Hey there,

I posted this on one other forum looking to get some help.

Long time listener, first time caller.

I'm looking for some advise on a fairly unique build I am doing.

I own some land but zoning is preventing me from building any more homes with a foundation. having a trailer parked on the land is fine. No building inspectors or anything like that where I live.

Buying a new trailer bed is in the $6000+ price range. I'm planing to build on top of a trailer bed that previously had a mobile home on it on this property. I demo'ed the mobile and roof that was built over/on-top of the mobile. The trailers tires are toast and I'm not able to move the trailer anyways. My water/power/septic is already in place to the trailer location, which is another reason i'm building on this spot. The trailer is in good shape I just need to nock off some surface rust and re-paint it. The main of the trailer is 2 steel C beams with 8 smaller cross sections tying it together. Not sure what the weight rating is (6-8000 maybe) but I would imagine that is for the tires and suspension only, not the frame.

Here is what I have so far.

The home is going to be 28x8 w/14x8 loft. About 12ft from trailer bed to peak with a shed style roof. I'm going to tie in the house to the trailer and I have some tiny home plans that I purchased and modified by lengthening and moving some windows around.

So I guess i'm looking for some input on how the best way to build on the trailer as a foundation would be?

My current plan is to do a sub floor with 6" joists @ 24" apart, crossing the C beams and with an overhang of 18" on either side. the spacing between the beams is 60". 2x4 framing and a metal roof. I would prefer to have a foundation of just concrete blocks supporting the beams which is what the previous owner did when it was a mobile home but i'm not sure if the 18" overhang is too much? should I double up the joists to give greater support to the over hanging walls?

Hope this all makes sense, obviously i'm a noobie so I apologize for my ignorance ahead of time.

Any input is very much appreciated.

ps. most tiny homes on trailers are smaller with a different style trailer bed and meant to be moved periodically. Otherwise I could just follow a classic tiny home build.

-Goatboy

sorry can't figure out how to insert image yet

Image insertion notes can be found at the following link
http://countryplans.com/smf/index.php?topic=11663.0


What you are proposing to build will likely be much heavier than what the trailer chassis was designed to carry. It's going to be difficult to tell if the main chassis channels will stand up to the new loads. The 18" cantilever on the floor joists is outside the realm of good building practice and any building codes. If the new structure width is only 8 feet with the 18" cantilever that frame is probably sort of light weight. That's my guess. What was the original width?

How many axles on it and what size are the tires. That could give a pretty good idea of what the old design load was. The manufacturer probably did not design the frame to have much more capacity than the running gear as every penny he spent raises the price.

Thanks for taking the time to reply Mountain Don,

The original width of the trailer was 8 ft. the walls were resting on welded outriggers.

I know what you are saying about the manufacturer designing the frame to capacity. I'm wondering though if I put concrete blocks every 24" running along the length of the frame.

Is there another reason that 18" cantilever on the joists is bad building practice other than the load being to heavy? What about 8" joists @ 16"?

Here is a link to the same topic I have going on another forum with 3 pictures posted.

I apologize ahead of time if i'm doing a no-no by posting links to other forums. 

http://www.small-cabin.com/forum/2_4358_0.html#msg64509

From your pic on the other forum this appears to be a true structural C channel rather than a lightweight bent C section? If so what are the dimensions... height, width of flange, thickness of flange and web? Is it in good condition rustwise and are there any sections of the channel notched or otherwise compromised?

Hi Don_P,

Yes I think it is a welded structural C channel. The width (flang) is 1 3/4", the hight (web) is 5", and the thickness of the flang and web seems to be a little over 1/8" more like 5/32" or 6/32" but i don't have callipers.

The channel seems to be in good condition, I still need to knock some light rust off to make sure it's just surface rust but everything seems to be fine. I just finished demoing the trailer so I haven't gone over it with a fine tooth comb yet.

Thanks for the help, much appreciated.

I'm not a fan of piers but here are some steel capacities. If you can weld we can get over my dislike with a bit of steel detailing.
The AISC Steel Construction Manual lists 2 types of 5" C Channel, one has a 1-7/8" flange the other has your 1-3/4" flange.  As a beam with lateral support on not more than 5.2' centers (To keep the top flange from buckling out of plane under load). At 4' span it has 11,000 lb capacity with .07" deflection. At 7' span it has  6,200lb capacity with .22" deflection, still passes deflection max of L/360... 84"/.22"=L/381, at that load.

For the overhanging joists, well, I've meant to make a point loaded overhanging beam calc but hadn't really had a reason... give me a bit.

This is great information Don_P I really appreciate it.

What is the steel detailing you speak of? I don't weld but it's not out of the realm of possibility that I could get someone to do it for me.

It seems a 7' span would work good at the 28' length.

I'm going to put down metal flashing between the joists and trailer bed to keep the critters out. I was thinking of re-using the aluminum walls from the mobile, but I seem to remember reading somewhere that building with aluminum directly in contact with steel is a bad idea. A quick google search has me even more confused. Any idea on this?

Again thanks for the help on this.

Well, I've been tied up dismantling a log springhouse for restoration so didn't have much time to play with a spreadsheet... let me talk and write my way through your loads for a minute.

I'm under the impression a 30 psf snowload on the roof would be plenty for your location, that should take care of a heavy wind load as well (they are called a live load). Add 10 lbs/sq ft for roof self weight (dead load), so 40 psf roof... figure a 1' overhang for a 10' total roof width, we'll figure for one side so 5'. If you frame at 16" on center that's 1.33'. Summing.. 40psf X 5' X 1.33' = 266 lbs heading down towards each overhanging joist.

