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Strengthening Vulnerable Soft-Story Buildings

By Thor Matteson

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. E 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.

Engineers are accustomed to designing MFs, but they are usually a poor fit – literally – for a retrofit.

Steel Cantilevered Columns

(CCs, see Figure 2) act like extremely strong and rigid flagpoles that extend from a new foundation to the structural members above the soft story. Unlike MFs or PMFs, it is possible to install a single CC on the most convenient side of an opening to provide necessary bracing. This helps avoid conflicts with existing electrical, plumbing or other obstructions.

Building codes severely penalize CCs by requiring a much bigger design “safety factor” compared to other systems. This can affect the entire earthquake retrofit design (footings, other connections, and potentially work in other building areas), not just the column itself.

The “SkinnyBrace” structural

fuse system (Figure 3) is the author’s invention that combines several advantages of the other systems, without the disadvantages. The specially designed structural fuses act similarly to fuses in an electrical system: they predictably absorb earthquake energy, before other parts of the system are overloaded. The fuses are precisely engineered and manufactured to maximize efficiency of the system and provide predictable performance.

SkinnyBraces do not require onsite welding like site-built MFs and many CCs do. A single SkinnyBrace can often brace the weak wall in a typical four to eight-unit apartment building. Finding room for a single SkinnyBrace on one side of a garage door is much easier than finding room for a MF’s two columns. Compared to CCs and most MFs, the predictable and reliable structural fuses allow designing SkinnyBraces with a more reasonable safety factor – this can reduce the required size of footings by 46 percent compared to MFs and 62 percent compared to CCs.

The structural fuses in SkinnyBraces are intended to be easily replaced after an earthquake. Most other bracing methods are “single-use” items, meaning that you may have to replace the entire MF, PMF or CC (and possibly the foundations that support them) after an earthquake. This would be extremely disruptive, expensive and time-consuming, and was another factor in developing my own bracing system. Replacing the structural fuses in a SkinnyBrace requires unfastening and replacing only five or six bolts.

CONCLUSION

If you need to strengthen a soft-story building, the bracing system you choose can have major impacts on parking and access, as well as initial cost and future repairability.

L TO R: Figure 1. Steel cantilevered column between two garage doors. The floor slab was cut out to allow digging a new footing trench to hold the column base. Figure 2 Classic “soft-story” building failure after the 1989 Loma Prieta earthquake. Figure 3 A single SkinnyBrace installed on one side of garage braces the apartment above. The replaceable yellow structural fuses at the top absorb earthquake energy.