A couple thoughts from assessing the long-track KY tornado last week with steer.network, and with and collaborating on a VORTEX-SE project. Mostly in Mayfield, Buena Vista, and Princeton, with some time in Bremen and Dawson Springs also.
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Overall, damage was on par with the worst I’ve personally investigated (Tuscaloosa EF4 and Joplin EF5). Was it an EF5? It could be argued so, but when considering context of immediately adjacent structures, I think the NWS did a great job as usual.
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While focus is usually on P(EF5 | Damage), we have to also consider P(EF5 | Surviving) for adjacent structures and the latter probability was too low in areas I saw to justify EF5 given construction quality. Obviously, EF Scale is an imperfect method, and the rating is debateable
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For example, in downtown Mayfield, immediately downpath of the collapsed water tower was a home with roof still intact (more on it later), others with only roof structure damage. Many other examples. Aerial panoramas can help give that context. windstorm.eng.auburn.edu/2021-12-10-qua
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Poor wind resistance in most construction, but on par with what you’d expect for older structures in regions with current design wind speeds of only ~110 mph. Remember that tornadoes were ignored in our minimum design load standard (ASCE 7) until this year enr.com/articles/53126
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Despite the best of warnings, many people still killed in buildings. How do we prevent that when we don’t design most buildings for tornadoes? Community shelters aren’t the silver bullet for a variety of regions, as has summarized well.
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In commercial structures, the fatality risk appears highest in collapse of long-span, large-volume buildings (Amazon facility, candle factory). These often have little redundancy (ability to absorb loads from damaged members into adjacent ones without them also failing).
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When they fail, heavy structural members are collapsing on occupants if they don’t have a designated sheltering option. Due to access restrictions and ongoing SAR, we unfortunately couldn’t make on-site assessments of the most critical failures from this event.
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For wood-frame structures, we must accept that failures will happen in an EF4, but ensure safer failure sequences in such extreme wind events. Safer is top-down, out-in failures – roof fails, then exterior walls, then interior walls.
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When the foundation connections are the weakest link, especially in homes not built on a slab, you short-circuit the failure sequence and expose occupants to the worst safety environment prematurely.
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This is essentially the same failure mechanism that makes manufactured homes so deadly (ascelibrary.org/doi/full/10.10). Before the roof can fail, homes are swept off the foundation and collapse. In several instances, homes failing in this way collapsed into the basement.
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Depending on construction method, the bottom plate to sub-floor, floor joists to sill plate, and sill plate to stem wall can all be critical weak points. Tim Marshall highlighted this issue well related to anchor bolts.
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Here’s an example where the failure occurs at the bottom plate to subfloor. We saw this repeatedly, creating catastrophic damage patterns and endangering occupants. You can’t always assume that these are the result of the typical failure sequence (roof then walls).
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We did see some successes - this home just downwind of the collapsed water tower had been revitalized, which included adding metal straps that wrapped over the truss top chord and connected to the wall top plates. Adjacent structures had roof structure damage and/or wall collapse
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Here’s another home (in between two leveled homes and just up-path of several demolished, unreinforced concrete masonry buildings) that also used metal straps, albeit atypically. Occupants left the home for an outdoor underground storm shelter and survived whas11.com/article/news/k
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Several "home-made" interior storm shelters performed well thankfully, despite using inadequate doors. Fatalities have occurred in shelters like these in the past due to using standard doors and door hardware that are inadequate to resist storm debris.
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Home made shelters like these provide better protection than many other structures, but not absolute unless the right door hardware is used. fema.gov/sites/default/
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I continue to see hurricane straps attached to only the upper top plate, with exterior sheathing only attaching to the lower top plate, creating a critical weak link. The hurricane straps only minimally improve resistance because the roof simply takes the upper top plate with it.
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In an adjacent structure to the previous one (which used hurricane straps at truss-to-wall conection), toe-nails were used instead of straps at the truss-to-wall connection, but straps were used to tie the studs and both top plates together. You need both!
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Finally, if you’ve made it this far, a couple closing big picture takeaways: (1) even in the most intense tornadoes, engineering still matters. A continuous load path will improve performance and provide a safer structure, even if it still suffers some damage.
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(2) the goal should not be to prevent all damage in the most intense tornadoes. The goal should be to shrink the damage path by minimizing damage in the outer regions, and have safe failure sequences in the most intense portions of the path.
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(3) I don’t blame building owners or occupants, and for the most part don’t blame contractors, for the building failures I see. As a society we’ve for decades simply assumed that tornadoes are not worth designing for, so we shouldn’t be surprised at the destruction.
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That philosophy is changing thanks to @DavidPrevatt2 and many others, but we as engineers need to do a better job of educating – what do we currently design for and why; and what could we be designing for, at what costs, and for what benefits.
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Still much work to do, and this thread is an imperfect and inadequate assessment of a complex issue for sure. But I’m happy to engage any questions, comments, or opposing viewpoints to any of the above that would help us move forward.
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Finally, beyond the engineering, there are so many hurting people that need our help and our prayers. The timing of this storm is especially difficult given the upcoming holidays, so please do not forget those affected.
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Random Side Note #1: tornadoes continue to humble me with how little I truly know about them, as I try to come up with explanations for specific post-storm conditions (e.g., pillow sucked in through the top of an exterior wall between studs and drywall).
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Random Side Note #2: I apparently lost 5 lbs over the three days I was in the field. Definitely didn’t help my strength conditioning goals.
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