Abstract

This paper concerns the applicability of the Weibull stress approach for brittle fracture initiated at butt welds of beam to diaphragm joints. Beamto- diaphragm joint models were tested under cyclic loads. The models were designed to represent a connection of an I-section beam to an rectangular hollow section (RHS) column member with throughdiaphragms. Both sides of weld termination had through-cracks or surface-cracks. An FE analysis was conducted for the prediction of brittle fractures with a variety of defect types. The applicability of the Weibull stress approach was verified in predicting brittle fracture from such defects. Furthermore, the effect of loading modes on prediction accuracy was examined.

INTRODUCTION

Many beam to column connections in steel frame buildings failed due to brittle fracture during the Kobe earthquakes. The causes were cracks growing from singularity zone at weld tab regions in the bottom flange groove welds or the corner of cope holes in the beam to column joints. Many of these fractures occurred around beam flange groove welds starting from tips of ductile cracks that grew from geometrical discontinuities according to post-earthquake investigations.

One of the post-earthquake proposals is to use improved profiles of weld access holes (AIJ, 1997). While these new details revealed other weld defects, e.g. lack of fusion, lack of penetration, slag inclusion. Weld defects that are sharp notch can lead to brittle fractures. Therefore, the influences of the weld defects on the integrity of welded joints must be assessed to determine a tolerable flaw size in quantitative terms. This study place emphasis on the assessment of susceptibility to brittle fracture from weld defects.

For building structures, Based on fracture mechanics parameters like J integral or stress intensity (BSI, 1999), it is difficult to predict the occurrence of the brittle fracture. A conventional method using Jc gave a conservative prediction of brittle fracture because of the difference of plastic constraint (Minami et al. 1997). The effect of plastic constraint is one of the factors of difficulty. The Weibull stress based on the local approach proposed by Beremin (1983) is a new parameter for predicting the initiation of brittle fracture near crack tips.

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