Ductile Detailing of RC Structures Questions and Answers

Answer: C

Moment reverses under cyclic loading, so both faces need continuous steel able to carry tension wherever the reversal places it. Two bars in each face also give the hoops something to be tied to along the whole length of the member, which is what keeps the cage in position while concrete is placed.

Answer: D

The expression makes the minimum depend on both the concrete and the steel grade, so that the section does not fail suddenly the moment the concrete cracks. It is more demanding than the corresponding provision for ordinary beams, which is calibrated for gravity loading alone.

Answer: A

An over-reinforced section crushes its concrete before the steel has yielded appreciably, which is a brittle failure and destroys the ductility the design assumed. Capping the ratio at two and a half per cent keeps the section under-reinforced and therefore ductile.

Answer: B

Under reversing load the beam must be able to hinge in sagging as well as in hogging at the same section, and the bottom steel is what provides for it. Detailing only for the gravity moment leaves the member unable to sustain the reversal an earthquake imposes.

Answer: C

That is the region in which the plastic hinge is expected to form, so it is where the concrete needs confinement and the bars need restraint against buckling. Away from it the hoop spacing may be relaxed to the ordinary shear requirement.

Answer: D

Tying the bars at close intervals stops them buckling outward once the cover spalls, which is what allows the hinge to keep its strength over repeated cycles. The standard also states that the spacing need not be reduced below one hundred millimetres.

Answer: A

Correct answer: 50 mm

The code requires the first hoop within 50 mm of the joint face. The reason is that the plastic hinge forms immediately adjacent to the joint, and it is exactly there that the concrete must be confined and the compression bars restrained against buckling - so leaving the usual spacing at the very end of the member would omit the confinement where it is most needed.

Answer: B

A hook turned into the core cannot open out when the cover spalls, whereas a right-angled hook straightens and releases the confinement just when it is needed. The extension is specified as six bar diameters and not less than sixty-five millimetres.

Answer: C

Very thin hoops cannot restrain the longitudinal bars once they begin to buckle, and longer members carry larger bars that need more restraint. The step at five metres is the standard's way of tying the hoop size to the scale of the member.

Answer: D

The same threshold separates the two sets of rules from the other side: below it the member is treated in flexure, above it as a column. Axial load reduces the rotation a section can sustain, which is why the confinement requirements become more demanding.