Search

This paper presents the results from an experimental pushover test on a one-third-scale model of a special inverted-V- braced steel frame with zipper struts. Zipper elements are vertical elements added at the intersections of the braces above the first floor and designed to carry upward the unbalanced loads resulting from buckling of the braces. As far as the writers know, this is the first test on such a configuration that has been suggested as an alternate to conventional braced frames in the American Institute of Steel Construction seismic specification for many years. The model was pushed to a target roof drift ratio of +3.58%, which was achieved without strength degradation and followed a trilinear backbone curve. The load was then reversed until the bottom story brace in tension fractured at a drift of -0.78%. After this fracture, the frame still carried about 37% of the maximum base shear. The zipper elements demonstrated their ability to activate buckling in all stories except the top one, redistributing the loads in the structure and minimizing strength losses. Two-dimensional and one three-dimensional frame models were analyzed and found to reproduce satisfactorily the experimental results up to the beginning of tearing in the tension brace. DOI: 10.1061/(ASCE)0733-9445(2008)134:10(1619) CE Database subject headings: Steel frames; Seismic design; Bracing; Struts.

The wind moment frame (WMF) method has been and continues to be widely utilized in the central and eastern United States, resulting in a very large population of structures having been designed without a rigorous analysis with respect to lateral loads. The primary objectives of this paper are to determine whether moment-resisting frames designed by the WMF method are adequate to handle the increased demand on lateral resistance made by modern seismic codes, and to evaluate and compare the seismic performance of existing and new moment-resisting steel frames with partially restrained (PR) connections. Three prototype frames were designed and analyzed utilizing synthetic ground motions for the Mid-America region. The seismic performance of these PR frames was evaluated in accordance with FEMA 273/356 criteria. In addition, studies were conducted on the confidence level of the performance results to take into account the uncertainties and randomness inevitably involved. This analysis and evaluation found that all PR frames designed in this study satisfied the selected performance objective with 99% confidence level for the global performance limit. Thus, the seismic performance of the PR frames in the Mid-America region will be satisfactory even under large ground motions. CE Database subject headings: Seismic effects; Frames; Earthquakes; Connections; Nonlinear analysis; United States.

To investigate the effectiveness of several seismic strengthening techniques, a full-scale unreinforced masonry (URM) structure was subjected to slowly applied lateral load reversals after the application of fiber reinforced plastic overlays, near surface mounted rods, and vertical posttensioning. Results showed that all techniques were effective for improving the seismic resistance of the previously tested URM building structure. Each system either increased the lateral in-plane strength and/or provided continuity of pier and spandrel elements over increased lateral displacements. In addition, the response of the test structure (overall height to base ratio of approximately one) showed that global issues such as flange effects, the effects of overturning moment, and global rocking can be substantial and must be considered. DOI: 10.1061/(ASCE)0733-9445(2007)133:9(1215) CE Database subject headings: Masonry; Seismic effects; Post tensioning; Full-scale tests.

Based on the results of a full-scale lateral load test of a two-story unreinforced masonry (URM) structure, modifications are suggested for the model implied by FEMA 356 for the in-plane analysis of perforated URM walls. These modifications allow the model to address global issues such as flange participation, overturning effects, and global rocking. In addition, changes are proposed to the pier model as well as to the definition of effective pier height. Following a description of each modification, the analysis results obtained using the proposed model and FEMA 356 are compared with experimental results. In terms of base shear resistance, the proposed model displayed an average error (absolute value) of 13% compared with an average error of 21% from FEMA 356. In addition, the proposed model also provided more accurate representation of failure modes. The improved accuracy of the proposed model is primarily attributed to the consideration of global issues. The results of a parametric analysis are also presented to provide insight into the relative influence of each modification proposed. CE Database subject headings: Masonry; Seismic effects; Retrofitting; Buildings, low-rise.

