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CROSS Safety Report

Post-fixed RC anchors - assumptions lead to unsafe design

Report ID: 443 Published: 1 January 2015 Region: CROSS-UK

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Overview

A reporter raises concerns that engineers may be using post-fixed anchors without complying with the manufacturer's guidance or ensuring that their design assumptions are applicable.

Key Learning Outcomes

For civil and structural design engineers:

  • All design assumptions should be clearly communicated in calculations to ensure they can be adequately checked, particularly if they are being checked by an external party

  • 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.

  • Manufacturers requirements and guidance for fixings should be followed

  • Where fixings are key components and part of the quality assurance procedure consider carrying out site testing to ensure their strength capacity

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Structural engineering consultants on a recent project have reported that a number of steel to reinforced concrete (RC) moment resisting connections were required. The steel fabricator proposed forming these connections using post-fixed anchors and the design was undertaken by their engineer.

The reporter's firm reviewed the connection calculations which gave reduction factors for the capacity of the anchors and, when queried, the fabricator's engineer confirmed that this factor accounted for both anchor spacing and concrete edge distance in accordance with the fixing manufacturer's design guidance.

Concrete edge distance

During construction the reporter became concerned about one of the fabricator's designs and undertook a check using the fixing manufacturer's proprietary design software. In doing this it was realised that several of the proposed fixings did not have the minimum concrete edge distance required. When these fixings were disregarded the software calculated that the design had only a small fraction of the required capacity.

In doing this it was realised that several of the proposed fixings did not have the minimum concrete edge distance required

The issue was raised with the fabricator's engineer who explained that they had specified an adhesive type of grout and that as this adhesion between the steel and the concrete meant that concrete edge distances could be ignored when calculating anchor capacity. There was no reference or mention of this assumption in any of the connection design calculations or on drawings.

Unpredictable capacity of fixings

The fixing manufacturer said that those anchors without the required edge distance would only have a small but unpredictable capacity and that there was no established design method for accounting for adhesion between the steel and concrete. Several of these connections had already been installed on site but fortunately had only had a small proportion of the full design loading applied.

Extensive strengthening works were required to achieve the required capacity in accordance with the manufacturer’s guidance. Had these issues not been identified then there was very real danger that part of the structure would have collapsed.

Communicating design assumptions

The reporter is concerned that engineers may be using post-fixed anchors without complying with the manufacturer's guidance or ensuring that their design assumptions are applicable. It was also very worrying to learn that such an important design assumption had not been communicated in calculations or on drawings.

The reporter is concerned that engineers may be using post-fixed anchors without complying with the manufacturer's guidance or ensuring that their design assumptions are applicable

Expert Panel Comments

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Fixing problems make up 10% of all reports to CROSS and of these many have related to post-drilled fixings. The Standing Committee on Structural Safety (SCOSS) Alert - Tension systems and post-drilled fixings - March 2014 gives details of several cases of failure and advice on inspecting existing installations and installing new fixings.

Manufacturer’s instructions

The importance of following manufacturer’s instructions is stressed. A review of the design process for many proprietary fixing items will reveal the complexities of differing design processes. It is of course very important not to assume that recommendations for one product will suit another. Many failure studies highlight that they result from errors in apparently small items or that what one party thought was being built was not actually so.

Site Testing

A feature in some of the ceiling collapses previously reported was failure of the anchorages. A lesson might be that where these are key components and part of the quality assurance  procedure should be site testing to ensure their strength capacity.

The selection and installation of top fixings for suspended ceilings published by AIS gives advice on all aspects including testing for smaller fixings. The relevant British Standard is: BS EN 13964:2014 – Suspended ceilings.

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