National Program to Improve Seismic Performance of Steel Frame Buildings

This paper reviews the performance of welded steel moment frame buildings during the Northridge earthquake. Some of the studies being undertaken in the United States as part of the FEMA-funded SAC Steel Project are described. The intent of these studies is to devise improved methods for designing new steel frame structures; for inspecting, evaluating, and repairing seismic damage to these types of structures following a major earthquake; and for inspecting, evaluating, and retrofitting existing at-risk steel frame buildings. General observations resulting from these studies are highlighted and the overall format for the new design provisions is presented.     

钢框架刚性连接的抗震性能分析

通常认为钢框架具有良好的抗震性能,但在最近的几次大地震中许多高层钢结构房屋的梁柱刚性连接节点受到严重破坏.这说明传统的刚性连接框架在某些方面存在不足之处.文章对刚性连接框架的形式以及抗震性能进行了深入的分析和总结,并在设计、构造、及施工三方面对提高刚性连接框架的抗震性能提出一些意见和建议.     

Causes of Collapse and Damage to Low-Rise RC Buildings in Recent Turkish Earthquakes

This study assesses performance objectives defined in the Turkish Earthquake Code (TEC) in order to make a realistic evaluation related to heavy damage and collapse reasons of reinforced concrete (RC) buildings that experienced severe earthquakes in Turkey. A series of three-dimensional RC buildings with different characteristics and representing low-rise structures damaged and collapsed in the earthquake areas is designed according to Turkish codes (Turkish Design Standards and Turkish Earthquake Code). Pushover analyses are carried out to determine nonlinear behavior of the buildings under earthquake loads. Building performances are determined by using the displacement coefficients method, which is a commonly used nonlinear static evaluation procedure for different seismic hazard levels defined in the TEC. The stipulated performance objectives in the TEC are checked in terms of plastic rotations and maximum story drift. From the results of this research, it can be concluded that low-rise RC buildings designed according to Turkish codes sufficiently provide for the performance objectives stipulated in the TEC. Reasons for the heavy damages and collapses of RC buildings during severe earthquakes are explained by commonly occurring themes (i.e., project errors, poor quality of construction, modifications of buildings, etc.).     

Responses of Buried Corrugated Metal Pipes to Earthquakes

This study describes the results of field investigations and analyses carried out on 61 corrugated metal pipes (CMP) that were shaken by the 1994 Northridge earthquake. These CMPs, which include 29 small-diameter (below 107 cm) CMPs and 32 large-diameter (above 107 cm) CMPs, are located within a 10 km2 area encompassing the Van Norman Complex in the Northern San Fernando Valley, in Los Angeles, California. During the Northridge earthquake, ground movements were extensively recorded within the study area. Twenty-eight of the small-diameter CMPs performed well while the 32 large-diameter CMPs underwent performances ranging from no damage to complete collapse. The main cause of damage to the large-diameter CMPs was found to be the large ground strains. Based on this unprecedented data set, the factors controlling the seismic performance of the 32 large-diameter CMPs were identified and framed into a pseudostatic analysis method for evaluating the response of large diameter flexible underground pipes subjected to ground strain. The proposed analysis, which is applicable to transient and permanent strains, is capable of describing the observed performance of large-diameter CMPs during the 1994 Northridge earthquake. It indicates that peak ground velocity is a more reliable parameter for analyzing pipe damage than is peak ground acceleration. Results of this field investigation and analysis are useful for the seismic design and strengthening of flexible buried conduits.     

Potential for Structural Failure in the Seismic Near Field

This study analyzes the possibility that large distortions and distortion rates due to wave-propagation phenomena within structures were responsible for unexpected cracking at connections of steel-frame buildings in the seismic near-field region during the Northridge (1994) and Kobe (1995) earthquakes. Since such internal wave propagation is characteristic of a structure with a continuous distribution of mass, the problem is studied by numerically simulating the structural response for both discrete and continuous models of a 20-story building, using ground motion time histories from the Northridge earthquake. The time histories are chosen from the far-field and near-field regions of the earthquake to determine if wave-propagation effects within the structure are especially significant in the near field. A truncated modal analysis is also performed using only the first vibrational mode to see if significantly lower response levels result. It is found that the continuous model gives higher response levels—indicating that wave propagation may have been a factor—but the discrepancy is not limited to the near field. Strain rates are higher from the continuous model than from the discrete model and much higher than from the truncated modal analysis, but the magnitudes are too low to be a significant factor in the observed damage. The explanation for the connection cracking may simply be high-intensity ground motion in the near field.     

