Load Combinations

Courses > Load Calculation > Structural Loading > Load Combinations

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Introduction

All structures must be designed to support their own weight along with any superimposed forces, such as the dead loads from other materials, live loads, wind pressures, seismic forces, snow and ice loads, and earth pressures (if buried underground). Because various loads may act on a structure simultaneously, load combinations should be evaluated to determine the most severe conditions for design (worst case scenario). These load combinations vary from one document to another, depending upon the jurisdiction. There are a set of combinations for the allowable stress design and another set that incorporates load factors for strength design.

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Concepts and Formulas

 

Various load combinations:

2012 IBC:

1. load combinations for strength design: Section 1605.2
2. two sets of load combinations for allowable stress design 1605.3
   1. basic load combinations: Section 1605.3.1 (aligns closely with the load combinations found in ASCE 7)
   2. alternative basic load combinations: 1605.3.2

 

 

Basic load combinations of IBC Section 1605.2 (strength design):

1. 4 times (dead load plus lateral fluid pressures), [1.4(D+ F)]
2. 1.2 times (dead load plus lateral fluid pressures) plus 1.6 times (live load plus hydrostatic lateral soil) plus 0.5 times either roof live load, or snow load, or rain load, [1.2(D+ F) +1.6(L+ H) + 0.5(Lr or Sor R)]
3. 1.2 times (dead load plus lateral fluid pressures) plus 1.6 times either roof live load, or snow load, or rain load plus 1.6 times the hydrostatic soil load plus either factor f₁ times the live load or 0.5 times the wind  load, [1.2(D+ F) + 1.6(Lr or Sor R) + 1.6H+ (f₁ L or 0.5W]
4. 1.2 times (dead load plus lateral fluid pressures) plus wind load plus factor f₁ times the live load plus 1.6 times the hydrostatic soil load plus 0.5 times either roof live load, or snow load, or rain load, [1.2(D+ F) + 1.0W + f₁ L+ 1.6H+ 0.5(Lr or Sor R)]
5. 1.2 times (dead load plus lateral fluid pressures) plus earthquake load plus factor f₁ times the live load plus 1.6 times the hydrostatic soil load plus f₂ times the snow load, [1.2(D+ F) + 1.0E+ f₁L+ 1.6H+ f₂S]
6. 0.9 times (dead load plus lateral fluid pressure) plus wind load plus 1.6 times hydrostatic lateral soil [0.9(D + F)+ 1.0W+ 1.6H]
7. 0.9 times (dead load plus lateral fluid pressures) plus earthquake load plus 1.6 times hydrostatic lateral soil [0.9(D+ F) + 1.0E+ 1.6H]

Read notes below.

 

Notes on basic load combinations of IBC Section 1605.2:

• Include lateral earth pressures in the design where they result in a more critical combination.
• The floor live load should not be included if its inclusion would result in lower stresses for the structure or member being designed.
• For places of public assembly, areas with live loads in excess of 100 psf and in parking garages, the factor f₁ in the above equations shall be taken as 1. For other conditions, f₁ shall be taken as 0.5.
• For roof configurations that do not shed snow (such as saw tooth), the factor f₂ in the above equations shall be taken as 0.7. For other conditions, f₂ shall be taken as 0.2.

 

Basic load combinations of IBC Section 1605.3.1 (allowable stress design):

