Optimum design of steel frames with semirigid connections and nonlinear behaviour (Spanish text)

The thesis presents a computer numerical method for the optimum design of steel planar frames with non-linear behaviour due to semi-rigid connections and geometric non-linearities. The optimization algorithm achieves the design with minimum combined cost of connections and members. With cost and stiffness of real semi-rigid connections a right objective function is proposed. It is justify the choice of design variables of members and semi-rigid connections of the frame. Constraints of slenderness, lateral sway, deflection of beams, member resistance, member stability and global stability, are proposed.; The semi-rigid connections are modeled as springs, with rotational stiffness, at the ends of the beams. A hybrid planar element with six degrees of freedom, obtained by static condensation, is used for the beams. The presence of semi-rigid connections increase the lateral sway of the frame and the P-delta and P-Delta effects are accounting for the use of stability functions and by considering the frame equilibrium in the deformed configuration. The stiffness of semi-rigid connections may be determined by considering linear or non-linear behaviour. There are several mathematical models to describe the non-linear moment-rotation relationship of the connection by fitting a curve to experimental data. Among these the polynomial model proposed by Frye and Morris is adopted in this study due to its easy implementation.; During the optimization process, when the non-linear analysis of the frame is carried out, an instability structural behaviour may appear. To withstand this situation in the optimization process, proper global stability constraints are used.; The analysis of the frame by the Newton-Raphson method has shown efficiency when it is combined with a RQP mathematic programming optimization algorithm. To achieve the efficiency and reliability of the optimization method using finite difference derivatives, some techniques are proposed.; Four examples are presented to illustrate the features of the design method. The results suggest that the proposed semi-rigid design method realistically accounts for the non-linear connection behaviour and produced more appropriate and safety member sizes than does the traditional fully rigid or the linear semi-rigid design methods.