Estimation of Maximum Roof Displacement Demands in Regular Multistory Buildings

For approximate estimation of the maximum inelastic roof displacement demand for existing multistory buildings, the current displacement coefficient method uses the maximum linear elastic displacement of the first-mode single-degree-of-freedom (SDOF) system corresponding to the buildings. The statistics of the SDOF system estimate of the roof displacement on the following topics are presented by three SAC steel buildings subjected to 72 earthquake ground motions recorded on firm sites. (a) How uncertain is the first-mode SDOF system estimation of the roof displacement? (b) What are the main sources of uncertainty in the estimation of the roof displacement? (c) How can the uncertainty be reduced? It is shown from the study that the maximum inelastic roof displacement demand of multistory buildings can be more accurately and more directly estimated by the maximum inelastic displacement of the first-mode elastoplastic SDOF system. This is especially true for low-rise buildings.     

Seismic Performance of Hillside Fills

Permanent ground deformations in unsaturated, compacted hillside fills under seismic loading conditions are discussed, with emphasis given to fill performance during the 1994 Northridge earthquake. These movements represent a significant yet often unrecognized hazard to developed hillside areas, as relatively modest deformations induced widespread damage totaling hundreds of millions of dollars during the Northridge event. The development of grading standards in the Los Angeles area is reviewed to place the seismic fill deformation problem in context with other issues that have shaped design and construction practices for hillside fills. Field observational data on fill performance during the Northridge earthquake is presented, and typical ground distress patterns are found to include cracking near cut/fill contacts, lateral extension and settlement of fill pads, and bulging of fill slope faces. For most sites, the prevalent mechanism of permanent ground deformation responsible for the fill movements is contractive volumetric strain accumulation within the unsaturated fill soils during strong earthquake shaking (that is, seismic compression).     

Earthquake Response and Energy Evaluation of Inelastic Structures

A computational method is derived to characterize the energy in inelastic structures and the transfer among various energy forms over the duration of an earthquake. This computational method is based on the force analogy method, which uses a change in displacement field to represent the inelastic behavior of structure instead of the traditional method of changing stiffness. The evaluation of plastic energy due to inelastic deformation in the structure becomes very simple using the force analogy method, where the accumulation of plastic energy due to plastic rotations is exactly equal to the elastic moment multiplied by the change in plastic rotations. In addition, this plastic energy formula can be used for any material with predefined stress–strain relationship, and therefore the transfer of energy among various forms can be calculated at any specific time. Once the energy equation is derived, numerical analyses are performed on a single degree of freedom system to study the characteristics of energy transfer. This is then extended to study the transfer of energy among various forms in a multidegree of freedom system. These two studies show that the analytically derived equation for plastic energy is accurate in studying the structural energy response due to earthquake excitations.     

Investigation of effect of near-fault motions on substandard bridge structures

The objective of this study was to investigate the effects of near-fault ground motions on substandard bridge columns and piers. To accomplish these goals, several large scale reinforced concrete models were constructed and tested on a shake table using near- and far-field ground motion records. Because the input earthquakes for the test models had different characteristics, three different measures were used to evaluate the effect of the input earthquake. These measures are peak shake table acceleration, spectral acceleration at the fundamental period of the test specimens, and the specimen drift ratios.For each measure, force-displacement relationships, strains, curvatures, drift ratios, and visual damage were evaluated.Results showed that regardless of the measure of input or response, the near-fault record generally led to larger strains,curvatures, and drift ratios. Furthermore, residual displacements were small compared to those for columns meeting current seismic code requirements.     

Analytical Evaluation of Seismic Performance of RC Frames Rehabilitated Using FRP for Increased Ductility of Members

Many reinforced concrete (RC) frame structures designed according to pre1970 strength-based codes are susceptible to abrupt strength deterioration once the shear capacity of the columns is reached. Fiber composites are used to increase the shear strength of existing RC columns and beams by wrapping or partially wrapping the members. Increasing the shear strength can alter the failure mode to be more ductile with higher energy dissipation and interstorey drift ratio capacities. The objective of this study was to analytically evaluate the effect of varying distributions of fiber-reinforced polymer (FRP) rehabilitation on the seismic performance of three existing RC frames with different heights when subjected to three types of scaled ground motion records. The FRP wrapping is designed to increase the displacement ductility of frame members to reach certain values representing moderate ductility and high ductility levels. These values were assumed based on previous experimental work conducted on members wrapped using FRP. The study also investigates the effect of the selected element’s force–displacement backbone curve on the capacities of the structures with respect to maximum interstory drift ratio, maximum peak ground acceleration, or peak ground velocity resisted by the frames, maximum storey shear-to-weight ratio and maximum energy dissipation. It was found that for low-rise buildings, the FRP rehabilitation of columns only was effective in enhancing the seismic performance; while for high-rise ones, rehabilitation of columns only was not as effective as rehabilitation of both columns and beams. Ignoring representing the postpeak strength degradation in the hysteretic nonlinear model of FRP-rehabilitated RC members was found to lead to erroneous overestimation of the seismic performance of the structure.     

