To evaluate whether it is also necessary to consider the second-order analysis in a dynamic calculation, the sensitivity coefficient of interstory drift θ is provided in EN 1998‑1, Sections 2.2.2 and 4.4.2.2. It can be calculated and analyzed using RFEM 6 and RSTAB 9. The coefficient θ is calculated as follows:$$\mathrm\theta\;=\;\frac{\displaystyle{\mathrm P}_\mathrm{tot}\;\cdot\;{\mathrm d}_\mathrm r}{{\mathrm V}_\mathrm{tot}\;\cdot\;\mathrm h}\;$$
For the ultimate limit state design, EN 1998‑1, Sections 2.2.2 and 4.4.2.2 require a calculation considering the second‑order theory (P‑Δ effect). This effect may be neglected only if the interstory drift sensitivity coefficient θ is less than 0.1.
In RFEM 6, seismic analysis can be done by using the Modal Analysis and the Response Spectrum Analysis add-ons. Once the spectral analysis has been performed, it is possible to use the Building Model add-on to display story actions, interstory drifts, and forces in shear walls.
The story drift of a building provides valuable information about its structural behavior under seismic loads. These could cause large horizontal deformations and even instabilities. Some standards, therefore, call for a check of the story drift in its center of gravity. It indicates, for example, if a second-order analysis (P-Δ effect) is necessary.
For the ultimate limit state design, EN 1998 1, Sections 2.2.2 and 4.4.2.2 [1], requires the calculation considering the second-order theory (P-Δ effect). This effect may be neglected only if the interstory drift sensitivity coefficient θ is less than 0.1. The coefficient θ is defined as follows: $$\mathrm\theta\;=\;\frac{\displaystyle{\mathrm P}_\mathrm{tot}\;\cdot\;{\mathrm d}_\mathrm r }{{\mathrm V}_\mathrm{tot}\;\cdot\;\mathrm h}\;(1)$$ where θ is the interstory drift sensitivity coefficient, Ptot is the total gravity load at and above the story considered in the seismic design situation (see Expression 2), dr is the design interstory drift, evaluated as the difference of the average lateral displacements dS at the top and bottom of the story under consideration; for this, the displacement is determined using the linear design response spectrum with q = 1.0, Vtot is the total seismic story shear determined using the linear design response spectrum, h is the interstory height.