Current Seismic Design Code Research Articles

Effective Assessment of Inelastic Torsional Deformation of Plan-Asymmetric Shear Wall Systems

Torsional deformation may occur in plan-asymmetric wall structures during seismic events. In most current seismic design codes, the torsional deformation is handled using the concept of design eccentricity, in which design loads may be excessively amplified. This approach has limitations in accurately estimating the torsional deformation of the plan-asymmetric structures, mainly because it is based on linear elastic material behavior. In this paper, we propose a simple method that can accurately evaluate the inelastic lateral displacement and rotation of plan-asymmetric wall structures and is suitable for use in the displacement-based design method. The effectiveness of the proposed method is verified by comparing its predictions with the results of rigorous time history analyses for a model problem. The comparison shows that the proposed method is able to provide accurate estimates of the inelastic torsional deformation for the plan-asymmetric wall system, while requiring less computational cost than the time history analysis.

Seismic Retrofit of Steel Truss Bridge Using Buckling Restrained Damper

Buckling Restrained Bracings (BRBs) are widely used to improve the seismic behavior of buildings. They are employed for bridges as well, but their application in this respect is limited. BRBs can also be used as a function of the individual damper rather than the structural component or the bracing, in which case the device may be called a Buckling Restrained Damper (BRD). Yet, such application has not been explored much. There are quite a few bridges designed according to the old design codes in Japan. Their seismic resistance may not be satisfactory for the current seismic design codes. Against this background, the behavior of a steel truss bridge under a large seismic load was investigated by nonlinear dynamic finite element analysis. Some members were indeed found to be damaged in the earthquake. Retrofitting is needed. To this end, the application of BRD was tried in the present study: a parametric study on the seismic behavior of the truss bridge with BRD was conducted by changing the length, the cross-sectional area, the location and the inclination of BRD. The effectiveness of BRD was then discussed based on the numerical results thus obtained. In all the analyses, ABAQUS was used.

Experimental and numerical study on seismic performance of deficient interior RC joints retrofitted with prestressed high-strength steel strips

This paper presents the results of an experimental and numerical study on strapping method for retrofitting deficient interior RC beam-column joints, which do not meet the requirements of current seismic design codes due to some deficiencies including low strength of concrete, inadequate transverse reinforcements, and low construction quality. In order to explore the seismic behavior of the retrofitted RC joints, six interior joint specimens, including two deficient RC joints with inadequate transverse rebar, three RC joints retrofitted with prestressed high-strength steel strips and one RC joint retrofitted with adhesively-bonded steel and prestressed high-strength steel strips, were designed and tested under cyclic loading. The main variables included the retrofit methods, axial compression ratio and volume of steel strips. The test results indicated that the prestressed steel strips worked together well with the deficient RC joints, and the retrofitted specimens exhibited better seismic behavior than the deficient control specimens, especially in deformability and energy dissipation. On the basis of the experimental study, the corresponding numerical model was established using the Open System for Earthquake Engineering Simulation (OpenSEES) and then verified to be valid. Finally, the modified softened strut and tie model was proposed in this paper to calculate the shear capacities of the specimens, and the calculated results agreed well with the test results.

Development of Model Parameter Prediction Equations for Simulating Load-deformation Response of Non-ductile RC Columns

Many reinforced concrete (RC) buildings constructed prior to 1980's lack important features guaranteeing ductile response under earthquake excitation. Structural components in such buildings, especially columns, do not satisfy the reinforcement details demanded by current seismic design codes. Columns with deficient reinforcement details may suffer significant damage when subjected to cyclic lateral loads. They can also experience rapid lateral strength degradation induced by shear failure. The objective of this study is to accurately simulate the load-deformation response of RC columns experiencing shear failure. In order to do so, model parameters are calibrated to the load-deformation response of 40 RC column specimens failed in shear. Multivariate stepwise regression analyses are conducted to develop the relationship between the model parameters and physical parameters of RC column specimens. It is shown that the proposed predictive equations successfully estimated the model parameters of RC column specimens with great accuracy. The proposed equations also showed better accuracy than the existing ones.

