Ductile Detailing of RC Structures Questions and Answers

Answer: A

IS 13920 sets out the confinement, anchorage and joint requirements that let a reinforced concrete frame yield in a controlled manner. It is the document that justifies the response reduction factor claimed under IS 1893, and it is used alongside IS 456 rather than in place of it.

Answer: B

A factor of five is reserved for a frame that can yield reliably at its beam ends, and only the ductile detailing standard delivers that capacity. Designed for a fifth of the elastic force and detailed for none of the ductility, such a frame has neither the strength to stay elastic nor the ability to deform safely.

Answer: C

Below M20 the concrete cannot be relied on to confine reliably or to develop bond over the anchorage lengths the standard assumes. Taller buildings in those zones are required to use M25 or better, because the axial stresses and the confinement demands both rise.

Answer: D

Higher axial stress in the lower columns and greater confinement demand together justify the stronger concrete. The step at fifteen metres is a simple proxy for the number of storeys and the load they accumulate, and it applies across all three of the higher zones.

Answer: A

Ductility in the member depends on the bar being able to stretch well past yield and to strain harden rather than snap, so both elongation and the strength ratio are specified. Grades such as Fe500D and Fe550D are admitted precisely because they meet those requirements.

Answer: B

A rising branch after yield spreads the yielding along the bar rather than concentrating it at one crack, which greatly increases the rotation the member can sustain. This is why a minimum ratio of ultimate to yield strength is specified rather than only a minimum yield.

Answer: C

Above that stress the member is treated as a column and the more demanding confinement rules apply. The threshold separates members whose behaviour is dominated by flexure from those in which axial load governs, since the two fail in quite different ways.

Answer: D

A narrower beam cannot accommodate the confining hoops, the longitudinal bars and the cover with enough room for the concrete to be placed and compacted. Two hundred millimetres is the practical floor below which the detailing cannot be built as drawn.

Answer: A

A very narrow deep beam is prone to lateral instability of its compression zone once the concrete cover spalls under cyclic loading. Keeping the width to at least three tenths of the depth gives the compression zone enough lateral stiffness to survive that condition.

Answer: B

Beyond that proportion the member behaves as a deep beam in which shear rather than flexure governs, and the flexural hinge the design relies on cannot form reliably. The limit keeps the member within the range for which the standard's rules were calibrated.