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CROSS Safety Report

Failure of blind bolts

Report ID: 71 Published: 1 April 2007 Region: CROSS-UK

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Overview

There was a partial collapse of a steel frame building due to the failure of some blind bolts during the construction period. Loads at the time were well below anticipated failure levels.

Key Learning Outcomes

For civil and structural design engineers:

  • Connections can often be the weak link in structures and attention to detail is required to ensure what is designed can be fabricated

  • Careful consideration is required for connections, particularly at interfaces between different materials. The role of tolerances should not be overlooked.

  • Helpful guidance can be found in the alert that has been published by the Standing Committee on Structural Safety (SCOSS) - The selection and installation of construction fixings

Full Report

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A reporter has experience of the partial collapse of a steel frame building due to the failure of some blind bolts (Figure 1) during the construction period. Loads at the time were well below anticipated failure levels. Tests showed that the hardness of the blind bolts used on the site varied considerably. Many were extremely hard and had become hydrogen embrittled during the manufacturing process prior to delivery to site.

Sketch image of a blind bolt in use.
Figure 1: blind bolt

The design of the bolts was such that in their embrittled state, these were extremely susceptible to failure on loading. It was reported that many bolts failed on installation or over a period time (up to several days) afterwards. It is clear that bolt failure ultimately led to the collapse of part of the building.

The quality of the manufacture of the bolts in this case was variable, but the hardness of the embrittled bolts was far higher than it should have been, and the reporter says that this is easy to test in situ. From this experience the reporter’s concern is whether others using such bolts have experienced any similar problems of brittle failure of the bolts on installation or afterwards or have tested the hardness of the blind bolts being used to confirm they are in accordance with the manufacturer's specification.

Expert Panel Comments

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This is not the first time that concern has been expressed about blind bolts, or indeed about other bolts, and sometimes the sources of supply are thought to be doubtful. BCSA has a Model Specification for Structural Bolts and Holding Down Bolts which will be included in the supplement to the NSSS Commentary which is currently in preparation (2007).

This does not specifically deal with blind bolts. One of the issues with blind bolts is that they are not governed by any British Standard. CROSS is considering how to best promote the necessary quality control of these products. It is of particular concern that bolts were failing after a few days and in the meantime, until further guidance is forthcoming, engineers should ensure that bolts comply with the necessary properties

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