Effect of design spectral shape on inelastic response of RC frames subjected to spectrum matched ground motions

In current seismic design codes, various elastic design acceleration spectra are defined considering different seismological and soil characteristics and are widely used tool for calculation of seismic loads acting on structures. Response spectrum analyses directly use the elastic design acceleration spectra whereas time history analyses use acceleration records of earthquakes whose acceleration spectra fit the design spectra of seismic codes. Due to the fact that obtaining coherent structural response quantities with the seismic design code considerations is a desired circumstance in dynamic analyses, the response spectra of earthquake records used in time history analyses had better fit to the design acceleration spectra of seismic codes. This paper evaluates structural response distributions of multi-story reinforced concrete frames obtained from nonlinear time history analyses which are performed by using the scaled earthquake records compatible with various elastic design spectra. Time domain scaling procedure is used while processing the response spectrum of real accelerograms to fit the design acceleration spectra. The elastic acceleration design spectra of Turkish Seismic Design Code 2007, Uniform Building Code 1997 and Eurocode 8 are considered as target spectra in the scaling procedure. Soil classes in different seismic codes are appropriately matched up with each other according to VS30 values. The maximum roof displacements and the total base shears of considered frame structures are determined from nonlinear time history analyses using the scaled earthquake records and the results are presented by graphs and tables. Coherent structural response quantities reflecting the influence of elastic design spectra of various seismic codes are obtained.

Seismic response of RC frames under far-field mainshock and near-fault aftershock sequences

Engineered structures built in seismic-prone areas are affected by aftershocks in addition to mainshocks. Although aftershocks generally are lower in magnitude than that of the mainshocks, some aftershocks may have higher intensities; thus, structures should be able to withstand the effect of strong aftershocks as well. This seismic scenario arises for far-field mainshock along with near-field aftershocks. In this study, four 2D reinforced concrete (RC) frames with different numbers of stories were designed in accordance with the current Iranian seismic design code. As a way to evaluate the seismic response of the case-study RC frames, the inter-story drift ratio (IDR) demand, the residual inter-story drift ratio (RIDR) demand, the Park-Ang damage index, and the period elongation ratio can be useful engineering demand parameters for evaluating their seismic performance under mainshock-aftershock sequences. The frame models were analyzed under a set of far-field mainshock, near-fault aftershocks seismic sequences using nonlinear dynamic time-history analysis to investigate the relationship among IDR, RIDR, Park-Ang damage index and period ratio experienced by the frames. The results indicate that the growth of IDR, RIDR, Park-Ang damage index, and period ratio in high-rise and short structures under near-fault aftershocks were significant. It is evident that engineers should consider the effects of near-fault aftershocks on damaged frames that experience far-field mainshocks as well.

Influence of Rotating Strong Ground Motions on the Response of Doubly Symmetrical RC Wall Structures in Romania and Its Implication on Code Provisions

This research focuses on the evaluation of the seismic response of three doubly symmetrical RC wall structures having three height regimes and subjected to a series of bi-directional ground motion recordings. The recorded strong ground motions are rotated in a clockwise direction and the effects of this procedure on several engineering response measures (e.g., wall-bending moments, wall shear forces, and storey drifts) are evaluated. The three doubly symmetrical RC wall structures are designed according to the current Romanian seismic design code for a peak ground acceleration of 0.25 g. The strong ground motions used in this study consist of five pairs of recordings from the Vrancea intermediate-depth earthquakes of 1977, 1986, and 1990 and from the Erzincan and Aigion earthquakes in Turkey and Greece. The investigations show differences between the response measures as a function of the orientation of the strong ground motion horizontal components and significant differences between the responses of each individual structure, as well. Thus, the maximum response obtained from the rotation procedure can be regarded as dependent on both the ground motion characteristics and on the ground motion recording, as well.

Monotonic and cyclic response of bolted connections with welded cover plate for modular steel construction

Attributable to the superiority in construction speed and quality, modular steel constructions (MSCs) have been increasingly used for mid-to-high rise buildings with repetitive units. An architecturally pleasing bolted connection with welded cover plate was proposed in this paper to settle the current research limit. Tests on seven full-scaled tee-shape connections made of hollow structural section (HSS) members were conducted under monotonic and cyclic load. The results indicated that all the specimens under monotonic load were capable of developing the full plastic strength of the twin-beams. The twin-beams were found to have individual bending behavior. The current seismic design code GB 50011-2010 and AISC 341-10 were used to evaluate the seismic performance and verify the applicability for seismic design of the proposed connection, demonstrating the satisfactory deformation capacity and ductility. Furthermore, EC3 Part 1–8 was adopted to evaluate the stiffness characteristic of the proposed connection, indicating that the proposed connection can be classified as “semi-rigid” connection. In addition, the design consideration was recommended to predict the bending capacity of the proposed connection. The presented research work will provide useful references for expanding the application of MSCs.

