ABSTRACT

This paper concerns the applicability of prediction approach of brittle fracture initiated at toes of weld access hole. In order to find suitable shape of beams copes in the field-welded joint, the cyclic loading tests using beam-to-diaphragm joint models, which were designated to represent beam-to-column connections with through diaphragm, were conducted. Experimental results showed the improved beams copes had sufficient deformation capacity. The applicability of the TSM approach, in predicting brittle fracture from cracks was verified.

INTRODUCTION

The 1994 Northridge and 1995 Kobe Earthquakes both took structural engineering professionals by surprise in that many of the welded connections in modern steel building frames sustained brittle fractures. (Azuma et al. 2001) Both the Kobe earthquake and post-earthquake investigations using many replicate specimens demonstrated that brittle fracture could occur in the beam-to-RHS column connections with through diaphragms. These fractures most frequently occurred in regions around toes of beam copes or defects at weld tab regions in the beam-to-column joints. Although many experimental researches carried out to avoid brittle fracture at weld tab regions and some proposals have been put into practical use, remaining challenges are how to prevent brittle fractures at toes of beam copes. As a solution for these issues, one of the post-earthquake proposals is to use improved profiles of cope holes or no access hole. These new details, however, revealed other weld defects. The field-welded connections can be more economical than shop-welded connections, because beam splices may be omitted if the connections are field-welded. However, since the field welding of the bottom flange required the flat welding, the no weld access hole procedure can't be used because the beam web interrupts the weld line.

The critical fracture toughness, Jc is one of important parameters for evaluating the brittle fracture. Jc was obtained from fracture toughness test, such as a single edge notched bend (SENB) tests (BSI, 1999). A deep notch in SENB test pieces, which was approximately a half of length of the thickness of those, was subjected to high plastic constraint compared to cracks and defects found in welded connections. A conventional method using Jc gave a conservative prediction of brittle fracture (Minami et al. 1997). The plastic constraint condition which varies depending on the size and location of the defect affects the occurrence of brittle fracture.

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