Live and roof live loads (ASCE 7)
Live load and roof live load reductions
Reductions can be applied to live and roof live loads to take account of the unlikelihood of the whole building being loaded with its full design live load. The reduction is calculated based on total floor area supported by the design member. 'Live' and 'Roof Live' load types each have their own reductions applied in accordance with either Section 4.8 and 4.9 of ASCE 7-05, or Section 4.7 and 4.8 of ASCE 7-10, -16, -22 as appropriate.
Due to the complications associated with live load reduction when considering beams at an angle to the vertical or horizontal, reductions are only applied to:
- Horizontal steel beams with vertical webs (major axis horizontal) which are set to be “gravity only” pin ended only
- Columns of any material
- Concrete walls, mid-pier or meshed
Tributary Area, AT
The reduction factor is related to the ‘tributary area’ of load carried by each particular member. This is established separately for live load reductions and roof live load reductions.
The 'hidden' procedure for doing this in Tekla Structural Designer is to apply a unit load to floor and roof areas in order to establish the reactions and axial forces in the beams and columns respectively. This provides a direct determination of the tributary area.
The live and roof live load reduction factors are then calculated for each beam and each column stack as described below.
Live load reduction factor
The live load reduction factor, R is calculated as follows:
If working to US Customary units:
- R = (0.25 + 15 / √(KLL*AT)) - where R <= 1.0
If working to metric units:
- R = (0.25 + 4.57 / √(KLL*AT))
KLL comes from Table 4-2 in ASCE 7-05, -10, or Table 4.7-1 in ASCE 7-16, -22. 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
- For all beams and column stacks supporting one floor: R ≥ 0.5
- For all column stacks supporting two or more floors: R ≥ 0.4
Note: As it is not possible to automatically assess where cantilever slabs are and what
they are attached to - the KLL factor can be manually specified for
individual columns, wall stacks and beam spans.
Roof live load reduction factor
The roof live load reduction factor is calculated as follows:
R = R1 * R2
Where R2 = 1.0 (conservatively assumes roofs < 18 degs)
If working to US Customary units:
- R1 = 1.2 - 0.001 * AT , where 1.0 ≥ R1 ≥ 0.6
If working to metric units:
- R1 = 1.2 - 0.011 * AT
Note:
Roof live load reduction factors are only calculated for those beams and columns that are loaded by roof panels.