DEPARTMENT OF BUILDING INSPECTION
Expert Source: Raymond Lui, S.E., Plan Review Services
San Francisco Building Code
All new building construction in San Francisco is governed by the 2001 San Francisco Building Code (SFBC), which is comprised of the 2001 California Building Code (CBC) and 2001 San Francisco Amendments. The 2001 CBC is based on the 1997 Uniform Building Code (UBC), whose purpose is:
101.2 Purpose. …to provide minimum standards to safeguard life or limb, health, property and public welfare by regulating and controlling the design, construction, quality of materials, use and occupancy, location and maintenance of all buildings and structures within this jurisdiction and certain equipment specifically regulated herein.
1626.1 Purpose. The purpose of the earthquake provisions herein is primarily to safeguard against major structural failures and loss of life, not to limit damage or maintain function.
Recommended Lateral Force Requirements and Commentary, known as the “Blue Book,” was last published in 1999 by the Seismology Committee of the Structural Engineers Association of California (SEAOC). It forms the basis for the seismic provisions of the 1997 UBC. The Blue Book Commentary states:
C101.1 Scope and Objectives. … This Commentary provides the users of this Blue Book with an interpretation of the intent of these Requirements, the basis for these provisions and their limitations, and suggested procedures for cases and conditions outside the limits of regular practice…
These Requirements provide minimum standards for use in building design regulation to maintain public safety in the extreme ground shaking likely to occur during an earthquake. These Requirements are intended primarily to safeguard against major failures and loss of life, not to limit damage, maintain function, or provide for easy repair. It is emphasized that the purpose of these recommended design procedures is to provide buildings that are expected to meet this life safety objective…
C101.1.1 Damage Levels. Structures designed in conformance with these Requirements should, in general, be able to:
- Resist a minor level of earthquake ground motion without damage.
- Resist a moderate level of earthquake ground motion without structural damage, but possibly experience some nonstructural damage.
- Resist a major level of earthquake ground motion – of an intensity equal to the strongest earthquake, either experienced or forecast, for the building site – without collapse, but possibly with some structural as well as nonstructural damage.
It is expected that structural damage, even in a major design level earthquake, will be limited to a repairable level for most structures that meet these Requirements. In some instances, damage may not be economically repairable. The level of damage depends upon a number of factors, including the intensity and duration of ground shaking, structure configuration, type of lateral force resisting system, materials used in the construction, and construction workmanship.
C101.1.2 No Guarantee. Compliance with these Requirements does not constitute any kind of guarantee or assurance that significant structural damage will not occur in the event of design level earthquake ground motion. To fulfill the life safety objective of these Requirements, there are requirements that provide for structural stability in the event of extreme structural deformations and requirements that protect the vertical load-carrying system from fracture or buckling at these extreme states. While damage to the primary structural system may be either, negligible or significant, repairable or virtually irreparable, it is reasonable to expect that a well-planned and constructed structure will not collapse in a major earthquake. The protection of life is reasonably provided, but not with complete assurance.
In regions of high seismic activity, such as the San Francisco Bay Area, the Maximum Considered Earthquake (MCE) is taken based on conservative estimates of the ground motion from a deterministic event, representing the largest magnitude event that the nearby faults are believed capable of producing. The Design Basis Earthquake (DBE) ground motion is taken as two-thirds of the MCE ground motion.
San Francisco Department of Building Inspection
The San Francisco Department of Building Inspection (DBI) is responsible for enforcing the SFBC. We accomplish this through plan check review of construction documents, issuance of permits, inspection of construction, and through code enforcement procedures that may include prosecution of code violations.
By enforcing modern building codes in new buildings, we can reduce significantly the seismic risks. However, with older, existing buildings, it is difficult to require retroactively higher seismic design standards. San Francisco has initiated seismic retrofit triggers for existing buildings that undergo alterations, modifications, or additions. We also encourage property owners to develop and implement voluntary seismic strengthening measures in seismically vulnerable buildings.
For example, DBI has implemented the Unreinforced Masonry Building Seismic Hazard Reduction Program and requires property owners of unreinforced masonry buildings (UMB) to retrofit their buildings. As of today, more than two-thirds of these have been retrofitted. Many UMBs have collapsed in past earthquakes, including the 1989 Loma Prieta Earthquake and the 2001 San Simeon Earthquake.
We also are initiating a Soft-Story Wood-Frame Seismic Hazard Reduction Program to require property owners to retrofit this class of buildings. Soft-story wood-frame structures are those that have little or no lateral-load-resisting system at the first floor and stiff, relatively strong upper floors – the kind of design you might find in some San Francisco neighborhoods, such as the Marina, where car garages below residential units are inadequately braced to resist earthquakes. Buildings with soft first stories have collapsed during the 1989 Loma Prieta and the 1994 Northridge earthquakes.
There are no reliable statistics on the potential number of damaged or collapsed buildings in the event of a major earthquake. Based upon 2007 Assessor Office records, there are more than 190,000 buildings in San Francisco -- and the vast majority of these buildings are old and not built to current code standards.
Building codes evolve constantly. As earthquakes occur, we learn more. For example, when steel moment resisting frame structures were first introduced, engineers believed this was an excellent system. But after the 1994 Northridge Earthquake, it was discovered that many of these structures had cracked welds, columns and beams. Although no buildings collapsed, many were damaged. As a result, the building code was revised and now moment frame beam-column joints are built differently (and better).
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