Seismic performance of mid-rise code-conforming X-braced steel frames

Prolixity and complexities inherent in the nonlinear dynamic analysis (NDA) impel engineers to lean towards nonlinear static analysis (NSA) in practice. This paper partly explores differences and similarities in responses obtained from these two simulation techniques. The scope of the study is narrowed down to ubiquitous regular code-conforming mid-rise steel X-braced frames. Different common lateral load patterns are coupled with NSA to study their interactive effects on responses. The modal nonlinear static analysis is also carried out, where deemed necessary. NDA using three different earthquake records are conducted as well, to validate NSA results. Results of nonlinear analyses are undertaken to compare provisions of seismic rehabilitation code with those of seismic design codes. Base shear, story drift, lateral displacement profile obtained for each model are independently thoroughly discussed. Eventually, some suggestions to improve design code requirements are accordingly provided, as applicable

Effect of Wind Load on Structural Performance of Dimensionally Regular & Irregular High rise Buildings with different Outrigger Systems

Outriggers system is used as one of the best way of increasing the lateral stiffness and has been generally utilized in tall building structures. The outrigger system is very effective in increasing structure’s flexural stiffness. It is likewise viable in decreasing the horizontal top deflection and bending moments in the core of high-rise frame-core structures. This study presents the detailed analysis on how the top drift, inter-story drift, base moment in the core are influenced by the outriggers stiffness, outriggers optimum locations, columns axial stiffness and foundations flexibility. outrigger braced structures can strengthen a structure without disturbing its aesthetic appearance and this is a significant advantage over other lateral load resisting systems. The thesis compare between many outriggers systems including both concrete & steel Outriggers. The Rectangular shape & L- shaped building structures are taken for analysis. The study also presents simplified procedure to optimize the location of the outriggers that will result in a maximum reduction in the lateral displacement at the top of the building. The modeling of the structure is done using “ETABS” program. The investigation of the model is completed by comparable static technique. Finally, the thesis proposed a simplified analysis for outriggers structures with core for both regular and irregular high rise building

Evaluation of Seismic Performance of Improved Rocking Concentrically Braced-frames with Zipper Columns

Concentrically braced frames (CBFs) as one of well-known stiff and common lateral force resisting systems often show limited ductility capacity under severe earthquakes. This study proposes rocking zipper braced frame (RZBF) to improve the drift capacity of CBFs which is based on combination of rocking behavior and zipper columns. In the RZBF system, rocking behavior permit the braced frame to uplift during the earthquake and then restoring force induced through post-tensioned bars self-center the frame to its initial state. Also, zipper columns can decrease the concentration of damage by distributing the unbalance force at the mid bay over the frame’s height. To assess the performance of RZBF, a comparison study is carried out considering CBF, rocking concentrically braced frame, zipper braced frame and RZBF. For this purpose, some frames structures are designed and nonlinear time history analysis conduct under a set of earthquake records. Seismic responses such as roof drift ratio, gap opening at the column-base interface, forces of top story braces and post-tensioned bars are taken into consideration. The results show that the proposed RZBF has better performance among the others and zipper columns can improve the behavior of rocking systems

Framework to Explore the Design Space for Design of Tall Buildings

Design of tall buildings is undergoing a resurgence that is driven by a variety of factors – economical growth, scarcity of land in urban areas, high land costs, increased population density, technological advancements and man’s desire to build taller structures. Considerable research work has been done in the last two decades to meet this demand. Computer-based tools that help design engineers explore design alternatives are indispensable in tackling this complex problem. In addition, a framework that finds the near optimal design, adds value to this exploratory work. In this paper, we develop a general framework for the design optimization of buildings using sizing, shape, and topology design variables. Sizing optimization can be carried out using discrete design variables (from a database of available sections) or continuous design variables (cross-sectional dimensions of custom wide flange sections). Similarly, shape optimization can be carried out using either discrete or continuous design variables. And finally, topology optimization can be carried out using boolean design variables. Allowable stress design guidelines are used as constraints along with displacement, inter-story drift, total structural weight, and frequency constraints. The finite element model is made of three-dimensional beam elements. A typical function evaluation involves a linear, static analysis with multiple load cases, a linear, modal analysis to extract the lowest few eigenpairs, and a linear, buckling analysis to find the buckling capacity. An optimization toolbox that contains gradient-based and population-based optimizers, is a part of the framework. Numerical results how that the framework is capable of producing efficient designs effectivel

Numerical comparison on the efficiency of conventional and hybrid buckling-restrained braces for seismic protection of short-to-mid-rise steel buildings

