Soft-Story Retrofit With Added Load and a Liquefiable Site
A five-story, 1964 wood-frame apartment building has 40 units over a one-story tuck-under parking level with an open front at grade. The city has adopted a mandatory soft-story retrofit ordinance keyed to ASCE 41 with a Collapse Prevention objective. While the retrofit is being designed, the owner adds scope: a roof deck over the rear half, replacement of the front spandrel walls with new sheathing and stucco, and conversion of two ground-floor storage rooms into two dwelling units. The 1998 geotechnical report is the only soils document available; a 2023 update notes 12 feet of loose sandy fill over soft bay mud, groundwater at 8 feet, and a mapped liquefaction hazard zone.
1. The owner's consultant argues the project is 'just a retrofit' and that the added scope is separate. Analyze what the added scope triggers for the lateral-force-resisting system and for the rest of the code.
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- A retrofit ordinance is a minimum for the existing deficiency, not a ceiling. The added scope changes the building. The roof deck and new spandrels add dead load and change mass distribution; two new dwelling units change occupancy and egress, and converting storage to habitable units adds floor loading, plumbing, and a change-of-use question under the existing building code.
- Under the California Existing Building Code, an addition plus a change of occupancy plus alteration work pushes the project up the alteration levels. The existing lateral system generally must be evaluated for the added seismic demand and for continuity and load path of the new work, and the new elements - deck framing, new unit partition walls, new shear transfer into the podium - are designed to current code (ASCE 7-16 / CBC 1613), not to the ordinance's reduced criteria.
- The two new units at grade are also a life-safety and accessibility event: egress and travel distance from a single open-front parking level, fire separation from parking, sprinklers, and the accessible-route and dwelling-unit provisions that the ground floor did not previously have to meet.
- The correct sequence is to design the retrofit and the added scope together, in one model, once. Designing the retrofit first and adding load later yields a retrofit the owner pays for twice.
2. What performance objective actually governs each part of the work, and how should that be expressed on the drawings?
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- Two criteria coexist. The pre-existing elements being strengthened only because the ordinance requires it can be evaluated to the ordinance's objective - Collapse Prevention at the BSE-1E hazard level, with the acceptance criteria of the referenced ASCE 41 edition.
- New elements and anything carrying the added load are designed to current code: new steel moment frames or cantilever columns at the open front, new foundations, new unit shear walls, and the deck. When new and existing elements interact, the connection and load path must satisfy the higher of the two demands.
- Expressing it: a design basis report in the permit set stating the standard, edition, hazard level, performance objective by element, and the assumptions about existing material strengths; a plan sheet keyed to the report; and a clear note that the ordinance scope and the owner's scope are combined into one analysis.
- Where the two criteria produce different demands at the same joint, design to the governing one and say so. Reviewers reject retrofit sets that are silent about which criterion controls where.
3. What should the architect do about the soils information, and who is responsible for the foundation and ground-improvement design?
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- The 1998 report is not a design basis. The mapped liquefaction hazard and the loose fill over soft bay mud put the site in the category requiring site-specific evaluation; under ASCE 7-16, liquefiable soils fall outside the simple site-class table and require site-specific ground motion and response analysis. Base shear cannot be finalized on the old report.
- Commission an updated geotechnical investigation scoped to the actual questions: liquefaction triggering and post-liquefaction settlement, lateral spreading toward the open parking face, downdrag on any new piles, and the feasibility of ground improvement under an occupied building. Tell the owner this is additional scope and fee - it is not covered by a retrofit fee proposal written before the added scope appeared.
- Ground improvement, deep foundations, and shoring are designed by the geotechnical engineer and the structural engineer of record, not by the architect, and must be coordinated and shown. The architect's responsibility is to see that the recommendation is carried into the drawings and that the contractor's means and methods (vibration, underpinning, temporary excavation on a 1964 wood building) are not allowed to undermine the existing structure.
- Do not let the contractor mobilize on the assumption that the foundation stays as-is. On a liquefiable site with an occupied building, that assumption is the expensive one.