Report ID: 789

Published: Newsletter 54 - April 2019

Report Overview

This event concerns the temporary stability of a 4-storey steel frame structure with precast concrete planks and a structural topping.

Report Content

This event concerns the temporary stability of a 4-storey steel frame structure with precast concrete planks and a structural topping. A reporter says that during the erection of the structure, the contractor had provided temporary steel bracing to a number of the bays to stabilise the structure and prevent it from swaying. The bracing was in the form of flat steel plates arranged diagonally.

Whilst attending site, the visiting structural engineer found one of the bracing members unbolted at the base of a column. The column where the bracing should have been fixed to was located at the perimeter of the building where the outside ground level was lower than the internal slab level. To protect operatives from falls, edge protection had been provided. But when installing the edge protection, the bracing had been unbolted.

The situation was plainly unsafe and indicated both a disregard for safety by the operative who unbolted the bracing and a lack of control, supervision and oversight from the main contractor.

For the same project, several concerns were found about the temporary works design for the temporary stability of the frame:

1. During the project, the engineer had communicated that until the concrete planks were grouted together, the diaphragm for distributing the lateral loads should be assumed to be incomplete. The implication of this was that the stability of the structure was the responsibility of the contractor until the diaphragm was complete. The steel frame subcontractor was responsible for both the erection of the steel frame and the landing of the precast concrete planks. Whilst they were willing to take on design responsibility for the temporary stability of the steel frame without planks, they passed on design responsibility for the steel frame when the precast planks had been landed to the main contractor. This division of responsibility was unexpected and led to confusion between the two temporary works designers, with the steel frame subcontractor using moment fixity from the beam end-plate connections and the main contractor assuming perfectly pinned beam-column connections that required cross-bracing. The latter approach eventually proved to be very conservative and had to be revised to be cost effective.
2. When the main contractor began the temporary stability design for the steel frame with concrete planks, they requested to see the calculations that had been completed by the steel frame subcontractor for the frame without planks. Stability was justified based on the moment capacity of the end-plate connections and whilst this principle was sensible, there were no accompanying calculations. The structural engineer deemed this was insufficient to demonstrate that the frame would be stable during construction.

There are, says the reporter, two lessons learned from the above experiences. The first is that temporary works designers for main contractors may not have adequate experience to undertake the temporary works design for the stability of a steel frame. Given the size of the structure, the lack of calculation initially provided by the steel subcontractor was also of concern and may be indicative of a more widespread problem within the industry.

The second lesson is that splitting design responsibility for temporary works inevitability provides opportunity for confusion, but thankfully did not endanger safety in this case.

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