The load combination for reinforced concrete design is 1.4Gk + 1.6Qk where Gk and Qk represent dead and imposed loads respectively. 1.4 and 1.6 is the factor of safety for dead and 1.6 is the factor of safety for imposed loads.
Other local combinations include; 1.2 Gk+1.2Qk+1.2Wk where Wk represents wind load.
For full information on load combinations see table 2.1 page 9 of BS 8110 part 1, 1997.
In reality however, various load combinations of dead, imposed and wind loads should be considered. This is to ensure that the structure is designed for the worst possible case.
While it is ok to load all parts of the structure using 1.4Gk + 1.6Qk, it is also to be appreciated that theoretically, these loads can vary i.e from 1.0 Gk to 1.4 Gk and 0.0 Qk to 1.6 Qk, in the light of these, various load cases need to be considered to achieve an accurate design.
For example, assuming we are working on a 3 span beam with Gk of 25kN/m and Qk of 10kN/m. It is good practice to consider the following load cases;
Case 1: loaded even spans with 1.4Gk + 1.6Qk and odd spans with 1.0Gk.
Case 2: loaded odd spans with 1.4Gk + 1.6Qk and even spans with 1.0Gk.
Case 3: All spans with 1.4Gk + 1.6Qk
Each load case is analyzed for bending moments and shear forces.
Bending moment diagrams for each load case. (all values in kNm);
CASE 1. (Even spans loaded):
CASE 2. (Odd spans loaded):
CASE 3. (All spans loaded)
The results of load cases are then compared. So for any beam span, the highest value of bending moment and shear forces of the 3 load cases is taken for design.
Mosley & Bungey, Reinforced concrete design. 5th Ed. Page 30.
BS 8110 part 1, 1997.