Liam O’Brien

PV & Electrical Design Engineer

Engineering a µHouse

A blog following my progress designing and fabricating a modern ~200sqft tiny home

Its all about connections

Securing your tiny house to the trailer (if your fabricator screwed up with the threaded rods)

bolts close up

Sadly these rods had to go since my walls wouldn’t have fit around them

If you read my previous post on trailer woes you will know that due to some errors on the part of my trailer company and their fabricator my threaded rods (for each corner) got placed in the wrong spot, meaning I had to chop them off in order actually have my walls fit properly. Not optimal, but the bigger problem was it left me to find a way to securely attach the bottom plate and walls to my steel subframe.

In the sprit of thoroughness, I thought I would go over a few popular options including my ultimate solution (caution some math involved!)

1) Full length threaded rods 

While it seems like a good idea in theory, what you really want is a steel shaft with threaded ends, the reasoning being, that by threading the rod along its full length you create hundreds of stress points (each thread counts) as well as making the overall diameter thinner since you remove material to create the thread profile. To make matters worse, you then proceed to place the rod in tension (further pulling at each stress point), meaning deformation and creep of the material. Once a bolt is not under preload it is as good as useless. The slightly better way, since all you really need is a few inches of screw thread on the top and bottom, is to use a stud or threaded shaft, where only the last 2-3in of each end are threaded. This leaves you with the integrity of the steel shaft in your wall cavity but a screw thread where you need it. In my case due to the geometry of where my bottom plate lies in relation to my steel frame this solution wouldn’t work for me

2) Threaded rods with a Simpson Holddown

hdu5

Simpson Strongtie HDU5 hold-downs

This was what I was supposed to do and would have done had my trailer been made correctly. This is a really nice option because you are bolting the corner walls, trailer frame, and bottom plate all together, increasing the rigidity of the corners and making sure your house stays where you put it.

3) Steel Construction Bolts

While I initially wanted to go this route using ASTM325 structural bolts, I found that I would need to place the bolt at the edge of the bottom plate in order to go through the steel of the trailer, not optimal from a structural standpoint. Yet another option out.

4) Self Tapping metal screws

90420a667lLink to buy them on McMaster

Finally we get to what my ultimate solution was, and while I’m not a certified structural engineer (just a mechanical one) I think the math works out, if I have done something egregious and you can show me (using alternative calculations) a better way please let me know. I would like to my house to stay attached to the trailer just as much as the guy driving behind me.

 

Calculation for pullout force of a screw in steel (strength formulas from this paper)

FOS (Safety Factor) = 1.5

Futs = Ultimate tensile strength of the material (~45,000psi for this particular steel)

d = nominal screw diameter

Tc = thickness of material, i.e. how much material is the thread grabbing into

Fpout (each screw) = 0.85*Tc*d*Futs

Fpout (each screw) = (.85*.120*.25*45000) = 1,147.5lbs (the weight of 1 grand piano)

 

Number of screws required

Wall Perimeter  = 25.5ft + 25.5ft + 7.58ft + 7.58ft = 66.16ft

Quan = Perimeter/2ft spacing = 33 screws

Total Pullout force lbs = (Fpout × Quan)/FOS = 25,245lbs (in pure tension) or about the weight of just under 19 Formula 1 cars

 


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