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414 Blockwork lateral restraint
Report ID: 414
Published: Newsletter No 37 - January 2015
Report Overview
Blockwork walls were being constructed to form basement plant rooms. The wind posts were not built into the wall as normal, but were on the face, and ties were built in to the wall and welded to the posts.
Report Content
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 in to 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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There are at least three lessons that could be learned:
a) 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,
b) any masonry wall that does not have substantial vertical loading needs to be looked at carefully,
c) 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. Also by making the changes, the sub-contractor may have attracted designer duties under CDM2007.