Current Seismic Design Provisions Research Articles

Numerical assessment of TMS 402/602-22 and CSA S304-14 seismic design provisions for partially grouted reinforced masonry shear walls failing in flexure

Despite the economic benefits of using partially grouted reinforced masonry shear walls (PG-RMSWs) as a seismic force resisting system (SFRS) in low-rise buildings, using such a system in mid- and high-rise buildings is still questionable. This is because the literature lacks comprehensive studies investigating different seismic design provisions adopted for rectangular and flanged PG-RMSWs of aspect ratios greater than 2.0 and failing in flexure. Therefore, this study aims to conduct a comprehensive numerical assessment of the seismic design provisions of the North American masonry standards (i.e., TMS 402/602–22 and CSA S304–14) for designing different categories of rectangular and flanged PG-RMSWs failing in flexure. In this regard, 108 PG-RMSWs were designed and modeled using the Extreme Loading for Structures (ELS) software. Afterward, some changes were proposed to the current seismic design provisions in CSA S304–14 and TMS 402/602–22 for rectangular and flanged PG-RMSWs. In addition, a comparison was made between the seismic design provisions for PG-RMSWs and FG-RMSWs, designed according to each RMSW category, to investigate the expected differences in the behavior based on the applied seismic design provisions. Accordingly, additional 36 FG-RMSWs were designed according to CSA S304–14 and TMS 402/602–22. These additional walls were then modeled on the ELS and compared with their PG counterparts. The results show that both standards can be safely used to design rectangular and flanged PG-RMSWs only when using the two proposed provisions for each standard. For CSA S304–14, the possibility of updating clause 16.8.5.2 is concluded, allowing the use of PG-RMSWs without limitations. New seismic force modification factors were also proposed for TMS 402/602–22 for the design of rectangular and flanged PG-RMSWs. Finally, the comparison between rectangular PG and FG-RMSWs failing in flexure and designed according to CSA S304–14 and TMS 402/602–22 shows that the behavior of PG-RMSWs failing in flexure is comparable to FG-RMSWs under certain circumstances. This study is considered a significant step to prove the potential for more economical and safe usage of PG-RMSWs in mid- and high-rise buildings.

Elastic and inelastic acceleration amplification of nonstructural components characterized using recorded building responses

This paper presents a comprehensive study of the acceleration (or force) amplification behavior of building nonstructural components characterized using a large set of recorded building responses available in the Center for Engineering Strong Motion Data (CESMD) repository. The elastic acceleration amplification demands of nonstructural components are analyzed statistically and subsequently compared with the corresponding inelastic demands characterized by the constant-ductility floor response spectra. Different simplified hysteretic rules are adopted for evaluating the inelastic acceleration responses of nonstructural components. The numerical study demonstrates that the elastic acceleration amplification characteristics of the nonstructural components are highly dependent on their natural periods relative to the specific vibration modes of the supporting structures as well as their attachment locations relative to the building height. The presence of component ductility effectively reduces their seismic acceleration demands, particularly in period ranges tuned with their supporting structures. Furthermore, the hysteretic behavior of nonstructural components may moderately modify their acceleration amplification effects. Findings from this study substantiate current seismic design provisions of nonstructural components with evidence derived from building floor responses recorded during real earthquakes.

Quantification of seismic performance factors for modular corner-supported steel bracing system

