A quick plaster’s often not enough

This Issue This is a part of the Remediation feature

By - , Build 131

With repair work to minor damage on Canterbury houses well under way, some are asking if the repairs are adequate. Sometimes, only cosmetic makeovers have been done when screw fixing of wallboard bracing is needed, writes a Christchurch architect.

Figure 2: Severely racked bracing element. The corner of the plasterboard sheet has broken off. A new sheet will be required, fixed in a bracing pattern.
Figure 1: Considerable movement and damage indicate wallboard fixing failure – the wallboard fixings must be renewed.
Figure 3: Diagonal cracking of wallboard at a door head. This wall will have reduced resistance to horizontal forces, and the structure will no longer perform as expected.

There has been considerable debate following the Canterbury earthquakes about the effectiveness of wallboard sheet bracing. Homeowners and occupiers generally say their houses are now noisier and move and creak more than before. Although wallboard bracing has performed well, given the severity of the events, some structural damage must be expected.

From what I’ve seen, some project management office (PMO) or earthquake recovery (EQR) remedial providers are ignoring some obvious structural issues and classifying damage as merely cosmetic. In some houses that have had a quick rake out, stop and paint makeover, cracks have opened up again after minor aftershocks.

Wallboard bracing

NZS 3604:1978 Code of practice for light timber frame buildings not requiring specific design introduced the use of a combination of plasterboard-based wallboards and diagonal timber or steel cut-in braces as bracing elements. As NZS 3604 developed, tested systems such as plasterboard-based wallboards have been permitted to provide the total bracing resistance of light timber-framed buildings.

Figure 1: Considerable movement and damage indicate wallboard fixing failure – the wallboard fixings must be renewed.

Once correctly fixed, taped, stopped and the framing held down, plasterboard wall linings become the stiffest element in a wall’s construction and will be subjected to horizontal forces first, even if not designed to do so.

Wallboard bracing uses specialised screw fixings and spacing. When stressed excessively, fasteners compress or damage the core plaster around their fixing holes and the bracing element loses some of its stiffness. Considerable deflection is now needed before the wallboard will start to resist wind or earthquake forces.

Failure of the bracing element

In addition to detailed panel fixing, some wallboard bracing systems require specialised hold-down brackets to transfer the resisted forces to the floor and foundation structure. Failure of these elements is usually evident by diagonal cracking or movement between the wall and floor skirting board (see Figure 1). Diagonal cracking from opening corners (see Figures 2 and 3) indicates failure of the bracing element.

Figure 2: Severely racked bracing element. The corner of the plasterboard sheet has broken off. A new sheet will be required, fixed in a bracing pattern.
Figure 3: Diagonal cracking of wallboard at a door head. This wall will have reduced resistance to horizontal forces, and the structure will no longer perform as expected.

Damage must be repaired to provide at least the bracing resistance that was available prior to the earthquake. When bracing requirements are designed, only the elements needed to achieve the necessary resistance are normally calculated, with lined walls not designated as bracing elements being ignored. These other walls will have attempted to resist the load at the same time as the bracing walls and will also have sustained some damage. They must also be refixed.

Bracing resistance needs reinstating

As the earthquake remediation work ramps up in Christchurch, contractors and project managers, acting on behalf of EQC and private insurance companies, need to acknowledge that house bracing resistance may have been compromised. Important bracing elements must be remediated to restore the bracing resistance to the level it was before the earthquakes.

As noted, structural failure is often being ignored or treated as cosmetic. It may be far from cosmetic and can be a serious structural failure. Where damage is evident, walls must be repaired to reinstate bracing resistance.

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Figure 2: Severely racked bracing element. The corner of the plasterboard sheet has broken off. A new sheet will be required, fixed in a bracing pattern.
Figure 1: Considerable movement and damage indicate wallboard fixing failure – the wallboard fixings must be renewed.
Figure 3: Diagonal cracking of wallboard at a door head. This wall will have reduced resistance to horizontal forces, and the structure will no longer perform as expected.

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