Question

Provide three reasons for applying factors to the loads when combining them for the purpose of structural design. (explanation in greater detail for the 3 reasons explaining why these factors are used and anything else relevant to it would be highly appreciated)

Answer 1

Ans) Three reasons for applying load factors to the loads in structural design are :

1) To account for uncertanities in the estimation of different loads. Load factor increases with increase in uncertanities.For example,The load factor for live load will always be more than live load because we can estimate the dead load precisely but it is tough to estimate accurate live load on structure.

Also, load factor will decrease with decease in uncertanity. For example, as per ACI code, load factor for wind load is taken as 0.90 for dead load because larger dead load will support thestructure agaist overturning, increasing the stability of building.Hence, load factor is less  

2) To meet the requiremnt that design load should always be more than maximum load on structure , loads are increased with the load factors. These load factors are roughly a ratio of theoretical design strength to maximum load expected in service

3) To consider the effect of sesmic loads, loads due to temperature variation and shrinkage of concrete. The earthquake can lead to sudden increament in service load of structure so for stability of structure, it is neccessary to consider the sesmic loads. Since, it is hard to estimate the sesmic load. The design load is increased with the help of load factors for durable design of structure.