Modular construction, using steel corner-supported modular units, is an impressive and environmentally friendly construction technique. It leverages off-site factory-controlled fabrication of volumetric units, including beams, columns, braces, and their intra-connections, as well as their on-site assembly through the use of so-called inter-connections, to form a building as a whole. The seismic detailing of both inter-connections and intra-connections deviates from current seismic design provisions, thereby affecting the seismic behaviour of a modular structure as a whole. Despite their widespread application, seismic design and detailing of corner-supported modular structures mostly relies on the available provisions, adopted for traditional lateral load-bearing systems due to their limited research data available. This paper aims to evaluate the seismic performance factors of modular corner-supported steel bracing systems using a rational nonlinear simulation technique. A nonlinear mathematical model which simulates the stiffness/strength deteriorated and pinched hysteretic response of both intra-connection beam-column elements and braced elements is adopted according to available data in literature, whereas the capacity-protected behaviour is assumed for inter-connections. The modelling technique is implemented into nonlinear mathematical models of corner-supported modular steel buildings with four, eight, and twelve storeys. A parametric study is firstly performed to evaluate the influence of inter-connection stiffness parameters on the nonlinear response of the structures. A set of incremental dynamic analyses is secondly conducted to generate data required to quantify seismic performance factors. The results are finally compared with the seismic performance factors recommended by the ASCE/SEI standard for code-approved connections. It is found that a static overstrength factor is close to 2.5 for the four-storey building whereas for taller buildings lower values than 2.5 is achieved. However, the dynamic overstrength factor showing inelastic force redistribution due to dynamic loading achieves values well above 2.5 regardless of building heights. Further, the response modification coefficient of 9.5 is suggested for braced corner supported modular systems which is well above the value for the equivalent code-specified system.

System overstrength factor induced by interaction between structural reinforced concrete walls, floors and gravity frames: Analytical formulation

In multi-storey structural wall buildings, the structural walls are required to resist additional shear force due to their interactions with the floors and gravity-resisting system, which is not fully catered for in current seismic design provisions and assessment guidelines. This paper scrutinizes the mechanics of the interaction between structural reinforced concrete (RC) structural walls, floors and gravity frames in multi-storey RC structural wall buildings during elastic and nonlinear response phases. It also investigates the implications of this interaction on design of multi-story RC wall buildings. Generic expressions are derived to predict the drift and rotation profiles of multi-storey RC wall buildings. Then, a simple hand calculation method is developed to estimate the system (moment) overstrength of multi-storey RC wall buildings due to system (wall-floor-frames) interaction. The proposed method is applied to a prototype building with different slab dimensions and stiffness, and verified by comparing with the system overstrength factor obtained using finite element analysis. The simplified method estimates, and the nonlinear finite element analyses results agree, that a system overstrength factor of 1.7 can be used to account for the 3D interaction between the structural walls, floors and gravity frames in design and assessment of typical ductile RC wall buildings.

Shake table test program of cold-formed steel in-plane partition walls

Cold-formed steel (CFS) partition walls accounting for a significant portion of the nonstructural inventory have been vulnerable to seismic events. The seismic damage to CFS partition walls may occur at even low-intensity ground shaking leading to negligible structural damage and, in turn, results in significant losses. Regarding this, current seismic design provisions require that CFS partition walls on egress paths should be designed to accommodate interstory drifts of a structure. However, there is little information on the seismic performance of CFS partition walls built at low- and moderate seismicity regions although previous studies emphasize that they could be damaged at even low-intensity ground shaking. In order to evaluate the seismic performance of CFS partition walls constructed in low-and moderate seismicity regions, shake table test with CFS partition walls was carried out in this study. The interior partition wall specimens with different stud spacing were built according to the Korean common construction practices and were planned to suffer in-plane deformations. Physical and measured responses of the CFS partition walls observed during the shake table test demonstrated that decreasing the stud spacing can improve the seismic performance of CFS partition walls subjected to in-plane deformation. From the statistical investigation characterizing the damage states of the CFS partition walls and their correlation with engineering quantities, the construction practice of CFS partition walls built in low- and moderate seismicity regions might not provide acceptable seismic performance for life safety even though the structure installing them was assumed to be design to meet the codified limitation related to its engineering quantity.

