LRFD Load Combinations

When LRFD design is selected in the Design Settings, the load combinations for design are automatically generated by Sizer from:

ASCE 7	Section 2.3.1: Strength Design - Basic Combinations
ICC IBC	Section 1605.1: Refers to ASCE 7 2.4 and adds three exceptions

Even with LRFD selected, the load combinations used for deflection are those for ASD design.

The LRFD deflections are

1.4D
1.2D + 1.6L + 0.5(Lr or S or R)
1.2D + 1.6(Lr or S or R) + (L or 0.5W)
1.2D + 1.0W + L + 0.5(Lr or S or R)
1.2D + 1.0E + L + 0.2S
0.9D + 1.0W
0.9D + 1.0E

Sizer incorporates the additional types Ls and I as described in Load Input, so the combinations in Sizer are

1.4D
1.2D + 1.6(L + Ls + I) + 0.5(Lr or S or R)
1.2D + 1.6(Lr or S or R) + (L + Ls or 0.5W)
1.2D + 1.0W + L + Ls + 0.5(Lr or S or R)
1.2D + 1.0E + L + Ls + 0.2S
0.9D + 1.0W
0.9D + 1.0E

D = dead, L = floor live, Lr = Live roof, S = snow, W = wind, E = Earthquake

I = impact ( = live load with time-effect actor for impact loads)

Ls = Storage Live ( = live load with time-effect factor for storage loads)

ASCE 7 Exception 1

Sizer implements Exception 1 to ASCE 2.3.1 pertaining to the reduction to 0.5 of the live load factor used in wind and earthquake combinations 4 and 5 for certain occupancies.. This is controlled by an option in Load Input view and is described more fully in Load Options from Design Codes and Standards.

Subsets of Combinations

Sizer examines subsets of these load combinations if they could possibly govern for design. Whether they can govern for design is also affected by the time-effect factor.. The following is a list of all subsets of the load combinations generated by Sizer for LRFD design

		Load Type
ASCE No.	No. in Sizer	L	Ls	Lr	S	I	W	E	Load combinations if all the loads in the combination listed exist on the member

Load Combination 1 (Dead only)									1.4D
1	1								1.4D

Load Combination 2 (L principal)									1.2D + 1.6 L + 0.5 (Lr or S )
2	2	x	x						1.2D + 1.6(L + Ls)
2	3		x						1.2D + 1.6 Ls
2	4	x	x	x					1.2D + 1.6(L + Ls) + 0.5Lr
2	5		x	x					1.2D + 1.6 Ls + 0.5Lr
2	6	x	x		x				1.2D + 1.6(L + Ls) + 0.5S
2	7		x		x				1.2D + 1.6 Ls + 0.5S
2	8	x	x		x	x			1.2D + 1.6(L + Ls + I) + 0.5S
2	9	x	x	x		x			1.2D + 1.6 (L + Ls + I) + 0.5Lr

Load Combination 3 (Lr or S principal)									1.2D + 1.6 (Lr or S ) + ( L or 0.5W)
3	10	x	x	x					1.2D + 1.6Lr + 1.0(L + Ls)
3	11			x					1.2D + 1.6Lr
3	12	x	x		x				1.2D + 1.6 S + 1.0(L + Ls)
3	13				x				1.2D + 1.6S
3	14			x			x		1.2D + 1.6Lr + 0.5W
3	15				x		x		1.2D + 1.6S + 0.5W

Load Combination 4 (W principal)
4	16	x	x	x			x		1.2D + 1.0W + L + Ls + 0.5Lr
4	17	x	x				x		1.2D + 1.0W + L + Ls
4	18	x	x		x		x		1.2D + 1.0W + L + Ls + 0.5S
4	19			x			x		1.2D + 1.0W + 0.5Lr
4	20						x		1.2D + 1.0W
4	21				x		x		1.2D + 1.0W + 0.5S
Load Combination 5 (E principal)									1.2D + 1.0E + L + 0.5 (Lr or S)
5	22	x	x		x			x	1.2D + 1.0E + L + Ls + 0.2S
5	23				x			x	1.2D + 1.0E + 0.2S
5	24								1.2D + 1.0E + L + Ls
5	25							x	1.2D + 1.0E
Load Combination 6 (Counteracting W)									0.9D + 1.0W
6	26				x				0.9D + 1.0W
Load Combination 7 (Counteracting E)									0.9D + 1.0E
7	27				x				0.9D + 1.0E

Notes

To avoid unnecessarily complicating the list of load combinations with rarely used loads that are unlikely to govern, impact loads are incorporated only into load combination 2, where they have a different time-effect factor than other loads.
Live storage loads are incorporated into all load combinations that have live loads, although the special time-effect factor is applied only to load combination 2.

In This Section

Load Combinations for Deflection