Strength Loading Combinations of the 2009 IBC
Strength loading combinations from Sec. 1605.2.1 of the 2009 IBC are given
below:
1. 1.4 (D + F)
2. 1.2 (D + F + T) + 1.6 (L + H) + 0.5 (Lr or S or R)
3. 1.2D + 1.6 (Lr or S or R) + (L or 0.8W)
4. 1.2D + 1.6W + f1L + 0.5 (Lr or S or R)
5. 1.2D + 1.0E + f1L + f2S
6. 0.9D + 1.6W + 1.6H
7. 0.9D + 1.0E + 1.6H
where f1 = 1.0 for floors in places of public assembly, for live loads in excess of 100 lb/ft2, and for parking garage live load = 0.5 for other live loads
f2 = 0.7 for roof configurations (such as saw tooth) that do not shed snow off the structure = 0.2 for other roof configurations
D = dead load
E = combined effect of horizontal and vertical earthquake induced forces as defined in Sec. 12.4.2 of ASCE 7
F = load due to fluids with well-defined pressures and maximum heights
H = load due to lateral earth pressure, ground water pressure, or pressure of bulk materials
L = live load, except roof live load, including any permitted live load reduction
Lr = roof live load including any permitted live load reduction
R = rain load
S = snow load
T = self-straining force arising from contraction or expansion resulting from temperature change, shrinkage, moisture change, creep in component materials, movement due to differential settlement or combinations thereof
W = wind load due to wind pressure
Basic Allowable-Stress Loading Combinations of the 2009 IBC
Basic allowable-stress loading combinations from Sec. 1605.3.1 of the 2009 IBC are given below:
1. D + F
2. D + H + F + L + T
3. D + H + F + (Lr or S or R)
4. D + H + F + 0.75 (L + T) + 0.75 (Lr or S or R)
5. D + H + F + (W or 0.7E)
6. D + H + F + 0.75 (W or 0.7E) + 0.75L + 0.75 (Lr or S or R)
7. 0.6D + W + H
8. 0.6D + 0.7E + H
where D = dead load
E = combined effect of horizontal and vertical earthquake induced forces as defined in Sec. 12.4.2 of ASCE 7
F = load due to fluids with well-defined pressures and maximum heights
H = load due to lateral earth pressure, ground water pressure, or pressure of bulk materials
L = live load, except roof live load, including any permitted live load reduction
Lr = roof live load including any permitted live load reduction
R = rain load
S = snow load
T = self-straining force arising from contraction or expansion resulting from temperature change, shrinkage, moisture change, creep in component materials, movement due to differential settlement or combinations thereof
W = wind load due to wind pressure
Note on 1/3 Increase Permitted by the 2008 MSJC Code
The allowable-stress provisions of the 2008 MSJC Code permit allowable stresses to be increased by 1/3 for loading combinations involving wind or earthquake, for load standards that do not specifically prohibit such increase. This 1/3 increase is based on experience only. No data exist to justify it based on rate effects, and because dead load always acts, there is no statistical justification for a reduction in wind or earthquake loading in combination with dead load. The 1/3 stress increase is in effect simply an extra increase in allowable stress for wind or earthquake loads. It might be justified in some cases by historically low allowable stresses for flexural reinforcement, but it does not seem justified in general. It is currently being reviewed by the MSJC. The 2009 IBC specifically prohibits use of the 1/3 increase in conjunction with basic allowable-stress loading combinations (IBC 2009, Sec. 1605.3.1.1), while permitting use of the 1/3 stress increase in conjunction with the alternative allowable-stress loading combinations (IBC 2009, Sec. 1605.3.2). ASCE 7-05 specifically prohibits the use of the 1/3 increase. Because it is permitted in only limited circumstances, the 1/3 increase is not used in this book.
