ASCE 7-05 Seismic Provisions


Earthquake Protective Design Philosophical Issues

  • High probability of “Failure”
  • “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 only.

How to deal with huge seismic force?

  • Isolate structure from ground (Base isolation)
  • Increae damping (Passive energy dissipation)
  • Allow inelastic response

Interim 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)

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