Applying new FEMA P-807 Weak Story Tool to Soft-Story Retrofit

This week’s blog was written by Louay Shamroukh, P.E., S.E., who is a regional engineering manager working out of the Simpson Strong-Tie Stockton branch. Louay is a licensed Structural Engineer in the State of California. He started his career with Simpson Strong-Tie in 1999 as a R&D engineer responsible for testing, improving and developing products for the light frame construction industry and he holds several wood construction connector patents. Louay serves as the liaison between the engineering department, customers, sales and manufacturing. He supervises a department that is tasked with providing technical and application support for Simpson Strong-Tie products to sales, specifiers and building officials. He explores market opportunities for developing new products through interaction with customers in the field and at industry events. Here is Louay’s post.

We have written about San Francisco’s Soft-Story Retrofit Ordinance and Soft-Story Retrofits before on the blog. I wanted to discuss in more detail the issues with soft story buildings and FEMA’s new tool for addressing them. Under the San Francisco Ordinance, wood-framed residential structures that have two or more stories over a “soft” or “weak” story require seismic retrofit. So far, more than 6,000 property owners have been notified about complying with the mandate.

Multi-unit wood-frame buildings with more than 80% open area on one first story wall or more than 50% on two adjacent walls are considered weak story buildings. During the 1971 San Fernando earthquake, 1989 Loma Prieta quake and the 1994 Northridge earthquake, this type of building often sustained major damage or completely collapsed. One cause for this structural weakness is the mixed use of the buildings, which dictates an open space and less partition walls on the first story than the upper stories.

Soft Story building
Figure 1: Multi-unit wood-frame building with first weak story.
Soft story building after an earthquake
Figure 2: Near collapse of typical weak-story wood-frame building.

The lack of exterior walls or partition walls on the first story leads to a considerable difference in lateral strength, stiffness and stability between the first story and the upper stories. During an earthquake, this difference exposes the first story to a concentrated lateral deformation in lieu of distributing it over the height of the structure. In the presence of large openings in the exterior walls, the concentrated lateral deformation is superimposed with the building’s tendency to twist.

Rotation of first story of a corner building
Figure 3: Rotation of first story of a corner building with openings on two side walls.

Buildings built prior to 1978 were constructed of materials and finishes that are archaic, non-ductile, with low displacement capabilities and poor detailing that can lead to earthquake damage, and in some cases, to building collapse. Some of these materials are stucco, diagonal sheathing, plaster on wood lath and plaster on gypsum lath that possess a maximum inter-story drift ratio of 2% or less.

unit load drift curves
Figure 4: Unit load drift curves for sheathing material with low displacement capacity vs plywood panel siding.

The Federal Emergency Management Agency (FEMA) has developed the FEMA P-807 guideline, “Seismic Evaluation and Retrofit of Multi-Unit Wood-Frame Buildings with Weak First Stories.” FEMA P-807 provides procedures for the analysis and seismic retrofit of weak first story buildings built with structurally archaic material.

FEMA P-807 Guideline
Figure 5: FEMA P-807 Guideline.

The guideline’s design philosophy is to provide a cost-effective seismic retrofit method limited to the first story without disrupting the occupancy of the upper stories. The guideline limits the retrofit to the first story by introducing sheathing materials or structural elements with high lateral displacement capacity. This is designed to improve seismic performance and reduce the risk of collapse without driving the earthquake forces into the upper stories and exposing them to the risk of damage or collapse.

nit load drift curves for sheathing material with high displacement capacity
Figure 6: Unit load drift curves for sheathing material with high displacement capacity used for retrofitting weak first story of a multi-unit wood frame building.
Unit load drift curves for Simpson Strong-Tie® Strong Frame® special moment frame with high displacement capacity
Figure 7: Unit load drift curves for Simpson Strong-Tie® Strong Frame® special moment frame with high displacement capacity used for retrofitting weak first story of a multi-unit wood frame building.

FEMA’s Weak Story Tool, an electronic tool developed for FEMA P-807, tabulates the walls in a building graphically. Each wall in the building has its inherent material capacity to provide resistance during an earthquake. The tool applies the rules of the provisions and performs the analytical calculation to evaluate the building before and after the retrofit. Performing the analysis manually and iteratively requires a considerable amount of time and calculation. On the other hand, the tool is a convenient mean that aids in the analysis and keeps checking the input as the assemblies, special moment frames and walls are added for seismic retrofit. A report summarizing the data and formulas is available once the retrofit meets the provisions of FEMA P-807.

Recently, the Simpson Strong-Tie® Strong Frame® special moment frame was added to the Weak Story Tool. The Strong Frame® special moment frame is a 100% field bolted connection frame that does not require field welding for the retrofit of an existing building. It has a unique replaceable patented Yield-Link™ structural fuse that provides the ductile lateral resistance with high lateral displacement capacity. In close quarters of an existing building, such as a parking garage or commercial space, the Strong Frame footprint is considerably smaller than other retrofit assemblies. It also eliminates the need for beam bracing normally required for special moment frames, which was discussed in a previous post.

