13/04/2026
What is the P-Delta Effect (Second-Order Moment)? When a horizontal force (like wind or an earthquake) pushes a building, it sways. The vertical gravity loads (the weight of the structure), now acting at a slight offset from the building's base, create an extra overturning moment. This additional moment is the second-order moment and its effect is called the P-delta (P-Δ) effect. Eurocode 8 (EN 1998-1) uses the interstory drift sensitivity coefficient, θ (theta), to decide whether P-delta effects need to be considered in seismic analysis. It is calculated per story as:
θ = (Ptot × dr) / (Vtot × h)
Where:
Ptot is the total gravity load at and above the story.
dr is the design interstory drift.
Vtot is the total seismic story shear.
h is the interstory height. The required actions based on θ are:
θ ≤ 0.1: P-delta effects can be neglected.
0.1 < θ ≤ 0.2: P-delta effects must be considered. A simplified approach is allowed, where all seismic action effects are multiplied by the amplification factor 1/(1-θ).
θ > 0.2: The structure is too flexible. A full non-linear second-order analysis is mandatory 👉 Follow me so you don't miss it:
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