A variety of elastic and inelastic analytical approaches were used to investigate the response of a full-scale unreinforced masonry (URM) structure tested in the laboratory. Elastic analyses, employing a three-dimensional finite element model, revealed little coupling between parallel walls, and pointed to the appropriateness of two-dimensional analytical tools for further simulation of the test structure. The nonlinear analytical methods employed included, in order of increasing complexity, a rigid body analysis, a two-dimensional nonlinear pushover analysis, and a three-dimensional nonlinear finite element analysis. All methods considered important structural characteristics such as failure modes of perforated walls, flange effects, and global overturning moment effect. All methods predicted the experimental behavior fairly well. Strengths and weaknesses of these analytical approaches are discussed, and recommendations for analyzing old URM buildings are presented. CE Database subject headings: Masonry; Seismic effects; Buildings; Low-rise; Structural analysis.

A full-scale two-story unreinforced masonry (URM) building was tested in a quasistatic fashion to investigate the nonlinear properties of existing URM structures and to assess the efficiency of several common retrofit techniques. This paper presents the main experimental findings associated with the nonlinear properties of the original URM structure. The test structure exhibited large initial stiffness and its damage was characterized by large, discrete cracks that developed in masonry walls. Significant global behavior such as global rocking of an entire wall, and local responses such as rocking and sliding of each individual pier were observed in the masonry walls with different configurations. In addition, formation of flanges in perpendicular walls and overturning moments had significant effects on the behavior of the test structure. A comparison between the experimental observations and the predictions of FEMA 356 provisions shows that major improvements are needed for this latter methodology. CE Database subject headings: Masonry; Seismic effects; Failure investigations; Full-scale tests; Retrofitting.

The commentary to the AISC-LRFD Specification addresses the design of framing systems containing partially restrained (PR) connections, but it does not provide specific guidance on how to analyze these types of systems to determine appropriate internal forces for design. In addition, it provides little information on bow to select an appropriate connection stiffnesses for design calculations, such as the calculation of effective length factors. This paper presents an approach that allows for the consideration of nonlinear connection response using commonly available elastic analysis software. In addition, the application of the direct analysis approach to PR framing systems is presented in the context of the proposed method for handling the connection nonlinearity. The direct analysis approach allows for stability to be assessed using the AISC-LRFD beam-column interaction equations with the axial strength term P, determined based on the actual member length. This eliminates the complexity of determining effective length factors for partially restrained framing. CE Database subject headings: Design; Steel frames: Connections.

This study evaluates the feasibility of a new class of partially restrained connections using shape memory alloys. In the martensitic form, shape memory alloys (SMAs) have the ability to recover large residual deformations by heating the alloy above its transformation temperature. The proposed connection consists of four large diameter NiTi SMA bars connecting the beam flange to the column flange and serve as the primary moment transfer mechanism. Two full-scale connections were tested using the SAC loading protocol. The connections exhibited a high level of energy dissipation, large ductility capacity, and no strength degradation after being subjected to cycles up to 4% drift. Following the initial testing series, the tendons were heated to recover the residual beam tip displacement. After initiating the shape memory effect within the tendons, the connections were retested, displaying repeatable and stable hysteretic behavior. An additional test was performed under dynamic loading to examine the strain rate effects on the performance of the connection. The dynamic tests showed similar behavior, except for a decrease in energy dissipation capacity when compared to the quasi-static tests. CE Database subject headings: Beam columns; Steel; Connections, semi-rigid; Shape memory effect; Seismic design; Cyclic tests.

Partially restrained connections and their interaction with the floor slab have received recent attention due to numerous failures of welded steel moment frames during the 1994 Northridge and 1995 Kobe earthquakes. After the earthquakes, it was apparent that the 'simple' connections, including the slab, served as a 'backup' structural system after the full moment connections failed. These connections significantly influenced the behavior of the lateral load resisting system. This paper reports on the results of a unique, full-scale bidirectional test on a composite connection. The connection behaved very well up to a uniaxial drift of about 2%. After that drift level, composite action began to deteriorate and the connection strength and stiffness declined. The results indicate that two previously unreported failure modes may need to be considered in the design of composite buildings if large cyclic bidirectional forces are present. One is a 'punch through' of the column web due to the force transfer from the 'pinned' side of the connection. The other is the formation of a concrete wedge as a result of the large bearing forces between the slab and steel column flange. As a result of this observation, a suggestion that the full depth of the slab be maintained for a region 150 mm (6 in.) around the joint has been incorporated in the commentary to Part II of the 2002 AISC Seismic Specification. DOI: 10.1061/(ASCE)0733-9445(2004)130:2(320) CE Database subject headings: Connections; Composite columns; Beam columns; Slabs; Seismic design; Cyclic loads.

