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Strengthening Vulnerable Soft-Story Buildings By Thor Matteson
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Thor Matteson has practiced structural engineering since 1990. He has designed hundreds of earthquake retrofits. He wrote two books relating to earthquake retrofits: “Earthquake Strengthening for Vulnerable Homes” and “Wood-Framed Shear Wall Construction – an Illustrated Guide.” Matteson was frustrated that a better system for bracing soft-story buildings was not available, so he invented his own system, patented it, ran full-scale tests, and had it independently evaluated for building code acceptance. To date the system has been permitted and installed in over a dozen Bay Area cities. He established a manufacturing company in 2018 to produce SkinnyBraces and now limits his engineering practice to projects using SkinnyBraces. Learn more at www.quakebracing.com. 16 WINTER Q1 2022 / EBRHA.COM
arthquakes happen, and they happen unexpectedly. You don’t want to be the property owner whose building collapses and threatens human life. Today, building earthquake preparedness has taken leaps and bounds over yesterday’s techniques. In the case of soft-story apartment buildings (see Figure 1), many owners aware of the risks or those adhering to city requirements, will be pleased to know advanced strengthening techniques can spare their buildings and most importantly protect human life. For 10 years I owned a structural engineering firm that focused on strengthening buildings to better resist earthquakes. This article shares some of what I learned. For brevity, I have simplified the engineering concepts.
THE CHALLENGE
Typically garage door or carport openings weaken one wall of a building, leading to the soft-story condition. Nobody wants to give up width in a parking space, driveway or garage-door opening. New structural elements need to be as narrow as possible. This article discusses four varieties of narrow structural systems: moment-frames, pre-manufactured steel frames, cantilevered columns, and the author’s patented “structural fuse” system. Each of these four systems share some basic traits, most notably the need for a new footing to support the bracing system.
THE CONTENDERS
The following methods are the most common when trying to preserve access and parking space. Steel Moment Frames (MFs) are the system of choice for new buildings where large wall openings are desired. Typical MFs consist of two vertical columns, one on each side of the wall opening that creates the soft-story weakness. A steel beam connects the tops of the columns. The joints between the steel members are usually welded together. On-site welding poses a fire hazard and requires costly special inspections. Engineers are accustomed to designing MFs, but they are usually a poor fit – literally – for a retrofit. Existing buildings often have electrical panels or gas meters (sometimes both) that obstruct the MF installation. In many cases, the beam going across the top of the door opening will conflict with garage door openers. On the plus side, in addition to resisting earthquakes, MFs can support the building above; this can allow removing posts to make parking areas more open. Pre-Manufactured Moment Frames (PMFs) are available. Manufacturers include PACO Steel, Simpson Strong-Tie and Hardy-Mitek. As one contractor said about PMFs, “They are fine if you don’t mind taking the existing building apart and rebuilding it to fit around them.” PMFs can also support the building above if you wish to remove posts.
