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.

Changes in IBC from 2009 to 2012: Seismic Design

The transition from one building code to the next always begs the question, “how is the newer code different?” There are several changes between the 2009 IBC and 2012 IBC that will change the way designers approach seismic design. This blog post is a broad overview of some of the changes. Since it’s not practical to cover all the changes between the previous and new codes in detail in one post, the discussion will be mainly on 2012 IBC and the corresponding ASCE7-10 reference standard.

Seismic ground motion map
Seismic ground motion map

The seismic ground motion maps have been updated to match ASCE7-10. The titles of the maps in IBC were revised from “Maximum Considered Earthquake Ground Motion” to “Risk-Targeted Maximum Considered Earthquake (MCER) Ground Motion Response Accelerations” in order to reflect the titles in 2009 NEHRP and ASCE 7-10. As in previous editions, some areas will prove difficult to read due to the contour lines, so the USGS site and GPS coordinates are recommended (http://earthquake.usgs.gov). Additional information about changes made for 2009 NEHRP is available at www.nibs.org or www.bssconline.org.

The term “occupancy category” was replaced with “risk category” in the 2012 IBC for consistency with the term used in ASCE 7-10. This change was made because it was decided that the use of the word “occupancy” implied the category was directly tied to occupancy classifications in the code, while the word “risk” more accurately communicates that the category is based on acceptable risk of failure.

Seis-pic 2ASCE7-10 revised the way designers use the corresponding Drift amplification, Cd, and Overstrength factor, Ωo, of the Response modification factor, R.  In ASCE7-05, when there is a vertical combination of different R-values, the Cd, and Ωo cannot decrease as you go down each level of a building. In ASCE7-10 (12.2.3.1), the Cd and Ωo always correspond to the R-Value as you go down. The adjacent figure illustrates the new provision to use the corresponding Cd, and Ωo with the R-value at each level.

ASCE7-10 (12.3.4.1) added a clarification for out-of-plane anchorage forces where the redundancy factor, p = 1.0.  The intent of the redundancy factor was to ensure the vertical seismic-resisting system with insufficient redundancy had adequate strength. The design forces for out-of-plane wall loading are not redundancy requirements. ASCE7-10 (12.11.12) revised the out-of-plane wall anchorage force equation where the anchorage forces are reduced for shorter diaphragm spans.

Seis-pic 3Light-frame construction structures are no longer exempt from amplification of accidental torsion in ASCE7-10 (12.8.4.3).  There are many structures vulnerable to torsional effects including some “tuck under” parking buildings that are often light-frame structures. See posts  titled Soft-Story Retrofits and City of San Francisco Implements Soft-Story Retrofit Ordinance for more discussion of soft-story, light-frame buildings.

This is just a brief summary of changes related to seismic design found in the 2012 IBC.  What are other changes that will modify your approach to seismic design?

Changes Made to ACI 318 With Respect to Adhesives Anchors in Concrete: What Engineers Need to Know

For the first time, ACI 318 – 11 includes a design provision for adhesive anchors in concrete.  Previously, adhesive anchors were designed according to provisions found in both ICC Evaluation Service (ICC-ES) AC308 and ACI 318 – 08. A relatively new standard, ACI 355.4, must be used to qualify adhesive anchors in concrete. This new standard, along with ACI 318 – 11, contains important changes that will affect anchor systems designed to the 2012 IBC.  Not all changes are discussed here. I will only focus on what you – the engineer – should be aware of.

ACI 355.4 requires that adhesive anchors in concrete be evaluated using a bond strength (measured in terms of psi and used with the surface area of the embedded portion of the anchor) that corresponds to a long-term temperature (LTT) of 110 degrees F to account for potential elevated temperature exposure conditions. This wasn’t necessarily the case previously where, for example, the engineer could elect to use a temperature category that listed bond strength values based on a LTT of 75 degrees F. The issue here is creep.

Creep, in the world of adhesive anchors, looks at how well the anchor can resist load without too much axial displacement over a period of not minutes, not hours, not even years but decades. As a general rule, it’s no surprise that creep worsens as the temperature rises for almost any material.  In our case, the bond strength is effectively reduced. Most adhesives, if not all, currently list bond strength values that correspond to a LTT of 110 degrees F. Make sure to select the temperature category that meets this minimum requirement. Some adhesives will experience a reduction in bond strength at an LTT of 110 degrees F, some won’t.

What about applications involving short-term-only loading? Is creep still relevant? Generally, you’ll find that adhesive anchors negatively impacted by the higher LTT requirement will gain back much of their load for seismic/wind-only load applications. So creep becomes irrelevant.

While adhesive anchors used solely for the purpose of resisting short-term loads will remain largely unaffected by this code change, significant changes have been made to the design and installation of adhesive anchors when used for sustained loading applications (e.g. dead load, live load, etc.).

