CROSS Safety Report
Safety concerns with twin wall system
This report is over 2 years old
A reporter raises safety concerns over the temporary stability of twin wall systems after they observed failures during construction.
Key Learning Outcomes
For the construction team:
Quality control and competent supervision on site can help to ensure that the structure is built in accordance with the design
Having a competent temporary works designer/adviser in place to supply an engineered solution can ensure all temporary works are carefully considered and planned
Verification of temporary works erection by a competent person who can oversee and coordinate the whole process can also ensure the works are installed correctly
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Twin wall systems are hybrid in-situ/precast products used to construct concrete walls quickly. Panels are typically 3m high, up to 8m long and the leaves are in the range 65-80mm thick with a 50-100mm void. A panel can weigh 10T or more. Some failures have been observed on civil engineering projects by a reporter. They say that starter bars must be coordinated with the lattices which hold the two leaves together. Any damage to the lattices will radically reduce the ability of the panel to resist the pressure of the wet concrete tending to push the two leaves apart.
Temporary support of panels
Raking props are used to give lateral restraint to the head of the panels. Properly engineered equipment is needed to meet the minimum 2.5% lateral restraint load, or the calculated wind load, whichever greater. A particular risk is that the fixings at the bottom of the props lack strength, because they are connected to an immature concrete slab. Then, for the props to form a rigid triangulated structure with the panel, the base of the panel must be restrained against horizontal movement. Often a panel is shimmed to level, and the shims have very little friction. If there are two props along the length of the panel and one is adjusted in length, but the others is not, the panel will rotate in plan adding an over-turning moment to the system (Figure 1).
Failure of panel
Also shims may be kicked out or work loose. A combination of the two effects above was identified as the cause of a panel over-turning on a recent project; fortunately no one was injured (Figure 2). The concrete mix, its temperature and the rate of rise of the pour, all influence the shutter pressure. Because the void is so thin, the rate of pour may need to be very low. The way in which the twin-wall panels are made affects their performance as the ability of the panel to resist the pressure of the site-placed concrete is therefore entirely reliant on the 20mm, or so, embedment of the lattice into the (interior) cover zone of the panels. It is suggested by the reporter that this capacity is somewhat uncertain.
Expert Panel Comments
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The reporter is right that the starters need to be positioned to not clash with the lattice and there may be a temptation to force the panel into place or bend bars to make it fit. Sometimes kickers are used for positioning and to form a shear key. Because this type of construction is inherently quick the bolts which fix the bottom end of the inclined prop are likely to be into immature concrete.
There is a role here for the Temporary Works Co-ordinator (TWC) who is responsible for ensuring that the contractor’s procedures for the control of temporary works are implemented on site. The TWC is responsible for ensuring that a suitable temporary works design is prepared, checked and implemented on site in accordance with the relevant drawings and specification. The principal activities of the TWC are listed in Clause 7.2.5 of BS5975:2008. This, in conjunction with appropriate Designer identification of risk (CDM 2007), should reduce the risk. The TWC would be expected to require a base tie to fully triangulate the system and not rely on marginal base connections.
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