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I live in an old rented apartment on the 4th floor without a fire escape. (It's legal, unfortunately, all checked out). I would like to have an emergency window exit but I measured my windows and they seem too wide for the commercial ones that hook around the window frame. So I'd like to install something permanent.

Just for my own peace of mind, I'd like to know how you actually know for certain what is the capacity of steel screws in wood studs. I read on this site that the strength depends on the density of the wood substrate, the diameter of the fastener and the threads; likewise this Stackexchange about a wall hanging gives other numbers. Am I right in understanding this Stackexchange to say that it takes about 300 lbs to rip a screw out of the stud itself?

I've been trying to figure out also kind of professional that is qualified to determine or install this but I don't know or can't find. It seems like engineers are for much bigger projects.

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You ask valid questions and have very valid concerns about a 4th floor fire escape (I presume there is some sort of fire escape, but that's a different question).

I'd recommend that you use the hardware provided and follow the installation instructions of whatever escape you purchase.

I installed window escapes in my house when we moved our kids to upstairs bedrooms. The kit came with a mounting bracket that mounted below the window trim and I used the four (provided) screws with each bracket. IIRC they were roughly a #10 screw about 3" long and went into studs.

After installing them, I showed the kids how to unfurl the escape ladder, then I climbed down first to make sure everything was safe before having each of them climb down so they'd know they could do it. There was no issue whatsoever with the screws holding.

You might need different size screws/bolts for a 4th-floor capable fire escape, but anything that's passed any sort of UL/CPSC testing (if you're in the US, or whatever sort of national testing lab there is in your country), you can be pretty certain that you'll be safe using the included hardware.

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  • Thanks so much, this is really helpful. I saw an installable kit by X-it escape ladders, that was recommended by the NY times. I was thinking to get it & would definitely just follow the directions. But I guess the whole thing is so scary (4 stories is a lot!) that I was wondering why or how does one know that a certain set of screws in a certain kind of wood are or are not enough to a 100+ pound human. I see what you're saying about passing the UL/CPSC testing. But how do they know what kind of wood is in my building, below the window trim, for that to apply? Is it all standardized?
    – Lisa
    Jan 15 at 19:02
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    Most houses are built out of 2x4 SPF (that's spruce/pine/fir - i.e. whatever was cheapest on the day the lumber futures were purchased). It's a pretty well known commodity. Despite variances in each and every piece of lumber, engineering data will provide a minimum expected strength. Four (or more) story buildings, however, often are not built out of lumber, but from steel and/or concrete, brick, or other material. It will be important for you to know what your walls are actually made of. If they're not wood, you'll need to use appropriate anchors, and that should be a new question.
    – FreeMan
    Jan 15 at 19:06
  • Thank you!! I will work on figuring that out…
    – Lisa
    Jan 16 at 0:24
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If you're uncomfortable with the ladder's installation after following the manufacturer's instructions, then I suggest a low risk proof test: hang it out the window, take the stairs down to the bottom, and put a bunch of weight on it. What's the proof test load? That's an interesting question in that I assume that this ladder is for escaping fires instead of drug raids. In the fire scenario, I can easily imagine your ladder looking very inviting to occupants of the two apartments between you and the ground level. An upper bound on the design weight, then, is me, any family, any dinner guests, and all of my downstairs neighbors and their dinner guests. With the height constraint of 4 stories, 5 average weight people sounds like the most who could descend at the same time. Be careful not to break the bottom rung of the ladder with proof test weight. Weight in the center stresses the rung itself, whereas weight on the ends just stresses the connection to what I assume is a rope support.

Alternatively, wood connection design is unfortunately the most difficult from among common construction materials. The National Design Specification (NDS) is the US authority on design values for fastening into sawn lumber like wall framing (although other materials than sawn lumber do get used in wall framing). Chapter 12 covers dowel-type fastener design (nails, wood screws, bolts, and lag screws). My mental image of your use case is just a ladder hanging from a bar, so I suspect that withdrawal strength isn't relevant to your project, where instead you want to compute lateral strength. If there's a weak layer of stuff between your brackets and the wall studs, however, then the NDS's lateral load design values will be no good (withdrawal loads would still be okay, though).

The institutional structure behind the NDS (the American Wood Council) has a technical report providing lateral load design values for my "weak layer of stuff" cases like window trim, but it's near incomprehensible for non-engineers. I don't know of any online calculators that cover the scope of that technical report. The technical report provides the fundamentals to answer your linked StackExchange question correctly (or compute a strength given an intermediate layer of poorly fastened trim), whereas the NDS isn't up to the task; that 300# is total BS.

You're right about wood density being a common variable in fastener strength computations. Technically the NDS uses specific gravity (with "G" as its variable) instead of density, but they're effectively the same thing (you multiply specific gravity by the density of water, (62.4#/ft³)(1ft/12in)³, to get density).

Among the common US softwood species used in construction (Douglas Fir, Hem-Fir, Southern Pine, and Spruce-Pine-Fir), Spruce-Pine-Fir (South) (or SPFs, where the little s is for "South," not plural) has the lowest specific gravity (0.36 from page 87 of the NDS's chapter 12), making its fastener strength values conservative when working with sawn lumber.

For your single shear connection, there are 7 failure modes visualized below from the Appendix chapter of the NDS (page 184). Standard practice is to compute all 7 and take the smallest as the strength. If you're anchoring through a piece of wood trim and want to use the NDS instead of that technical report, then you would need to attach the trim sufficiently so it acts as a structural member. It can't shift before the ladder's fasteners reach the smallest failure load among the 7 modes. Given such structural trim, you can pretend that the framing thickness is the actual framing thickness plus the trim thickness. I really don't like this strategy, because for all I know your trim is some engineered wood product without even the strength of SPFs. And I've seen trim weathered into dirt, although maybe that's an unreasonable expectation for a structure with 4+ levels.

7 failure modes

Assuming that the trim is at least as strong as SPFs and attached sufficiently, you can use the NDS to compute your fastener design strength. Section 12.3 of the NDS is where to find the lateral strength prescription (starting on page 83). The 7 failure mode computations are nasty, but don't worry about that. The American Wood Council maintains a connection calculator that will compute them. Take note of Fem and Fes in the expressions, though, and observe Table 12.3.3 on page 86 to see how the specific gravity plays a role in the failure mode values. For the connection calculator, my instinct is that your ladder has thin brackets. I would use steel for the calculator's side member, choosing its thickness based on the bracket's thickness. Use load duration factor CD = 1.25. Depending on your climate and rain exposure, you might use wet service factor CM = 0.7 to account for high moisture. The other 2 factors in the connection calculator should be 1.0. When in doubt, use Spruce-Pine-Fir (South) as your lumber species.

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  • You write good, well researched answers. Unfortunately, I fear they go waaaay over many (most?) people's heads. (I know I glaze over when reading them.) Possibly consider adding an "executive summary" or "tl;dr:" section at the top to boil it down to to a fairly simple calculation/answer/solution with all the supporting technical info after that.
    – FreeMan
    Jan 17 at 14:59
  • I have no doubt that this answer is correct and accurate, I just can't find it here and get lost trying. I mean, "For your single shear connection, there are 7 failure modes that you can see in the Appendix chapter of the NDS... compute all 7 and take the smallest as the strength". Perfect for the design engineer. Not so helpful for Joe Average home owner...
    – FreeMan
    Jan 17 at 15:00
  • I hope that this comes across as constructive and not criticism, because that's the way it's intended. I have no doubts about your knowledge and sincerity in sharing it, just that it can be really hard for non-engineering types to sort through.
    – FreeMan
    Jan 17 at 15:01

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