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866 Portal frame design and fabrication

Report ID: 866

Published: Newsletter 57 - January 2020

Report Overview

A reporter is concerned that, in some cases, no structural design is carried out for steel portal frames.

Report Content

A reporter, who is a chartered engineer, regularly designs commercial, domestic and agricultural steel portal frame buildings. They are also regularly asked to prepare structural calculations to support retrospective building control applications, which is usually as a result of a farmer or commercial building owner being reported to a local authority for not having applied for regulatory approval.

In such cases, the reporter always asks the fabricator who prepared the original design for calculations and is invariably told that "this is the way we always do it". In other words, there has been no structural design carried out and the fabricator has simply used the section sizes that they normally use for a particular shed size.

The reporter says that these sizes are often suitable for an enclosed shed but can be much too light for sheds with one or more open sides and the connections are more often than not inadequate. From the reporter’s experience, a significant number of fabricators consider structural calculations as an optional extra which can be provided, but only if they are asked for to support a building control application.

The reporter recently met with a fabricator and erector after preparing calculations which demonstrated that a completed commercial portal frame was completely inadequately designed. Some of the issues were:

  • haunches comprising simple flat plates i.e. no flanges;
  • no full depth stiffeners which were required to prevent column web buckling;
  • inadequate bolting arrangement at the eaves, and
  • an unsuitable bracing arrangement.

The fabricator for this project agreed to carry out the required remedial work but stated that if they always had to work to ‘a proper specification’ they would never win any work. The reporter finds this very concerning as it may mean that, in many cases, portal frame sheds are being fabricated to a cost rather than to a proper design.

The reporter is regularly told by fabricators that there have never been any problems with their sheds, to which the reporter reminds them of the heavy snow of 2012 when there were a large number of portal frame failures largely due to web buckling because of lack of full depth stiffeners and inadequate purlins.

Comments

What is being reported here is an approach where there is a real risk of a structural failure and building collapse. There is other evidence from previous CROSS reports that proper structural design and verification may be grossly inadequate for routine work such as domestic extensions and minor works. Inadequate structures in this market sector, along with agricultural buildings, pose a risk to life safety.

Portal frames need special attention, particularly for overall sway stability which depends on the rafters that are usually very slender. Furthermore, the members need consideration of the stability of the inner flange because this, or any plates on the inner face, may be subject to compression under either gravity loads or wind loads, so frequent restraints are needed. Restraint to the inner flange is commonly provided by diagonal braces to the purlins which must be of sufficient stiffness. There also needs to be adequate bracing so that the purlins are held in position.

Excellent guidance on the above and other aspects of portal design is given in Steel Construction Institute (SCI) publication P397 Elastic Design of Single-span Steel Portal Frame Buildings to Eurocode 3.

It is usually most economical and safest to procure steel portal frames from a steelwork contractor who has design offices that regularly undertake the calculations for portal frames in addition to the drawings. The vast majority of structural steelwork in the UK is supplied by reputable steelwork contractors with recognised accreditations. This is to ensure that the contractor is deemed competent with robust systems in place under 4 main headings:

The issue highlighted by the reporter is that less than competent suppliers are cutting corners and failing to design steelwork. It is implied that some companies think it is acceptable to miss-sell the product and compromise their clients. In addition to the worst-case scenario where a building collapses, there are the risks of a prosecution and insurance policies being invalid due to a lack of structural design resulting in business or livelihood failure.

Advice for clients looking to buy structural steelwork is to use a reputable steelwork supplier. The British Constructional Steelwork Association (BCSA) is a trade organisation whose members are accredited, audited and employ structural engineers who design steelwork to the structural design codes required by the building regulations. There are other competent contractors who are not members of BCSA but this is readily shown by their CE Certificate - Factory Production Control to EN 1090-2. The challenge is to deliver this advice to clients who are not involved in the construction industry. It is understood that the BCSA are taking steps to address the situation.

 

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View other CROSS reports published in Newsletter 57


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