When should a concrete building be classed as non-sway?
ACI 318-11 clause 10.10.5.2 states that a story can be considered as non-sway when the stability index Q is less than or equal to 0.05. (Basically, second order effects can be ignored below this value.)
Tekla Structural Designer does not report Q directly, but the Drift report does calculate an indicative stability coefficient (Δ2/Δ1) which is effectively the same as a moment magnifier.
From ACI Eq. (10-20):
Moment magnifier, δs = 1/(1-Q)
Equating this to the stability coefficient gives
1/(1-Q) = (Δ2/Δ1)
1-Q = 1/(Δ2/Δ1)
Q = 1 - 1/(Δ2/Δ1)
For Q < 0.05
1 - 1/(Δ2/Δ1) < 0.05
(Δ2/Δ1) < 1/0.95
(Δ2/Δ1) < 1.0526
Therefore, if the Drift report determines an indicative stability coefficient (Δ2/Δ1) less than 1.05 it can be assumed non-sway; whereas if greater than this value a second-order analysis is required.
Note: The accuracy of determination of the second-order effects for concrete structures is dependent upon a reasonable estimation of the concrete long term properties. It is therefore important that appropriate member type specific modification factors have been specified - see
Use of modification factors.