Toward advanced analysis in steel frame design

Abstract

The trends for analysis and design of steel frames are indicated in this dissertation. The current practice consists of applying the first-order elastic analysis with amplification factors or second-order elastic analysis in combination with the AISC-LRFD interaction equations. Determination of the effective length factors and individual member capacity checks are necessary to select adequate structural member sizes. The direct analysis method is a second-order elastic analysis approach that eliminates the determination of effective length factors from the current AlSC-LRFD method. Unsupported member length is used to calculate the axial strength of a member. Equivalent notional loads and/or modified stiffness are applied together with the external loads to account for the effects of initial out-of-plumbness and inelastic softening. For both AlSC-LRFD and direct analysis methods, a structure is analyzed as a whole, but the axial and flexural strength of each member is examined individually. Inelastic redistribution of internal forces in the structural system cannot be considered. As a result, determined member forces are not correct and more conservative member sizes will be obtained. Moreover, member-based approaches cannot predict structural behaviors such as failure mode and overstength factor. The advanced analysis method considered in this work is a second-order refined plastic hinge analysis in which both effective length factors and individual member capacity checks are not required. In addition, advanced analysis is a structure systembased analysis/design method that can overcome the difficulties of using member-based design approaches. Thus, advanced analysis is a state-of-the-art method for steel structure design. Several numerical examples are provided to show the design details of all three methods. The design requirements corresponding to each analysis approach are illustrated in these examples. The pros and cons of each method are discussed by comparing the design results. Advanced analysis is also a computer-based analysis and design procedure consistent with the features of performance-based design. Applying advanced analysis to performance-based fire resistance and seismic design are proposed. This dissertation shows advanced analysis is efficient in predicting the duration that structures could support load under elevated temperature and capable of determining the performance level of a structure subjected to seismic forces.