CROSS Safety Report
Scaffold support overturns
This report is over 2 years old
A 5m high support scaffold screened with fabric overturned when airflow was introduced to a building.
Key Learning Outcomes
For civil and structural design engineers:
The design and construction of scaffolding requires the same degree of competence and quality as does permanent works
Give attention to the whole design of scaffold support systems and the safety-critical aspects of their fixings and anchors
Be aware that all internal structures can be subjected to horizontal loads, including wind pressures, and should be designed to accommodate these
For construction professionals:
Having a competent temporary works designer/adviser in place to supply an engineered solution can ensure all temporary works are carefully considered and planned
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This concerns a 5m high scaffold screened with fabric that was erected to protect against debris and dust from adjacent demolition works of arched structures below a building. When the demolition broke through into the arched structure, airflow was introduced producing horizontal load on the scaffold which overturned (Figure 1). It fell onto a section of a pedestrian route for the site team and fortunately there were no injuries.
The root cause of the event, says the reporter, was misinterpretation of the scaffold standard TG20:13 Good Practice Guidance for Tube and Fitting Scaffolding, in that the need for a designed scaffold was not appreciated and wind loading was not considered. The scaffold had inadequate ties to the masonry arch.
A procedure was in place that stated all classes of temporary works/scaffold should be on a register which this was not. The scaffold requirement was discussed on site at a temporary works planning meeting however, this did not cover the scaffold screens.
Expert Panel Comments
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All internal structures can be subjected to horizontal loads, including wind pressures, and should be designed as such. This is almost a classic case of not considering the safety attribute of ‘sensitivity’. The original assumption of no wind was wrong so even a slight pressure would have been serious.
Achieving robustness always requires a presumption of some horizontal forces but there is always also danger in assuming low wind speeds for temporary works designs. The force difference in say going from 2m/sec to 4m/sec is (4/2)2 = an increase of 400%.
It is essential that temporary works are designed and constructed to BS5975 and associated Temporary Works forum (TWf) guidance. Fixings needed for stability should be designed and constructed to BS8539 and the Construction Fixings Association (CFA) guidance.
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