Structural design is nothing but a selection of structural system and type, the material of construction and proportioning of structural members. So that the building is strong enough to sustain all the loads that it might have to resist over its life.
Members of the structure are to be designed for sufficient capacity so that it can sustain in all conditions without excessive [simple_tooltip content=’Change in shape due application of load’]deformation[/simple_tooltip] or failure or collapse.
The design loads are always multiplied by a factor which is known as Factor of Safety.
The different factor of safety is applied to different load combinations in accordance with the probability of occurrence of loads. E.g.: (1.5) x Dead load + (1.5) x Live load
There are different load combinations according to different Standards and some are listed as below:
According to Indian Standard load combinations are as given below:
01. Limit state of collapse
- 1.5 DL ± 1.5 LL
- 1.5 DL ± 1.5 WL
- 1.5 DL ± 1.5 EL
- 0.9 DL ± 1.5 WL
- 0.9 DL ± 1.5 EL
- 1.2 DL ± 1.2 LL ± 1.2 WL
- 1.2 DL ± 1.2 LL ± 1.2 EL
02. Limit state of serviceability
- 1.0 DL ± 1.0 LL
- 1.0 DL ± 1.0 WL
- 1.0 DL ± 1.0 EL
- 1.0 DL ± 0.8 LL ± 0.8 WL
- 1.0 DL ± 0.8 LL ± 0.8 EL
Note: When snow load is to be considered, replace Live load on the roof with snow load in the above combination, as snow is present on the roof.
According to American Standard load combinations are as given below:
01. Allowable Strength Design
- 1.0 DL + 1.0 FL
- 1.0 DL + 1.0 HL + 1.0 FL + 1.0 LL + 1.0 SF
- 1.0 DL + 1.0 HL + 1.0 FL + 1.0 (RLL or SL or RL)
- 1.0 DL + 1.0 HL + 1.0 FL + 0.75 (LL + SF) + 0.75 (RLL or SL or RL)
- 1.0 DL + 1.0 HL + 1.0 FL + (WL or 0.7 EL)
- 1.0 DL + 1.0 HL + 1.0 FL + 0.75 (WL or 0.7 EL) + 0.75 LL + 0.75 (RLL or SL or RL)
- 0.6 DL + WL + HL
- 0.6 DL + 0.7 EL + HL
02. Load Strength Design
- 1.4 DL + 1.4 FL
- 1.2 (DL + FL + SF) + 1.6 (LL + HL) + 0.5 (RLL or SL or RL)
- 1.2 DL + 1.6 (RLL or SL or RL) + (LL or 0.8 WL)
- 1.2 DL + 1.6 WL + LL + 0.5 (RLL or SL or RL)
- 1.2 DL + 1.0 EL + 1.0 LL + 0.2 SL
- 0.9 DL + 1.6 WL + 1.6 HL
- 9 DL + 1.0 EL + 1.6 HL
Where:
- DL : Dead Load
- LL : Live Load
- WL : Wind Load
- EL : Earthquake Load
- SL : Snow Load
- RL : Rain Load
- FL : Load due to fluids with well defined pressures and maximum heights
- HL : Load due to lateral earth pressure, ground water pressure, or pressure of bulk materials
- SF : Self Straining Force
- RLL : Roof Live Load
Calculation of loads and a right factor of safety are therefore very important for the safety of a house.
