMA P695 [38]) and incorporate response reduction factors, since they are based on the capacity spectrum method [34], the displacement coefficient method [34], or the N2 method [39]. In general, the reduction factor depends on the parameters that may affect the nonlinear response of the building: mechanical behaviour of materials, design procedures, typology of the structure, construction details, structural regularity, and so on. However, the response modification factors provided by the different seismic codes may differ in both meaning and magnitude. The behaviour factor of Eurocode 8 [10] is based on the typology of structure, structural regularity, and ductility class but does not explicitly account for the overstrength. On the contrary, US [7, 35], Canadian [40], New Zealand [41], and Japanese [42] codes include this parameter in the force reduction factor definition. In Eurocode 8 [10], for a highly ductile frame structure, q = 4.5 αu/α1, where αu/α1 is the ratio of the seismic action that causes the development of a full plastic mechanism to the seismic action at the formation of the first plastic hinge. Since the ratio αu/α1 is 1.2 for single-bay multistorey frames and 1.3 for multibay and multistorey frames, the behaviour factor will range between 5.4 and 5.85. On the other side, the American seismic codes (UBC97 [7], IBC [8], and ASCE-7 [30]) provide a fixed value of the reduction factor depending only on the structural resisting system. For example, a reduction factor R = 8 is assumed in ASCE-7 [30] for special (i.e., highly ductile) RC-frame structures. The National Building Code of Canada (NBCC) [40] for ductile moment-resisting RC-frame buildings prescribes a response modification factor that is the product of two factors, a ductility-related factor Rd = 4 and an overstrength-related factor Ro = 1.7. However, it should be underlined that a direct code comparison between European and American seismic code provisions is not consistent if only the level of force reduction is considered. A reliable comparison should involve not only the reduction factor but also the full design process. 3. Methods for Evaluating Behaviour Factor The response modification factor is generally expressed as the products of different factors [27–29]:where Rμ, Rρ, RΩ, and Rξ are ductility, redundancy, overstrength, and damping reduction factors. The damping reduction factor Rξ is used to characterize energy dissipation given by viscous damping or hysteretic behaviour. It is included only if supplemental viscous damping devices are used [28]; otherwise Rξ = 1. The meaning of the other parameters may be highlighted based on the pushover curve obtained by nonlinear static (pushover) analysis. Figure 1 shows the base shear force (F) versus roof displacement (δ) relationship and its elastoplastic idealization according to EC8 [10] and NTC-2018 [21]. The parameters in this figure are defined as follows: design base shear force (Fd), first significant yield strength (F1), idealized yield strength (Fy), elastic response strength (Fe), ultimate displacement (δu), and yield displacement (δy). Concerning Figure 1, the behaviour factor may be expressed as follows:where

Many studies have demonstrated that the design of structures in the region through the uniform hazard principle does not guarantee the uniform collapse risk. Even in those regions with similar PGAs corresponding to the same mean return period, the seismic risk in terms of failure probability will be significantly different due to the structural capacity uncertainty. In this paper, the newly introduced method, known as risk targeting , is being explored in Spain using the recently updated seismic hazard map. Since risk targeting involves multiple input parameters such as model parameters of fragility curves, their variability is considered through their probability distribution corresponding to the RC moment frame building, which is the most common typology in Spain. The influence of variation of these parameters on the risk results are investigated and different assumptions for estimating the model parameters of fragility curves are illustrated. These assumptions are included in a fixed fragility curve (generic) or building-site-specific fragility curves. Different acceptable risk levels (i.e., collapse and yielding) were considered concerning the Spain seismicity level. Finally, the maps for risk-targeted design ground motion and risk coefficient are presented. It was outlined since the shape of the hazard curve across Spain is different and considering the uncertainty of structural capacity; the employment of risk-targeted analysis led to the modifications for existing design ground motions. Moreover, it was found that using the building- and site-specific fragility curves could provide better results.

• Xinzheng Lu
• Hong Guan

Following Chap. 3 which primarily focuses on the overall structural performance of supertall buildings, this chapter is developed on the basis of increased demand on seismically resilient tall buildings. It details the structural performance and seismic resilience of typical concrete and steel tall buildings designed based on the Chinese and US codes, using the computational models developed in previous chapters and the new generation performance-based design method.

• Hua Huang
• Min Huang
• Wei Zhang
• Boquan Liu

Due to the frequent occurrence of natural disasters and terrorist attacks, research on resisting progressive collapse has attracted increasing attention from the engineering and academic communities. Numerous experimental studies conducted in recent decades have focussed on single-story 3D structures; however, there are many differences between single-story 3D structures and multistory 3D frames due to spatial effects. Therefore, a 1-bay-by-2-bay two-story reinforced concrete (RC) frame with the loss of one edge column was constructed, tested and analysed in this study. The collapse phenomena, load–displacement curves, lateral displacements and axial load distributions were recorded and discussed. Furthermore, the effect of the slab on the progressive collapse resistance of the multistory 3D frame without one bottom edge column was investigated using ABAQUS software. Moreover, a previously presented dynamic resistance model was modified in this article, and the contributions of the beams and slabs to the progressive collapse resistance of the multistory 3D frame were theoretically analysed. The slabs and beams in these frames could contribute approximately 1/3 and 2/3 of the load resistance, respectively.