Considerations of Multimode Structural Response for Near-Field Earthquakes

This paper presents an investigation of multimode effects of tall buildings idealized as a continuous shear-beam model subjected to near-field pulse-like ground motion. The investigation is based on three analytical approaches: a damped wave solution approach, a fundamental-mode approach, and a modal summation approach. In the modal summation approach, all modal damping ratios are assumed to be equal and a set of Green’s functions for the shear strain response is explicitly derived. The multimode effects on the base-level shear strain/force demands are compared by using an effective response spectrum for shear-beam systems. The study results show that the occurrence of major spectral differences is conditioned on the ratio of the fundamental structural period to the duration of the predominant excitation pulse. Seismic analyses for a set of recorded near-field earthquake data indicate a strong correlation between the characteristics of effective response spectra and the ground pulse parameters.     

Predictive Optimal Linear Control of Inelastic Structures during Earthquake. Part II

The predictive optimal linear control (POLC) algorithm derived in the companion paper is extended to analyze the controlled responses of inelastic structures by incorporating the force analogy method (FAM). While POLC is very effective in compensating for the negative effect of time delay for elastic structures, the FAM is very efficient in performing the inelastic analysis. Different from conventional inelastic analysis methods of changing stiffness, the FAM analyzes inelastic structures by varying the structural displacement field, and therefore the state transition matrix needs to be computed only once. This greatly simplifies the computation and makes inelastic analysis readily applicable to the POLC algorithm. Numerical simulation is performed on a single-degree-of-freedom system to demonstrate the applicability of the POLC algorithm. Results show that the proposed control algorithm has feasibility for any inelastic structures for various control gains. Even though the control efficiency deteriorates with increase in time delay magnitudes, POLC maintains structural stability over a relatively wide range of time delay magnitudes. Finally, a computer model of a six-story moment-resisting steel frame is analyzed to show that POLC has a good control result on real inelastic structures, particularly in reducing the structural damages experienced during the earthquake.     

Wave Passage and Ground Motion Incoherency Effects on Seismic Response of an Extended Bridge

This paper investigates the implications of ground motion spatial variability on the seismic response of an extended highway bridge. An existing 59-span, 2,164-meter bridge with several bearing types and irregularity features was selected as a reference structure. The bridge is located in the New Madrid Seismic Zone and supported on thick layers of soil deposits. Site-specific bedrock input ground motions were selected based on a refined probabilistic seismic hazard analysis of the bridge site. Wave passage and ground motion incoherency effects were accounted for after propagating the bedrock records to the ground surface. The results obtained from inelastic response-history analyses confirm the significant impact of wave passage and ground motion incoherency on the seismic behavior of the bridge. The amplification in seismic demands exceeds 150%, whereas the maximum suppression of these demands is less than 50%. The irregular and unpredictable changes in structural response owing to asynchronous earthquake records necessitate in-depth seismic assessment of major highway bridges with advanced modeling techniques to realistically capture their complex seismic response.     

Evaluation of Seismic Performance of Gravity Load Designed Reinforced Concrete Frames

The seismic performance of reinforced concrete frames designed for gravity loads is evaluated experimentally using a shake table. Two 1:3 scale models of one-bay, three-storied space frames, one without infill and the other with a brick masonry infill in the first and second floors, are tested under excitation equivalent to the spectrum given in IS 1893-2002. From the measured response of the models during excitation, the shear force, interstory drift, and stiffness are evaluated. The effect of masonry infill on the seismic performance of reinforced concrete frames is also investigated. Then, the frames are tested to failure. Severe damage is observed in the columns in the ground floor. The damaged columns are strengthened by a reinforced concrete jacket. The frames are again tested under the same earthquake excitations. The test results showed that the retrofitted frames could sustain low to medium seismic forces due to a significant increase in strength and stiffness.     

Dynamic Testing of a Masonry Structure on a Passive Isolation System

Lightly reinforced and unreinforced masonry buildings have not performed well in earthquakes. Evaluation of past performance of masonry structures has led to more stringent design and construction requirements in the current building codes, and has raised concerns about the performance of existing lightly reinforced and unreinforced masonry buildings in future earthquakes. Base isolation has been shown to be effective in reducing damage to large building structures, and appears to be particularly effective in protecting stiff masonry structures. Using the base isolation principle, Kansas State University’s stiffness decoupler for the base isolation of structures (SDBIS) was designed to effectively reduce the acceleration and force transferred into a building superstructure during a seismic event. The sliding system uses a passive method to provide damping and to dissipate some of the kinetic energy to reduce relative displacements. In addition, the SDBIS system includes a self-centering element that will recover the majority of the induced displacement and provide resistance to overturning. In order to apply the SDBIS system to the masonry building industry, dynamic testes were performed to evaluate the structural response of a full-size one-story masonry model that was supported by the SDBIS system. Acceleration time-history results are presented for dynamic tests using the July 21, 1952 Kern County earthquake, Station 1095 Taft Lincoln School record, the May 19, 1940 Imperial Valley earthquake, Station 117 El Centro Array #9 record, the February 9, 1971 San Fernando earthquake, Station 279 Pacoima Dam record, and the January 17, 1994 Northridge earthquake, Station 24436 Tarzana Cedar Hill record ground motions. Test results show the system is effective when used with a masonry structure.     

