Zipper Frames Research Articles

Evaluation of the seismic performance of suspended zipper column concentrically braced steel frames

The performance of the most common type of concentric braced configuration, i.e. Chevron braces, is governed by buckling of first story braces in compression, resulting in a localization of the failure. To improve the performance of chevron concentric braced frames, zipper columns were introduced to transfer the unbalanced forces; over-strength braces were further introduced to avoid the complete collapse of the frame. Such concentric braced frames are called suspended zipper concentric braced frames.The objective of this study is to evaluate the seismic performance of suspended zipper concentric braced frames designed according to Eurocode 8 and to compare their performance with conventional concentric braced configurations. It is important to highlight that this study introduces a novel design methodology to size braces, zipper columns, beams and columns in suspended zipper frames. For this purpose, two concentric braced frame structures from each suspended zipper configuration and stud-to-ground configuration are designed and analysed for a ground motion with a probability of exceedance equal to 10% in 50 years, i.e. a return period of 475 years, with peak ground acceleration of 0.3 g.Based on the comparison of results, it can be concluded that the performance of suspended zipper frame is better than that of conventional concentrically braced frames in medium-rise buildings, but not in low-rise buildings. Furthermore, the novel design methodology is proven to be satisfactory in sizing the structural elements of suspended zipper concentric braced frames when considering the inelastic time history analyses results.

Lateral cyclic behavior of zipper braced frames-considering connection details

During Northridge earthquake in USA in 1994, a variety of failures occurred in welded steel connections. Studying these structural failures has led to development of more reliable moment resisting connections and new ways of using braced frames as seismic load resisting systems. This article investigates through numerical simulations, the lateral capacity and seismic behavior of two of these newly-thought braced frames, zipper braced frames and suspended zipper braced frames. The overall seismic behavior of these frames is investigated through displacement-based pushover analyses considering the effect of connection elements such as gusset plate and shear tab. To study the efficiency of these two types of concentrically braced frames, a numerical investigation on their behaviors for low-, mid- and high-rise buildings was conducted. Three zipper braced frames and three suspended zipper braced frames with different number of stories have been modeled using OpenSEES software. For each simulation, frame maximum strength, maximum drift capacity, and weight are determined and compared with each other. It is concluded that connection modeling has significant effects on the lateral behavior of these frames. Furthermore, the suspended zipper braced frames show higher ductility when compared with the ductility of zipper braced frames. Finally, the suspended zipper braced frames are recommended to be used in high-rise buildings, however, for the lowand mid-rise buildings it is recommended to use zipper frames due to economic efficacy.

An Overview of Pros and Cons of Zipper Braced Frames

The chevron bracing system provides the highest level of stiffness and average ductility by using the yield or buckling of braces. This system demonstrates weak post-buckling behavior while other structural members such as beams, columns and joints show elastic behavior. As a result of bracing buckling at a story, the unbalancing force acts perpendicular on the middle of the beam span and results in the concentration of damage on that story. Consequently, the affected story becomes prone to the soft story mechanism and the structure collapses eventually. In order to prevent this problem, it is possible to place vertical elements (zippers) between beams so that the head of bracings are connected at height and the resulting unbalancing force in the story is transferred to upper stories. Such a frame is known as the zipper frame. With a proper zipper configuration, it is possible to address the problems and weaknesses of the chevron bracing frame. Therefore, zipper frames can become proper substitutes for this type of bracing systems. By using the chevron element in the chevron bracing and turning it into a zipper frame, it is possible to increase the strength, ductility and energy absorption capacity of such frames.