Seismic experimental study on a concrete pylon from a typical medium span cable-stayed bridge

According to the current seismic design codes of bridges in China, cable-stayed bridges have been usually required to remain elastic even subjected to strong earthquakes. However, the possibilities of pylon plastic behavior were revealed in recent earthquake damages. The lack of due diligence in the nonlinear seismic behavior of the pylon has caused a blurry understanding about the seismic performance of such widely built though less strong earthquake experienced structures. In light of this point, a 1/20 scaled concrete pylon model which from a typical medium span cable-stayed bridge was designed and tested on the shaking table longitudinally. The dynamic response and seismic behavior of the pylon were measured, evaluated and compared to reveal its vulnerable parts and nonlinear seismic performance. The results show that most parts of the concrete pylon remain elastic even under very strong excitations, which means a sufficient safety margin for current pylon longitudinal design. The most vulnerable parts of the pylon appeared first at the pylon bottom region, cracks opening and closing at the pylon bottom were observed during the test, and then extended to the lower column and middle column around the lower strut.

From Design to Earthquake Loss Assessment of Steel Moment-Resisting Frames

Steel moment-resisting frames (MRFs) are well known for their ductile and stable hysteretic behaviour. For this reason, they are an attractive and effective structural system for seismic resistance. Current seismic design codes, namely Eurocode 8, provide system performance factors that should be used in the seismic design under different ductility classes. However, recent research studies have shown that the use of the code-prescribed performance factors lead to stiffer and heavier structural solutions that are not consistent with the performance-based design assumptions. A new methodology, Improved Force-Based Design (IFBD), has recently been proposed with the aim of a more rational determination of the adopted value of the behaviour factor, q, instead of using the upper bound reference values provided by the design code. This paper investigates if the obtained values of q for both EC8 and IFBD concerning steel MRFs are not only adequate, but also provide sufficient margins against collapse under maximum considered earthquake (MCE) ground motions. To this end, the methodology proposed in FEMA P695 was used. Additionally, the expected direct economic seismic losses are computed according to the PEER-PBEE methodology.

Fragility curves for steel-concrete hybrid tall buildings

Fragility curves of steel-concrete hybrid tall buildings designed according to the current Chinese seismic design code were derived by analytical methods, taking into account the uncertainty of earthquake ground motions. In total, 45 analytical models were analysed considering all combinations of three design parameters, i.e., seismic protection intensity, site soil type and design group. On the basis of a large number of nonlinear time history analyses, fragility curves were derived reflecting exceeding probabilities corresponding to each performance level. The influence of different engineering demand parameters (EDPs) and design parameters on fragility curves was analysed. The result showed that fragility curves based on different EDPs varied significantly, indicating that different structural components played different roles in seismic performance of the structure. While seismic protection intensity had little impact on seismic vulnerability, structures tended to be more vulnerable on the site with softer site soils and longer characteristic periods.

Seismic Behavior of Turbine-Generator Foundation under Strong Earthquake Action in Different Directions

In order to study seismic behavior of half-speed turbine-generator foundation under horizontal earthquake loading in different directions, the 1/10 scaled model was designed and fabricated. The rigid foundation of half-speed turbine-generator sets can be seen as a complex space frame system. The tests were conducted under eight earthquake waves in two directions separately. The loading directions were along the axis of longitudinal and transverse. The seismic response of displacement and story drift was investigated by a pseudodynamic test. The hysteresis behavior and crack propagation were analyzed. From the research, it is shown that the maximum displacement of the foundation under the earthquake of intensity 7 is 15.20 mm (longitudinal), basically in the range of elastic deformation. The seismic response of earthquake input in different directions is obviously different. Under the same earthquake input, the seismic displacement along the axis of longitudinal is larger than that of transverse. Under the rarely earthquake of intensity 8, the foundation still keeps good working condition. The maximum elastic-plastic story drift is 1/191 under the limit value 1/50 provided in the Code for Seismic Design of Buildings. The deformation capacity of the structure meets the requirements of the current seismic design code of China.

