CROSS Safety Report
Contractor uses incorrect fixing bolts for masonry support angles
This report is over 2 years old
Overview
A correspondent noticed that proprietary masonry support angles specified to support the outer leaf of masonry had been incorrectly connected to the steel beam using carriage bolts.
Key Learning Outcomes
For construction professionals:
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Quality assurance and competent supervision on site can help to ensure that the structure is built in accordance with the design
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Consider introducing a quality control procedure for the inspection of safety critical elements being delivered to ensure they are the correct ones as specified on the construction drawings
For civil and structural design engineers:
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Consider attending site and inspecting the installation of critical structural elements
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If you are unable to attend site, consider asking the contractor for site photos of the installation of critical structural elements
Full Report
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Whilst carrying out a routine inspection of a three-storey apartment building under construction, a correspondent noticed that the proprietary masonry support angles which had been specified to support the outer leaf of masonry over a large corner window opening had been incorrectly connected to the steel beam using carriage bolts.
Incorrect bolt grade
They were alerted to this by the cupped head of the bolt. On further investigation they noticed that the square section of the bolt shank located under the cupped head was clearly preventing the bolt from seating properly onto the serrated washer. They also noted that the bolt grade was 4.6, as would be normal for the bolting of timber, but not appropriate for the steel to steel connection in this instance.
They also noted that the bolt grade was 4.6, as would be normal for the bolting of timber, but not appropriate for the steel to steel connection in this instance
Fixings required for slotted connections
Of particular concern to the correspondent was that the proprietary masonry support angle was detailed in such a way that there is a high degree of setting out tolerance in the vertical plane via long slotted sections on the supporting bracket which are locked in place by serrated sections. Clearly, the bolt which had been used on site was not capable of being tightened up owing to the cupped head.
They believe that the contractor had mistakenly ordered the masonry support angle for a concrete framed building which came without any bolts. The correspondent states that the contractor rectified the issue on site with the correct kit for a steel connection being provided by the supplier.
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This is yet another report which demonstrates the value of site inspections to ensure that design intent is realised on site.
The learning benefits of site visits
As well as improving quality and safety, site inspections are also a valuable learning tool for designers, which is something that is often overlooked. By attending site and speaking to the contractors building their design, designers at the start of their careers might realise weaknesses in their ideas which were difficult to envisage from their office.
This could help them to develop an improved way to conduct their design next time around. At the very least, site visits will help all designers to develop a relationship with the contractors which should result in better collaboration to resolve any related issues that arise during construction.
There are similarities in this report with report 634 (Contractor installs incorrect steel grade), as it is not always possible to distinguish different grades of steel simply by looking at them, although bolts are typically marked on the head to show their grade.
Nonetheless, the comments on report 844 (Defects in tapered thread reinforcement bars for coupling) about the importance of quality assurance (QA) and quality control (QC) systems are particularly pertinent.
Procurement and installation issues
Regrettably UK procurement practice of masonry supports can lead to errors on site because:
Structural engineers typically provide generic concept details for incorporation by the architect into their drawings so that waterproofing and joints are shown on a single architect’s drawing. However, projects do not always have a final coordinated drawing.
Installation drawings may not exist for the particular manufacturer used and their system may not incorporate the tolerance needed.
If bricklayers are installing the masonry supports, they may modify the installation to suit the brickwork programme, without consideration for the effect this change might have. The importance of bricklayers understanding what they are being asked to do was highlighted in the Edinburgh Schools Inquiry.
Structural redundancy is inevitably limited when shelf angles may only have a single bolt fixing in each of the two serrated brackets typically used to support 1200mm or 1500mm angle lengths. Bolt tightness on the main brackets back to the structure are not often checked.