| Summary: | 碩士 === 國立交通大學 === 土木工程系所 === 107 === Concentrically Braced Frame (CBF) system has been one of the most common structural systems. Braces are the primary members to dissipate energy by their nonlinear behavior and provide the system with lateral stiffness and strength efficiently. One of the typical failure modes of CBF is due to the concentration of large plastic deformation. Such failure modes in steel braces are usually associated with low-cycle fatigue and lack of ductility capacity leading to the rupture of braces and the development of soft-story mechanism. To improve such behavior in CBF, this study used wide-flange spliced mid-segment to replace part of square HSS braces. Under tension, the mid-segment stays elastic without cumulating excessive plastic strains; the side-segments, on the other hand, yield and play the role of energy dissipation predominantly. Under compression, plastic behavior of local and overall buckling take place at the mid-segment instead of side-segment. By doing so, one can prevent the local section from cumulating large plastic deformation caused by both tension and compression under cyclic loading. Therefore, we can increase the energy dissipating regions on the braces and delay the rupture due to low-cycle fatigue.
In this study, eight specimens with different length and section in the mid-segment were designed for static cyclic loading tests. There are two series of specimens, namely, Series 89 and Series 10. Series 89 represents the reinforcement of braces that were designed under 1989 specifications (AISC Allowable Stress Design and Plastic Design, 1989) but did not satisfy the requirements in 2010 AISC Seismic Provisions (2010). Series 10 represents the reinforcement of braces that were designed under 2010 specifications. According to the experimental results, critical parameters are investigated including the hysteretic behavior, strength (yielding strength, maximum tensile strength and maximum compression strength), energy dissipation, ductility, post-buckling strength, residual strength, effects of mid-segment length, etc.
Test results showed that the specimens with longer mid-segment generally have better ductility, but the ability of energy dissipation, although increased by large amount with mid-segment, is less sensitive to the length of the mid-segment. One can increase the ductility of Series-89 specimens from 3.67 to 5.49, and the total cumulative energy to 723% of the conventional HSS braces. For Series-10 specimens, although the ductility was not significantly improved, the total cumulative energy was increased to 356% of the conventional HSS braces. The change of the axial stiffness was between 9% and 16% for Series-89 specimens and between 10% and 13% for Series-10 specimens. Such change implied that the structures retrofitted with the proposed strategy were able to keep their original stiffness and structural periods without increasing the seismic force. This study also proposed shape factor to modify the evaluation of buckling strength for different parts in the braces and improve the adequacy of the design criteria. Such modification successfully ensures the preferable failure modes for the specimens.
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