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796 Bridge bearing design and installation – more

Report ID: 796

Published: Newsletter No 53 - January 2019

Report Overview

A reporter says that the issue of successful co-ordination between designers, contractors and specialist bearing suppliers is one they have experienced.

Report Content

Newsletter No. 52 carried report 781 Quality of design and construction of a major bridge structure describing problems around bearing design and installation. A reporter says that the issue of successful co-ordination between designers, contractors and specialist bearing suppliers is one they have experienced.

Report 781 focused more on the design deficiencies but the present correspondent can see that some of the issues relate to lack of "push-back" from the contractor. They are concerned over bridge installation skills and experience in the UK supply chain.

As designer and client of a steel footbridge, the reporter produced a bearing specification for tender without naming specific products. The main contractor had little bridge’s experience and sub-contracted the fabrication and installation to a national supplier, who claimed to have extensive bridge’s experience; although it turns out this was largely in refurbishment.

The sub-contract was worded not to include "design elements" and the sub-contractor tried to avoid proposing bearings that met the performance specification as they viewed this as "design".

After some contractual wrangling they agreed to engage with the bearing supplier. They then proposed a set of bearings which were physically too large for the bearing shelves. In the end, the reporter liaised directly with the bearing supplier to resolve the site issues.

However, there were several possible installation methods for the whole bridge which comprised several in-line spans and multiple supports. The contract stipulated that the contractor should provide an overall installation method for the bridge in advance and in writing, so that the designer could review it, and so that certain elements of tolerance and adjustment could be taken up in relevant taper plates and layers of epoxy bedding mortar around the bearings, at the relevant stages of construction.

The sub-contractor proved entirely incapable of creating a written installation method that approached a sufficient level of detail and elected to install the bearings themselves instead of getting the bearing suppliers teams to do it - for cost reasons.

Upon meeting the operatives sent to site, it quickly became clear that they did not understand that the bolted guided and fixed bearings were only for horizontal restraint and would not take vertical load. They did not identify the elastomeric pads as bearings at all, asking "what are these for?".

The reporter had to provide the full installation briefing at the last minute. Successful installation of the bridge was due in significant part to luck, tolerances built into the design, and the full-time availability of the designer on site.

The reporter recommends that contracts must stipulate that bearing installation is to be undertaken by the bearing supplier themselves as "supply and install" to ensure the rest of the works are planned around getting this critical, precision operation correct. They would also tighten up the vetting process for client approval of bridge fabrication and installation sub-contractors to ensure they have the relevant skills, up to and including interviewing key staff.

Comments

A competent design was not matched by a contractor who had the necessary competence and experience.

Even with extensive design effort and communication from the designer, the execution was compromised through serious deficiencies in the supply chain (contractor/fabricator/erector). It is fortunate that these were overcome by the designer providing extensive support and supervision; beyond what should have been necessary.

It highlights once more that safety can be compromised by confusion at the interfaces between design and construction. No construction of a bridge is safe unless matched to the design overall assumptions, and this includes bearings which can accommodate the imposed forces and rotation and are properly installed. A universal safety attribute is to ensure that all design assumptions must be realised in practice.

Members of the site team (designer, contractor, fabricator and erector) must take ownership and responsibility for a well executed structure, as success depends not only on the right design but also the right people. Contracts should be written to facilitate this situation, not frustrate it.

The case shows again how the fragmentation of the industry and the contractual relationships are not conducive to good safe design. Having said this, where was the CDM control?

 

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View other CROSS reports published in Newsletter No. 53


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