ASD Load Combinations

When ASD design is selected in the Design Settings, and for deflection calculations for both LRFD and ASD selections, the combinations are automatically generated by Sizer from:

The following table lists the load combinations with combination factors:

1. A factor followed by the load type letter e.g .6D factors that load type, otherwise the factors are 1
2. The combination factor applies to all loads except dead loads, and only applies when there are more than one of these loads. The combination factors also apply to patterned loads
3. Load combinations shown in the table are constructed from those in ASCE 7 Section 2.4.1 and by creating load combinations from the ASCE 7 combinations with any of the load types removed, even if the load type is on the member. By removing a load type the combination can become critical due to load duration factor or counteracting forces.
4. A combination is only considered when all of its component load types are present on the member.
5. Impact loads are not included in ASCE 7 2.4.1, but load combinations are generated with impact loads considered as a separate load type because these combinations have a different load duration factor and a shear reduction factor for glulam members.

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

I = impact ( = live load with load duration factor for impact loads)

Ls = Storage Live ( = live load treated as permanent load for deflection)

IBC Exceptions

Sizer does not implement Exception 2 to IBC 1605.1 which specifies that snow or roof live loads on flat roofs that are less than or equal to 30 psf need not be combined with earthquake loads, and for those in excess of 30 psf, 20% of the load is combined with the earthquake loads.

Exception 3 pertains to crane hook loads and is not implemented in Sizer.

Deflection Load Combinations

Sizer implements notes d and f in IBC Table 1604.3 pertaining to special load combinations for long term creep deflection and component and cladding wind loads, respectively. These are controlled by options in Load Input view that are described more fully in Load Options from Design Codes and Standards.

Example

For example, the following basic combinations are listed in the analysis output when dead, live, snow and wind loads are applied to a beam for the ASCE 7 design standard.

 LOAD COMBINATIONS: ASCE 7-10

===================================================================================

 1: D only                               7: D+.75(S+W)

 2: D+L                                  8: .6D+W

 3: D+.75(L+S)                           9: D+W

 4: D+.75(L+S+W)                        

 5: D+.75(L+W)                          

 6: D+S    

(D=dead  L=live  S=snow  W=wind  I=impact  C=construction  Lc=concentrated)                          

Combinations for Concentrated Load Check

The previous example would generate the following load combinations for a Concentrated Load check:

LOAD COMBINATIONS: ASCE 7-10

===================================================================================

 1: D only                               8: .6D+W

 2: D+L                                  9: D+W

 3: D+.75(L+S)                          10: D+Lc

 4: D+.75(L+S+W)                        11: D+.75(Lc+S)

 5: D+.75(L+W)                          12: D+.75(Lc+S+W)

 6: D+S                                 13: D+.75(Lc+W)

 7: D+.75(S+W)                          

(D=dead  L=live  S=snow  W=wind  I=impact  C=construction  Lc=concentrated)

See also: Pattern Loads