Numerical Analysis of RC Column Failure Due to Blast and Collapse Scenarios for an Irregular RC-Framed Structure

This paper focuses on the numerical analysis of collapse due to blast action for a reinforced concrete (RC) framed building. The analyses are conducted using the applied element method. This numerical method is able to model accurately all the structural behaviour stages leading up to the collapse itself. In the first part of the study, the numerical method is validated through the use of experimental data available in the literature. Subsequently, the collapse resistance of an asymmetric RC-framed structure, designed according to the prescriptions from the current Romanian seismic design code, is evaluated by employing several column removal scenarios and even a demolition scenario. The results show that due to the seismic design, the structure is able to resist the loads occurring in the scenarios in which one or two columns are considered as failed. The demolition scenario is introduced to demonstrate the utility of the applied element method to observe the load redistribution mechanism in the case of failure of several columns.

Evaluation and prediction of cyclic response of Q690D steel

High-strength structural steel of nominal yield strength greater than 690 MPa is difficult to produce in the hot-rolled condition without heat treatment. Quenching and tempering are thus commonly used to trade ductility for high strength. However, in seismic regions, good ductility of steel structural members is required to endure expected inelastic deformation under severe earthquake actions. This paper describes an experimental investigation on the mechanical properties and hysteretic behaviour of Q690D steel through 29 tensile coupon tests and 14 cyclic loading coupon tests subjected to four different cyclic patterns. The 16 mm and 40 mm quenched and tempered Q690D steel plates used in this study are equivalent to S690Q steel according to EN 10025-6. The test results of Q690D steel were compared with the ductility requirements of current seismic design codes. Under cyclic loading, Q690D steel showed cyclic softening. Based on analysis of the test results, a combined isotropic and kinematic hardening hysteretic model was developed to predict the cyclic behaviour of Q690D steel, with consideration of the Bauschinger effect and cyclic softening behaviour.

Seismic Performance of Exterior RC Beam\u2013Column Joints Retrofitted using Various Retrofit Solutions

Beam–column joints in existing concrete buildings might not satisfy the design requirements for seismic reinforcement details specified in current seismic design codes. Thus, in this study, various retrofit solutions for existing exterior beam–column joints with non-seismic details were developed: head re-bars anchoring, carbon fiber reinforced polymer (CFRP) wrapping, haunch retrofit element, and steel jacketing with various shapes and sizes. To investigate the seismic performance of exterior joints strengthened with the proposed retrofit solutions, seven half-scale exterior reinforced concrete beam–column joints including one control specimen and six retrofitted specimens were fabricated and tested under cyclic loading simulating earthquake loading. The test results showed that the proposed retrofit solutions could partially enhance the seismic capacity of the beam–column joints: steel jackets could increase deformation and load-carrying capacities; steel haunch elements could increase the load-carrying capacity, stiffness, and dissipated energy; and head re-bar anchoring and CFRP wrapping did not significantly effect on the seismic capacity of the beam–column joints.

Hysteretic Behaviour of a Piston Based Self-centering (PBSC) Bracing System Made of Superelastic SMA Bars – A Feasibility Study

Buildings designed and constructed using current seismic design codes are expected to experience permanent deformation during high seismic events. Nonlinear material properties generate high hysteretic damping forces and reduce stiffness, which are often necessary to reduce seismic load effects on regular structures. Hence, traditional structural systems experience high residual deformation after an earthquake. In order to resolve this issue, smart structural systems have been proposed by many researchers. One such system is the proposed novel piston-based self-centering bracing (PBSC) system. This study investigates the feasibility of such a system and its cyclic behaviour in finite element environment and predicts its load-deformation hysteresis during seismic events. This newly developed bracing system utilizes superelastic shape memory alloy (SMA) bars inside a sleeve-piston assembly for its self-centering mechanism. During cyclic tension-compression loading, the bars are pulled from opposite directions in order to avoid compressive loading on the bars. The energy dissipation is achieved through nonlinear load deformation hysteresis. Furthermore, the PBSC bracing system is designed to be fully buckling restrained. Several solid element based brace models were built in ABAQUS software and a quasi-static cyclic axial loading was applied to determine their hysteretic response. From the analysis, the resultant load-deformation behaviour shows flag shaped hysteresis with negligible residual deformation.

