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Load factors for reinforced concrete design

In the strength design method, we used load factor to increase the amount of applied load to a structure, the use of these factors is to account for uncertainties in the estimation of different loads. load factor value should be more than 1.0. load factor value will decrease with increasing confidence of estimated load, dead load factor will be less than live load because we can calculate the dead load for a structure precisely but for the live-load, there will some uncertainties because it is a moving
and changing load.

Figure 1







ACI section 9.2 providing us with 7 equations, these equations represent load combination for different conditions and loads, the combination of loads will produce ultimate load, the ultimate load will be used for reinforced concrete design. 

U = 1.4D                                                                                     (ACI Equation 9-1)
U = 1.2D + 1.6L + 0.5(Lr or S or R)                                          (ACI Equation 9-2)
U = 1.2D + 1.6(Lr or S or R) + (L or 0.5W)                               (ACI Equation 9-3)
U = 1.2D + 1.0W + L + 0.5(Lr or S or R)                                   (ACI Equation 9-4)
U = 1.2D + 1.0E + L + 0.2S                                                        (ACI Equation 9-5)
U = 0.9D + 1.0W                                                                         (ACI Equation 9-6)
U = 0.9D + 1.0E                                                                          (ACI Equation 9-7)

where
U = the design or ultimate load the structure needs to be able to resist
D = dead load

L = live load
Lr = roof live load
S = snow load
R = rain load
W = wind load
E = seismic or earthquake load effects


ACI 9.2.2 stating that impact load should be treated as a live load in the situation where it applied quickly such as elevator, garages, and cranes, etc..


in equation 9-7 and 9-8, we notice that ACI code use 0.9 load factor for dead load, this should be used when the larger dead load will reduce the effects of other loads such as in tall building, in the event of wind and lateral load, lateral load will attempt to overturn the building, in the opposite the dead load will stabilize the building, therefore dead load shall be reduced by 10% to prevent overestimation of dead load.

ACI codes allowing reduction for some load factors in some conditions:

  • ACI allowing reduction of live load factor to 0.5 for live load for load combinations from 9-3 to 9-5 except for garages, the area used for public assembly and if love load is more than 100 psf(4.80kn/m2)

  • If the load W is based on service-level wind loads, replace 1.0W in load combination 9-4 and 9-6 with 1.6W. Also, replace 0.5W with 0.8W in load combination 9-3.


  • building codes and design references frequently converting seismic load to strength level values and this what ACI assumed in equation 9-5 and 9-7, but if the service load seismic force specified and used then load factor shall be changed from 1*E to 1.4*E in both equations.
  • self-restraining effects such as a change in temperature, shrinkage, and creep can be eliminated if an expansion joint used to eliminate the self-restraining effects.


  • Fluid loads, F, resulting from the weight and pressure of fluids shall be included with the same load factor as D in ACI Equations 9-5 through 9-7.
  • if soil load is present (H), they must be added to load combinations as follows:
  1. the load factor of 1.6 should be used when soil load is present and acting alone or add to other load effects
  2. when the effect of soil load is permanent and counteracting other loads, a load factor of 0.9 should be used.
  3. where the effect of H is not permanent but, when present, counteracts the effects of other loads, H shall not be included.


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