Allowing Damage to the Foundation: Is It All That Bad?

September 29, 2013
Publication: SEAOC 2013 Convention Proceedings, Structural Engineers Association of California p 392-400

Abstract: In typical code-conforming construction, building foundations are usually designed for forces derived from elastic analysis and reduced by a Response Reduction or R factor of 5 to 8. The R factor, is selected based on the type of lateral system used and has historically resulted in foundations that have performed well in past earthquakes. Engineers are increasingly using the ASCE 41-06 Standard for seismic evaluation and design. In its concrete chapter, ASCE 41-06 requires evaluation and retrofit (if necessary) of existing building foundations as force-controlled elements. The classification of foundations in ASCE 41 as force-controlled results in design forces that are 2 to 3 times larger than those typically designed in accordance with the building code resulting in a deficient tag in many evaluations, even for recent buildings. The approach taken by ASCE 41-06 is intended to assure that foundations will generally remain undamaged while most hysteretic energy dissipation and damage will occur in the superstructure. This paper highlights the mismatch in foundation design philosophy between ASCE 7-05 and ASCE 41-06, in terms of the forces used for design and evaluation of foundations, and proposes an alternative strategy whereby foundations are allowed to yield. The engineer then evaluates the extent of inelasticity just as he/she does for the main lateral force resisting elements and ensures compliance with the intent of ASCE 41-06 for the chosen performance limit state. Use of this approach typically circumvents expensive foundation retrofits saving valuable construction costs.

Services: Structural Design