Frame systems

Section 5 defines a frame system as one in which, according to the results of the analysis, 65% of the seismic base shear is (or rather should be) resisted by frames of primary seismic beams and columns. A key feature of frames is that they develop earthquake resistance mainly through normal action effects: bending moments with opposite sign develop at column ends, to give the column shears that resist the storey shear demand; the global overturning moment is resisted by axial forces (mainly) in the columns of the perimeter. As frame members normally have a shear span ratio (ratio of moment-to-shear divided by member depth) not less than 2.5, their resistance and ultimate deformation capacity are governed by flexure, and hence they are very ductile. Moreover, by dimensioning their columns in flexure to meet the strong-column-weak-beam rule and all members against pre-emptive shear failure, and by detailing plastic hinge ('critical') regions for ductility, frame systems can be reliably designed for a controlled and very ductile inelastic response.

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