Comparative Assessment of Two Rocking Isolation Techniques for a Motorway Overpass Bridge

Rocking isolation of structures is evolving as an alternative design concept in earthquake engineering. The present paper investigates the seismic performance of an actual overpass bridge of the Attiki Odos motorway (Athens, Greece), employing two different concepts of rocking isolation: (a) rocking of the piers on the foundation (rocking piers); and (b) rocking of the pier and foundation assembly (rocking footings) on the soil. The examined bridge is an asymmetric 5-span system having a continuous deck and founded on surface foundations on a deep clay layer. The seismic performance of the two rocking isolated bridges is comparatively assessed to the existing bridge, which is conventionally designed according to current seismic design codes. To that end, 3D numerical models of the bridge–foundation–abutment–soil system are developed, and both static pushover and nonlinear dynamic time history analyses are performed. For the latter, an ensemble of 20 records (10 ground motions of 2 perpendicular components each) that exceed the design level are selected. The conventional system collapses in 5/10 of the (intentionally severe) examined seismic excitations. The rocking piers design alternative survives in 8/10 of the cases examined, with negligible residual deformations. The safety margins of the rocking footings design concept are even larger, as it survives in all cases examined. Both rocking isolation concepts are proven to offer increased levels of seismic resilience, reducing the probability of collapse and the degree of structural damage. Nevertheless, in the rocking piers design alternative high stress concentrations at the rotation pole (pier base) are developed, indicating the need for a special design of the pier ends. This is not the case for the rocking footings concept, which however is subject to increased residual settlements but no residual rotations.

EFFECT OF USING HIGHLY CONFINED SHORT LAP-SPLICE REINFORCEMENT ON SEISMIC PERFORMANCE OF EXTERIOR BEAM-COLUMN JOINT

The present paper aims to examine the behavior of exterior beam-column joints with different reinforcement details of their columns: continuous and lap splice longitudinal bars were adopted as design parameter. The experimental program was carried out at the Concrete Research Laboratory at Housing and Building Research Center in Egypt; included three exterior reinforced beam column joints that were tested under the effect of quasi-static cyclic loading. One sample was designed according to the provisions of the current seismic design codes and served as a control sample. Columns of the other two samples were detailed with well confined shorter lap spliced longitudinal reinforcement. The overall behavior of the beam-column joints and conclusions are discussed in this paper.

Site characterization of the strong motion stations and evaluation of site effects in the Ankara region during the December 2007 and March 2008 moderate Bala earthquakes

Local site conditions may significantly affect the amplitude and frequency of ground motion during earthquakes. Recordings of recent major earthquakes have demonstrated that soil conditions can generate prominent amplification of ground shaking and can be compared with the amplification predicted by numerical simulation. This study analyzes the ground motion data from the December 2007 and March 2008 moderate Bala earthquake series considering the effect of soil conditions on ground shaking in the southern part of Ankara, the capital city of Turkey. Initially, shear wave velocity profiles of the strong ground motion stations were evaluated to define site classifications for each station in Ankara and its surroundings. Strong motion data collected during the Bala earthquake series (Mw range 5.6–4.8) were used to develop event-specific attenuation relationships for peak ground acceleration and spectral acceleration at various periods and different site conditions. Site amplification factors were derived from the regression results from the event-specific attenuation relationships for the Bala earthquake series. The implications of these ground motion amplification factors were also discussed and compared using the current seismic design codes. Finally, the response spectra from the recording stations were used to determine site amplification factors for different site conditions to evaluate the variability of the site response in localized regions in Ankara.

Site Classification and Design Response Spectra for Seismic Code Provisions - (I) Database and Site Response Analyses

【Korea is part of a region of low to moderate seismicity located inside the Eurasian plate with bedrock located at depths less than 30 m. However, the spectral acceleration obtained from site response analyses based on the geologic conditions of inland areas of the Korean peninsula are significantly different from the current Korean seismic code. Therefore, suitable site classification scheme and design response spectra based on local site conditions in the Korean peninsula are required to produce reliable estimates of earthquake ground motion. In this study, site-specific response analyses were performed at more than 300 sites with at least 100 sites at each site categories of $S_C$ , $S_D$ , and $S_E$ as defined in the current seismic code in Korea. The process of creating a huge database of input parameters - such as shear wave velocity profiles, normalized shear modulus reduction curves, damping curves, and input earthquake motions - for site response analyses were described. The response spectra and site coefficients obtained from site response analyses were compared with those proposed for the site categories in the current code. Problems with the current seismic design code were subsequently discussed, and the development and verifications of new site classification system and corresponding design response spectra are detailed in companion papers (II-development of new site categories and design response spectra and III-Verifications)】