Seismic Forces—Irregular Structures

 

"Irregular structures...

...have one or more significant discontinuities in its configuration or in its LFRS."  (lateral force resisting system)

• big
• bigger
• biggest
• hmmm

 

 

ASCE 7 section 12.3.2

• examples
• examples
• Rapid Visual Screening of Buildings for Potential Seismic Hazards

 

 

Horizontal (Plan) Irregularities

1. torsional

notable separation between center of mass and center of rigidity

 

2. reentrant corners

L, T, U-shaped buildings with both projections > 15% 

seismic separation joint 1, 2, 3

 

3. diaphragm

“. . . When openings are relatively small, chord forces do not increase significantly and it is usually sufficient to simply reinforce perimeter framing and assure that it is continuous. Continuous framing should extend from each corner of the opening both directions into the diaphragm a distance equal to the largest dimension of the opening.”

 

4. out-of-plane offset

particularly serious - special seismic force, overturning

shearwalls not stacked

 

5. nonparallel systems

 

nine design penalties

1. load multiplier
2. torsional amplification factor
3. 3D model is required
4. story drift limits at all edges
5. dynamic analysis
6. response history modeling
7. prohibits extreme torsional 
8. special seismic force, overturning
9. ??

 

 

Vertical Irregularities

1. soft story

example

2. mass

fundamental mode (first mode) of vibration

3. geometric

4. in-plane offset

particularly serious - special seismic force, overturning

5. discontinuity

 

five design penalties

1. analysis technique not yet developed for light-frame wood structures (torsional)
2. prohibit extreme soft story
3. special seismic loads
4. special seismic loads
5. prohibit extreme weak story

 

example  (see straps in last photo)