Ali Imanpour

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.

Analysis and Design of Two-Tiered Steel Braced Frames under In-Plane Seismic Demand

AbstractA seismic design strategy, which is intended to be implemented within the framework of the U.S. seismic design provisions for steel structures, is presented for single-story steel concentrically braced frames that are divided into two tiers. In this method, the columns are designed to resist the axial loads acting in combination with the in-plane flexural demand resulting from uneven distribution of brace inelastic deformations over the frame height. This design procedure, which establishes enhanced requirements beyond the 2010 edition of the U.S. seismic design provisions, prevents concentration of deformation in one tier and causes frame nonlinear deformation to be distributed between the tiers. The column bending moments depend on the story shear resistance that develops in each tier when the bracing members are at buckling and in the postbuckling range. The method also aims to control tier drifts to protect the bracing members from excessive inelastic demand, which could cause brace fracture. ...

Seismic design and performance of multi-tiered steel braced frames including the contribution from gravity columns under in-plane seismic demand

This paper examines the possibility of mobilizing gravity columns in the resistance of in-plane bending moments imposed on the columns of multi-tiered steel braced frames subjected to seismic loading. This would be the case when horizontal struts are used to connect gravity columns to braced frames at every tier level, as is often seen along exterior walls. A seismic design strategy based on the AISC Seismic Provisions is presented for four-tiered prototype steel concentrically braced frames. Three different approaches are proposed for the design of the braced frame and gravity columns. A set of 12 four-tiered X-braced frames, ranging from 15 to 30m in height and located in a high seismic area were designed based on the proposed design approaches. The seismic behavior of the frames is evaluated using nonlinear response history analysis. The results show that the seismic performance of the braced frames is improved as nonlinear seismic demand on the bracing members is reduced when mobilizing the gravity columns for lateral resistance. Furthermore, gravity columns bending moment demands are distributed between braced frame and gravity columns in proportion of their relative flexural stiffness. Adequate seismic performance and cost-effective design can be achieved when columns of both types are designed to resist their respective share of the flexural demand. Alternatively, satisfactory response was achieved when the gravity columns are verified for the seismic induced bending moments acting together with concomitant axial loads.

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.

Development of a Hybrid Simulation Computational Model for Steel Braced Frames

Hybrid simulation is an economical structural testing technique in which the critical part of the structure expected to respond in the inelastic range is tested physically whereas the rest of the structure is modelled numerically using a finite element analysis program. The article describes the development of a computational model for the hybrid simulation of the seismic collapse of a steel two-tiered braced frame structure due to column buckling. The column stability response in multi-tiered braced frames is first presented using a pure numerical model of the braced frame studied. The development of the hybrid simulation computational model is then discussed. Effects of initial out-of-straightness imperfections and axial stiffness, P-Delta analysis approach, and gravity analysis technique on the hybrid simulation results are evaluated using a numerical hybrid simulation model. Finally, the results of a continuous pseudo-dynamic hybrid simulation of the seismic response of the steel multi-tiered concentrically braced frame are presented. The test showed that failure of columns by instability is a possibility and can lead to collapse of multi-tiered braced frames, as was predicted by numerical analysis. Furthermore, suitable modeling methods are proposed for hybrid simulation of steel braced frame structures.

Numerical Evaluation of New Reduced Beam Section Moment Connection

Reduced beam section (RBS) moment resisting connections are among the most economical and practical rigid steel connections for high rise buildings that developed after the 1994 Northridge, earthquake. Section weakening concept in the plastic hinge region of beam cause to reduction of beam plastic section modulus in this region, and force plastic hinge to occur within the reduced section. In the ordinary RBS connection mostly portions of the beam flanges are selectively trimmed in the region adjacent to the beam-to-column connection that causes the plastic hinge to form in weakening section. This paper presents a new RBS connection that has been used aforesaid weakening concept in it, with this difference that corrugated plate webs instead of beam flange cutting were used in limited specific length near the column face. Corrugated plates because of their accordion effect don’t have bending rigidity. Then using of these plates in plastic hinge region reduces the beam plastic section modulus and plastic hinge is formed in corrugated region. For investigating the seismic behavior and performance of new RBS moment connection, finite element analyses were executed. All of the analytical results of new RBS connection were compared with ordinary RBS connection results. This comparison shows that the corrugated webs will improve the plastic stability and provide capability of large plastic rotation at the plastic hinge location without any appreciable buckling and brittle fractures in this region. Comparison of the seismic performance of new RBS connection with ordinary reduced beam section connection (dogbone connection) led to the conclusion that new RBS connection allows for the most stable motonic response at large story drift levels.

Numerical Evaluation of Through Plate Beam to Box Column Connection

Box column is used as one of the most desirable sections between the designers in Iran and most Asian countries .this section as column has considerable benefits such as high biaxial bending strength and higher plastic to elastic section modulus ratio which is an effective factor for force controlled components .In typical beam to column connection for this type of column section , one need to provide horizontal continuity plates in box section which is challenge in construction phase and there is no chance to provide them in hot rolled box section . This paper elucidates the seismic behavior of through-plates moment connections to box columns for use in steel moment resisting frames. This connection has lot of economical benefits such as no need to horizontal continuity plates and satisfying the weak beam-strong column criteria in the connection region .they might serve as panel zone plates as well. According to high shear demand in panel zone of beam to column joint one should use the doublers plates in order to decrease the shear demand in this sensitive part of structure but this plates have no possibility to mobilize the load transfer mechanism in column web and transfer them to column flanges. Finite element analysis was conducted to elucidate the seismic behavior of this connection; the results of finite element analysis established the effectiveness of the through plate in mitigating local stress concentrations and forming the plastic hinge zone in the beam away from the beam to column interface. The moment rotation hystersis graphs from sub-assemblage show a desirable seismic performance of this connection .the seismic behavior of this connection also has been investigated in diamond box columns.

Experimental Evaluation of Beam to Diamond Box Column Connection with Through Plate in Moment Frames

Moment resisting frames with built up section have very enhanced features due to high bending stiffness and strength characteristics in two principal axes and access to column faces for beam to column easy connections. But due to proper transfer of beam stresses to column faces there were always some specific controvertibly issues that how to make the load transfer through and in plane manner in order to mobilize the forces in column faces. Using diamond column instead of box column provide possibility to mobilize the load transfer mechanism in column faces. This section as a column has considerable benefit such as high plastic to elastic section modulus ratio which is an effective factor for force controlled components. Typical connection has no chance to be applied with diamond column.This paper elucidates the seismic behavior of through‐plates moment connections to diamond box columns for use in steel moment resisting frames. This connection has a lot of economical benefits such as no need to horizonta...