Seismic Design Philosophy and Codes Questions and Answers

Answer: A

The set required is \( 1.5(DL + IL) \), \( 1.2(DL + IL \pm EL) \), \( 1.5(DL \pm EL) \) and \( 0.9DL \pm 1.5EL \). The reduced factor on gravity in the combinations with earthquake acknowledges that the two are unlikely to peak together, and the last case covers uplift where dead load relieves rather than adds.

Answer: B

Where the seismic effect tends to lift a footing or reverse a moment, counting on the full dead load would be unsafe, so it is reduced to nine tenths. The case frequently governs the design of foundations to shear walls and of bracing connections in tension.

Answer: C

Each is a rare event in its own right, and the joint probability of both peaking together is small enough to disregard. Both must nevertheless be checked separately, since either may govern depending on the height, mass and exposure of the building.

Answer: D

Where one mode dominates and the geometry is regular, distributing an empirically computed base shear over the height is adequate. Height, setbacks, soft storeys and plan irregularity all bring higher modes and torsion into play, and only a dynamic analysis captures those.

Answer: A

Computed periods depend on modelling assumptions that may overstate the flexibility, particularly where infill stiffness has been left out, so the code sets a floor. Scaling preserves the distribution the dynamic analysis produced while restoring the total to a value the code is prepared to accept.

Answer: B

Once a building has swayed, its gravity load acts at an eccentricity and generates additional moment, which increases the drift further. The effect grows with flexibility and with axial load, and drift limits are one of the means by which codes keep it small.

Answer: C

The parabolic distribution approximates the shape of the fundamental mode, placing more force at the top where the displacement is greatest. Distributing the shear uniformly would understate the demand on the upper storeys badly, particularly in a tall building.

Answer: D

Because the drift concentrates in the flexible storey, the code multiplies the forces in its columns and beams by a factor determined from the analysis, or requires the storey to be stiffened with braces or walls. Ignoring the irregularity has repeatedly produced collapses of exactly that storey.

Answer: A

The parts divide the field by structure type: Part 1 covers general provisions and buildings, Part 2 liquid retaining tanks, Part 3 bridges and retaining walls, and Part 4 industrial structures. Part 1 carries the zone map and the zone factors that the other parts refer to.

Answer: B

Tanks need special treatment because the contained liquid sloshes, adding an impulsive and a convective component to the response. Part 2 sets out how those components are computed and combined, which the general building provisions do not address.