A New Approach to Predict Cyclic Response and Fracture of Shear Links and Eccentrically Braced Frames

Abstract

In eccentrically braced frames (EBFs) subjected to large lateral demands, inelastic actions are mostly concentrated in shear links. The links vary in size and, when employed in frames, are known to be subjected to combined tension/compression and shear stress states that influence their strength, low-cycle fatigue behavior, and fracture characteristics. Despite their significance as the main energy dissipation elements in a structure subjected to seismic demand, simulating their full response, including the number of cycles to failure, of these links as individual components, or when employed in full frames, is lacking. This is primarily because until recently, most low-cycle fatigue models did not allow link failures under complex stress states to be captured. In this study, by means of a well-established ultra-low cycle fatigue (ULCF) criterion, the behavior of these links is fully assessed. The focus is on short shear links since they are widely used in comparison to intermediate or long links. Results of the simulations are compared with their experimental equivalents and excellent comparisons are achieved, confirming the validity of the simulation methodology and providing, for the first time, a framework for simulating the ULCF behavior of shear links. The verified response prediction methodology is then applied at the structural level, and non-linear pushover analysis on EBFs is conducted. Unlike the existing numerical approaches where failure is indicated through a prescriptive target performance, such as 5% interstory drift for collapse prevention, the pushover analysis is conducted until complete fracture of the links and failure of the system. Seismic design parameters, such as deign and elastic base shears as well as force reduction factors, are also determined based on the pushover curves. The results demonstrate that the proposed approach can reliably predict the performance of EBFs and can potentially be used in future design and analysis of such frames.

Fracture prediction for square hollow section braces under extremely low cycle fatigue

“…The post-initiation softening rule is described by a linear relationship between the material degradation and the equivalent plastic strain, as shown in Eqs. ( 13)- (15). This can help to simulate fracture propagation and the correct cracking modes [17,48,49] and achieve better convergence without a sudden degradation of stress.…”

“…counting technique applied through fibre-based elements [12], (2) post-processing of FE simulation data [2] using the "rain flow" counting technique in conjunction with the Coffin-Mason strain-life relationship [13,14] and (3) finite element (FE) simulations with a fracture criterion [3,15,16]. The first approach is frequently employed in structure-level simulations, i.e.…”

Prediction of ductile fracture for circular hollow section bracing members under extremely low cycle fatigue

“…Numerical simulations are therefore becoming a popular means to examine structural problems, offering an efficient and cost-effective alternative to laboratory tests. Different fracture prediction methodologies have been developed for use in parallel with numerical simulations to predict earthquake-induced fracture at a member-level, such as: (1) the cycle counting technique through fibre-based elements [8] without consideration of buckling effects, (2) post-processing of finite element (FE) simulation data [9] using the "rain flow" counting technique in conjunction with the Coffin-Mason strain-life relationship [10,11] and (3) FE simulations with a fracture prediction model and corresponding damage evolution rule [2,12].…”

“…(11) and an isotropic hardening component as a function of equivalent plastic strain as given in Eq. (12).…”

“…Bai [15] and Algarni et al [16] extended the MMC model [25] which only accounted for monotonic loading, by adopting two weighting functions to account for the nonlinear damage accumulation and the reverse loading effects. Wen and Mahmoud [12,18] developed an analogous nonlinear damage evolution rule to take account of the coupling effects of nonlinearity of material properties caused by load reversal and load history. They also employed this nonlinear damage evolution rule to extend the ductile fracture model [40] to include reverse loading conditions.…”