Imposed load reduction (Eurocode)
Reductions can be applied to imposed loads to take account of the unlikelihood of the whole building being loaded with its full design imposed load. Reductions can not however be applied to roof imposed loads.
Imposed loads are only automatically reduced on:
- Columns of any material
- Concrete walls, mid-pier or meshed
The method used for determining the reductions is dependant on the National Annex:
- In the Base Eurocode a formula is given in cl 6.3.1.2(11), this is also used if the Irish, Finish, Norwegian, Swedish or Singaporean National Annex is selected.
- In the UK, and Malaysia the NA permits an alternative method of reduction using NA 2.6.
Although the code allows for imposed load reductions to be applied to floors, Tekla Structural Designer does not implement this automatically. For steel beams, concrete beams, slabs and mats it is however possible to define the level of imposed load reduction manually via the beam/slab item properties.
This is particularly relevant for the design of transfer beams/slabs:
- The imposed load reduction for beams, slabs and mats is intended to work with loads applied from columns acting on the beam or slab when the slab is acting in transfer or for a mat foundation supporting a column. (The theory being that if you want to design the columns for the reduced axial load, you should also design the supporting member for the reduced axial load applied by the column.)
- The engineer would need to work out the reduction of the axial load in the column and apply this as a the reduction percentage, i.e. if the raw axial load in the column is 100kN and the reduced load is 60kN, the reduction is 40%. You would than apply the 40% reduction to the transfer beam/slab or mat as well.
- The reduction is not applied to loads for analysis - it is a post-analysis process which does not affect the analysis results. It does not get applied solely to the imposed load applied directly to the beam or slab panel, but instead is applied to the design moment used in the beam/slab or mat design process.