Earthquake Protective Design Philosophical Issues- High probability of

  • “Failure” redefined to permit behavior (yielding) that would be considered
    failure under other loads.
  • High Uncertainty
  • Importance of Details> In dealing with earthquakes we must contend with appreciable probabilities that
    failure will occur in the near future.  Otherwise, all the wealth of the world
    would prove insufficient� We must also face uncertainty on a large scale� In a
    way, earthquake engineering is a cartoon� Earthquakes systematically bring out
    the mistakes made in design and construction, even the minutest mistakes. 
    Newmark & Rosenblueth

Performance Levels

  • Incipient Collapse
  • Life Safety
  • Immediate Reoccupancy
  • Fully Operational

Hazard Levels

  • Occasional
  • 50% in 50 years
  • Rare
  • 10% in 50 years
  • Very Rare
  • 5% in 50 years
  • Max Considered
  • 2% in 50 years

Design Objective Defined

  • A specific performance level given a specific earthquake hazard level
  • Stated basis of current codes:
  • Life safety (+some damage control) at 10% in 50 year event (nominally)

Purpose of the Provisions

FEMA 302 Section 1.1

The design earthquake ground motion levels specified herein could result in both
structural and nonstructural damage.  For most structures designed and
constructed according to these Provisions, structural damage from the design
earthquake ground motion would be repairable although perhaps not economically
so.  For essential facilities, it is expected that the damage from the design
earthquake ground motion would not be so severe as to preclude continued
occupancy and function of the facility.

For ground motions larger than the design levels, the intent of these Provisions
is that there be a low likelihood of structural collapse.

This example uses an old version of both the NEHRP and the ASCE 7 Wind Load
Criteria.  It is used for illustrative purposes

How to deal with huge seismic force?- Isolate structure from ground (Base

  • Increae damping (Passive energy dissipation)
  • Allow inelastic responseInterim Conclusion (The Good News)

The frame, designed for a wind force which is 15% of the ELASTIC earthquake
force, can survive the earthquake if:

  • It has the capability to undergo numerous cycles of INELASIC deformation
  • It has the capability to deform at least 5 to 6 times the yield deformation
  • It suffers no appreciable loss of strength

Interim Conclusion (The Bad News)

As a result of the large displacements associated with the inelastic
deformations, the structure will suffer considerable structural and
nonstructural damage.

  • This damage must be controlled by adequate detailing and by limiting
    structural deformations (drift)