Retrofitting of Structures Questions and Answers

Answer: A

Correct answer: connect the two faces of the wall together

Random rubble walls are two skins of stone with a loose fill between, and under shaking the two skins separate and each collapses independently. Through-stones or through-bolts tie the faces together so the wall acts as one thickness, which is a precondition for any other strengthening to be worth doing.

Answer: B

Corners are where two walls must act together and where the shaking pulls them apart, so cracking begins there. Stitching with bars or with mesh restores the connection and is normally combined with bands at lintel level.

Answer: C

Cement or lime grout injected under low pressure fills the interstices and bonds the loose fill to the skins. It raises strength and stiffness modestly and, more importantly, stops the wall from disintegrating during shaking.

Answer: D

Correct answer: removes mass from the top of weak walls

Inertia force is mass times acceleration, and mass at the top of a building is the worst place for it - it produces the largest overturning moment and the greatest demand on the walls beneath. Replacing a heavy earthen roof reduces the demand rather than increasing the capacity, which is often the cheaper of the two.

Answer: A

Piers between wide openings are often too slender to carry the storey shear, and infilling part of an opening lengthens them. The new masonry has to be properly toothed and bonded into the old, or it contributes nothing.

Answer: B

Correct answer: can overturn and fall on people below

A parapet is an unrestrained cantilever above roof level, so it sees amplified motion and has almost no capacity to resist it. It carries no structural load, so its collapse does not endanger the building - but it falls onto the pavement, which is where the casualties occur. Anchoring it is cheap and removes a large share of the life risk.

Answer: C

Falling ceilings, cladding, glazing and toppling equipment injure people and put buildings out of use even when the frame is undamaged. They are also cheap to fix relative to structural work, which makes them the first thing to do.

Answer: D

A hospital whose imaging equipment has shifted and whose supplies are on the floor cannot treat casualties however sound its frame. Restraint of contents is therefore part of a performance objective of continued operation.

Answer: A

Life safety, immediate occupancy and continued operation cost very different amounts, and the choice belongs to the owner. Codes for existing buildings generally permit an objective below that required of new construction.

Answer: B

Bringing an old building fully up to current new-build requirements is often impossible without rebuilding it, so a reduced objective is accepted in recognition of the remaining life and the cost. Essential facilities are the exception and are held to a higher standard.