Contributions on the lamellar tearing and fracture resistance of heavy sections for use in seismic loading conditions
Authors: Lindley C , Bateson P H , Bannister A C , Pike Dr T J 
Published: August 2002

Major earthquakes occurring in Northridge, USA in January, 1994 and Kobe, Japan, in January 1995 focused attention on the seismic performance of steel structural sections, particularly thick‐flange ‘jumbo’ columns used in conjunction with welded beam–column connections. Subsequent investigations identified joint design, poor fabrication, and low weld metal toughness as significant factors in the instances of cracking which occurred in and around the special moment‐resistant frame (SMRF) welded beam–column connections. The importance of high‐quality welding was highlighted and some concern was directed at through‐thickness properties in large sections. Consequently, a programme of research has been carried out at Corus Group's Swinden Technology Centre to evaluate the lamellar tear, fracture resistance and structural performance of typical sections, and confirm their suitability for use in seismic zones.

Laboratory and full‐scale test methods were devised to evaluate the capability of different sizes of section to resist the formation of cracks and tears during the welding of connections made to accepted US practice. These tests involved typical fabrication welding followed by ultrasonic examination, sectioning and metallographic examination of ∼ 40 sections from the welds to detect any evidence of cracking. The results from >30 tests show that, provided normal fabrication practice to control restraint levels is followed, the risk of cracking in Corus sections is very low.

An experimental programme incorporating a small number of ‘structural’ tests was carried out to assess the performance of sections subject to the types of stresses generated during earthquakes. A method, based on the fracture mechanics ‘wide‐plate’ test was used to measure the resistance to fracture of typical 125‐mm‐thick ‘jumbo’ column flanges under extreme through‐thickness tensile stresses. Through‐thickness fracture resistance was more than adequate, with fracture occurring in the welds at high strains (2–5%) and overall stresses well in excess of the columns' yield strengths. In addition, two cyclic loading tests, to a standard US test protocol, were carried out on SMRF welded beam–column connections. The loading sequence consisted of incrementally increasing fully reversed cycles under displacement control, and the tests were continued until general yielding and eventual failure of the connection occurred. In common with results from other test programmes and the Northridge experience, the joints failed by fracture initiating at the beam–column welds after significant yielding of the beams.

Further to the weldability and structural testing programmes, an assessment has been carried out using probabilistic fracture mechanics procedures to determine toughness requirements for sections under seismic loading. Finite element models were used to determine applied stress distributions, residual stresses from welding were estimated from published profiles, several potential flaw sizes and locations were considered and material property data were obtained from Corus databases. The results indicate that Charpy impact toughness requirements of around 27 J at +20°C are generally appropriate.