First, the bond strength must be reduced by a factor of 0.55 as compared to 0.75 under the previous code (following ICC-ES AC308). New to the code, section D.9.2.2 of ACI 318 App. D requires that adhesive anchors used for resisting sustained loads be installed by someone who has taken the Adhesive Anchor Installation Certification (AAIC) program. The installer must show proof that he/she is certified by passing both a written and performance examination. Installing adhesive overhead requires some skill. So it’s no surprise that the installer must satisfactorily demonstrate proficiency by blindly installing adhesive overhead into an inverted test tube that will later be cut in half and graded for the presence of voids.  Figure 1 shows no voids, so the installer passed.

ACI-CRSI Installer Workbook Publication
Figure 1 [from ACI-CRSI Installer Workbook Publication CP-80 (12)]
However, exceptions do exist. If you’re working on a hospital or school in California, the 2013 CBC (Table 1705A.3 footnote c) requires that all horizontal and overhead adhesives anchors – irrespective of load condition – be installed by a Certified Adhesive Anchor Installer (CAAI). This deviates from ACI 318 D.9.2.2.

Arguably, with AAIC, there’s an added cost to using adhesives for anchorage designed for sustained loading. However, for sustained loading applications best suited for adhesive anchors it should come as peace of mind to the engineer, owner, contractor and other parties involved with the construction project that a certified installer has been employed to ensure that the adhesive anchor has been installed in accordance with the manufacturer’s printed installation instructions.

While the engineer should be aware of the above limitations placed on adhesive anchors, by no means should it hamper their design. There are several options available to the engineer. Table 1 compares the tensile design strength of three common types of anchors – two adhesives, two mechanical anchors (one screw and one expansion type) – determined using the new design provision ACI 318 -11. While the creep test results show a reduced capacity for adhesive A, it does show a significant increase in load for seismic-only applications because , as we discussed earlier, creep is no longer an issue. Some adhesives, like adhesive B, will do well under the creep test (at an elevated LTT of 110 degrees F), so any capacity increase for seismic-only applications will be small.

Tensile Design Strength between 3 types of anchors.
Table 1

What three important points can we glean from Table 1? First, all things being equal, mechanical anchors will typically achieve higher “code values” for sustained loading applications relative to adhesives. Second, mechanical anchors are easier to install overhead. Third, AAIC is not required for mechanical anchors. While these reasons support using mechanical anchors for overhead anchorage, doing so is nothing new. The bulk of overhead attachments have almost always been made with mechanical anchors mainly because it’s just easier to do it that way.

Perhaps up to 95% of adhesives are used to secure rebar to concrete – we’ll call them rebar dowels. Like any anchor, rebar dowels can be used to resist seismic and/or sustained loads. While the exact breakdown is hard to determine, arguably, the bulk of rebar dowels in the west coast are found in seismic retrofits and renovations used to thicken walls, tie-in new concrete shear walls, connect new drag struts, strengthen existing concrete elements, etc., all for the purpose of strengthening the lateral capacity of the existing structure to withstand greater earthquake and/or wind loads. These typically won’t require a CAAI, but it might if it’s a school or hospital project that requires overhead or horizontal anchors. Some rebar dowels are used for enlarging footings to withstand greater dead and live loads, so these would require a CAAI. Remember: the bond strength can be lower than expected for sustained loading applications, so you may want to use an adhesive that does well at a LTT of 110 degrees F if that’s what your design requires.

One new benefit of ACI 318 is that the engineer can now design adhesive anchors to go into lightweight concrete using the factors found in section D.3.6.

One significant change engineers should include in their specification is that the concrete must be aged at least 21 days before installing an adhesive. Previously, the industry standard was to wait seven days. For additional information regarding adhesives installed into younger normal-weight concrete, read the following Simpson Strong-Tie engineering letter: http://www.strongtie.com/ftp/letters/generic/L-A-ADHGRNCON14.pdf

What are you experiencing in the design of your anchors in your jurisdictions? Leave a comment down below because we would like to know.

Soft-Story Retrofits

In February 2007 I had the opportunity to volunteer for a Soft-Story Sidewalk Survey for the San Francisco Department of Building Inspection. The purpose of the survey was to inventory buildings in San Francisco that appeared superficially to have soft or weak first stories. The volunteers were given a list of addresses to review and we recorded if the building was more than three stories tall, had five or more dwellings, and estimated what percentage of the ground level had openings in the walls. No structural analysis going on, just counting stories, mailboxes, doors and windows.

San Francisco soft-story structure. Photo credit: USGS.
San Francisco soft-story structure failure. Photo credit: USGS.
A collapsed house in San Francisco from the 1989 Loma Prieta earthquake. Photo credit: Adam Teitelbaum, AFP, Getty Images.
A collapsed soft-story in San Francisco from the 1989 Loma Prieta earthquake. Photo credit: Adam Teitelbaum, AFP, Getty Images.

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