As we are starting on the retrofit design of several apartment buildings in San Francisco in accordance with the “Soft Story” ordinance, for the last several months I have been studying a new methodology of evaluating and retrofitting of the Soft Story buildings. It is a FEMA document number P-807, and is unsurprisingly entitled “Seismic Evaluation and Retrofit of Multi-unit Wood-Frame Buildings with Weak First Story”. It is specifically designed to be used under the ordinance.
The most significant feature of this methodology is that it doesn’t require the engineer to develop and analyze a complete nonlinear model of the specific building, which is extremely time consuming. Instead, it relies on the statistical representation of hundreds of surrogate models already analyzed by the authors of the document. This allows the engineer to account for important nonlinear behavior without requiring difficult and expensive modeling. At the same time, P-807 is intended to represent an improvement over current code-based provisions and offers a balance of engineering precision and cost-effective practice. It uses a performance based approach, allowing the owner to understand the probability of the building collapse given an earthquake of certain strength.
The authors, and several engineers at the SF DBI, indicated that the design using these guidelines may result in a more cost effective retrofit solution as compared to alternate evaluation and design methods allowed by the ordinance. These alternate methods are:
1. International Existing Building Code (IEBC), Chapter A4, the most conventional method that has been used so far. Both Berkeley and Alameda ordinances are based on this code. IEBC A4 approach is code based and inasmuch requires an engineer to design the first story in conformance with the current code, albeit for slightly reduced loads. It focuses on the main deficiency that can cause a building collapse, requiring retrofit only at the first story thereby minimizing disturbance to upper floor occupants and reducing costs.
Unlike IEBC chapter A4, the FEMA method considers consequences of over-strengthening of the first story. In some buildings, adding too much strength and stiffness to the first floor may yield pushing more damage to the upper floors. The FEMA method, checks for these cases and limits the retrofit of first floor accordingly.
2. ASCE 41, Seismic Evaluation and retrofit of existing buildings – a “Mercedes-Benz” of Seismic building analysis. It is normally used for much larger buildings and requires the analysis of the entire building, not just the first story.
ASCE 41 design approach requires costly and sophisticated non-linear analysis of the materials and structural response. It is intended to be used for the entire building and not just for the first floor (even though the ordinance does limit the retrofit to first story only). It allows a more complete understanding of the existing structure as compared to IEBC A4 and may yield to a more cost effective design. However, it requires a good set of “as built” drawings and\or a thorough investigation of the existing structure as well as much more engineering time. The initial investment in engineering for these types of buildings may not be worth it.
Like ASCE 41, the FEMA method uses sophisticated non-linear analysis technique. Unlike ASCE41, the method uses a simplified procedure for the particular type of vulnerable buildings with weak story deficiency. An engineer need only to specify the structure materials and geometry, then compare key parameters to the results of nonlinear analysis of surrogate structures already performed and summarized by FEMA document authors.
In conclusion, the FEMA method, while requiring more engineering than more conventional IEBC code based approach, may ultimately yield a cheaper retrofit. A small investment in engineering will potentially save on overall project construction costs.
