Question

Identify and briefly discuss the two main factors in the Strength Limit State Design Method which are used for safety purposes. Briefly discuss the significance of both these factors in the context of a real-world structural steel design example of your own choosing.

Answer 1

Ans a) Two main factors in strength limit state design method which are used for safety purposes are :

1) Loading - It allows for possible deviation of the actual behavior of structure from the analysis model . It also allows for deviation of loads from specified values It reduces probability that the various loads acting together will simultaneously reach the characteristic value

2) Material strength - It takes account the possible deviation of the material strength in structure from that which assumed in design.It also account possible reduction in strength of material from its characteristic value , manufacture tolerance and mode of failure (brittle or ductile)

Ans b) Due to both of these factors the safety format used in codes is based on probable maximum load and probable minimum strength,so that consistent level of safety can be achieved.Safety factor for loading reduces the possibility of all loads acting together. The safety factor for material strength of steel is 1.10 for yielding and buckling while for ultimate resistance 1.25.

(Load x Load factor) < or = (Resistance) /(Resistance factor)