CROSS Safety Report
Blockwork lateral restraint
This report is over 2 years old
A recent building control inspection revealed issues with basement blockwork walls restrained by wind posts.
Key Learning Outcomes
For construction professionals:
Quality control and competent supervision on site can help to ensure that elements are installed in accordance with the design requirements
Alterations should not be made without approval from the design engineer
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A recent building control inspection revealed issues with basement blockwork walls restrained by wind posts. Blockwork walls were being constructed to form basement plant rooms. The walls were approximately 140 thick, 5m high and 20m long. Intermediate lateral supports were provided at 6m centres in the form of wind posts.
The masonry panels were designed as having support on all 4 sides. Vertical loading was from self-weight only, but horizontal loading was from differential internal pressures and a line load to account for pedestrians and trolleys moving around the basement. The wind posts were not built into the wall as normal, but were on the face, and ties were built into the wall and welded to the posts.
The reason was that M&E services in the corridor were also being supported by the posts. The blockwork contractor was concerned that movement in the wind post from the services dead load would crack the blockwork and inserted slip joints onto the ties so as to allow for some movement.
This, of course, also removed any effective restraint and was not considered by the subcontractor, nor spotted by the main contractor. After being raised as a concern by the building control officer, the slip joints were removed.
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Expert Panel Comments
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There are at least three lessons that could be learned:
All walls have lateral loads some of which might be poorly defined. Loads which are eccentric add to complexity so all walls need to be robust.
Any masonry wall that does not have substantial vertical loading needs to be looked at carefully
There is always a need to define a lateral load then have a clear (and stiff) load path back to ground
In this case, the subcontractor made amendments which altered the structural engineer’s original design strategy without reference back. What was a small change, could have led to failure, and it shows the importance of recognising design assumptions. By making the changes, the sub-contractor may have attracted designer duties under CDM Regulations 2007.