Weak Story Tool with Strong Frame Special Moment Frame.

Figure 8: Weak Story Tool with Strong Frame Special Moment Frame.

To use the Simpson Strong-Tie lateral system solution in the Weak Story Tool, go to the Assemblies Tab, where you can select Strong Frame special moment frame as a retrofit assembly. The frame is specified in the Simpson Strong-Tie screen functionality after inputting the frame’s dimensions and the ultimate target force. After selecting the frame, the functionality provides the initial stiffness, yield strength, ultimate strength and drift at ultimate strength for the tri-linear backbone curve, which are seamlessly inputted into the Weak Story Tool.

New Assembly button to specify retrofit assemblies
Figure 9: New Assembly button to specify retrofit assemblies and Strong Frame special moment frame.
Strong Frame Special Moment Frame Functionality
Figure 10: Strong Frame Special Moment Frame Functionality.

The Weak Story Tool is a convenient and powerful tool that can save the specifier several hours of mundane work and resources. Please try out the Weak Story Tool with the addition of the Simpson Strong-Tie® Strong Frame® special moment frame and let us know what you think. We always appreciate the feedback!

FEMA’s Weak Story Tool can be downloaded here.

If you’re in the Bay Area, please join us for hands-on training on the use of the FEMA Weak Story Tool. Register here, bring your laptop, and join us in the Weak Story Tool workshop presented by Simpson Strong-Tie engineers on Wednesday, October 22 at Oakland City Hall, 1 Frank H. Ogawa Plaza, Oakland, California 94612.

Home Seismic Retrofit

The 6.0 magnitude earthquake that struck Napa, CA, in August caused more than 200 injuries and structural damage to many homes and businesses throughout the area. The quake was the largest to hit the San Francisco Bay Area since the Loma Prieta earthquake (6.9 magnitude) in 1989, prompting the governor to declare a state of emergency.

I have done several posts about San Francisco’s Soft-Story Retrofit Ordinance and some of NEES-Soft testing related to soft-story retrofits. The soft-story ordinance only addresses multi-unit residential units and does not require retrofit of single-family homes. Cities are reluctant to mandate seismic evaluation and retrofit of single-family homes for a number of reasons that I won’t discuss here. The draft Earthquake Safety Implementation Program (ESIP) for San Francisco will not recommend mandatory retrofit of single-family homes until 2030.

CAPSS Implementation Priority Worksheet

The good news is homeowners can retrofit their homes without waiting for the government. A couple years ago in this post, I discussed some of the tools available to retrofit existing buildings.

One of these tools is the 2012 International Existing Building Code (IEBC). The IEBC has provisions for repair, alteration, addition or change of occupancy in existing buildings and for strengthening existing buildings. For alterations, these provisions may not comply with current IBC requirements, but they are intended to maintain basic levels of fire and structural life safety. The IEBC also provides prescriptive provisions for strengthening existing buildings against earthquake damage, which include strengthening residential houses on raised or cripple wall foundations.

Cripple Wall Reinforcing Schematic

Cripple Wall Retrofit Schematic and Installation
Cripple Wall Retrofit Schematic and Installation

Cripple wall failures are a common type of damage observed in older homes, caused by inadequate shear strength in the cripple wall. An additional failure point is the attachment of the wood sill plate to the foundation. Having a strong connection between the wood structure and the concrete foundation is critical in an earthquake. Since the work required to strengthen these connections is typically performed in a crawlspace or unfinished basement, it is a relatively low-cost upgrade that is extremely beneficial to structural performance.

Retrofit with UFP Foundation Plate in Napa
Retrofit with UFP Foundation Plate in Napa

Our website has information for retrofitting your home. The Seismic Retrofit Guide has information about how earthquakes affect a home and the steps to take to reinforce the structural frame of a house. The Seismic Retrofit Detail Sheet is intended to help building departments, contractors and homeowners with seismic retrofitting. It includes common retrofit solutions for reinforcing cripple walls and foundation connections.

One business owner in Napa chose to retrofit her building when she purchased it. You can see her video narrative here.

Ignore Seismic Requirements When Wind Controls?

Prior to joining Simpson Strong-Tie, my career involved the design of projects in California’s San Francisco Bay Area. When designing the primary lateral force resisting system, I would have several pages of seismic base shear calculations and, oh yeah, a one- or two-line calculation of the wind forces – just to show that seismic governed. There was no need for complete wind analysis, since the seismic design and detailing requirements were more restrictive. Of course, building components such as parapets, cladding or roof screens needed a wind design. Unfortunately, when wind appears to control, meeting the seismic requirements is not so simple.

Continue Reading