Evaluation of floor acceleration demands from the 2017 Mexico City code seismic provisions using a continuous elastic model and records of instrumented buildings

This study presents an evaluation of floor acceleration demands for the design of rigid and flexible acceleration-sensitive nonstructural components in buildings, calculated using the most recent Mexico City seismic design provisions, released in 2017. This evaluation includes two approaches: (1) a simplified continuous elastic model and (2) using recordings from 10 instrumented buildings located in Mexico City. The study found that peak floor elastic acceleration demands imposed on rigid nonstructural components into buildings situated in Mexico City might reach values of 4.8 and 6.4 times the peak ground acceleration at rock and soft sites, respectively. The peak elastic acceleration demands imposed on flexible nonstructural components in all floors, estimated using floor response spectra, might be four times larger than the maximum acceleration of the floor at the point of support of the component for buildings located in rock and soft soil. Comparison of results from the two approaches with the current seismic design provisions revealed that the peak acceleration demands and floor response spectra computed with the current 2017 Mexico City seismic design provisions are, in general, adequate.

Strength resistance factors for seismic design of exposed based plate connections in special steel moment resisting frames

This study presents a critical assessment of the reliability of current base plate connections in steel special moment resisting frames (SMRFs). Using a probabilistic outlook, this research evaluates the reliability of exposed column base (ECB) connections in SMRFs designed based on the current seismic design provisions; it suggests (and implements) a statistical approach to compute resistance factors for three modes of failure (concrete bearing, base plate yielding at tensile interface, and anchor bolt fracture) of ECB connections to achieve a target reliability index, β, of 4.5. Since ECB connections are limited to short buildings, therefore, this study is conducted on two-story and four-story SMRFs which are analyzed using a suite of 120 ground motions originating from strike-slip and reverse faults. ECB connections for the two-story building are designed to simulate pinned connection, while the bases of the four-story building represent moment connections. Detailed methodology for calculating the β of ECB connections is presented considering the three limit states in a moment–axial load interaction curve. Results indicate that the implementation of current seismic provisions results in β ~ 3.3 for non-moment resisting ECB connections for all tried combinations of resistance factors. For moment resisting ECB connections, however, only the designs based on a resistance factor for concrete bearing failure mode less than the current 0.65 result in an acceptable reliability factor of β > 4.5.

Reliability Analysis of Steel SMRF and SCBF Structures Considering the Vertical Component of Near-Fault Ground Motions

AbstractThis paper assesses the effects of the vertical component of ground motions on steel structures and evaluates the current seismic design provisions of ASCE 7 on the basis of a structural re...

Monotonic and cyclic behaviour of wood frame shear walls for mid-height timber buildings

The Chilean forestry industry has a significant presence in the economy of the country. Due to pollution problems and the high seismicity of the region, timber is a suitable material for new buildings. However, because of cultural customs and high demands of regulations, nowadays it is difficult to construct buildings higher than three stories in Chile. However, several international projects have shown that is feasible to construct mid-height timber buildings. Sturdy end studs and strong hold-downs are needed in mid-height wood buildings (up to 6 stories high) to resist large vertical and horizontal loads. However, design parameters provided by current seismic design provisions for those shear walls do not consider the effects of sturdy end studs and strong hold-downs in lateral strength and stiffness of the walls. In order to address this issue, a multidisciplinary team at the Catholic University of Chile has conducted an extensive experimental and numerical research program. This paper presents the results of seventeen in-plane monotonic and cyclic shear tests in wood frame shear walls of different lengths (1200, 2400 and 3600 mm) and 2470 mm height. The walls have five 2 × 6″ end studs, strong hold-down anchorages and standard 11.1 mm structural OSB panels (1200 × 2400 mm) on both faces of the wall and with nails in the edge of the OSB panels spaced at 50 or 100 mm. The main objectives of this research are to evaluate the seismic response of these shear walls and to assess the current code expressions applied to shear walls with sturdy end studs to be used in mid-height timber buildings. The results show that, while cyclic loads reduce the monotonic shear strength of walls, cyclic loads do not influence the ultimate displacement and stiffness. The main benefits of a smaller nail spacing are the increase of the strength and delay of stiffness degradation. The unit shear was influenced by wall length: 1200 mm walls presented a better unit shear capacity than 2400 and 3600 walls, and there were not observable differences between 2400 and 3600 mm walls. The characteristic damping of the walls varied between 7 and 10%. Finally, the current design provisions underestimate the shear strength and overestimate the stiffness of walls to be used in mid-height timber buildings.

