Ductile Detailing of RC Structures Questions and Answers

Answer: A

Being outside the hoops, the cover is unconfined and is lost early in the response, which is why confinement calculations count only the core. It still matters for durability and for bond, so it is not on that account reduced.

Answer: B

Once the cover has gone, a longitudinal bar in compression is a strut whose effective length is the hoop spacing. Tying it at close intervals keeps that length short, which is why the confinement spacing rules are tied to the bar diameter.

Answer: C

Under reversing load the bar is pulled from one side and pushed from the other, and progressive slip destroys the moment connection the frame depends on. Standard hooks bent into the joint core are the usual way of guaranteeing the anchorage.

Answer: D

Because the hinges will develop their real capacity rather than the design moment, the shear that accompanies them can be considerably larger than the analysis suggested. Designing the stirrups for that capacity-based shear is what keeps the brittle mode from arriving first.

Answer: A

Flexural overstrength means the real hinge moment exceeds the design moment, and the shear rises with it. If the stirrups were proportioned only for the smaller value, the brittle mode reaches its capacity first and the ductile mechanism never forms.

Answer: B

Only the first gives large rotation with a slow loss of strength; the other three are sudden and give no warning at all. Capacity design exists precisely to make the ductile mode occur first and to keep the brittle ones well in reserve behind it.

Answer: C

A honeycombed joint has neither the strength nor the bond the design assumed, and the defect is hidden once the formwork comes off. Larger columns, fewer larger bars and careful detailing of bar layout are the usual ways round it.

Answer: D

The columns were proportioned for a factor times the beam capacity, so a stronger beam can invite the column to hinge first. Extra steel is not automatically conservative in a capacity-designed frame, which is a point often missed on site.

Answer: A

The first two are correct statements of the standard. The third is wrong: splices are confined to the central half of the member, because the ends are where hinging and the largest bar stresses occur and a lap there would be the weakest point at the worst location.

Answer: B

Confinement acts on the core alone, and the cover spalls away at the strains where it matters, so the second statement is false. The hook detail in the first and third statements is what keeps the confinement in place after spalling.