Abstract
While quite slender cross bracing may suffice to maintain rigidity in a braced structure under static loading conditions, during an earthquake progressive slackness develops in the bracings, resulting from overstrain in tension, which gives rise to increasing horizontal movements and degradation. In this paper tests are described on devices, fabricated from round bars and located at the centre of the bracings, which allow repetitive overstrain to occur without the development of slack. It is postulated that, for a stable condition of cyclic overstrain, the device shall be of the same shape as the frame to be braced, and most usually fabricated in steel rod or bar. Successful tests were carried out on such a device in a square frame.
Highlights
Cross bracings are often employed in structures to resist in-plane shear forces
The ring could behave as an energy absorbing element, for the usual design, with a ring of flat strip pierced to take the rods, it could have a short fatique life
While the usual design of cross bracings behaves satisfactorily under predictable loading conditions, during earthquakes severe cyclic loading will produce progressive overstrain in tension, which results in the development of slack
Summary
Cross bracings are often employed in structures to resist in-plane shear forces. This has the particular advantage that, during load reversals, one member of a pair of cross bracings will always be in tension, allowing compression buckling to be ignored. Up to the time of first yield at points a, b, c and d (Fig. 7 ) , when deflections are not large, the loading condition is as shown, with the jack load Q producing a force P in a single diagonal, with the other one unloaded For this condition the elastic moments at the corners of the frame, both for small and large deflections, are all equal (Appendix 1) so that any overstrain occurring at a corner resulting from tension in one diagonal will be reversed when tension develops in the other, ie. The observed increment in peaking up (Fig. 11) is about 7 kN It is pointed out in Appendix 2, that the reaction R at right angles to the jack axis will not be provided in the free condition of a structure under earthquake attack, so locking up will be provided by bending at the joints only, which may lessen the effect. If a short-stroke energy absorber is needed for minimum interstorey drift, the overall size of the device would need to be reduced to induce overstrain at small deflections of the frame
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