This alert is produced to highlight the potential collapse hazard associated with freestanding masonry walls such as gable ends or party peaks.
An incident on a local construction site in August 2013 led to a worker being trapped by falling masonry from the peaks of a party cavity wall, resulting in significant injuries.
Masonry walls are often left freestanding during construction or demolition work. Freestanding walls not attached to any other structure are particularly vulnerable to wind loading or to other lateral forces. Their ability to withstand lateral forces such as wind loading greatly reduces as the height of the wall increases.
Newly constructed masonry walls are subjected to wind loads before the final design strength is achieved. This means that they can fall down under less severe wind conditions compared with a fully cured masonry wall.
Designers, principal contractors and subcontractors need to consider the possible instability of masonry walls during construction or demolition, and put appropriate control measures in place.
A comprehensive risk assessment of the design and proposed construction sequence must be carried out before construction work starts, to identify masonry walls which may be in a weakened state during the construction phase. The following should be considered:
- whether there are freestanding walls without returns or other structural elements providing support in the design
- the reduced mortar strength while the walls are curing particularly when retardant is used in the mortar
- the effect that damp proof membranes have on the stability of walls
- the wall height
- the likely wind conditions prior to completion of the building
- in demolition work- the fragility of lime-mortar walls
Control measures may include using temporary works such as props or shoring. These are the parts of a construction project that are needed to enable the permanent works to be built and are usually removed after use. It is very important that the same degree of care and attention is given to the design and construction of temporary works (TW) as to the design and construction of the permanent works. As TW may be in place for only a short while there is a tendency to assume they are less important. This is incorrect.
British Standard 5975 sets out one way of managing temporary works (TW) that has been found to work well on medium and large projects and uses the job title Temporary Works Coordinator (TWC). On projects with relatively simple TW needs, you may choose not to appoint a TWC. However, you must still make sure that TW are properly managed to ensure safety. Those individuals chosen to design, erect and dismantle TW must be competent in the work they are carrying out.
Where it is not possible to eliminate the risk of unintended collapse of a masonry wall the area should be fenced off to exclude workers and the public. The precautions taken to secure the site perimeter should reflect the level of risk. Only authorised people should enter and even their access should be controlled to avoid times when there is a risk of them being struck.