CROSS Safety Report
Incorrectly designed safety system
This report is over 2 years old
A reporter raises concerns about the way in which the structure for a safety system had been modelled using an inappropriate software package.
This was exacerbated by the fact that no hand calculation checks had been made to verify the design.
Key Learning Outcomes
For civil and structural design engineers:
- A structural model should reflect the design and what is to be built on site
It is good practice to check and validate outputs from design and analysis software
- It is important to recognise and know the boundaries of your expertise and work within the limits of your competence
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A reporter is a checking engineer for the client on a tall building. If tenants of the building wish to make structural alterations, they are obliged to send the reporter the details of the proposed changes and they review the effects on the main structure. One tenant introduced a stair between two floors. This meant that there was a double height section of facade to keep clean adjacent to the stair, and so a ‘mansafe’ system was proposed.
Inadequate structural modelling
This consists of a pair of small cantilever posts welded to base plates; the plates are bolted to the underside of the primary structure edge beam above the stair. A cable is strung between the posts and the window cleaners attach a lanyard to the cable and to a harness that they wear. There were several critical concerns with the way in which the structure had been modelled using an inappropriate software package exacerbated by the fact that no hand check had been made.
The cantilever posts were specified as circular hollow sections, but they were modelled as solid bars that were monolithic with the baseplate. The base plate was modelled as fully fixed over its entire area, and so no bending in the plate was considered. If the connection had been constructed according to the original submission, and if the window cleaner had fallen, the connection would have failed, risking injury or worse to the cleaner.
If the connection had been constructed according to the original submission, and if the window cleaner had fallen, the connection would have failed, risking injury or worse to the cleaner
Are unqualified individuals carrying out structural designs?
In the opinion of the reporter, the number of errors in the submission indicated that the person who modelled the connection was not a qualified engineer. Their concerns are that a supplier of mansafe systems should produce an unsafe design and that safety-critical equipment should be designed by someone who appeared to have so little grasp of engineering. Furthermore, there were no hand calculations to check the computer output. The errors were pointed out to the contractor and the installation redesigned.
Expert Panel Comments
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Components such as these are often not viewed in the same way as primary structure, and often do not get the level of attention they deserve. Proprietary mansafe systems fixed to primary structures normally lead to a structural engineer’s check based on loads provided by the supplier. Similar issues apply to bolts and fixings as witnessed by the number of reports sent to CROSS.
This case highlights issues around inadequate modelling of structures in design but also shows the value of an independent third party check. It is clear that the posts were a bespoke item and not part of a tried and tested system, and as such they should have been dealt with in a robust way.
The importance of validating design outputs
Many engineers have noted this growing tendency to design even simple structures by software and not to realise the output is critically dependent on the validity of the modelling. Whenever a computer process is carried out, common prudence requires some form of verification. Software packages in the hands of unqualified and inexperienced individuals are of increasing concern within the industry but what steps are being taken to reduce the risks?
Many engineers have noted this growing tendency to design even simple structures by software and not to realise the output is critically dependent on the validity of the modelling
Mansafe (fall restraint) systems need to be differentiated from abseiling/maintenance access systems and the appropriate loads and impact factors used. Relevant standards, some with different minimum anchorage values are:
BS 8437:2005– Code of practice for selection, use and maintenance of personal fall protection systems and equipment for use in the workplace
BS EN 795– Protection against falls from a height —Anchor devices —Requirements and testing
BS 7883:2005– Code of Practice for the design and selection, installation, use and maintenance of anchor devices
BS 7985:2013– Code of Practice for the use of rope access methods for industrial purposes
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