Live and roof live loads (ASCE 7)
Live load reductions
Reductions can be applied to roof live and 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. Roof live and 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 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
Live load reduction factor
The live load reduction factor, R is calculated as follows:
| R |
|
(0.25 + 15 / Sqrt(KLL*AT)) - where R <= 1.0 |
|
US-units |
| R |
|
(0.25 + 4.57 / Sqrt(KLL*AT)) |
|
metric-units |
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 | |
| 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 | ||||
| R1 |
|
1.2 - 0.001 * AT , where 1.0 ≥ R1 ≥ 0.6 |
|
US-units |
| = | 1.2 - 0.011 * AT |
|
metric-units | |
| R2 |
|
1.0 (conservatively assumes roofs < 18 degs) | ||