CROSS Safety Report
Stability of tenants' mezzanine
This report is over 2 years old
A reporter is concerned about structures where cold-rolled sections depend upon the chipboard for their lateral torsional restraint, and to act as a horizontal diaphragm.
Key Learning Outcomes
For clients and design engineers:
The design and installation of an internal mezzanine should be given the same degree of attention as the primary structure to improve safety, reliability and longevity
Consider appointing a single entity (or Chartered Engineer) to have overall control of the stability system
The IStructE publication Practical guide to structural robustness and disproportionate collapse in buildings October 2010, provides a very similar example of a mezzanine floor that was added within a factory and used to store heavy cable drums
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A reporter’s firm is employed as the checking engineer on a retail park where mezzanine floors are installed by the tenants. The mezzanine floors are not allowed to connect to the existing steel framed buildings and so must be self-sufficient for their stability. Typically, these structures are approximately 5m x 8m on plan (Figure 1) and the quality of the submissions is highly variable both for the calculations and the drawings. Often the fit-out contractor sub-contracts the design of the mezzanine to another company who then sub-contract it again to a structural engineer.
Lateral torsional restraint
Generally, the structures consist of four square hollow section (SHS) columns, four hot-rolled UBs around the perimeter, cold-rolled sections for the joists and a chipboard deck. The cold-rolled sections depend upon the chipboard for their lateral torsional restraint, and to act as a horizontal diaphragm.
The reporter has always had concerns regarding the stability of these structures when vertical bracing is not used. In one example, as shown in Figure 2, the UBs over-sailed the SHS columns via a 4 bolt cap plate - this provides questionable stability. However, in the other direction the stability was, apparently, provided by the UBs being connected via fin plates in to the web of the beam over-sailing the SHS columns - this has no chance of achieving the required stability.
After rejecting this design on the basis that it was unstable, the reporter was contacted by the engineer of the mezzanine supplier. Their primary concern was that he needed to be on site quickly and he admitted that he had not been allowed to use vertical bracing so was not really sure what to do! They were clearly not a Chartered Structural Engineer.
The reporter is aware that the market for such structures in the UK is probably significant, but that the design is probably rarely carried out by Chartered Structural Engineers. They would like to see clear guidance as to whether over-sailing beams can be used for stability - less so for their benefit, but rather as a document for mezzanine suppliers.
Perhaps a joint SCI and BRE Digest as an update to BRE Digest 437 giving clear guidance as to the limitations of when over sailing beams can be used i.e. only when vertical bracing is employed.
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The SCI has published documents about how stability of these frames should be handled. It is possible to design a mezz floor with no bracing and with over-sailing beams. However the columns must be designed as vertical cantilevers with fixes bases. This can significantly increase column sizes however.
Expert Panel Comments
Expert Panels comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-UK Expert Panels page.
Mezzanines are popular additions in shopping centres warehouses and the like but, as is indicated here, are not always adequately designed and the use of non-structural components is also a problem. All structures, regardless of any identifiable lateral loading, should be designed against sway as a code requirement and as a measure to ensure robustness.
Experience suggests that mezzanines are often designed by specialist subcontractors without the involvement of qualified structural engineers, and that these often use standard designs without reference to the actual site conditions. Stability checks should be made in each direction taking a percentage of the vertical load acting as a horizontal load. Potential problems may be picked up by building inspectors but this is not a substitute for competent design.
The IStructE publication Practical guide to structural robustness and disproportionate collapse in buildings October 2010, provides a very similar example of a mezzanine floor that was added within a factory and used to store heavy cable drums.
Clearly, says the report, a slight lean of the columns would have generated significant lateral force. It is generally unsatisfactory to build unbraced frames using standardised components intended solely for use where lateral stability is provided either by braced bays or by external means such as connections to the main building.
Just assembling standard components using nominally pinned connections with limited and unquantified stiffness is unreliable and potentially unsafe. As ever clients should be aware of the importance of using competent designers on all aspects of their projects and having potentially unstable mezzanines with high live loading in buildings with public access carries obvious risks. It may be that fresh guidance is needed and this is being considered by BCSA.
As ever clients should be aware of the importance of using competent designers on all aspects of their projects