Mezzanine floors: Baseplate design
July 29, 2010
How are the loads from mezzanine floors dispersed?
Loads from freestanding mezzanine floors are all supported through columns and base plates by the concrete floor slab, which is in turn supported by the subsoil below it.
The object when designing a mezzanine floor is to ensure that the loads applied down the columns are adequately distributed over the subsoil so that they are less than its ‘subsoil bearing capacity’. The load is then safely dispersed through the ground without settlement, subsidence, shear or any associated building damage.
A typical mezzanine floor supports a design load of around 5kN (approx. half a tonne) per square metre, uniformly distributed load, on a column grid of around 4m x 5m. This means that a column in the middle of such a floor will support half a tonne on each square metre of the 20 square metres that it supports, making a total of 10 tonnes down the column. Typically this will be applied to the concrete floor on a base plate 300mm (1 foot) square.
It is generally assumed when designing mezzanine floors that the load will be evenly applied over the whole area, and such a load is referred to as a uniformly distributed load or UDL.
Load dispersal is affected by the construction of the slab itself. The thickness of the slab, the amount of reinforcement and the depth of hardcore below the slab affect how the load applied by the column is distributed to the subsoil.
The subsoil in turn can vary from made up ground, through clays, sands and gravels to chalk, limestone and granite which have greatly varying subsoil bearing capacities, and therefore the mezzanine floor loads may need to be distributed over different sized areas in order to ensure they are kept below the ‘subsoil bearing capacity’.
Mezzanine floor base plates are designed by first calculating the load applied down each column, and based upon the subsoil and slab construction a suitable mezzanine floor base plate size can then be calculated to safely disperse the applied load. Plate sizes will vary to suit the load applied to each column, depending on the area of mezzanine flooring that it supports.
Depending on the site location, history and age, information regarding the slab construction and type of subsoil may be readily available, or may need to be obtained by investigation and test. An old riverside location with an old slab is likely to need some trial holes or cores and penetration tests to obtain the relevant data, whilst a brand new site in a chalk downland location would be expected to come with full specifications.
The first port of call for documentation is the landlord or owner, followed by the local authority. If no documentary information is available, conservative assumptions may be acceptable to approved inspectors and building control officers, failing which visual inspection of trial holes and testing of subsoil by independent geotechnical engineers may be necessary.
The poorer the ground quality and the higher the mezzanine flooring loads, the more likely trial holes and testing will be necessary, and the more likely that the slab alone will not be adequate to accommodate the applied loads and independent foundations may be needed, though the percentage of mezzanine floor applications where these factors combine is low.
This general information relates to the design of mezzanine floor base plates and is intended for guidance only. Each project needs to be assessed on its own merits.
It is always prudent to discuss your specific project with an approved inspector or building control officer prior to commencing work, a task with which your mezzanine floor contractor will be prepared to assist.
If you would like advice regarding the specific requirements for your project, someone to liaise with building control on your behalf, or a quotation for your project call Llonsson Ltd on 01883 622068.