Historical Performance Objectives of design
- life safety in rare earthquake
- serviceability
serviceability
building remains functional after frequent events
Modern Performance Parameters
- annual probability of collapse
- expected annual monetary loss (EAL)
- downtime
downtime
time building will likely remain unoperation following a certain intensity event
ELF
Equivalent Lateral Force Method
1. estimate Tn
2. read off spectral accn demand
3, Identify mu or Ry and calculate design base shear force
4. Distribute base shear as a set of equivalent lateral forces and identify design actions on structural members
Variation of ELF
first develop an inelastic spectrum for the target ductility level using R - mu - T relationship
Limitations of ELF
- period of vibration is required at start (can be affected by strength and section sizes)
- ductility is set/assumed to be purely a function of structural typology (not checked)
Uncertainties affecting dynamic response in EQ
- seismic intensity
- uncertainty in period
- uncertainty in damping
Impact of Foundation Flexibility
- system stiffness reduces … natural period increases
- causes decrease in Sa but increase in Sd
- may not be conservative to neglect effects of foundation flexibility
- can also change system ductility capacity
non structural elements can be
- acceleration sensitive (sprinklers, chimneys)
- drift sensitive ( windows)
Three possible retrofit strategies
- increasing strength and stiffness
- improving connection details - increase displacement capacity
- adding damping
increasing strength and stiffness
adding new structural elements
- reduces Tn
- increases Sa (bad for accn sensitive non-structural elements)
- decreases Sd (helps reduce both structural and non-structural drift damage)
improving connection details
- means that hopefully disp capacity > disp demand
- does not change period or seismic demands
- check that drift-sensitive non-structural elements can resist new increased structural capacity
adding damping
- not as common bc EXPENSIVE
- energy dissipation
- reduce Sd and Sa for elastic system by a factor n
n
n = sqrt ( 0.10 / (0.05 + damping) )