Influence of Inelastic Tower Links on Cable-Supported Bridge Response

A new concept for bridge tower designs in seismic zones incorporates sacrificial link schemes that enable the tower shafts to remain elastic under large seismic excitation. In order to study the influence of inelastic tower links on the seismic response of cable-supported bridges, global seismic time history analyses were performed on models of the new San Francisco-Oakland Bay Bridge East Span self-anchored suspension bridge (SASB) and a cable-stayed bridge (CSB) alternative. The addition of inelastic links to the signature tower improved the behavior of both structures. The tower and overall bridge demands were reduced, including the tower drift and moments as well as the suspension cable, cable stay, and superstructure drifts and axial loads. The inelastic tower links protected the SASB and CSB tower shafts from nonlinear behavior under the 1,500-year Safety Evaluation Earthquake (SEE) event as well as a 2,500-year event. When the inelastic tower links were removed, the SASB tower shafts yielded under the SEE. It was shown that the inelastic tower links could be used to tune the dynamic response of bridge towers in regions of high seismicity.     

Protecting Base-Isolated Structures from Near-Field Ground Motion by Tuned Interaction Damper

This paper deals with the combined use of a low-damping base-isolation system and a semiactive control system referred to as a tuned interaction damper (TI damper). The TI damper generates friction-type forces (rigid-plastic behavior) through interactions between the primary isolated structure and an auxiliary structure. Because of its energy-dissipation nature, a base-isolated structure controlled by a TI damper is inherently stable, and as a semiactive control device, its operation requires only minimal external power. The efficacy of the proposed hybrid system is examined through a numerical simulation for a five-story scaled building subjected to near-field ground motions. A sensitivity analysis on the parameter dependence of the structural response on control force limit, stiffness ratio, and frequency ratio is presented. By tuning these parameters to optimal values, the performance of the base-isolated structure equipped with a TI damper can be enhanced. Based on the numerical simulation results, it is concluded that a TI damper is capable of suppressing the base drift of base-isolated structures subjected to near-field earthquake ground motions while maintaining the superstructure interstory drift and accelerations at small levels.     

Investigation of Effects of Nonlinear Static Analysis Procedures to Performance Evaluation on Low-Rise RC Buildings

The aim of the study is to compare and evaluate structural response demands obtained from nonlinear static analysis procedures (NSPs) which are displacement coefficient method (DCM) recommended in FEMA 356 and capacity spectrum method (CSM) recommended in ATC 40. For these reasons, three of three-dimensional low-rise RC buildings with different characteristics are investigated. In order to determine nonlinear behavior of the buildings under lateral loads, the base shear-roof displacement relationships (capacity curves) are obtained by pushover analysis including P-delta effects. Then by considering four different seismic hazard levels, building performances are determined by using the CSM and by using from DCM results determined in a previous study. In order to determine performance levels of the buildings, maximum beam and column plastic rotation demands and maximum story drift demands are determined in the related maximum displacement demands. Plastic strains in the equivalent diagonal struts, representing the nonstructural infill walls, are also determined, similarly. Comparing structural response quantities (such as plastic rotations, story drifts, etc.) obtained from the NSPs for considered low-rise RC buildings, effects of different NSPs in performance evaluations of the buildings are investigated comparatively, as well.     

轻质混凝土拼装墙板填充钢框架协同抗震性能试验研究

为了研究墙板与钢框架结构之间的协同抗震性能,对采用不同墙框连接节点的轻质混凝土拼装墙板填充钢框架进行了低周往复荷载试验。通过对比试件的承载力、滞回性能、刚度、耗能以及延性性能,探讨了轻质混凝土拼装墙板及其整体性对结构抗震性能的影响。结果表明:填充墙板钢框架结构的最终破坏形态以墙板挤压开裂,框架梁柱端部翼缘屈曲为主;轻质混凝土拼装墙板与钢框架协同工作,有利于提高结构整体的承载力和变形能力,减轻钢框架在平面内的屈曲破坏;与刚性节点相比,采用柔性节点连接墙板与钢框架对结构的承载力、层间刚度和耗能能力更为有利;增强拼装墙板的整体性,有助于提高结构整体刚度、变形和耗能能力。研究结果可为轻质混凝土拼装墙板填充钢框架结构的抗震设计提供参考。