Gregory G. Deierlein

Seismic Collapse Safety of Reinforced Concrete Buildings. II: Comparative Assessment of Nonductile and Ductile Moment Frames

This study is the second of two companion papers to examine the seismic collapse safety of reinforced concrete frame buildings, and examines nonductile moment frames that are representative of those built before the mid-1970s in California. The probabilistic assessment relies on nonlinear dynamic simulation of structural response to calculate the collapse risk, accounting for uncertainties in ground-motion characteristics and structural modeling. The evaluation considers a set of archetypical nonductile RC frame structures of varying height that are designed according to the seismic provisions of the 1967 Uniform Building Code. The results indicate that nonductile RC frame structures have a mean annual frequency of collapse ranging from 5 to 14×10-3 at a typical high-seismic California site, which is approximately 40 times higher than corresponding results for modern code-conforming special RC moment frames. These metrics demonstrate the effectiveness of ductile detailing and capacity design requirements,...

Accounting for Ground-Motion Spectral Shape Characteristics in Structural Collapse Assessment through an Adjustment for Epsilon

One of the challenges of assessing structural collapse performance is the appropriate selection of ground motions for use in the nonlinear dynamic collapse simulation. The ground motions should represent characteristics of extreme ground motions that exceed the ground-motion intensities considered in the original building design. For modern buildings in the western United States, ground motions that cause collapse are expected to be rare high-intensity motions associated with a large magnitude earthquake. Recent research has shown that rare high-intensity ground motions have a peaked spectral shape that should be considered in ground-motion selection and scaling. One method to account for this spectral shape effect is through the selection of a set of ground motions that is specific to the buildings fundamental period and the site hazard characteristics. This selection presents a significant challenge when assessing the collapse capacity of a large number of buildings or for developing systematic procedures because it implies the need to assemble specific ground-motion sets for each building. This paper proposes an alternative method, whereby a general set of far-field ground motions is used for collapse simulation, and the resulting collapse capacity is adjusted to account for the spectral shape effects that are not reflected in the ground-motion selection. The simplified method is compared with the more direct record selection strategy, and results of the two approaches show good agreement. DOI: 10.1061/(ASCE)ST.1943-541X.0000103.

Evaluation of ACI 318 and AISC (LRFD) strength provisions for composite beam-columns

Abstract The development of an interactive computer program for modeling biaxial bending of encased composite steel-concrete columns and its application to design are presented. Included is a description of an analytical procedure for modeling inelastic behavior based on the fiber element method. Results of fiber element analyses are used to evaluate nominal uni- and biaxial bending strengths of composite columns calculated according to the ACI-318 and AISC-LRFD specifications. Slender column behavior and the effect of residual stresses and built-in forces (resulting from the construction sequence) in the steel sections are included. In addition to providing insight into behavior and assessing current design provisions for composite columns, the implementation of the fiber element method demonstrates the feasibility of using inelastic simulation programs in design.

Inelastic analyses of a 17-story steel framed building damaged during Northridge

A series of two- and three-dimensional static and dynamic inelastic frame analyses are performed for a 17-story steel moment frame building damaged by the 1994 Northridge earthquake. The primary objectives of the study are to: (1) exercise state-of-the-art inelastic static and dynamic analyses for the evaluation and design of steel buildings; (2) establish to what degree frame analyses can be used to predict the types of brittle connection damage that occurred during the Northridge earthquake; and (3) investigate the reliability of the analyses and the influence of modeling parameters on computed performance indices. In general, this study shows that calculated interstory drift ratios and curvature demands obtained from inelastic time history analyses correlate reasonably well with the pattern of connection damage observed in the building. However, there is significant scatter in the computed deformation demands that are strongly dependent on the degree to which three-dimensional torsion, secondary structural elements and strength/stiffness degradation (associated with connection fractures) are modeled in the analyses. Further, comparisons of static and dynamic analyses indicate that for this building static pushover analyses do not capture higher vibration modes that are significant.

Simulation of Seismic Collapse in Nonductile Reinforced Concrete Frame Buildings with Masonry Infills

AbstractImproved analysis methods and guidelines are presented to simulate the seismic collapse of nonductile concrete frame buildings with masonry infills. The analysis tools include an inelastic dual-strut model that captures the post-peak behavior of the masonry infill and its interaction with the surrounding frame. The dual compression struts capture the column-infill interaction that can cause shear failure of the columns and loss of their vertical load carrying capacity. A rigid softening shear degradation model is implemented in the beam-column elements to capture the shear failure of nonductile RC columns. Guidelines are presented to determine the strut model parameters based on data from 14 experimental tests on infill frames. The models are applied in three-dimensional nonlinear dynamic analyses of a three-story nonductile concrete frame prototype building with infills. The incremental dynamic analyses technique is utilized to understand the effect of the infill-column interaction and the rockin...

