Technical Note Design Loading Combinations

©COMPUTERS AND STRUCTURES, INC., BERKELEY, CALIFORNIA SEPTEMEBER 2002

STEEL FRAME DESIGN BS 5950-2000

Technical Note

Design Loading Combinations The design load combinations are the various combinations of the load cases for which the structure needs to be checked. According to the BS 5950-2000 code, if a structure is subjected to dead load (DL), live load (LL), wind load (WL), and earthquake load (EL), and considering that wind and earthquake forces are reversible, the following load combinations may need to be considered (BS 2.4): 1.4 DL 1.4 DL + 1.6 LL

(BS 2.4.1.1, Table 2)

1.0 DL ± 1.4 WL 1.4 DL ± 1.4 WL 1.2 DL + 1.2 LL ± 1.2 WL

(BS 2.4.1.1, Table 2)

1.0 DL ± 1.4 EL 1.4 DL ± 1.4 EL 1.2 DL + 1.2 LL ± 1.2 EL These are also the default design load combinations whenever the BS 59502000 code is used. The user should use other appropriate loading combinations if roof live load is separately treated, other types of loads are present, or if pattern live loads are to be considered. Live load reduction factors can be applied to the member forces of the live load case on an element-by-element basis to reduce the contribution of the live load to the factored loading. In addition to the above load combinations, the code requires that all buildings be capable of resisting a notional design horizontal load applied at each floor or roof level. The notional load should be equal to the maximum of 0.01 times the factored dead load and 0.005 times the factored dead plus live loads (BS 2.4.2.3, 2.4.2.4). The notional forces should be assumed to act in any one direction at a time and should be taken as acting simultaneously with

Design Loading Combinations

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Steel Frame Design BS 5950-2000

Design Loading Combinations

the factored dead plus vertical imposed live loads. They should not be combined with any other horizontal load cases (BS 2.1.2.4). It is recommended that the user define additional load cases for considering the notional load in the program and define the appropriate design combinations. When using the BS 5950-2000 code, the program design assumes that a Pdelta analysis has already been performed. It is suggested that the P-delta analysis be performed at the factored load level corresponding to 1.2 dead load plus 1.2 live load. See also White and Hajjar (1991).

Reference White, D.W. and J.F. Hajjar. 1991. Application of Second-Order Elastic Analysis in LRFD: Research to Practice. Engineering Journal. American Institute of Steel Construction, Inc. Vol. 28. No. 4.

Reference

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