The Problem of Wall Tie Failure
Wall ties are an important structural component ensuring the stability of the outer leaf of a cavity wall against wind loads. The condition and number of ties in the structure are important and where corrosion has set in or wall ties are missing or broken, then remedial ties will be required. Solid masonry may also delaminate through broken headers or deteriorated mortar, requiring remedial tying to ensure future stability.
Wall tie failure
Corrosion of galvanised metal cavity wall ties is a widespread problem and has been well documented in Australia since the Newcastle earthquake in 1989. The earthquake resulted in 13 deaths and over 150 injured. Many buildings collapsed and many lost large portions of their external cavity brick walls which were found to be unsupported, largely due to corroded wall ties. Fatalities and injuries resulted in many instances from the direct impact of falling brickwork, in others injury and death resulted from brickwork collapsing onto awnings which then collapsed under the impact of the extra weight. The event alerted building owners and professionals to the problems and risk associated with wall tie failure in cavity brick buildings.
Since 1990, codes and practices have been changed requiring the use of stainless steel and other non-corrosive materials for cavity wall ties in coastal zones where corrosion risks are greatest. However, many older buildings continue to suffer brickwork collapses during strong winds as a result of corroded wall ties. Most cavity brick buildings in Australia built before 1990 contain wall ties made from galvanised mild steel. Unexpected wall collapse can be extremely dangerous and require costly repairs.
Many older buildings, like this one in Sydney continue to suffer brickwork collapses during strong winds as a result of corroded wall ties
Corroded wall ties cause brickwork collapse on a building in Sydney
Wall tie corrosion
Indicators of wall tie failure
The majority of wall tie corrosion occurs within the bed joints of the external leaf, and not in the cavity itself, as is commonly suggested. This means that the extent of the problem of wall tie failure can be difficult to ascertain. The most assured method of correctly diagnosing the problem is to use a tie locator or metal detector to locate and mark a number of ties in each elevation, to then remove one or more bricks in the external skin and to then visually inspect the portion of the tie end embedded in the mortar joint. Other indicators include:
- Out of plumb, bowed, cracked or collapsed brickwork
- Corroded arch bars and lintels
- Rust stains in the walls
- Gaps between timber reveals and brickwork at openings
- The age and location of the building and known problems with other similar buildings in the area
