Date of Award

1997

Degree Type

Thesis

Degree Name

Master of Engineering (ME)

Department

Civil Engineering

Supervisor

Dr. K.S. Sivakumaran

Abstract

W-shaped steel members are widely used in various structural applications, such as buildings, bridges, and industrial complexes. The recent trend is to produce W -shaped sections using higher and higher strength steel as a replacement for mild-carbon steel, such as 300W steel. However, the width-to-thickness ratios (b/t or w/t) specified in the current CSA steel design standard (1995) for local buckling strength and ductility are based on studies using 300W steel. An investigation was carried out to study the local buckling behavior, with an emphasis placed on flange buckling of compression members, of W-shaped sections made of high strength steel. First, stress-strain characteristics of high strength steel of 350W, 480W and 700Q steel, along with the mild-carbon 300W steel, were determined using standard tensile tests. Based on these experimental material properties, analysis material models were derived for the finite element analysis. Analysis material models used in the finite element modeling were the multi-linear, modified tri-linear, and modified bi-linear models. The next part of the investigation included tests on stub columns of these selected steel grades having flanges at Class I limit based on the current design standard (CSA 1995). Based on the experimental results from these stub column lests, the applicability of b/t Class 1 limits for high strength steel sections were assessed. The experimental results showed that the reserve capacity (fu/fy) of all the stub columns were close to the sanle, regardless of the steel grade. However, the ductility of the W-shaped sections corresponding to each grade differed substantially. The 300W and 350W steel displayed much more ductility than the 480W and 700Q steel. It was concluded that the bIt limits of a Class I section, which was based on studies on the 300W steel, is also applicable to the 350W steel, but are not transferrable to the higher strength steels. The third part of this study used the analysis material models determined from the tensile tests into a finite element modeling, where a 9-node "assumed strain" shell element was employed. Due to the symmetric behavior of a stub column, a quarter of a stub column was used to simulate the W -shaped section subjected to uniform compression. Meanwhile, an idealized residual stress with parabolic distribution across the web and flange of the W-shaped section was assumed and incorporated in the finite element modeling. Comparing the finite element analysis results with the corresponding experimental results from the stub column tests, an appropriate material model was selected to be used for further finite element analysis. It was found that the results obtained from the tri-linear and multi-linear models were similar to each other, and the modified bi-linear material model best represented the experimental results. Using this bi-linear material model in the finite element analysis, strength and ductility of W-shaped sections with varying flange b/t ratios but constant w/t ratio were determined from the finite element analysis. Results showed that the 350W steel demonstrated characteristics similar to the 300W steel. However, the 700Q steel possessed very little ductility and reserve capacity. The 480W steel was excluded in this part of the study due to the uncertain results from its tensile test. In all of these steel grades, it was found that ductility and reserve capacity decreased as the bIt ratio increased. From both the experimental investigation and the finite element analysis, the b/t limits in the current design standards were found to be applicable to both 300W and 350W steel, but not to the 700Q steel.

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