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CROSS Safety Report

Steel stud failures during bridge construction

Report ID: 332 Published: 1 October 2012 Region: CROSS-UK

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Overview

This report concerns the failure of a steel studs during construction works on a new bridge associated with major road improvements.

Key Learning Outcomes

For the construction team:

  • Quality control and competent supervision along with a well documented inspection regime on site can help to ensure that the structure is built in accordance with the design

  • Consider introducing a quality control procedure for the inspection of safety critical elements being delivered to ensure they are the correct ones as specified on the construction drawings

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This concerns the failure of a stud during construction works on a new bridge associated with major road improvements. The reporter says the form of the bridge is of steel girders with a reinforced concrete deck and parapets. In order to pour the deck and parapets, cantilevered formwork was installed. As one of the support rods was being tightened to take the load of a table unit, a stud which attached the forged beam bracket to the steel girder failed (Figure 1). The table unit was still supported by the crane and no one was injured as a result of the failure.

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Figure 1: weld failure of stud

Independent investigation of studs

The stud was sent for independent investigation and it was found to be made from Gr.8.8 instead of Gr.4.8 steel and that different welding procedures would have been needed to achieve a sound connection. It was also noted that the shank on the stud was 19mm and not 22mm diameter as detailed on the manufacturer’s data sheet. It was recommended that on-site hardness testing should be done to confirm the steel grade of the remaining studs. A visual survey was carried out to identify the presence of 19mm shanks on studs at other sites with similar beams and a number were found.

Incorrect steel grade

The studs can be distinguished not only by the shank size and length of thread but also by the beveled edge at the top of the top of the 22mm diameter stud (Figure 2). The faulty 19mm stud had a thread which goes all the way to the top and has no chamfer (Figure 3). The stud failed because it was of incorrect steel grade and as it looked the same as the rest of the studs, the installer had no way of knowing this was the case. The origin of the 19mm shank studs has not been proven and as a consequence the reporter’s firm has made improvements to the supply chain, installation and testing procedures as well as the communication of material parameters by including them on the design drawings, then checking again on site.
The reporter concludes:

  • Don’t assume what you have been sent is the correct product – always measure/check against specification

  • Ensure safety critical elements supplied by third parties on temporary works drawings are fully specified on the drawings so that there is no need to cross reference to data sheets and can be easily checked

  • Ensure test certificates are explicit to the item being tested and not one certificate for a batch of items

  • Ensure specific weld procedures are in place for third party supplied items

  • Shear studs have a manufacturer’s stamp on top of the head. It may be possible for threaded stud manufacturers to do the same. Consignment notes and conformity certificates could then be easily checked against the installed product.

  • Always check the installed stud against the specified stud as part of the Temporary Works inspection

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Figure 2: 22mm diameter stud with chamfered end
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Figure 3: 19mm diameter stud with full threads

Expert Panel Comments

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There have been many failures, some with fatal consequences, due to the wrong grade of bolt being inadvertently adopted and this is a similar situation. It is important that any safety-critical item is subjected to adequate checks. Design can help to prevent the chance of using the wrong component – such as using one size/grade only (all 22mm grade 8.8 studs for example). The stud diameter can be checked by attempting to fit the correct nut but checking of steel grade of every one is not practicable.

The issue of marking is important as there is no way of telling one grade of steel from another just by looking at it. Stamping every stud might be possible and colour coding could work. In design and build where the contractor does not want to pay for the designer to be on site in the Engineer role, the rigour of self-certification is often an issue. There is always the need for vigilance, especially when details are small.  All contracts need a proper quality management procedure to assure what has been designed is what gets built.

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