Racking resistance of bracing walls in low-rise buildings subject to earthquake attack

Technical Abstract

Lateral loads such as those produced by the effects of wind and earthquake can be resisted in buildings by cantilever action, by moment resisting frames, by shear walls, by diagonal bracing or a combination of these.

In New Zealand light timber frame construction, the resistance provided entirely by shear walls. The total resistance of a wall is determined by summing the dependable strengths of individual full height panels located between openings. The standard method for assessing the racking resistance of wall bracing elements between openings, since 1978, has been the BRANZ P21 test. It has been known for some time that there are deficiencies with the P21 test and evaluation procedure with major problems being whether the test loading regime can adequately identify severely degrading elements, and in the assessment of wall ductility.

A detailed literature survey of wall racking tests carried out around the world and the factors which contribute to bracing panel behaviour is given. Taking this into account, a three phase experimental programme was carried out on bracing panels under various loading protocols, including monotonic and reverse cyclic loading. The end studs to the specimens were either fully held down with tie-rods, or restrained from uplift by the application of a vertical load or use of a partial restraint. A series of experiments was also carried out with no restrain to the end studs. The test specimens were lined with sheathings commonly found in New Zealand construction.

Both the onset of damage to the panels and the displacements at which a significant drop off in load occurred were investigated.

Methodology is presented in this report to enable an accurate computer model to be matched to the test element response. Once matched, the model may then be used to analyse the performance of the element under dynamic seismic loading and to generate seismic response spectra. The result from this analysis is quantification of the mass that the test panel can dependably restrain without the necessity to assess wall ductility.