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Load Combinations

Courses > Load Calculation > Structural Loading > Load Combinations


Introduction on Load Combinations :

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.



Concepts and Formulas of Load Combinations:

 

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

 

Note! The selection of the load combinations to be used for design is largely a matter of personal preference.

 

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:

 

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:

 

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:

 



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