1. Dead load plus lateral fluid pressures, [D + F]
2. Dead load plus hydrostatic lateral soil plus lateral fluid pressures plus live load, [D + H + F + L]
3. Dead load plus hydrostatic lateral soil plus lateral fluid pressures plus either roof live load, or snow load, or rain load, [D + H + F + (Lr or Sor R)]
4. Dead load plus hydrostatic lateral soil plus lateral fluid pressures plus 0.75 times live load plus 0.75 times either roof live load, or snow load, or rain load [D + H + F +0.75(L) + 0.75(Lr or Sor R)],
5. Dead load plus hydrostatic lateral soil plus lateral fluid pressures plus (0.6 times wind load or 0.7 times earthquake load) [D + H + F +(0.6Wor 0.7E)],
6. Dead load plus hydrostatic lateral soil plus lateral fluid pressures plus 0.75 times (0.6 times wind load or 0.7 times earthquake load) plus 0.75 times live load plus 0.75 times (either roof live load or snow load, or rain load) [D + H + F +0.75(0.6W or 0.7E) + 0.75L+ 0.75(Lr or Sor R)],
7. 0.6 times dead load plus 0.6 times wind load plus hydrostatic lateral soil [0.6D + 0.6W + H],
8. 0.6 times (dead load plus lateral fluid pressures) plus 0.7 times earthquake load plus hydrostatic lateral soil [0.6(D+ F) + 0.7E + H]

Read notes below.

 

Notes on basic load combination of IBC Section 1605.3.1:

• Include lateral earth pressures in the design where they result in a more critical combination.
• The IBC does not require crane hook loads to be combined with roof live loads nor with more than three fourths of the snow load or one-half of the wind load.
• For flat roof snow loads exceeding 30 psf, 20 percent of the snow load shall be combined with the seismic loads. Flat roof snow loads of 30 psf or less need not be combined with seismic loads.
• The floor live load should not be included if its inclusion would result in lower stresses for the structure or member being designed.
• Increases in allowable stresses shall not be used with the load combinations given in this section of the IBC.
• Multiply hydrostatic lateral soil pressures by 0.6 when they are permanent and they resist the primary variable load effects. If the hydrostatic lateral soil pressures are not permanent, do not consider that they provide any resistance to other load effects.
• In Load Combination 8 (IBC load combination 16-16), the dead load factor may be increased to 0.9 for special reinforced masonry shear walls.

 

Alternative basic load combinations of IBC Section 1605.3.2 (allowable stress design):

1. Dead load plus live load plus either roof live load, snow load, or rain load, [D + L +(Lr or Sor R)]
2. Dead load plus live load plus 0.6 times the coefficient ωtimes the wind load, [D + L +0.6 ωW]
3. Dead load plus live load plus 0.6 times the coefficient ωtimes the wind load plus onehalf times the snow load, [D + L+ 0.6 ωW+ S/2]
4. Dead load plus live load plus snow load plus 0.6 times one-half coefficient ωtimes the wind load, [D + L + S + 0.6 ωW/2]
5.  Dead load plus live load plus snow load plus (1/1.4) times the earthquake load, [D + L + S + E/1.4]
6. 0.9 times the dead load plus (1/1.4) times the earthquake load, [0.9D + E/1.4]

Read notes below.

 

Notes on alternative basic load combinations of IBC Section 1605.3.2:

• Include lateral earth pressures in the design where they result in a more critical combination.
• The IBC does not require crane hook loads to be combined with roof live loads nor with more than three-fourths of the snow load or one-half of the wind load.
• For flat roof snow loads exceeding 30 psf, 20 percent of the snow load shall be combined with the seismic loads. Flat roof snow loads of 30 psf or less need not be combined with seismic loads.
• When using these alternate basic load combinations that include wind or seismic loads, allowable stresses are permitted to be increased or load combinations reduced, where permitted by the IBC or by the referenced standard of IBC, provided that when wind loads are calculated by Chapters 26 through 31 of ASCE 7, the coefficient ω in the above equations shall be taken as 1.3. For other wind loads ωshall be taken as 1.0.
• If allowable stresses are not increased, the coefficient ω may be taken as 1.
• When these combinations are used for foundations for loads including seismic, the vertical seismic effect, Ev, in Equation 12.4-4 of ASCE 7 is permitted to be taken as zero.
• When these combinations are used to evaluate sliding, overturning, and soil bearing at the soil-structure interface, the reduction of foundation overturning from Section 12.13.4 of ASCE 7 shall not be used.
• For load combinations that include counteracting effects of dead and wind loads, only two-thirds of the minimum dead load that is likely to be in place during the designed wind event shall be used.

 

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