Steel Structures Research Update:Seismic Performance and Design of Steel Panel Dampers for Steel Moment Frames

Ongoing work on the seismic performance and design of steel panel dampers for steel moment frames is highlighted. Dr. Keh-Chyuan Tsai, professor in the Department of Civil Engineering at National Taiwan University, leads the team from National Taiwan University and the National Center for Research on Earthquake Engineering (NCREE) in Taipei. In 2018, At NCREE, one recent collaboration with the University of Washington included cyclic tests of a three-story chevron special concentrically braced frame (SCBF). Current seismic design provisions require large beam sizes to resist the unbalanced forces from the chevron braces after brace buckling. The research has explored options for alternative ductile mechanisms and reduced beam sizes. Steel research at NCREE has also included studies on steel beam-to-box-column moment connections and electro-slag-welded (ESW) joints in those connections. Meanwhile, research into premature fracture of ESW diaphragm-to-column joints in beam-to-box-column connections has revealed sensitivity to eccentricity in loading between the beam flange and the diaphragm. Further, a micromechanical-based stress-modified critical stress (SMCS) model has been developed and shown to be capable of predicting the crack initiation of the diaphragm-to-column ESW joint (Li et al., 2018).

Overestimation of liquefaction hazard in areas of low to moderate seismicity due to improper characterization of probabilistic seismic loading

This study evaluates the impact of characterizing probabilistic ground motions for liquefaction hazard analysis (defined and quantified in this paper as liquefaction triggering and free-field post-liquefaction settlements) in areas of low to moderate seismicity. Both pseudo-probabilistic and probabilistic (i.e., performance-based) methods are assessed and compared. Results of the comparative study suggest that pseudo-probabilistic methods can significantly overestimate liquefaction hazards in areas of low seismicity. Performance-based probabilistic methods are shown to predict between 5.2 cm and 16.5 cm (approximately 36–47%) less post-liquefaction free-field settlement on average than pseudo-probabilistic methods in areas of low seismicity at a return period of 2475 years, and to predict 9 cm to 19.7cm (approximately 96%) less post-liquefaction free-field settlement on average than pseudo-probabilistic methods at a return period of 475 years. Soil site classification is shown to have substantial impact on the estimated liquefaction hazards in areas of low seismicity due to soil amplification, potentially increasing design accelerations by up to 56%. Consequences of inconsistencies regarding design ground motions in current seismic design provisions are also discussed. To avoid potential for overpredicting liquefaction hazards, engineers should apply a performance-based approach when assessing liquefaction triggering and its effects in areas of low to moderate seismicity.

Computationally effective and accurate simulation of cyclic behaviour of old reinforced concrete columns

To evaluate the seismic performance of existing old reinforced concrete (RC) buildings, it is important to use an accurate analytical model. Most columns in these buildings do not satisfy the reinforcement details specified by current seismic design provisions. Columns that possess insufficient reinforcement details can exhibit significant pinching and cyclic deterioration in their strength and stiffness. They can also experience flexure-shear and shear failure during large earthquakes. The objective of this study is to simulate the cyclic behaviour of old RC columns accurately and efficiently with the Pinching4 model considering pinching and cyclic deterioration. Modelling parameters are calibrated based on the test results of 40 collected flexure-shear and shear critical columns. Columns failed by other failure modes rather than shear and flexure-shear failure are not considered in this study. Forward stepwise regression analyses are conducted to determine statistically significant variables from 34 candidate predictor variables and to propose the empirical equations of the modelling parameters. It is shown that the Pinching4 model with the proposed empirical equations accurately simulates the cyclic behaviour of both flexure-shear and shear-critical columns including pinching and cyclic deterioration in strength and stiffness.

