CROSS Safety Report
Masonry panels rock in wind due to missing wall ties
Wall ties designed to connect masonry partitions to adjacent steelwork framing were not installed leaving the masonry walls unrestrained. Walls were seen to be rocking in high winds.
Key Learning Outcomes
- Consider the significant benefits derived from independent inspection of works in progress
For architects, engineers, and other building designers:
- Ensure correct wall tie densities, embedment lengths and all fixing details are shown on construction drawings
- Be aware that ‘buildability’ can significantly impact what is built
- During site visits check that the correct wall ties are being installed adequately
- Be aware that incorrectly installed wall ties can lead to wall failures
For builders and client’s site representatives:
- Quality assurance processes and competent supervision can help to ensure that the structure is built adequately
- Consider introducing a quality assurance procedure for the inspection of safety critical elements such as wall ties and masonry restraints
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A reporter saw that partition walls in an industrial unit under construction were rocking in high winds. The 215mm thick masonry walls were being built between 6m high steel columns which were acting as windposts.
An investigation found that wall ties specified to fix the masonry to the windposts had not been installed leaving the walls unrestrained above floor level. It transpired that the main contractor’s representative had witnessed some wall ties being fixed to the steel posts but had not witnessed all areas of the masonry erection. At the time of building the walls, the mason had apparently protested that it was difficult to install the wall ties. The reporter, therefore, thought it likely that the mason simply did not install ties when not being observed. Other walls of various heights within the building were found to be similarly unrestrained.
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Expert Panel Comments
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A lack of appropriate ties or appropriately fixed ties has been the cause of many wall failures. The lack of embedment of ties between leaves of cavity walls contributed to a collapse at a school in Edinburgh and major remedial work to many others. The Standing Committee on Structural Safety (SCOSS) Alert Inquiry into the construction of Edinburgh Schools published in 2017 provided an overview of the failings investigated and a number of the recommendations that followed.
The absence of wall ties is a critical failing that should never happen. As in this reported example, the ties connect the masonry to the structure and without these ties, the masonry may be inadequate and unstable. Windposts in walls, externally or internally, provide support to the masonry dividing the walls into panels which are then adequate to resist wind and lateral loads. Without the windposts (and adequate ties between the masonry and the windposts) the masonry may collapse. Internal masonry, such as the partition walls reported, will still likely be exposed to significant lateral loadings during its life. The walls will be subjected to wind loads during construction when the external shell is incomplete, as appears to have been the case here. The walls will also be subjected to wind loadings in the finished building (especially if there are dominant openings) or indeed, be subjected to lateral loadings arising out of the use of the building. The installation of wall ties as intended under the design is therefore a critical activity.
the installation of wall ties as intended under the design is a critical activity
The designer’s intent must be clearly communicated to the persons building the walls. The masons on-site must have clear and understandable information showing all the details of what they are to build. Details of all masonry units, mortars, ties, masonry reinforcement, windposts, flashings and DPCs must be provided in a suitable format.
In the case reported here, the mason had apparently protested that it was difficult to install the wall ties. Whilst difficulty of installation was absolutely no excuse not to install the ties, designers should note that ‘buildability’ is a key part of design, and that more buildable designs will be less difficult to construct as the designer intends.
designers should note that ‘buildability’ is a key part of design, and that more buildable designs will be less difficult to construct as the designer intends
Contractors should be encouraged to engage competent masons. Supervision of all works by the contractor must be adequate and appropriate to the works in hand. Where works are hidden from view on completion (such as wall ties built into masonry) then the supervision arrangements must be sufficient to establish adequacy as works proceed, or compliance testing undertaken.
Clients should consider inspection arrangements
Independent inspection of work, through a clerk of works, resident engineer, or other site inspectors, can lead to the early identification of inadequate work but can also change the mindset and approach of those working on-site. This change in mindset and approach can improve both the quality and effectiveness of completed work. Designers visiting sites can help ensure that their design intent is understood and implemented. For all projects, regardless of scale, clients should consider what inspection arrangements are required taking into account safety criticality, complexity and many other aspects of the work, including the impact of poor quality work and subsequent failings on their organisation’s standing and reputation.
All clients, designers, contractors and operatives, particularly masons, should be very aware that poorly built walls can collapse. In the event of a failure of the wall causing death, there would likely be investigations for gross negligence manslaughter for an act by an individual and for corporate manslaughter against any organisations involved.
In the event of a failure of the wall causing death, there would likely be investigations for gross negligence manslaughter