Steel Design: Loads And Forces

LOADS AND FORCES

Clause 3.2 of IS 800:2007 specifies the various loads and forces that has to be considered while performing the design of steel structures. As per Cl. 3.2.1 of IS 800:2007, for the purpose of designing any element, member or a structure, the following loads (actions) and their effects shall be taken into account, where applicable, with partial safety factors and combinations (Cl. 5.3.3 of IS 800:2007). (a) Dead loads; (b) Imposed loads (live load, crane load, snow load, dust load, wave load, earth pressures, etc); (c) Wind loads; (d) Earthquake loads; (e) Erection loads; (f) Accidental loads such as those due to blast, impact of vehicles, etc; and (g) Secondary ffects due to contraction or expansion resulting from temperature changes, differential settlements of the structure as a whole or of its components,eccentric connections,rigidity of joints differing from design assumptions.

1. Dead loads (Cl. 3.2.1.1 of IS 800:2007)

Dead loads should be assumed in design as specified in IS 875 (Part 1).

2. Imposed Loads (Cl. 3.2.1.2 of IS 800:2007)

IS 800:2007 specifies in Cl.3.2.1.2 that imposed loads for different types of occupancy and function of structures shall be taken as recommended in IS 875 (Part 2). Imposed loads arising from equipment, such as cranes and machines should be assumed in design as per manufacturers/suppliers data (Cl. 3.5.4 of IS 800:2007). Snow load shall be taken as per IS 875 (Part 4).

3. Wind loads (Cl. 3.2.1.3 of IS 800:2007)

Wind loads on structures shall be taken as per the recommendations of IS 875 (Part 3).

4. Earthquake loads (Cl. 3.2.1.4 of IS 800:2007)

Earthquake loads shall be assumed as per the recommendations of IS 1893 (Part 1).

5. Erection Loads (Cl. 3.3 of IS 800:2007)

All loads required to be carried by the structure or any part of it due to storage or positioning of construction material and erection equipment, including all loads due to operation of such equipment shall be considered as erection loads. The structure as a whole and all parts of the structure in conjunction with the temporary bracings shall be capable of sustaining these loads during erection.

6. Temperature Effects (Cl. 3.4 of IS 800:2007)

Expansion and contraction due to changes in temperature of the members and elements of a structure shall be considered and adequate provision made for such effect. The co-efficient of thermal expansion for steel is as given in Cl. 2.2.4.l of IS 800:2007.

7. Load Combinations

Load combinations for design purposes shall be those that produce maximum forces and effects and consequently maximum stresses and deformations. The following combination of loads with appropriate partial safety factors as given in Table 4 of IS 800:2007 may be considered. The table is reproduced here as Table 2 for ready reference. a) Dead load + imposed load, b) Dead load + imposed load + wind or earthquake load, c) Dead load + wind or earthquake load, and d) Dead load+ erection load. The effect of wind load and earthquake loads shall not be considered to act simultaneously. The load combinations are outlined in detail in Cl. 3.5 of IS 800:2007.