Seismic

Question 1

Explain how F=MxA applies to the effect of earthquakes on buildings

Answer 1

The larger the building (M) and the stronger the acceleration (A takes into account duration too), the more damage caused by Force. Acceleration is measured by the acceleration due to gravity or g (a parchutest experiences 1 g but even 0.001 g is perceptible). Acceleration is also affected by a buildings period.

Question 2

What is the main determinant of how long a building will sway? What can change this?

Answer 2

The height. Building period = # of stories / 10
The period may be changed by earthquake damage: a concrete structure may crack and become “softened” and increase the period of vibration. A steel structure may stiffen until the point where the steel yields and deforms.

Question 3

Resonance

Answer 3

When the ground period and frequency matches the building period and frequency which amplifies the vibrations dramatically by 4-5x. Quake in Mexico city caused most damage to buildings 6-20 stories because they matched period of ground.

Question 4

Damping

Answer 4

Decrease in amplitude of vibration allowing the structure to return to its original position. Heavy structures (concrete/steel structure) take longer to return than lighter ones (steel moment frame, curtain wall office building).

Question 5

Ductility

Answer 5

Think bending a metal spoon vs plastic.
Bend but not break.
Ability of a material to withstand deformation = elastic limit.
Buildings are designed to distort when subjected to forces higher than those required by code so that while building may deform, it will still stand.

Question 6

Strength vs. stiffness

Answer 6

Stiffness - measured by deflection; rigidity; ability to resist deformation
Strength - the capacity of the individual elements to withstand the loads that are applied to them. measured in force per unit area (psi)

Question 7

Best shape of building to resist torsion and week points

Answer 7

Symmetrical, with even-height columns or foundation walls to spread the loads evenly so the weakest point is not found. Center of mass and resistance are both in the center.

Question 8

Dampening methods for lateral loads in tall buildings

Answer 8

Viscous dampers - shock absorbers built into structure, used in seismic zones
Base isolators - used in seismic zones to separate building and ground
Tuned mass dampers - pendulum at top of building
Tuned liquid dampers - water tanks at top of building
Active dampers - sensors control hydraulic activators (need power so not best in seismic zones)

Question 9

Seismic issues covered by building code

Answer 9

Ground shaking, horizontal movement, vibration effects.
Also classifies 4 types of structural systems:
1. Bearing wall: shear wall or braced frame without a vertical load carrying space frame
2. Building frame system: compete space frame
3. Moment resisting frame: more flexible but horizontal drift is greater
4. Dual system: combination of the above

Question 10

L and u shaped buildings, ways to reduce stress at corner

Answer 10

1. Seismic joint
2. Strong connection
3. Splay corner