土木工程建筑外文文献及翻译 第6页
The longitudinal strains along the centerline of the bottom–flange outer face are shown in Figs. 22 and 23 for positive and negative cycles, respectively. From Fig.23, it is found that the strain on the RBS becomes several times larg- er than that near the column after cycles at –1.5_y; this is responsible for the
flange local buckling. Bottom flange local buckling occurred when the average strain in the plate reached the strain-hardening value (esh _ 0.018) and the reduced-beam portion of the plate was fully yielded under longitudinal stresses and permitted the development of a full buckled wave.
5.2.2. Cumulative energy dissipated
The cumulative energy dissipated by the specimens is shown in Fig. 24. The cumulative energy dissipated was calculated as the sum of the areas enclosed the lateral load–lateral displacement hysteresis loops. Energy dissipation sta- rted to increase after cycle 12 at 2.5 y (Fig. 19). At large drift levels, energy dissipation augments significantly with small changes in drift. Specimen 2 dissipated more energy than specimen 1, which fractured at RBS transition. However, for both specimens the trend is similar up to cycles at q =0.04 rad
In general, the dissipated energy during negative cycles was 1.55 times bigger than that for positive cycles in specimens 1 and 2. For specimens 3 and 4 the dissipated energy during negative cycles was 120%, on the average, that of the positive cycles.
The combined phenomena of yielding, strain hardening, in-plane and out- of-plane deformations, and local distortion all occurred soon after the bottom flange RBS yielded.
6. Conclusions
Based on the observations made during the tests, and on the analysis of the instrumentation, the following conclusions were developed:
1. The plastic rotation exceeded the 3% radians in all test specimens.
2. Plastification of RBS developed in a stable manner.
3. The overstrength ratios for the flexural strength of the test specimens were equal to 1.56 for specimen 1 and 1.51 for specimen 4. The flexural strength capacity was based on the nominal yield strength and on the FEMA-273 beam–column equation.
4. The plastic local buckling of the bottom flange and the web was not accompanied by a significant deterioration in the load-carrying capacity.
5. Although flange local buckling did not cause an immediate degradation of
strength, it did induce web local buckling.
6. The longitudinal stiffener added in the middle of the beam web assisted in transferring the axial forces and in delaying the formation of web local buckling. How ever, this has caused a much higher overstrength ratio, which had a significant impact on the capacity design of the welded joints, panel zone and the column.
7. A gradual strength reduction occurred after 0.015 to 0.02 rad of plastic rotation during negative cycles. No strength degradation was observed during positive cycles.
8. Compression axial load under 0.0325Py does not affect substantially the connection deformation capacity.
9. CGMRFS with properly designed and detailed RBS connections is a reliable system to resist earthquakes.
Acknowledgements
Structural Design Engineers, Inc. of San Francisco financially supported the
experimental program. The tests were performed in the Large Scale Structures Laboratory of the University of Nevada, Reno. The participation of Elizabeth Ware, Adrianne Dietrich and of the technical staff is gratefully acknowledged.
References
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