Base Isolation and Energy Dissipation Questions and Answers

Answer: C

Correct answer: \( F = Cv^{\alpha} \) with an exponent applied to the velocity

A viscous damper resists velocity, not displacement, so its force peaks when the structure is passing through its neutral position - a quarter cycle out of phase with the spring forces, which is why adding one does not increase the peak column force. The exponent is typically between 0.3 and 1.0, and a value below unity caps the force at high velocity.

Answer: D

Below the threshold the device is rigid and the structure behaves normally, and above it the connection slips at a constant force. The behaviour is displacement-dependent, so the damper force peaks at the same instant as the elastic force in the frame.

Answer: A

Concentrating the yielding into a replaceable device keeps the damage away from the structural members themselves. Such devices are inspected after a significant earthquake and changed if they have deformed beyond their rated range.

Answer: B

Correct answer: yields in compression as well as in tension without buckling

The yielding steel core is encased in a mortar-filled casing that restrains it laterally without carrying axial load, so the core reaches its yield force in compression just as it does in tension. A conventional brace buckles in compression and its hysteresis loop pinches badly, dissipating a fraction of the energy for the same drift.

Answer: C

A viscous device is at its largest when the structure is moving fastest, which is when the member forces are smallest, so the two do not add. A friction or yielding device peaks at maximum displacement and therefore adds directly to the member force.

Answer: D

The auxiliary mass moves out of phase with the structure and its damper absorbs the energy, which narrows and lowers the resonance peak. Mass ratios of around one per cent are typical, and the tuning has to be maintained if it is to keep working.

Answer: A

It needs several cycles at the tuned frequency to take up energy, which wind provides and a brief near-fault pulse does not. Its main use is therefore serviceability against wind, with some benefit in moderate shaking.

Answer: B

Correct answer: the sloshing of water in a tank tuned to the structure's frequency

The sloshing mass of water moves out of phase with the building and absorbs energy in the same way as the mass of a tuned mass damper, with the tank dimensions and water depth setting the sloshing frequency. Its appeal is that the water can serve a second purpose - fire reserve, for example - and that there is nothing mechanical to maintain.

Answer: C

The isolation plane normally sits just above the foundation or at the top of a basement, so that the whole superstructure rides on it. Placing the layer part way up the building is done occasionally but is a special case rather than the norm.

Answer: D

Displacements of a few hundred millimetres have to happen freely, so a clear gap is kept all round and bridged with sliding covers. Anything placed in that gap after the building is occupied defeats the isolation entirely.