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242 Stability compromised in school roof

Report ID: 242

Published: Newsletter No 25 - January 2012

Report Overview

The project in question is a new school hall; steel framed with infill cavity masonry walls. The building is highly and unusually glazed, and as such, is reliant upon the steel frame for overall stability and to provide restraint to the masonry panels.

Report Content

The project in question, says a reporter, is a new school hall; steel framed with infill cavity masonry walls. The building is highly and unusually glazed, and as such, is reliant upon the steel frame for overall stability and to provide restraint to the masonry panels. An initial 'oversight' by the foundation sub-contractor and the main contractor allowed a reinforced concrete pad to be cast without the reinforcement cage. The pad is one of two that provide overall stability to the building.

A further issue on the same project, continues the reporter, was cause for even greater alarm. As project structural engineer, one of his roles is to carry out an inspection of the work on site and he found that masonry panel cavity head restraint ties had not been installed by the brickwork sub-contractor. Furthermore the normal cavity wall ties did not span the full cavity width one tie to each leaf had been used, loosely hooked over each other in the cavity! The bricklayers admitted that they had run out of 140mm thick inner leaf blocks and had swapped to 100mm thick blocks. This resulted in the wall ties not being long enough and the reporter could, alarmingly, wobble a 3m wide x 2m high panel of cavity wall by hand! The site operatives had every intention of 'bodging' the installation of the head restraint ties because the primary steel rafter had not been provided with vertical cleats as detailed on the drawings. They had also further risked the safety of the building occupiers (small children) by making decisions on site well outside their remit, and trying to cover it up.

Comments

The steady reduction in site supervision and inspection and the changes from engineer to contractor over the years has resulted inexorably in the greater likelihood of serious shortcomings.  In this case the contractor took design decisions, wittingly or unwittingly, which were outside his remit. Design codes explicitly require an adequate quality control system to be in place during the construction phase in order that the design is not invalidated. This is frequently ignored with predictable results. Designers should provide clear guidance to Clients as to the necessary site QM required and ensure it is included in the contract. In this case the difference in lateral stability provided by an infill masonry panel when substituting a 140mm blockwork leaf with a 100mm blockwork leaf is substantial. Effectively omitting wall ties to the cavity wall leaves is also compromising lateral stability of the wall and there can be no lateral transfer of wind load between the two leaves. The designed load paths for all elements are often complicated and are not always obvious particularly to a non engineer. Details that might look minor may be critical and design instructions to site should make that clear. Had these defects not been found the stability of the school hall would have been severely compromised. There is never any excuse for contractors to make unauthorised changes and this incident is a warning to those who think that omitting site audits is a way of saving money.


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