• Taeo KIM
• Sang Whan Han

Building structures designed according to current seismic design codes should satisfy the seismic performance objectives specified in codes during big earthquake events. ASCE 7-16 specifies that risk category I and II structures should have a probability of collapse less than 10% against the maximum considered earthquake (MCE) shaking hazard. ASCE 7-16 provides four analysis methods to calculate the seismic demands on structures. In this study, 4-, 8-, 12-, and 16-story steel special moment frames (SMFs) are designed using the two most popular elastic analysis methods: the equivalent lateral force (ELF) method and the modal response spectrum analysis (RSA) method. The collapse probabilities of these structures are estimated against MCE shaking hazards according to FEMA P695. It is observed that the collapse probabilities of these structures vary according to analysis methods used for design. To improve the seismic collapse performance of SMFs, a modified method is proposed.

• Deepak Ottar Karattupalayam Palanisamy
• Vignesh Prabu Venkatesan
• Kothai Arjunan
• Visuvasam JosephAntony

The response modification factor (R) in IS1893 [5] is a significant parameter in the analysis of seismic design of structure by means of non-linear response of a structure. This paper enumerates on determining the actual values of R factor for RC special moment resisting frame by performing detailed non-linear static pushover analysis for various 2D framed structures of special moment resisting frames. A parametric study has been carried out by varying the time period up to 4 of the structure and zone factor and the corresponding R values are obtained, which primarily depends on three other factors. From the detailed analysis, a relationship between response reduction factor, Time period, Zone (R μ -T-Z) has been proposed. The results show that the values of R, given in the codes are pessimistic and hence, an optimistic value shall be adopted based on the analytical results. As an application, the values arrived from the proposed relationship is then compared with the values obtained from the structures analyzed.

Seismic performance of a representative single-story confined unreinforced masonry school building in Tehran, Iran is evaluated by means of incremental dynamic analyses according to FEMA P-58 framework. For this purpose, fragility curves are derived for each of the constituent walls of the building. The in-plane behavior of the walls is considered only. Both flexible and rigid diaphragm conditions are investigated. For comparison purposes, the corresponding unconfined building exactly duplicating the considered confined unreinforced masonry building is also studied. In order to analyze the effects of near-source seismic actions on the performance of masonry buildings, two separate far-field and near-field ground motion sets containing 326 records are used. By utilizing the results of the incremental dynamic analyses and the available data of unreinforced masonry school buildings in Tehran, a scenario-based risk assessment of this type of school buildings is performed considering all three adjacent faults for three different earthquake magnitudes. Considerable performance improvement is achieved by providing confinement to the walls which leads to over $100 million reduction in the damage costs for masonry school buildings in Tehran. Also, significant reduction in seismic vulnerability, especially for unconfined masonry buildings, is observed by providing the roof with more rigidity. The findings in this study can be of direct use for disaster management of masonry school buildings in Tehran and similar cities.

Many studies on the strength reduction factor mainly focused on structures with the conventional hysteretic models. However, for the self-centering structure with the typical flag-shaped hysteretic behavior, the corresponding study is limited. The main purpose of this study is to investigate the strength reduction factor of the self-centering structure with flag-shaped hysteretic behavior subjected to near-fault pulse-like ground motions by the time history analysis. For this purpose, the smooth flag-shaped model based on Bouc-Wen model which can show flag-shaped hysteretic behavior is first described. The strength reduction factor spectra of the flag-shaped model are then calculated under 85 near-fault pulse-like ground motions. The influences of the ductility level, vibration period, site condition, hysteretic parameter, and hysteretic model are investigated statistically. For comparison, the strength reduction factors under ordinary ground motions are also analyzed. The results show that the strength reduction factor from near-fault pulse-like ground motions is smaller. Finally, a predictive model is proposed to estimate the strength reduction factor for the self-centering structure with the flag-shaped model under near-fault pulse-like ground motions.

In the design of PCI-girder bridges, the application of various optimum design methodologies can result in significant cost savings and improved structural performance. However, most of the optimisation techniques focus on the individual components and the overall structural system of the superstructure of the bridge system. Limited studies are carried out in the context of longitudinal and transverse configurations of the members in a particular bridge system. This study identifies the optimum span for the PCI-girder expressway bridge system by adopting longitudinal and transverse arrangement of members as design variables while keeping the cross-section of the girder constant. Using an existing case study bridge structure located in Bangkok, selected parametric studies are carried out to achieve cost optimisation. It is observed that the optimum span range for the PCI-girder bridge is in the range of 25 m (82 ft) to 33 m (108 ft).

Any additional loads applied to a damaged structure can aggravate its instability and thus, the impact of successive earthquakes need to be considered. This study proposed a quantitative assessment model for the fragility of a damaged structure subjected to aftershock. Mean period and the strong motion duration were considered as characteristics of earthquake motions. Simulation models of two reinforced concrete structures and one steel structure were selected to examine the applicability of the model. Based on the suggested fragility and residual deformation coefficients, critical earthquake sequences for each structure were identified. The proposed model was efficient in selecting critical earthquake sequences by using the limited number of aftershocks, and these sequences are expected to be useful indicators in the establishment of a retrofit plan according to the predicted structural response and target performance levels.

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Source: https://www.researchgate.net/publication/276026813_Minimum_Design_Loads_for_Buildings_and_Other_Structures_ASCE_Standard_7-05