Abstract
Indiscriminate application of the capacity design philosophy can lead to unnecessary or indeed absurd conservatism in the earthquake resistant design of gravity load dominated ductile reinforced concrete frames. Low-rise framed buildings are typical examples. The origin of excessive potential strength with respect to lateral loading is discussed and proposals are made to establish an acceptable upper bound for lateral load carrying capacity in such frames. A technique is presented by which the successive formation of potential plastic hinges, involving partial beam sway mechanisms, can be conveniently assured. While retaining the requirements for energy dissipation in beams, it is postulated that at an acceptable high level of lateral loading the formation of storey mechanisms, necessary to complete the frame sway mechanism, should be tolerable. Examples are given to illustrate the determination of design quantities for bending moments, shear and axial forces for both, beams and columns.
Highlights
In low rise ductile reinforced concrete frames, in those with long span beams, often gravity rather than seismic load requirements will govern the design strength of beams.- In capacity design normally it is necessary to evaluate the overstrength flexural capacityof both potential plastic beam hinges in each span in order to evaluate the maximum attainable moment input into the columns of the frame
This procedure, applicable to low rise buildings with more than two storeys, in which the "weak beam - strong column" design philosophy is pursued, may produce column design moments and shear forces that are unnecessarily large. The reason for this is the large potential beam strength and the consequent lateral load required to produce in the beam sway mechanism plastic hinges with positive rotations in a region largely dominated by a negative gravity moment
Under these circumstances the magnitude of the lateral load, required to produce a complete beam hinge sway mechanism in each span of a bent, may be several times that intended by the loading c o d e ( 2 ) a if the designer insists on the full execution of capacity design in order to ensure that column yielding will generally occur after the formation of beam sway mechanisms, the columns would have to be designed for an even larger lateral load
Summary
In low rise ductile reinforced concrete frames, in those with long span beams, often gravity rather than seismic load requirements will govern the design strength of beams. This procedure, applicable to low rise buildings with more than two storeys, in which the "weak beam - strong column" design philosophy is pursued, may produce column design moments and shear forces that are unnecessarily large The reason for this is the large potential beam strength and the consequent lateral load required to produce in the beam sway mechanism plastic hinges with positive (sagging) rotations in a region largely dominated by a negative gravity moment. In certain cases it may be shown that with the development of the negative (hogging) plastic hinge in each span, that is with an incomplete beam sway mechanism, seismic strength well in excess of the required minimum can be developed At such high level of lateral load resistance, considerably reduced demand for energy dissipation is to be expected. This implies that at an acceptable high level of lateral load resistance, column hinges rather than additional beam hinges, with positive rotations, will need to form to complete the sway mechanism of the frame
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