Next add the loft.  (30psf Live + 10psf Dead) X 4' span X 1.33 =212.8 lbs heading down

Next the main floor, there's only going to be the 1.5' overhanging out there X (40 + 10) x 1.33 = 99.75 lbs

sum them up to get the load on the cantilever 266 + 213 + 100 = 579 lbs, call it 600 lbs bearing on each overhanging joist end.

Engineering time;
Maximum bending moment (think torque wrench here) is simply the point load x the lever arm length ( the overhang, 1.5') So 1.5' x 600 lbs = 900 ft-lbs maximum bending moment, multiply x 12 = 10,800 in-lbs

#2 SPF lumber has an allowable bending stress of around 1200 psi when used repetitively as in joists...
section modulus required is 10,800 in-lbs/1200 psi= 9 in³

Section modulus is (breadth of the lumber x depth squared)/6.. bd²/6... or working from this end get the square root of 9/.25... which is 6" deep

The minimum joist height is 6", since a 2x6 is only 5.5" deep and we just did that fast and loose I'd use 2x8's for the floor joists.

QuoteWhat is the steel detailing you speak of?

The problem with piers and beams is that typically the connection between pier and beam is not rigid, it's a hinge. Push on the building from the side, the pier rotates, and the building lands on the ground... or the trailer ends up as a wrecktangle at the far end of a K mart parking lot. Now if there were a pipe or something steel and rigid welded with a good gusset to the C channel and extending down through the pier and cast into the footing, youve just created a rigid frame, no hinge, a moment resisting frame.

QuoteI seem to remember reading somewhere that building with aluminum directly in contact with steel is a bad idea

You can google the galvanic series table, basically many dissimilar metals in contact with one another begin to exchange electrons, on the plus side this makes a battery, on the minus side if they are metals you want to be something other than a battery, they are arcing out and consuming one another. Put something non conductive between them. Tyvek tape, vinyl strips, anything that keeps them from contacting one another even if they sweat, and the metals will sweat on humid mornings.

QuoteIt seems a 7' span would work good at the 28' length.

Oops, missed this one, look back up we figured we had 600 lbs or so on the outboard side of the beam every 1.3' so (600/ 1.3) X 7' =3230 lbs.. we need to pick up the inboard main floor load for that side, half the 5' width, 2.5' x (40 + 10) x 7' = 875 lbs ... add them together 3230 + 875 = 4105 lbs on that 7' section of C channel. Looking back at last night's post there was 6,200 lbs capacity, we're good 

Just got schooled. This is really great Don_P to be able to figure out the math and know i'm not just winging it. Can't thank you enough.

So with the formula you showed (and IF (big if) i'm doing it right) I can use 8" joists @ 24" instead 16" and  I should still be good as this would require 7.2" deep as opposed to 6" deep on your formula. This was my original plan as my walls and rafters are 24". The over hang on the floor joists is actually 17" not 18" so that gives me a fraction more breathing room on top of that.

400+320+150=870 lbs

870x 1.5= 1305 x 12=15,660

15'660/1200 psi= 13 in

13/ .25  is 7.2" deep...?

thanks for the tip on the aluminum as well.

I hear you on the steel detailing but if i'm going to weld and sink rods into concrete I might as well just put the piers directly under the walls I would think.

Yup, section modulus of a 2x8 is 13.14"³ You're going to be cutting it (bending) and I imagine horizontal shear pretty fine... if possible step up to a higher grade or stronger species or hand select for stock with as clear a section as possible in and around the bearing areas. I'm just a carpenter who can look up and use beam formulas. Unfortunately I couldn't find this particular situation so have been using a point loaded beam overhanging a single support equation, close but not exact. In a situation like that I prefer more oops room than you're allowing. You can find it in the publications list at awc.org as publication DA-6, Beam Design Formulas.

On piers, trailer parks make me a believer in Darwin's theories. We ran with and from the motorhome more than once in the face of a tornado. The motorhome was also one of my first introductions into galvanic action. The alternator on those old big block chevies had an unbushed bolt hole through the aluminum housing. The long bolt through it was steel. My wife's dad who built the camper made captain Ahab look like a weekend fisherman, the camper stayed down at Cape Hatteras all summer every summer in the salt air. Salty water is an excellent conductor and just enhanced the corrosion between the steel and aluminum.  Flash forward several years to when he gave us the camper and subsequently the alternator died on the road. Actually in the parking lot at Mt Rushmore, but you can coast all the way down to the campground beside the NAPA in Rapid City from there. In that campground I spent several hours with duct tape around a hacksaw blade and then a pair of vice grips to get that darn bolt out of the block. You can also coast from Clingman's Dome to the first gas pump in Gatlinburg... but that's a whole nuther story 

Ya i'm actually going to be using Douglas Fir which I believe is a slightly stronger option than spf. Perhaps I should use 10" just to be safe...or double up joists in spots with a higher load..

Those are viable options. Fb on #2 DF is 1242 psi but shear strength went from 135 psi to 165 psi and stiffness went from 1,400,000psi to 1,600,000 psi... good increases. Select struc is the highest grade Dougfir and comes in at fb 2070 psi, MOE 1.9 million and Fv of 180 psi, not hard to find if you are out in it's area.