| Abstract: |
Second-order geometric nonlinearity, commonly termed the P-Delta effect, governs the true lateral stability of multi-storey steel frames once storey drifts move beyond a few tenths of a percent. This study examines how bracing configuration alters the magnitude and distribution of P-Delta effects in a representative ten-storey steel building subjected to seismic and wind lateral loading. A braced frame model incorporating concentric X-bracing and an unbraced moment-resisting frame of identical geometry, member sizes, and gravity loading were analysed under five lateral load cases using linear static and nonlinear pushover procedures. Storey drift ratios, stability coefficients, base shear-displacement response, and ground-storey column axial force amplification were extracted and compared against code-prescribed thresholds from ASCE 7 and IS 800. Results indicate that the unbraced frame exhibited stability coefficients exceeding the 0.10 threshold under two of five load cases, whereas the braced frame remained below 0.04 throughout, confirming a reduction in P-Delta sensitivity of roughly 65 to 70 percent attributable to bracing. Peak storey drift in the unbraced frame reached 1.08 percent at the seventh storey against the braced frame's 0.42 percent at the same level. These findings quantify the stabilising contribution of bracing beyond simple stiffness addition, demonstrating that bracing systems suppress the destabilising moment generated by gravity loads acting through lateral displacement, a mechanism that conventional first-order analysis cannot capture. The paper concludes that P-Delta checks should be treated as a mandatory verification step for unbraced steel frames taller than six storeys in moderate-to-high seismic zones, and proposes a simplified stability-coefficient screening threshold for preliminary design use [1]. |
