BS 5950-1:2000 Section 2.4.2.5 requires that all structures "have sufficient sway stiffness, so that the vertical loads acting with the lateral displacements of the structure do not induce excessive secondary forces or moments in the members or connections. Where such "second order" ("P-Δ") effects are significant, they should be allowed for in the design…".

BS 5950 defines two drift limits by which the engineer can determine whether they are required to consider second order effects on the design of their structure. Section 2.4.2.6 defines a parameter λ_cr that is used to define whether a structure is "non-sway" or "sway-sensitive" for drift in a particular direction. Note that in the RAM Structural System "non-sway" is sometimes referred to as "braced against sidesway", whereas "sway-sensitive" is sometimes referred to as being "unbraced (against sidesway)". When λ_cr is larger than 10.0 the code classifies the structure as a non-sway frame and the second order effects are deemed to be insignificant and do not need to be considered. When λ_cr is less than 10.0 the structure is classified as sway-sensitive in that direction and secondary effects should be considered. The engineer can determine whether the building, its frames, and members within those frames are to be considered "sway-sensitive" or "non-sway", or it can have the program make that determination. The engineer can create a global criteria for the sway state ("Non-sway", "Sway Sensitive", or "Calculate Per Combo") and override that global setting on a member by member basis (See BS5950 and Sidesway ).