Chapter 8Load Combinations

8.1 General

A judicious combination of the loads keeping in view the probability of,

a) their acting together; and

b) their disposition in relation to other loads and severity of stresses or deformations caused by the combinations of the various loads is necessary to ensure the required safety and economy in the design of a structure.

8.2 Load Combinations

Keeping the aspect specified in 8.1, the various loads should, therefore, be combined in accordance with the stipulation in the relevant design Codes. In the absence of such recommendations, the following loading combinations, whichever combination produces the most unfavourable effect in the building, foundation or structural member concerned may be adopted (as a general guidance). It should also be recognized in load combinations that the simultaneous occurrence of maximum values of wind, earthquake, imposed and snow loads is not likely:

a) DL

b) $DL$ and $IL$

c) $DL$ and $WL$

d) $DL$ and $EL$

e) $DL$ and $TL$

f) $DL, IL$ and $WL$

g) $DL, IL$ and $EL$

h) $DL, IL$ and $TL$

j) $DL, WL$ and $TL$

k) $DL,EL$ and $TL$

m) $DL,IL,WL$ and $TL$

n) $DL,IL,EL$ and $TL$

$(DL = \text{dead load}, IL = \text{imposed load}, WL = \text{wind load}, EL = \text{earthquake load and } TL = \text{temperature load}).$

1 When snow load is present on roofs, replace imposed load by snow load for the purpose of above load combinations.

2 The relevant design codes shall be followed for permissible stresses when the structure is designed by working stress method and for partial safety factors when the structure is designed by limit state design method for each of the above load combinations.

3 Whenever imposed load (IL) is combined with earthquake load (EL), the appropriate part of imposed load as specified in 5 should be used, both for evaluating earthquake effect and also for combined load effects used in such combination.

4 For the purpose of stability of the structure as a whole against overturning, the restoring moment shall be not less than 1.2 times the maximum overturning moment due to dead load plus 1.4 times the maximum overturning moment due to imposed loads. In cases where dead load provides the restoring moment, only 0.9 times the dead load shall be considered. The restoring moments due to imposed loads shall be ignored. In case of high water table, the effects of buoyancy have to be suitably taken into consideration.

5 In case of high water able, the factor of safety of 1.2 against uplift alone shall be provided.

6 The structure shall have a factor against sliding of not less than 1.4 under the most adverse combination of the applied loads/forces. In this case, only 0.9 times the dead load shall be taken into account.

7 Where the bearing pressure on soil due to wind alone is less than 25 percent of that due to dead load and imposed load, it may be neglected in design where this exceeds 25 percent, foundation may be so proportioned that the pressure due to combined effect of dead load, imposed load and wind load does not exceed the allowable bearing pressure by more than 25 percent. When earthquake effect is included, the permissible increase in allowable bearing pressure in the soil shall be in accordance with 5.

Reduced imposed load specified in 3 for the design of supporting structures should not be applied in combination with earthquake forces.

8 Other loads and accidental load combination not included should be dealt with appropriately.

9 Crane load combinations are covered in 3.6.4.