Strengthening commercial buildings

Like residential buildings, strengthening of commercial buildings is primarily concerned with improving the connections between structural elements, increasing the structure’s capacity to resist seismic loads and improving the connections between non-structural elements and the main structure.

Unlike residential buildings, where seismic strengthening improvements to existing buildings are voluntary, commercial buildings can pose a significantly greater risk to the public during an earthquake. Therefore, all buildings, not just new builds, are required to comply with a minimum level of seismic performance.

Unreinforced masonry

URM

Unreinforced masonry structures, in this case, a double-brick wall, are prone to damage during an earthquake. (BRANZ)

Unreinforced brick and concrete masonry (URM ) structures without strengthening are very likely to collapse in a major earthquake.

The majority of buildings constructed using URM consist of external structural masonry walls in conjunction with cast iron or steel columns and timber beams. Internal masonry walls may also be present that support flexible timber floor diaphragms and roof trusses. The seismic performance of a URM building is largely determined by the quality of the connections between the external URM walls and the flexibility of the internal timber diaphragm . These connections often consist of steel anchors, which are also used to restrain features such as parapets. The brittle nature of the URM walls means that, if they are overloaded, they will collapse, and the support for the floors and roof is lost.

It may be possible to add retrofit bracing to support URM structural walls and contain the out-of-plane movement. Retrofitted bracing must be carefully designed with professional engineering input to ensure it has a sufficiently stiff response to protect the URM before it is damaged.

Some of the more common types of URM retrofit include the addition of:

  • steel strongbacks, which restrain the wall and prevent out-of-plane failure
  • a sufficiently stiff steel moment frame, which supports the walls and increases the building’s ability to resist lateral movement
  • shotcrete to the face of the brick wall, which increases the strength of the walls.

Many URM buildings in Canterbury that had been retrofitted with these systems showed no sign of damage. However, others were still severely damaged, with poor connections between the old and new components being one of the common contributing factors. In most cases, however, the retrofits did prevent the building from collapsing.

figR3

An example of a steel moment frame retrofit used to support a URM building. (Jason Ingham, University of Auckland)