Activate reductions in live or imposed loadcases
When you create a loadcase whose type is Live (US) or Imposed (other regional codes), you can allow Tekla Structural Designer to automatically calculate load reductions in accordance with the percentages specified on the Load reductions page in Model Settings. To do so, see the following instructions.
Overview of live or imposed load reductions
Live load reductions (US), or imposed load reductions (other regional codes) are established during pre-analysis for use in subsequent column and wall design, and when the regional code is set to ACI/AISC - beam design also.
- For regional codes other than ACI/AISC, the level of imposed load reduction to consider for beams can be set manually in the beam properties. This is especially useful for more economic design of transfer beams supporting a number of floors.
Reductions are only applied to those live/imposed loadcases that have had the Live/Imposed Load Reductions box selected on the Loadcases page in the Loading dialog.
The reduction percentage to be applied is specified on the Live/imposed load reduction settings page in Model Settings. This percentage can differ depending on the number of floors being supported.
To cater for additional floors that are carried but that have not been included in the model an Assume extra floors supported value can be specified in the column and wall properties.
The methodology for live/imposed load reduction differs between national codes of practice:
Regional code: EC or BS
Levels can be designated either as "to be" or "not to be" included in the determination of the load reductions through Count floor as supported check boxes for each level in the column and wall properties. This feature enables what appears to be a roof to be counted as a floor, or conversely allows a mezzanine floor to be excluded from the number of floors considered for any particular column or wall. The moments from fixed ended beams framing into a column or wall are never reduced.
For more information, see: Imposed load reduction (Eurocode) and Imposed and roof imposed loads (British Standards)
Regional code: ACI/AISC
Before undertaking member design, Live and Roof Live loads are multiplied by a reduction factor R for roof live loads and other live loads independently. This reduced load is then used in combination to create design forces. The reduction factor is related to the tributary area of load, AT, carried by the particular member and also the KLL factor, where KLL comes from Table 4-2 in ASCE 7-05/ASCE 7-10 or Table 4.7-1 in ASCE 7-16 .
Essentially:
- Interior and exterior cols (no cantilever slabs) KLL = 4
- Edge and interior beams (no cantilever slabs) KLL = 2
- Interior beams (with cantilever slabs) KLL = 2
- Cantilever beams KLL = 1
- Edge cols (with cantilever slabs) KLL = 3
- Corner cols (with cantilever slabs) KLL = 2
- Edge beams (with cantilever slabs) KLL = 1
As it is not possible to assess where cantilever slabs are and what they are attached to - you are able to change the KLL factor for all column/wall stacks and beam spans as required in the Properties Window.
The default settings are:
- Columns/walls = 4
- Beams = 2
- Cantilevers = 1
For more information, see: Live and roof live loads (ASCE 7)
Regional code: AS
Before undertaking member design, imposed loads are multiplied by a reduction factor ψa.
This reduced load is then used in combination to create design forces. The reduction factor is related to the tributary area of load carried by the particular member.
For more information, see: Imposed and roof imposed loads (Australian Standards)