The results of tests on 48 T-stub specimens are presented and discussed. These tests were carded out as part of a SAC Phase II project in order to provide insight into the behavior, failure modes, and ductility of these components. The main variables tested include the size of the T-stub, the gauges of the bolts, and the type and diameter of the bolts. The primary intent was to develop a large database to calibrate simplified models suitable for design. In order to develop these models, a comprehensive instrumentation system that could identify the different components of deformation in the individual T-stubs was utilized. Most of the T-stubs failed by net section fracture through the stem and by tension fracture of the bolts, but generally the failure was after significant plastification had occurred. Other failure modes observed included bolt shear and block shear. The results indicate that current design equations provide conservative estimates of the ultimate strength of the T-stubs but that they are not necessarily good predictors of the governing failure modes.

This paper presents the findings from a computational investigation performed on the Borax corporate headquarters building, a four-story steel frame structure in which 75% of the steel moment-resisting connections suffered brittle fractures during the 1994 Northridge earthquake. A companion paper provides detailed documentation of the forensic investigation and repair design performed immediately following the earthquake on this structure, which was less than one year old when the earthquake struck. This paper describes a series of preliminary analyses conducted shortly after the forensic investigation and repair were completed. The main objectives were to investigate the behavior of the structure during the earthquake, and to determine whether different levels of established, advanced analysis techniques could estimate the distribution and extent of the damage. The results showed that a three-dimensional nonlinear dynamic analysis using a site-specific accelerogram provides strong correlation with the observed damage, while elastic static and dynamic analyses, two-dimensional nonlinear static and dynamic analyses, and three-dimensional nonlinear static analyses show less correlation. In addition, the results indicate that substantial redistribution of forces may have occurred, and that force distribution most likely had a noticeable effect on the pattern of failures observed in the structure.

The Borax corporate headquarters building, a low-rise steel moment frame, was less than one year old when it sustained significant structural damage during the 1994 Northridge earthquake. Seventy-five percent of its steel moment-resisting connections suffered severe brittle fractures. This paper presents a detailed documentation of the forensic investigation performed on this structure immediately following the earthquake. A companion paper presents the findings from a subsequent computational investigation performed on the structure. The two primary failure modes at the Borax building were column flange fracture and pull-out of the girder flange from the column. The failures are primarily attributed to a fundamental flaw in the standard code-prescribed welded-flange bolted-web connection and the extreme ground motion at the site. The paper also describes the repairs made to the structure. These repairs were completed before the SAC research that now guides repair and retrofit of these structures was available.

Riveted steel connections were used throughout most of the United States for many years. The connections were commonly encased in massive but lightly reinforced concrete for fire protection. These older structures were designed with little or no consideration of seismic behavior, and today these buildings sometimes require seismic rehabilitation and retrofit, but little information regarding this behavior is available. The present paper provides information regarding the seismic performance of these older steel structures. An experimental study which evaluates the seismic behavior of subassemblages with T-stub and clip angle connections is described in detail. The results are analyzed. The strength, stiffness and ductility of the connections are shown to be related to the properties of the steel and the concrete, the mode of failure of the connection, and the concrete encasement. The hysteretic behavior, energy dissipation and deformation capacity of the connections are shown. Approximate models for predicting the strength, stiffness, and deformation limits are presented as practical implications of the research.

The results of 48 T-stub component tests and eight full-scale T-stub connection tests are used to develop and calibrate a comprehensive T-stub connection stiffness model. The model, based on basic component spring theory, incorporates deformations from tension bolt elongation, bending of the T-stub flange, elongation of the T-stem, slip of the T-stub relative to the beam flange, bearing deformation of the T-stem, and bearing deformation of the beam flange. Nonlinear material models, for both the tension bolts and the T-stub base material, are included. A sophisticated slip/bearing routine accounts for the misalignment of shear bolt holes and variable slip conditions. Comparisons of multilinear model predictions with experimental data show that the model accurately predicts connection stiffness response and deformation at failure.