Zhichao Lai

Analysis and Design of Noncompact and Slender CFT Beam-Columns

AbstractConcrete-filled steel tube (CFT) beam-columns are categorized as compact, noncompact, or slender depending on the governing slenderness ratio (width-to-thickness b/t or D/t ratio, λ) of the steel-tube wall. The current AISC specification recommends the bilinear axial force-bending moment (P−M) interaction curve for bare steel members for the design of noncompact and slender CFT beam-columns. This paper compiles the experimental database of tests conducted on noncompact and slender CFT beam-columns, and demonstrates the overconservatism of the AISC P−M interaction curve. This paper also presents the development and benchmarking of detailed 3D finite-element models for predicting the behavior and strength of noncompact and slender CFT members. The benchmarked models are then used to evaluate the fundamental P−M interaction behavior of CFT beam-columns, and the influence of material and geometric parameters such as the tube slenderness ratio (λ), material strength ratio (Fy/fc′), member length-to-sec...

Experimental Behavior and Design of CFT-RC Short Columns Subjected to Concentric Axial Loading

AbstractConcrete-filled steel tube (CFT)–RC composite members consist of multiple CFT columns connected by RC plates or webs. CFT-RC composite members have been used in different types of structure...

Retrofit of Built-Up Steel Columns: Modeling and Fundamental Behavior

AbstractCover plates are often added to built-up steel columns as an effective retrofit method for increasing strength. For retrofitted columns, the original member carries all loads, whereas the added cover plates resist only the loads applied after their addition. As a result, the original member may yield before the added cover plates. Although some specifications provide guidance for evaluating the capacity of retrofitted columns, the approaches are conservative and do not address the behavior of retrofitted columns if portions of the original cross section yield first because of preload. In this paper, the behavior and strength of retrofitted built-up columns, as well as the influence of the cover plates, are evaluated using two analysis approaches: (1) three-dimensional finite-element analysis and (2) nonlinear inelastic column buckling analysis. Both approaches include the effects of geometric imperfections, column slenderness, cyclic loading, and temperature gradient on the behavior and strength o...

Retrofit of Steel Columns: Parametric Studies and Design

AbstractCover plates are often added to existing (loaded) steel columns as an effective retrofit method for increasing their compressive design strength. Current bridge design specifications provide equations for calculating the compressive design strength of steel columns. However, these equations cannot be extended to calculate the design strength of retrofitted columns because they do not account for the effects of cover plates added to the existing loaded members. The authors previously investigated the fundamental behavior of retrofitted built-up steel double-box columns using the finite-element analysis approach and also developed and benchmarked a nonlinear inelastic column buckling (NICB) analysis approach that accounts for the mechanics of existing (loaded) built-up steel columns retrofitted by added cover plates. This paper presents the results of comprehensive parametric studies conducted using the benchmarked NICB analysis approach to evaluate the behavior and strength of retrofitted steel col...