Design Concepts for Controlled Rocking of Self-Centering Steel-Braced Frames

AbstractThe self-centering rocking steel-braced frame is a high-performance system that can prevent major structural damage and minimize residual drifts during large earthquakes. It consists of braced steel frames that are designed to remain elastic and allowed to rock off their foundation. Overturning resistance is provided by elastic post-tensioning, which provides a reliable self-centering restoring force, and replaceable structural fuses that dissipate energy. The design concepts of this system are examined and contrasted with other conventional and self-centering seismic force resisting systems. Equations to predict the load-deformation behavior of the rocking system are developed. Key limit states are investigated including desired sequence of limit states and methods to help ensure reliable performance. Generalized design methods for controlling the limit states are developed. The design concepts are then applied to a six-story prototype structure to illustrate application of the rocking steel fram...

Cost-Benefit Evaluation of Seismic Risk Mitigation Alternatives for Older Concrete Frame Buildings

Cost-benefit assessment is used to evaluate the effectiveness of various seismic retrofit strategies to address concerns about earthquake safety and damage to older concrete frame buildings. The benefits of mitigation are quantified in monetary terms by assessing the reductions in risk of building damage (repairs) and occupant fatalities. When evaluated considering repair costs only, the maximum cost of retrofitting that offers a positive rate of return is 10% to 30% of the building replacement value. Since seismic retrofits generally cost more than this, this result suggests that reduced risk of damage and repairs alone does not warrant the expense of retrofitting. However, the added benefits of reduced fatality risks would make retrofitting cost-effective if it costs as much as 60% of the building replacement cost. The cost-benefit approach provides an important tool to evaluate the merits of seismic mitigation in light of competing demands for finite public and private resources.

Nonlinear analysis of three-dimensional steel frames with semi-rigid connections

Abstract With the ultimate aim of improved methods for the realistic limit state design of structures, a method has been developed for incorporating nonlinear connection response in the analysis of three-dimensional steel structures. The method is implemented in an interactive graphics analysis and design program which can model both geometric and material nonlinearities in framed structures. The connection model includes nonlinear moment-rotation response for both major- and minor-axis bending. Standardized models for different types of connections which are calibrated to existing test data and are amenable to design are presented. Application of the method is presented for a realistic case study consisting of a low-rise three-dimensional structure with partially restrained connections.

Calibration of the SMCS Criterion for Ductile Fracture in Steels: Specimen Size Dependence and Parameter Assessment

The stress modified critical strain (SMCS) criterion provides a local index for the initiation of ductile fracture in metals as a function of plastic strain and stress triaxiality. Previous research has confirmed the SMCS criterion to be an accurate index for fracture initiation in mild steels and demonstrated its application to civil/structural engineering. To facilitate practical implementation of the SMCS criterion, two key aspects of its calibration for steel materials are examined. The first pertains to the sensitivity of the measured SMCS material toughness parameter to the size of the test coupon. New results from 23 tests of cylindrically notched tension (CNT) specimens of various sizes and notch geometries indicate that the toughness parameter is relatively insensitive to calibration specimen size. This finding validates the use of miniature bar specimens to calibrate the SMCS model for thin plate steels and in-service structures, where extraction of larger coupons is impossible. The second aspec...

Framework for Incorporating Probabilistic Building Performance in the Assessment of Community Seismic Resilience

AbstractA framework is presented for incorporating probabilistic building performance limit states in the assessment of community resilience to earthquakes. The limit states are defined on the basis of their implications to postearthquake functionality and recovery. They include damage triggering inspection, occupiable damage with loss of functionality, unoccupiable damage, irreparable damage, and collapse. Fragility curves are developed linking earthquake ground motion intensity to the probability of exceedance for each of the limit states. A characteristic recovery path is defined for each limit state on the basis of discrete functioning states, the time spent within each state, and the level of functionality associated with each state. A building recovery function is computed accounting for the uncertainty in the occurrence of each recovery path and its associated limit state. The outcome is a probabilistic assessment of recovery of functionality at the building level for a given ground motion intensit...