Risks from off-site manufacture and hybrid construction

A reporter was recently investigating a ‘near miss’ involving concrete construction in which pre-cast and in-situ concrete were used in combination. This type of construction offers efficiencies and, as in this instance, can reduce the number of man-hours worked at height. It is growing in popularity. It does however bring its own risks, and these need to be understood. The works under investigation comprised a circular shaft 20m in diameter and 20m deep, which required an L-shaped shelf or balcony some 5m from the top.

The balcony is just under 4m wide at its widest point and has a chord shape in plan, being against the shaft wall on one side, and in free air on the other. Its overall length is around 16m. It should be noted that the plan shape and location do not cause the balcony to be trapped by its geometry, should it begin to move away from the shaft wall. The shaft wall is built from precast shaft lining units of standard design, with the exception that certain of the units feature a corbel, onto which the balcony is placed.

The main structure of the balcony itself is constructed from 3 precast planks, the first being of a chord shape to fit against the shaft wall, the second being rectilinear but with bevelled ends and the third, the edge-most unit, being L-shaped, again with bevelled ends. The purpose of the concrete construction is to provide a combined sewer overflow (CSO). In normal circumstances, water is to flow across the balcony and be contained by the it’s upstand. At periods of high flow, water is to build up on the balcony (the in-flow pipe being larger in diameter than the outflow pipe) until it weirs over the upstand into the shaft, which serves as a storage tank.

In-situ concrete was used to provide headwalls for the in-flow and out-flow pipes. The headwalls are in fact large blocks of concrete, each approximately 2,500mm long and 1,650mm high. They were built using the balcony upstand to contain the concrete. During concreting, the upstand of the L-shaped pre-cast beam therefore acted as a ‘shutter’ and, owing to the geometry, had on it the same horizontal force as would a shutter giving a force of around 220kN. It had been assumed that the weight of the L-shaped pre-cast unit, 460kN, plus the weight of the in-situ concrete itself (around 190kN) was sufficient to ensure enough friction between the beam ends and the corbel to allow the in-situ concrete to be contained without any need for the beam to be fixed in place.

This proved not to be the case and the edge-most unit slid towards the corbel’s edge, but did not quite fall off. It should be noted that, as the precast units are parts of the permanent works, the temporary state was not identified as Temporary Works and so the management procedures of BS5975 were not applied. In fact, no calculations at all were made in relation to the temporary condition of the permanent works. Figure 1: Isometric view of balcony slab Fig 2: Diagram illustrating the forces acting on the ‘Part C’ upstand beam during the temporary condition of placing wet concrete The reporter asks the industry to be mindful on the following points. While they are drawn from this case specifically, they often apply in all cases where off-site manufacture or hybrid construction are used.

• In developing the design, be clear about who is responsible for what aspects of the design and for what phase of the assets lifecycle. In this case The Principal Contractor was a JV with a design consultant as a member of the JV. It was assumed that this member of the JV would be responsible for all issues associated with the permanent works design. Any such assumption needs to be made explicit as typical JV arrangements will not review matters such as this and all parties will be jointly and severally liable.

• Aspects of the design and precast erection methodology were sub-let to the precast supplier. While responsibility for the detailing of the elements was clear enough, the responsibility for ensuring that the elements were able to fulfil temporary works, temporary condition and permanent works roles needed to be clearer. Sub-let contractors design portions need coordination, and both the Principal Contractor and the Lead Designer (under CDM2015, the Principal Designer) have responsibilities.

• Principal Contractors should refresh their memories that they are expected to co-ordinate all temporary works and construction methodology to ensure the safety and welfare of all on and adjacent the site; the role of the Temporary Works Coordinator is clear: it is to coordinate the work of all who have an influence on the temporary works irrespective of commercial boundaries. The involvement of sub-contractors does not detract from this duty; if anything the involvement of sub-contractors enhances it.

• Notwithstanding explicit design responsibilities, design management processes need to involve cross-checking (is what we are assuming to be happening, actually happening?) and double-checking (is what I am told to be correct, actually correct?)

• The edge-most pre-cast beam weighed 460kN and the weight of concrete it supported was around 190kN. While it might seem natural that these weights are sufficient to provide the necessary friction, this was not the case. The matter at hand concerns pressures not weights, the active pressure pushing the edge-most unit being around 220kN, and the frictional resistance being around 162kN. The frictional resistance would have been somewhat reduced by the bedding membrane. The industry is asked to be very careful when making assumptions concerning the adequacy of concrete (or other dead weights) to resist loads by friction: the structural forces and frictional behaviour must be fully understood and worked out, and a high factor of safety (at least 2) allowed for friction. (It is noted that even a concrete-to-concrete bearing, with a coefficient of friction of say 0.4, would have given too low a frictional resistance for a full FOS.)

•  The bedding membrane was introduced during construction in the spirit it being ‘industry good practice’ to place a membrane between heavily loaded pre-cast units. The construction team did not appreciate the criticality of the reduction of friction this would cause.

• It is noteworthy that the permanent works designer was not sufficiently au fait with precast detailing to include a bedding detail, or, knowing that it is common practice to have one but actively not want one, to specifically exclude it. The importance of designers being familiar with the craft, or site, aspects of what they are designing is again emphasised, as is the importance of the site team building to the design details or, if they think they should be changed, raising this matter formally. 

• This being an industrial structure, it was not tied, as would been the case for a residential or building structure. Tying would (i) probably have given the restraint needed in the temporary condition (ii) be good practice for the permanent works in any case. Designers are asked to give thought to robustness of all structures.

• It should perhaps be noted that the shaft is sited under a public carpark; in the long term a failure of the landing slab could have knocked out the columns supporting the roof slab which in turn could have brought down the roof; a gapping 20m diameter by 20m deep hole suddenly appearing in such a location certainly has the potential to be a catastrophic structural failure. Such consequences should be considered when determining the level of robustness required.