Assessment of Code-specified Ground Motion Selection Criteria with Accurate Selection and Scaling Methods – I Ground Motion Selection

Current seismic design provisions such as ASCE 7-10 provide criteria for selecting ground motions for conducting response history analysis. This study is the sequel of a companion paper (I – Ground Motion Selection) for assessment of the ASCE 7-10 criteria. To assess of the ASCE 7-10 criteria, nonlinear response history analyses of twelve single degree of freedom (SDF) systems and one multi-degree of freedom (MDF) system are conducted in this study. The results show that the target seismic demands for SDF can be predicted using the mean seismic demands over seven and ten ground motions selected according to the proposed method within an error of 30% and 20%, respectively

Confinement of ultra-high-performance fiber reinforced concrete columns

This study investigates the axial load response of ultra-high-performance fiber-reinforced concrete (UHPFRC) columns with compressive strengths of 163 and 181MPa (design strengths of 150 and 180MPa). The UHPFRC used in this study had 1.5% of hybrid micro-steel fibers (1.0% of 19.5mm fibers and 0.5% of 16.3mm fibers) and did not contain coarse aggregate. A total of nine UHPFRC columns confined by transverse reinforcement with volumetric ratios of 0.9–9.9% and two different configurations (Types A and C) were tested under pure axial load to investigate the influence of these variables. The overall behavior of the UHPFRC columns was compared with the response of similar strength ultra-high-strength concrete (UHSC) columns having coarse aggregate. Test results showed a pronounced effect of the volumetric ratio of the transverse reinforcement on the confinement. Hybrid micro-steel fibers controlled brittle cover spalling very well and assisted the transverse confinement reinforcement after the peak load. Applicability of the confinement reinforcement equations in the current seismic design provisions for developing ductile behavior of the UHPFRC columns was investigated. The analytical study examined the ability of the existing high-strength concrete (HSC) confinement models for predicting the axial load response of the UHPFRC columns, and a prediction model that accounts for the effects of steel fibers and the stress-strain relationship of UHPFRC is proposed.

Assessment of ground motion selection criteria specified in current seismic provisions with an accurate selection algorithm

It is important to select proper ground motions for accurately estimating seismic demands using linear and nonlinear response history analyses (RHA). Current seismic design provisions such as ASCE 7–10 provide criteria for selecting ground motions. In this study, an accurate and computationally efficient algorithm for selecting and scaling ground motions is proposed, which satisfies the ASCE 7–10 criteria. The ASCE 7–10 ground motion selection criteria with the proposed algorithm is assessed using the results of nonlinear response history analyses of twelve single degree of freedom (SDF) systems and one multi-degree of freedom (MDF) system. In order to investigate the effect of the numbers of selected ground motions, period ranges for spectral matching, library sizes, and structural periods on selection results, parametric studies are conducted based on the results of nonlinear RHA. This study shows that the target seismic demands for all model SDF and MDF systems can be predicted using the mean seismic demands over seven and ten ground motions selected according to the proposed method within an error of 30 and 20%, respectively.

Buildings with Rigid Walls and Flexible Roof Diaphragms. II: Evaluation of a New Seismic Design Approach Based on Distributed Diaphragm Yielding

AbstractA seismic collapse evaluation study of buildings with rigid walls and flexible roof diaphragms (RWFD) presented in the companion paper indicates that this type of structure, as designed to current seismic design provisions in the United States, does not satisfy FEMA P695 performance criteria for Risk Category II Buildings. The seismic performance of RWFD buildings is often characterized by large deformations and yielding in the roof diaphragm rather than in the vertical elements of the seismic force–resisting system (SFRS). In this paper, a new seismic design approach is proposed to account for flexible roof diaphragm response. The proposed approach relies on distributed yielding in the roof diaphragm as the predominant inelastic response under extreme ground shaking. This is obtained by strengthening the end diaphragm regions, thereby allowing yielding to spread deeper into the diaphragm. The basic steps of the design approach and its limitations are described. A validation study of the proposed ...

