A. Davaran

Cyclic experimental behavior of angles and applications for connection design and modeling

Recent work on the seismic behavior of low-ductility steel braced frames has suggested that adding top and seat angles to gravity framing connections can increase a building’s reserve capacity and hence its collapse performance. To this end a comprehensive suite of 133 tests has been developed and is currently in progress to establish a baseline of ultimate capacities under monotonic and cyclic loading. Angles range in size from L4x4x5/16 to L8x6x3/4, and will be fastened using 3/4” A325, 1” A325, and 1” A490 bolts. The distance from the heel of the angle to the bolt centerline in the vertical leg, referred to as the gage, has previously been shown to be an important parameter, particularly in relation to the thickness of the angle. Low gage-to-thickness ratios indicate stocky configurations, while high ratios indicate slender, flexure-controlled configurations. The ratios within this study range from 1.25 to 8.00. Based on the test results, this study aims to develop simple analytical models that can reasonably predict ultimate moment capacities and rotations of beamto-column connections reinforced with top and seat angles.

Experimental Behavior of Low-Ductility Brace Connection Limit States

In low-ductility concentrically braced steel frames (CBFs) with traditional fillet welds between the gusset plates and slotted HSS braces, fracture of the fillet welds or net section rupture of the tube are viewed as the most likely connection failure modes. After such failure, the braces lose all their tensile strength but may still re-engage in compression to provide some degree of reserve strength to the structure. A test program was recently completed to verify the behavioral limit states and probable reengagement strength of brace connections. Six specimens with slotted-HSS weldedgusset plate connections were examined in this research. The connection specimens were part of a 9-story prototype building with chevron CBFs located in eastern North America and designed with an R factor of 3.0. The connection specimens were proportioned to exhibit weld or net section rupture failure modes. The tests showed that connections can re-engage in compression after weld or net section rupture, when the edge of the HSS slot comes into contact with the edge of the gusset plate. For slender gusset plates, connection buckling occurred due to the eccentricity resulting from unsymmetrical damage modes, such as coupled weld and net section ruptures, before substantial connection re-engagement can develop in bearing.

Seismic Stability Response of Columns in Multi-Tiered Braced Steel Frames for Industrial Applications

This paper presents a study on the seismic response of multi-tiered braced steel frames. The seismic demand and stability of the columns in a 4-tiered special X-braced frame structure located in a high seismic area and designed according to current AISC seismic provisions is evaluated. The drift demand in the individual bracing tiers and the combined column axial force and in-plane flexural demand are evaluated through nonlinear time history analysis. Detailed finite element analysis of one column under demands extracted from time history analysis is carried out to investigate the buckling response of the column. The results show that high in-plane bending demand is induced in the columns. Flexural yielding caused by non-uniform tier drift demand may detrimentally affect the torsional-flexural stability of the columns.

Seismic behavior of low-ductility concentrically-braced frames

Current codes allow engineers in moderate seismic regions to ignore seismic detailing as long as they design the structure using R = 3. However, recent research has raised questions regarding the reliability of such systems. When subjected to the maximum considered earthquake seismic hazard, the collapse of these systems becomes inherently dependent on their reserve lateral load-resisting capacity. Several sources of reserve capacity in these structures have been identified: connections in the gravity framing system, connections in the braced frame system, column continuity, base fixity, and brace re-engagement. In this paper, the results of several parametric OpenSees studies are presented in order to evaluate the effect of these sources of reserve capacity in traditional R = 3 systems, with focus on a three-story prototype chevron concentrically-braced frame. Nonlinear inelastic static analysis as well as nonlinear inelastic incremental dynamic analysis for a suite of earthquake ground motions were performed. Various limit states for these structures were identified, and failure of the welded connection between the brace and gusset plate was identified as the prominent event affecting collapse performance.

Seismic Behaviour of Steel HSS X-Bracing of the Conventional Construction Category

This paper presents the results of an ongoing research project on the seismic performance of concentrically braced steel frames of the Conventional Construction category designed according to NBCC 2010 and CSA-S16 seismic provisions. The braced frames studied are of the tension-compression X-bracing configuration and special attention is devoted to the response of the connection at the brace intersection point. Numerical simulations were performed to determine the inelastic demand on the braces and brace connections. A test program including four full-scale quasistatic cyclic tests was carried out to verify the findings of the numerical simulations. The results indicate that the behaviour of the bracing members is influenced by the type of mid-connection. In particular, connections with single lap splices at the intersection of the braces may be prone to local instability. In all tests, failure occurred in the connections, indicating that more attention must be paid in design to prevent premature and brittle failure in connections.

Experimental Behavior of Bolted Angles and Beam-to-Column Connections

Low-ductility braced steel frames are widely used in moderate seismic regions of North America. A research project has been initiated to examine the possible benefits of considering the moment resistance of beam-to-column connections in the gravity load-carrying system to provide such braced frame building structures with minimum dependable lateral reserve capacity. Beam-to-column moment connections are constructed using top and seat angles acting together with beam web clip angles. A first test program has been developed to characterize the inelastic behavior of individual angles subjected to monotonic and cyclic demands. Test results are presented to illustrate the influence of several parameters, including the angle geometry and the loading sequence. A second test program has been undertaken to examine the cyclic inelastic response of the proposed beam-to-column joints. The test program is described and joint rotation capacity predictions based on individual angle test results are presented and discussed.

A New Seismic Design Method for Steel Multi-Tiered Braced Frames

A seismic design strategy is presented to design the columns of multi-tiered steel braced frames. Minimum strength and stiffness requirements are introduced for the columns under the seismic loads. The proposed method aims at reducing the concentration of ductility demand in one of the tiers and, thereby, the in-plane bending moment demand on the columns and the ductility demand on the bracing members. Nonlinear static and dynamic analyses are performed to validate the design procedure. Moreover, detailed finite element analysis is carried out to investigate the stability of the columns. The results show that the proposed design method can efficiently reduce the column in-plane flexural demand and the brace ductility demand.

Inelastic Buckling Analysis of Steel X-Bracing with Bolted Single Shear Lap Connections

AbstractAnalytical models are proposed to predict the compressive resistance of discontinuous bracing members in steel X-braced frames when governed by flexure of the connecting plates at the mid-connections. The connections are bolted single shear splice connections commonly used for HSS bracing members. An incremental analysis procedure is developed to predict the axial load-deformation response of the brace-connection assembly including geometric and material nonlinearities. The buckling strength depends on the thickness of the connecting plates, the length of the connection, the clear distance between the continuous and discontinuous bracing members at the brace intersection, and the length of the discontinuous brace segments. The method is validated against test results from full-scale X-bracing specimens. It is also used to verify the design procedure proposed in the AISC Design Guide 24 for Hollow Structural Section Connections for a series of 17 X-braces covering a wide range of properties. The AI...

Stability of X-Bracing Systems with Traditional Bolted Connections

This paper deals with the design for stability of steel concentrically X-bracing frames built with HSS bracing members. Traditional slotted-tube connections with bolted shear splices are used for the bracing members. Both single- and double-shear splices are investigated. Full-scale quasi-static cyclic test program showed that instability of the discontinuous braces initiated through bending deformations of the connection plates, prior to overall buckling of the bracing members. Elastic stability analysis is performed to assess the effective buckling length and compressive resistances of the bracing members with double shear connections. Brace resistances are also obtained from an analytical model that accounts for inelasticity effects and geometrical imperfections of the brace connections. Numerical predictions are compared to test results and a numerical example is included to illustrate the method.