UC Berkeley

The research in this dissertation describes simulations of the inelastic seismic behavior of steel braced frames including the effects of low-cycle fatigue. Various types of nonlinear behavior are considered: material inelasticity, low-cycle fatigue, and local and global geometric nonlinearities. The effects of suddenly started, quasi-brittle fracture are not considered herein. For steel braced frames, braces, columns, beams, and connections subjected to significant axial loads and deformations, as well as bending moments and shear. Under these complex-loading conditions, a wide variety of behavior mechanisms and failure modes may occur for each type of member and connection. Thus, numerical models that assess the initiation and propagation of failure during cyclic loading need to account for multi-axial states of material nonlinearity, local and global buckling, and the exhaustion of the ability of the material to deform inelastically caused by low-cycle fatigue.

Following a review of existing material models for simulating structural steel deterioration, a series of investigations are conducted using finite element modeling techniques. Finite element methods can directly account for complex states of stress and changes in deformed shape. And material models are critical for constitutive behavior at integration points of the finite element models. However, available material models tend to emphasize behavior associated with ideal ductile response or with failure occurring under monotonic loading conditions (\emph{e.g.,} during metal-forming processes or vehicle collision). These models are not suitable for progressive collapse analysis under cyclic loading where the consequence of this adverse behavior on the subsequent response or integrity of the structure is of interest.

Therefore, a new, numerically efficient continuum damage mechanics material model capable of simulating inelastic behavior and deterioration of mechanical properties because of low-cycle fatigue has been devised and implemented in a finite element software LS-DYNA (LSTC 2007). Computational results obtained with this new material model correlate well with test results for several beam-to-column connections, individual braces, and braced frame subassemblies. These applications of the finite element model to realistic cases involving progressive collapse illustrate the importance of material deterioration and rupture. Unfortunately, the ability of the material model to predict ultimate behavior depends heavily on the material modeling properties specified. Recommendations for characterizing material properties for these types of analysis are developed and presented.

A series of analyses are presented that evaluate and refine several requirements for detailing and analyzing special concentrically braced steel frame buildings, demonstrating that the fatigue life capacity of braces is heavily dependent on width-thickness ratios and deformation histories. Member slenderness ratios are shown to have negligible effect on fatigue life capacity. Therefore, recommendations are presented for developing fatigue life demand or loading protocols for use in numerical and experimental investigations. Next, damage evolution in gusseted beam-to-column connections is evaluated and compared for different connection details, and improved connection details are recommended to reduce the damage accumulation. The position of lateral bracing members for beams in V-type and inverted V-type braced frames are also examined. More appropriate positions and methods to compensate for problems detected for currently recommended lateral bracing member positions are suggested and evaluated. Finally, for low-rise braced frames that respond inelastically during strong earthquake ground shaking, an alterative method to estimate interstory drift demands is suggested based on the Modal Pushover Analysis procedure.