Response modification factor of suspended zipper braced frames

The suspended zipper bracing system is suggested to reduce the flaws of ordinary zipper braced and concentric inverted <i>V</i> braced frames. In the design procedure of suspended zipper bracing systems, columns and top story truss elements are strengthened. This bracing system show different performances and characteristics compared with inverted <i>V</i> braced and ordinary zipper frames. As a result, a different response modification factor for suspend zipper frames is needed. In this research paper, the response modification factor of suspended zipper frames was obtained using the incremental dynamic analysis. Suspended zipper braced frames with different stories and bay lengths were selected to be representations of the design space. To analyze the frames, a number of models were constructed and calibrated using experimental data. These archetype models were subjected to 44 earthquake records of the FEMA-P695 project data set. The incremental dynamic analysis and elastic dynamic analysis were carried out to determine the yield base shear value and elastic base shear value of archetype models using the OpenSEES software. The seismic response modification factor for each frame was calculated separately and the values of 9.5 and 13.6 were recommended for ultimate limit state and allowable stress design methods, respectively.

Research on the Seismic Performance of Zipper Frames

In order to investigate the seismic performance of zipper frames, three 10-storey structure models, including a zipper frame with tension zipper columns, a zipper frame with weak zipper columns and a chevron braced frame, were designed. Some indexes, including the natural periods, steel consumption and dynamic response under frequent and rare earthquakes, were analyzed and compared. The results show that, the dynamic responses of the three structures are similar to each other and all of them can meet the requirement of no-damage under frequent earthquakes and no-collapse under rare earthquakes provided by the GB50011 code, but the steel consumption for the zipper frames is less than that of the chevron braced frames, indicating the comprehensive seismic performance of the zipper frame is better than the chevron braced frame. However, it is also found that the design method of the zipper columns is conservative and a more reasonable design method of the zipper columns is needed to be provided.

Cyclic Behavior of Zipper-Braced Frames

Two one-third-scale three-story models of special inverted-V-braced frames with zipper struts were tested under quasistatic cyclic loads. Zipper frames utilize a vertical element at the intersection of chevron braces and floor beams to distribute the unbalanced vertical forces due to brace buckling. Two ground motions representative of a far-field (Chile 1985) and a near-field earthquake (Kobe 1995) were selected for the tests. Two tests with two different amplitudes of the 1985 Chile earthquake were conducted on one frame; three tests with three different amplitudes of the 1995 Kobe record on the other. For each specimen, a pre-experiment numerical model was used to generate the three floor displacement histories that were applied to the test frame. Comparison of the results for the hysteretic behavior of the braces, zipper struts, and overall frame validated the partial-height zipper mechanism envisioned in proportioning the specimens. The frames showed good dissipating-energy capacity and large deformation ductilities without significant strength losses. Moreover, they remained stable even after fracture of some braces occurred and showed significant residual strength.

Hybrid testing and modeling of a suspended zipper steel frame

AbstractAn analytical and experimental study has been conducted to evaluate the seismic performance of a three‐story suspended zipper steel frame. The frame was concentrically braced and had zipper struts to transfer the unbalanced forces induced on the beams due to the buckling of the lower‐story braces. The experimental study was conducted with the hybrid test technique, in which only the bottom‐story braces of the three‐story frame were physically tested, while the behavior of the rest of the frame was modeled using a general structural analysis software. The paper discusses issues pertinent to the calibration of the computer model for the analytical substructure as well as for the entire frame, including the selection of an appropriate damping matrix, and the modeling of the buckling behavior of the braces and bracing connections. The analytical model of the entire frame was validated with the hybrid tests and was able to accurately capture the material and geometric nonlinearities that developed when the braces yielded and buckled. This study has demonstrated the usefulness of hybrid testing in improving analytical models and modeling assumptions and providing information that cannot be obtained from an analytical study alone. The results have shown that the suspended zipper frame can distribute the brace nonlinearity over the first two stories as intended in the design and will not have catastrophic failure under the design‐level earthquakes considered in this study, despite the significant inelastic deformations. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Review of selected recent research on US seismic design and retrofit strategies for steel structures

AbstractThis paper provides a brief review of selected recent work on the development of solutions for the seismic design and retrofit of steel structures by various members of the US research community. In particular, this paper focuses on research on retrofit of beam‐to‐column moment connections, frame modifications at beams' mid‐span, self‐centering systems, zipper frames, buckling‐restrained braced frames, steel plate shear walls, plastic and rotation limits for buildings, and shear links and truss piers for bridges. Copyright © 2005 John Wiley &amp; Sons, Ltd.