Wood-Frame Soft-Story Structures

What is a wood-frame soft-story building?

Soft-story buildings are multistory structures constructed with large openings on one or more of the exterior wall lines, most commonly on the first floor. Examples of large openings include tuck-under parking, multiple garage doors, and large retail windows. In California, these buildings are typically of low-rise, wood-frame construction, and they are most frequently devoted to housing (apartments and condominiums). See below for simplified graphics and examples of wood-frame soft-story designs that have been adapted from a presentation prepared by the SEAOSC EBC Task Group entitled “Los Angeles Soft, Weak, or Open-Front Wall Line Ordinance Retrofit Example,” which was given at the 2015 SEAOSC Summit on November 4, 2015.

Classic Open Lines

 

Cantilever Upper Floor (Tuck-Under Parking)

Hillside Structures with Open Lines

Hillside Structures with Open Lines at
Partial Ground Floor Level

 

Why must they be retrofitted?

The presence of these openings causes the building to be soft, or weaker, at the ground level relative to the upper stories, making the building more susceptible to lateral (sideways) movement and potential collapse during an earthquake. See below for a simple graphic that illustrates the hazards of wood-frame soft-story construction and methods for retrofitting (illustration courtesy of Kathleen Cantner at American Geosciences Institute [AGI]).

The 1994 Northridge earthquake drew attention to the hazards of wood-frame soft-story construction. During this earthquake, wood-frame soft-story buildings in particular exhibited poor seismic performance, with several buildings experiencing partial collapses that resulted in the loss of human life, numerous injuries, and extensive property damage. As a result, retrofitting of wood-frame soft-story buildings was integrated into seismic ordinances to improve building performance, and more importantly, to address and improve the life safety requirements for such buildings.

What action is necessary?

Buildings categorized as wood-frame soft-story construction in preliminary surveys must undergo structural analysis by a qualifying individual. If the building does not meet the minimum earthquake requirements specified by the ordinance, it must be either retrofitted to increase the strength of the first floor, or demolished. Most ordinances require that the building owner notify both tenants and occupants of the structure’s status under the ordinance. Refer to your municipality’s ordinance for more information.

What is the compliance timeline?

Each municipality listed to the left has a unique timeline that specifies deadlines for assessment, tenant notification, and retrofit. See the links for more specific information on the deadlines for your municipality.

How can wood-frame soft-story buildings be retrofitted?

Retrofitting techniques come in a wide variety of forms, including the addition of plywood shear walls or steel moment-resisting frames at the “soft” wall line, with an appropriate load collector system. In most cases, a retrofit can be configured so that it will not alter the original use of the wall line (for example, tuck under parking spaces will still be vehicle-accessible after retrofit). It is recommended to seek engineering advice in order to select the most practical and economical solution for each unique structure.

 

The information provided on SeismicOrdinances.com is general in nature and should not be relied upon as fact. Persons requiring specific information on seismic ordinances should contact their municipality or e-mail seismicordinances@wje.com.