Direct Displacement-Based Seismic Design of Eccentrically Braced Steel Frames

A series of eccentrically braced frames (EBF) are designed and subjected to nonlinear analyses to highlight ambiguities and differences in current seismic design provisions for EBF structures. This provides motivation to implement better guidance for the checking of local displacement demand considerations and move towards a displacement-based design approach. A recently proposed direct displacement-based design (DDBD) procedure for EBFs is then described and further developed in this article through the calibration of a spectral displacement reduction factors that relate the displacement of an inelastically responding structure to that of the equivalent linear representation used in the DDBD of EBFs. Such an expression is calibrated as part of this study using an experimentally validated numerical model also proposed here for the EBF links such that the actual hysteretic behavior of the links is well represented. The DDBD guidelines are applied to EBF systems from 1–15 stories in height and their performance is verified via nonlinear dynamic analyses using two different sets of design spectrum compatible ground motions. The results of the study indicate the robustness of the proposed DDBD method in limiting the interstory drifts to design limits for a variety of EBF systems with short links, thus demonstrating that the proposed DDBD method is an effective tool for seismic design of EBFs.

Seismic Performance Evaluation of Intermediate Moment Frames with Reduced Beam Section and Bolted Web Connections

A reduced beam section with a bolted web (RBS-B) connection is permitted for use only in intermediate moment frames (IMF) according to the ANSI/AISC 358-05. This is because some RBS-B test specimens failed to achieve 4% total rotation capacity, which is the minimum story drift angle required for special moment frames (SMF). Several studies reported that some RBS-B connections could experience brittle connection fracture during earthquakes, which can also be detrimental to the seismic performance of IMF systems with RBS-B connections. For investigating whether IMFs with RBS-B connections provide a satisfactory seismic performance, this study evaluated the seismic performance of IMFs with pre-qualified RBS-B connections following the ATC-63 procedure. Twenty-four model buildings were designed according to current seismic design provisions. Several IMFs with RBS-B connections do not satisfy the acceptance criteria specified in ATC-63.

Reliability assessment of compression columns in seismic EBFs

In recent years, eccentrically braced frames (EBFs) have widely been used in steel structures in different countries. These systems are known to provide relatively high ductility, strength, and stiffness against seismic loads and to offer architectural flexibility by giving large openings. Seismic performance of EBFs is subjected to statistical uncertainties of both input excitations and performance of the system. This study aims at examining the current seismic design provisions and the reliability level of compression columns in EBFs with short shear links. An analytical model of minimally designed EBFs has been developed which is based on the LRFD design approach. For reliability analysis, analytical equations have been derived. The studies show that the current seismic provisions result in a less safe design relative to the target values for a wide range of the EBF columns. To cover this, an empirical equation is proposed for resistance factor of such columns. The study also includes a sensitivity analysis for five stochastic parameters that are presented and discussed.

HSS-to-HSS seismic moment connection performance and design

Abstract Hollow structural sections (HSS) provide an alternative to typical wide flange sections in low-rise seismic moment resisting frames. Their beneficial strength-to-weight ratio as well as bending, compression, and torsional resistance increases the versatility of moment frames and potentially improves performance under earthquake loads. With an understanding of current design requirements for seismic moment connections and static HSS-to-HSS connections, 39 fully welded unreinforced HSS-to-HSS connections are explored through finite element analyses. However, performance is limited due to column face plastification and inability to develop the plastic moment capacity of the beam. To rectify this problem, 24 through plate and 24 external diaphragm plate HSS-to-HSS moment connections are analyzed. Several important geometric parameters, such as the beam width–column width ratio, beam thickness–column thickness ratio, plate length, and plate thickness are considered to understand their effect on the connection moment capacity and sources of inelastic rotation. The through plate and external diaphragm reinforcement greatly improve the connection behavior moving yielding away from the column face and into the beam member. The results from the reinforced connections are used in combination with current seismic design provisions to develop a design procedure that optimizes the performance of these connections. The reinforced connections can be detailed to develop plastic hinging in the HSS beam member while minimizing the likelihood of a non-ductile weld failure.