Transmission towers play an important role in transmitting electricity in a power grid safely and reliably. In traditional design practice, the second-order P-Delta (frame side sway) and P-delta (member curvature) effects and joint effects such as load eccentricities, slippage effects, and semirigid connection are commonly ignored in analysis. Great discrepancy is frequently noted between full-scale tower tests and numerical simulations using first-order linear analysis. In this paper, second-order direct analysis is used and slippage of bolted joints as well as semirigid connection behavior are taken into account. Member initial bowing and frame out-of-plumbness imperfections are considered in the present study, which is verified by full-scale test on an ultrahigh-voltage (UHV) lattice transmission tower. The technique of semirigid design and simulation of joint stiffness for load eccentricity by simple modeling for transmission towers meeting the requirements for direct analysis with verification by a full-scale test is unavailable in literature and proposed in this paper. Furthermore, the influences of joint slippage on the deflection and load behavior of the studied towers are quantified and reported. (C) 2017 American Society of Civil Engineers.