Report ID: 615
Published: Newsletter No 46 - April 2017
When precast concrete stairs are installed, the landings are normally supported using either proprietary telescopic connectors (AKA ‘invisible connections’), or by means of an RSA bolted to the wall. When an RSA is used, the wall is drilled to fix the bolts, whether they be expanding fixings or chemical fixings, and frequently the drilling may hit reinforcement.
When precast concrete stairs are installed, the landings are normally supported using either proprietary telescopic connectors (AKA ‘invisible connections’), or by means of an RSA bolted to the wall. When an RSA is used, the wall is drilled to fix the bolts, whether they be expanding fixings or chemical fixings, and frequently the drilling may hit reinforcement. In such cases the hole should be moved, and re-drilled to miss the reinforcement. This may also entail re-drilling the RSA so there is a temptation not to move the hole, but to drill in a direction that misses the reinforcement. Doing this has two dangers: firstly, the hole will no longer be circular thus reducing the contact area for an expansion fixing, and secondly the bolt head will not sit squarely against the RSA thus reducing the clamping effect onto the RSA. Most fixings require tightening to a specific torque, and this is not happening. The photograph shows clearly that virtually all the bolts in this example are incorrectly installed. It is likely that the capacity of such fixings is far below the specified value, especially as the pull-out ‘cone’ effect has been very disrupted by the skewed direction of the installed fixings. These operations fall into a very grey area of responsibility. The RSA is probably shown on the drawings produced by the pre-caster, but has to be designed by ‘the Engineer’. The erection team will expect the RSA to be ready for them when they arrive to fix the stairs, so it may fall to the main contractor to source and fix it. The person fixing the RSA will have no knowledge of the loads to be catered for, and may not appreciate the need to install a structurally efficient fixing. The Engineer who originally specified the fixings will rarely have the time or inclination to visit site to check the installation. When the erection team arrives to install the stairs, they will assume that the RSAs are suitable, and land heavy, precast landings and stairs on them. It is far from clear just who is responsible for providing a support that is fit for purpose. RSA fixings for stair landings should be subject to a stringent quality regime, covering design, installation, and checking, before any loads are installed. They are very much safety critical, but seem to be treated almost as an afterthought, where nobody takes responsibility.
It would be thought that the Lead Designer must either design and detail such supports or formally delegate the task to another party. Almost all connections are safety critical and, as has been mentioned before, fixings make up the largest category of concerns reported to CROSS. There must be a responsible designer for fixings, and adequate design of these must take account of the practicalities of installation including potential clashes with rebar. The Designer must consider how loads are supported. If the retention of all rebar is necessary within a supporting RC structure, the fixing design will need to accommodate this and cast in supports should be considered. In any event, the fixing design will need to accommodate construction tolerances without imposing additional forces on the fixings. Proprietary fixings must be installed in accordance with the design and manufacturer’s installation instructions. Guidance is provided in CFA (Construction Fixings Association) publications.
A further point is that for class 2B buildings, UK Building Regulations require these to be anchored to the parts of the structure containing the main robustness ties. There is no evidence in the report to suggest this was a 2B structure and if it was, the ties may have been provided in another way. But the point is that this creates a more robust solution that has an alternative load path should there be other issues and should therefore be considered good practice for all building types. The progressive collapse of stairs has occurred where the stair below is not capable of supporting the dynamic load of the stair above failing on to it, the collapse progressing to the bottom of the stair well. Stairs may only be heavily loaded in emergency evacuation situations and early signs of failure are unlikely to be witnessed. A structural failure in these circumstances would be a terrible thing.
Incorrectly installed bolts