# For you do it your self guys and gals, something that I have learned after 30+ years



## Texasmark (Dec 20, 2011)

Merry Christmas and the "rush is over"...back to normalities.

I responded a few days ago to a thread about building your own and wood vs metal in the fabrication.

For whatever reason, I came out here with 4 chillins and a wife and just wanted a better environment in which to raise our offspring.

I have an electro-mechanical engineering/technical background, built onto my house twice in town doing slab, framing, electrical, plumbing and finishing out. Came out here and built our house on a minimum budget with a minimum of money, experience and tools doing it all with the family.

Today I still live in the house and other than foundation problems that are concurrent with heavy Houston (blackland) clay, it has been a maintenance free, pleasant home.

Ok. I started out doing my farming things; buildings and fences, in wood. Had reasons as I was familiar with wood and could work it alone with the tools that I had.

Well, things have changed; kids are grown, finances are better and all and so has my knowledge/experience with what works and what doesn't.

I am sharing my experience with you now.

Steel is the only way to go for fencing and buildings unless you have a specific reason for wanting wood. As I said, I had a shop built in '05 by BCI out of Muskogee, OK. out of wood framing, but I had a reason. Everything else on the place the last 10 years has been steel.

Had nothing to do today so I dug out 3 or 4 engineering texts and have some information for the guy like I was when I got here, wanting to do max with min.

The question came up on here about wanting to put a couple of trailers under a clear span shed of about 20-22' span across the front.

I said do it in steel and here is why:

Considering a Douglas Fir, or Southern Yellow Pine timber, a full dimension steel purlin of .08" thickness will weigh about the same.

Thinking about just how much beam you need to span a distance and not have it sag like "the old gray mule" I have two things to offer:

First of all all beams need to be vertical to take advantage of their load carrying ability with minimum deflection (sag in the middle). Secondly, the width varies directly...if you double the thickness of the beam, you double the load carrying ability (amount of sag in the middle for a given roof load).

However, height is extremely important as the height has the cube improvement as height of the beam is increased.

With everything else equal, load, span, material and all that, if you take a 2x6 (rough dimension of whatever....wood/steel) as a unit and talk about deflection of the beam (sag in the middle) the following is what you have:

Size of material___Ht incr ratio____Res to Sag_% gain

2x6 Ref.______________0___________216________0
2x8_________________1.3__________512_______2.4:1 
2x10________________1.67_________1000______4.6:1
2x12________________2.0__________1728______8.0:1

So, what this says is that the higher the beam (like 2x12 vs 2x6), the less deflection (sag in the beam) for a given span/load and all that....long spans, high beams....again, doubling thickness (like 2 ea 2x12's side by side) doubles your weight and only doubles your resistance to sag.....not the right answer.

Ok, last, I ran the numbers on a currently produced 2x6 thru 2x12 in wood using the finished dimensions and density of wood, vs a steel purlin that is full dimension (not shaved off like wood) and .080" thick.

The ability of the steel as described above, to resist deflection (sag) is 15 times better than wood.

So what that means to you is that you can take one steel purlin and support the same load as 15 wood counterparts....but 15 wood counterparts would weigh approximately 15x the weight of the steel beam so that in it'self causes sag.

Hope you can digest this. Like I said in the other answers on this site a day or two ago, go with steel, get the job done with minimum material, max strength, no wood rot, no split ends.....the benefits just go on and on.

Hope this helps you.

Mark


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## slowzuki (Mar 8, 2011)

Not quite sure what you mean, but its pretty normal here to build steel frame with wood purlins. Our heavy snow load, assuming steel roofing that is suitable for 2 ft span between purlins, spanning between moment frames (column/beam assembly) we can use 2x6 up to 8 ft, 2x8 to something like 10 ft, and I can't recall the rest. Double spanning slightly increases the distances.

It is the deflection criteria that limits the span, ie they won't break but will bow too much (I think 1/180 of the span max Can't recall at the moment)

Up here at least wood is cheap, and cheap for people to work with.

The other span criteria for sheet steel sections is because of the slim sections, you run into problems with instability where the purlins twists over in the middle and folds in half.

All put together they are pretty competitive with each other still. Steel building is still more expensive for same stiffness.

I know building my loft I needed a 6x10 spruce beam to span 16 ft and support the load where a 6x6 wide flange with 3/8" flanges could have done the same with a bit more headroom (both sized by deflection). The wood built up beam was what 45$ and a few hundred nails from the air nailer. The steel was going to be 400$ or so at the yard.


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## Texasmark (Dec 20, 2011)

Certainly snow loads are a consideration. I have a Morton brand building that used wood and the purlins are 2x6 vertical on 10' (span) centers 2' apart. The building was built in 1981 and I have noticed no measurable sagging....course I don't have snow loads down here....common roof pitch here is 1:3. The building I described that I had built in 2005 uses 2x4's horizontal on 5' centers and also 2' apart, same pitch.

Mark


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## Richardin52 (Aug 14, 2011)

Not quite following you as to where you want to put your beam but if you are trying to compare a timber beams to a steel I beams there is no comparison.

If however you want to save ton of money, not having to hire something to lift a beam in place and take the time to build a wood beam yourself, I would suggest a hollow plywood box beam. 1/2 inch ply wood has more shear strength than steel is much lighter and much cheaper.

A simple box beam is built by framing it just like you would frame a 2x6 wall only 2 feet high using a double top plate and bottom plate with short studs place 16 in. on center. Making sure the stagger any joints between the plates at least 4 feet and have two studs at each end of the beam. After the frame is done place glue and nail 1/2 in. ply wood on both sides. Nails should be 8-D staggered 3 in. in center. A very slight crown in the beam is nice but not necessary. If you want to save weight until the beam is in place sheath only one side until it is up then do the other side.

You can also make them out of 2x4's and not quite as tall depending on span, live load and dead load.

The beam I described above I made and used to support the top of a farmtek 36x 60 fabric topped building on 6 foot high walls. I used two beams each spanning 30 feet. My area has a 100 lb. snow load according to the ICC. If I had wanted to span the full 60 feet I would have made the beam deeper. I used angle iron bolted under the steel roof hoops and screwed into the sides of the beams so the beams would not move from side to side.


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