Buckling-restrained brace (BRB) is a specific kind of bracing system which has an acceptable energy dissipation behavior in a way that would not be buckled in compression forces. However, considerable residual deformations are noticed in strong ground motions as a result of the low post-yield stiffness of the BRBs. The seismic performance of a modern lateral load resisting system, which is called the hybrid BRB, and its conventional counterpart are assessed and compared in this paper. Multiple plates with different stress–strain behavior are used in the core of this new innovative system, and this is its difference with the existent BRBs. Nonlinear static and incremental dynamic analyses are carried out for three building frames with different structural heights, which use conventional and hybrid BRB systems. To carry out response history analyses, the FEMA P695 far-field earthquake record set was adopted in different hazard levels. The hybrid BRBs are shown to have superior seismic performance in comparison with the conventional systems based on the response modification factor and the damage measures including residual displacements and inter-story drift ratios

Optimal Location of Energy Dissipation Outrigger in High-rise Building Considering Nonlinear Soil-structure Interaction Effects

Buckling-restrained braces (BRBs) emerged to improve the seismic performance of high-rise structures as compared to the ordinary diagonal bracing. In this paper, the seismic performance of braced buildings with the BRB outrigger system is investigated to determine the optimal configuration of BRB outrigger, considering the nonlinear SSI effect. For this purpose, the nonlinear dynamic analysis is carried out on four braced buildings with a BRB outrigger system placed on three different soil types. The outrigger configuration changes from first to the top story to capture the seismic performance of different locations of BRB outrigger. It is observed that the outrigger location affects the seismic performance, which is measured in terms of inter-story drift ratio, story displacement, story shear, and energy dissipation capacity. The results are compared to the fixed base condition buildings, which proves considering SSI, shifts the optimal location to the upper story of the structure. Moreover, the effect of soil’s stiffness on the seismic responses of structures and the optimal BRB outrigger location is investigated. Finally, the merits of BRB outrigger are shown by comparing its seismic performance that of the conventional outrigger, under frequent, basic, and rare earthquakes. The results show that the optimal locations of different 2-D buildings rested on the dense soil, medium soil, and soft clay are obtained at 0.6, 0.65, and 0.7 of the building’s height (H), respectively. Also, the results show that the optimum location of the BRB outrigger system based on the energy dissipation criteria is 0.45H to 0.65H

Studies on the Design and Behavior of Strongback Braced Frames

When subjected to strong earthquake ground motions, conventional steel braced frames are vulnerable to soft-story mechanisms, whereby the weakest story accumulates more damage relative to the rest of the structure. This reduces the overall strength of the structure and increases the cost of repairs. One method for mitigating this behavior is the use of a stiff vertical “spine” with a more ductile, energy-dissipating system. The spine typically spans the height of the structure and is designed to remain elastic, distributing earthquake demands across the height of the structure and bridging weak stories. One proposed frame is the “strongback” braced frame (SBF), which merges a steel buckling-restrained braced frame and an elastic truss, using the buckling-restrained braces for energy dissipation and the truss for force distribution. Three studies are presented here on the behavior and design of strongback braced frames. The first considers the effect of the strength and stiffness of the spine in SBFs, and how methods to design the spine vary. Designing the spine is challenging, as higher mode effects and partial nonlinear mechanisms have been shown to be significant. The results show that two proposed SBF-specific design methods provide value over a control procedure. Both SBF-specific methods produce designs with reserve strength in the strongback, but one is much more conservative. The second considers the effect of the vertical distribution of the energy dissipators in the frame, and introduces a design method for the energy dissipators that supports the design of frames with non-standard dissipator configurations. The design method shows promising results, but the existing methods for design of the strongback members have inconsistent results when used with the non-standard dissipator configurations. The third considers the mitigation of higher-mode effects by using multiple pivoting segments in the spine, and how passive control devices between the segments impact the behavior of the frame. The results show that multiple segments reduce demands on the strongback braces, but increase other demands such as maximum story drift, and the use of passive control devices allows finding a medium configuration between one- and two-segment spines, with benefits over both

Seismic performance of mid-rise code-conforming X-braced steel frames

Prolixity and complexities inherent in the nonlinear dynamic analysis (NDA) impel engineers to lean towards nonlinear static analysis (NSA) in practice. This paper partly explores differences and similarities in responses obtained from these two simulation techniques. The scope of the study is narrowed down to ubiquitous regular code-conforming mid-rise steel X-braced frames. Different common lateral load patterns are coupled with NSA to study their interactive effects on responses. The modal nonlinear static analysis is also carried out, where deemed necessary. NDA using three different earthquake records are conducted as well, to validate NSA results. Results of nonlinear analyses are undertaken to compare provisions of seismic rehabilitation code with those of seismic design codes. Base shear, story drift, lateral displacement profile obtained for each model are independently thoroughly discussed. Eventually, some suggestions to improve design code requirements are accordingly provided, as applicable