San Francisco Building Inspection Commission Codes Part A General Requirements

A1. Compliance

A1.1. Required scope of evaluation and/or retrofit. Evaluation and/or retrofit shall address:

1. Vertical elements of the seismic force-resisting system (SFRS) in each target story in which the existing SFRS relies on wood-frame elements, and

2. Each floor diaphragm immediately above such a target story, and

3. Elements of the seismic force-resisting load path from each such diaphragm through the foundation. Exception: Where the existing vertical elements of the SFRS are shown to comply by evaluation, the diaphragm immediately above each target story need to be evaluated.

A.1.1.1. Floor diaphragm evaluation and/or retrofit. Floor diaphragms subject to evaluation and/or retrofit shall be shown to have adequate strength at the following locations:

1. For straight lumber sheathed diaphragms without integral hardwood flooring: Throughout the diaphragm. At the discretion of the code official, this requirement may be waived where the condition occurs only in relatively small portions of each dwelling unit.

2. For other diaphragms: At locations where forces are transferred between the diaphragm and a new or

strengthened vertical of the SFRS. Collector elements may be provided to distribute the transferred force over a greater force over a greater length of diaphragm.

Commentary: Evaluation of the diaphragm’s adequacy is only required when new vertical elements are added to the target story or existing vertical elements in the target story are strengthened. This allowance is considered appropriate for the limited objective of Chapter 5E. Therefore diaphragm adequacy need only be confirmed in two cases.

Case 1. Straight lumber sheathed diaphragms without integral hardwood flooring are thought to be considerably weaker and more flexible than other diaphragm systems. Though there are no known collapses due to this condition, expected poor performance could compromise the building’s ability to meet even the limited objective of Chapter 5E. Integral hardwood flooring - but not newer “floating” wood flooring - provides significant added strength and stiffness. Even in buildings with original hardwood flooring, some remodeled, carpeted, or tiled areas might have had the original wood flooring removed. Areas of the diaphragm that form a roof for the target story (such as the portion of a garage that extends beyond the wall line above, or at a lightwell or building setback) are also unlikely to have hardwood flooring to supplement the straight sheathing. These areas should be identified as part of the condition assessment (see Section A3) and evaluated. Small isolated areas without hardwood flooring are not expected to affect overall building performance, so the provision grants a waiver for these cases. As a rule of thumb, an area up to 150 square feet per unit might represent such an acceptable condition.

Case 2. Where vertical SFRS elements are added or strengthened, an adequate load path and diaphragm must be provided. This provision requires a local check for each such element but does not require an overall analysis of the full diaphragm. The unit shear demand at each vertical element is calculated as the force in the element divided by the length of the element plus any collector. The shear demand is then compared to the unit shear capacity of the diaphragm. Where demand is greater than capacity, either the diaphragm is strengthened or the collector is lengthened. An existing diaphragm can be strengthened by adding a wood structural panel soffit attached to the bottom of the floor joists in the vicinity of the vertical SFRS element. The rules given in Section B5.1.1.2 may be useful for combining the capacities of new and existing diaphragm components. Diaphragm capacity need not be checked at existing vertical elements that are not strengthened because (except for Case 1) it is assumed that the unit capacities of the existing vertical elements and the diaphragm are comparable.

A1.2. Other SFEBC requirements and Administrative Bulletins. Alterations and repairs required to meet the provisions of Chapter 5E shall comply with all other applicable structural requirements of the SFEBC unless specifically waived by those requirements, by this Bulletin, or by related Administrative Bulletins.

Commentary: See Administrative Bulletin 106 regarding procedural compliance with SFEBC Chapter 5E, specifically Section 506E.6.

A1.3. Qualified historic buildings. In addition to or in place of the criteria allowed by SFEBC Section 506E.2, qualified historical buildings shall be permitted to use structural engineering criteria provided in the latest edition of the California Historical Building Code (California Code of Regulations Title 24 Part 8), subject to the eligibility requirements of that code.

Commentary: SFEBC Section 504E.5 also mentions the CHBC, but that provision is about historic preservation in accord with San Francisco Planning Department guidelines and has no direct bearing on the structural engineering criteria.