CROSS Safety Report
Mandatory seismic retrofit construction with quality control problems
This report is over 2 years old
Overview
Standard seismic retrofit details were used for a building with a non-standard foundation, resulting in inadequate connection of structural shear walls to foundations.
Key Learning Outcomes
For structural design engineers:
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In retrofit of existing buildings designs and contract documents require knowledge of existing construction
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During construction periodic field visits and sign off that the construction generally conforms with the requirements of the contract document is typically mandatory, and where not mandatory is highly desirable
For contractors:
- In retrofit of existing buildings where design documents do not represent existing construction, consult the designer of record before proceeding
For building authorities:
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Careful plan checking and site visits to verify conformance of construction with the design documents can avert problems like the ones experienced in this case
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Project plans use common details for retrofit of a wood residential building with a ‘soft story’1 deficiency, including additional wood structural panel sheathing and tie-downs applied to existing studs over an existing concrete foundation. However, the actual building has brick foundations. Anchor bolts have been installed into the brick foundations per the concrete detail, but there is no record of a requirement for testing or inspection, which should have caught this discrepancy.
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The building that is the subject of this report was subject to a mandatory city2 retrofit program, which is specific in its evaluation and retrofit requirements.
The report describes a standard seismic retrofit being performed on a non-standard brick foundation. A competent retrofit engineer should have verified all existing conditions, or required the verification by a qualified third party, including the foundation type and condition, before beginning design work. Brick foundations, which are uncommon for the location and building type described, are typically unreinforced and have little or no strength except in compression.
They are typically inadequate to resist the shear and overturning forces associated with a retrofit design. The reporter correctly notes that a requirement for testing of the anchor bolts would have caught the discrepancy between the drawn concrete foundation and actual brick foundation. Testing is not always required if the code has default values for existing bolts, but it should have been required because of the atypical brick conditions.
1. For discussion of the term ‘soft story’ see footnotes of report ID 986.
2. For confidentiality, the city is not identified herein. For further discussion of soft-story ordinances, see footnotes of report ID 986.