# Calling all hearth.com engineers-strength of a wood beam?



## Badfish740 (Sep 16, 2011)

I am constructing a gate with a kind of archway over it because I want to be able to hang/hoist things from it if need be (pics of the construction in the picture forum). One of the reasons I'm doing this is so that I have a place to hang deer before butchering which it can certainly handle, but I have a habit of overbuilding things, so I wonder how much I weight it will support? For example, could I lift the front end of a compact utility tractor? Could I pull a small block Chevy with it, etc...? Here is a sketchup representation of what I'm building:







I've already got the 4 x 6 posts in the ground. They are buried about 3' 6" on what seems to be solid and relatively undisturbed heavy clay soil. I dug the holes 4' deep and added 6" of gravel to the bottoms to facilitate drainage. This weekend I'll be adding the lintel on top which will consist of a 4 x 6 sandwiched between two 2 x 8s. The 4 x 6 lintel will rest on the tops of the 4 x 6 posts (a deviation from the drawing-I just decided that last night) and Timberlock fasteners will be driven through the top down into each post. After that, the 2 x 8s will be screwed to each side using 3" coated deck screws. Part of the function of the 2 x 8s is to provide a finished look, but they should also provide quite a bit of stiffness also.

I tried looking online for information about the strength of wood beams, and found lots of charts and formulas, but I really don't understand them well enough to use the information. All of the lumber is pressure treated Southern Yellow Pine. The unsupported span of the beam is 8' 1". I'm just trying to get a rough estimate of how much weight I can safely support with this. For example, a SBC with a cast iron intake, exhaust manifolds, etc...comes in at just shy of 600lbs, vs. the 7.3L Powerstroke in my truck which weighs nearly 1000lbs fully dressed Any ideas?


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## woodgeek (Sep 16, 2011)

if you skipped the 4x6 lintel and just did 4 2x8s bolted through--it would be stronger.


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## Dune (Sep 16, 2011)

The cross beam is not your weak link. Keeping the posts exactly plumb is.


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## Badfish740 (Sep 16, 2011)

Dune said:
			
		

> The cross beam is not your weak link. Keeping the posts exactly plumb is.



The posts are plumb and should hold that way as they'll each be braced with a diagonal member holding them plumb in one direction.  The cross beam will also help keep them plumb in the other direction.


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## daveswoodhauler (Sep 16, 2011)

Dune said:
			
		

> The cross beam is not your weak link. Keeping the posts exactly plumb is.



+1...can you put up a few diagonal cross braces at the top? i.e. perhaps ones that go about 1 foot from the corner on the top beam and post.

edit....you posted while I was typing.


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## kettensÃ¤ge (Sep 16, 2011)

A layer of 1/2 or 3/4 exterior plywood on edge between the lintel and 2x8's will add a great deal of capacity and prevent nearly all sag that would occur once a load is applied. Glue and screw it all together.
It would be nice if all the top beam components rested directly the tops of the columns.

What is the distance between the columns?

I pulled a small block out of an old Chevelle I had in my Dads garage, 2x6 rafter with a second 2x6 screwed to it. Built 2 double 2x4 posts to the floor, set just wide enough to fit the car in between. In your case a temporary post could help support the top beam when pulling an engine shortening the span between the permanent columns and increasing it's capacity. I think as is or with a few minor changes 1000 or 1500 lbs won't be a problem. Just keep it from trying to tip over in either direction. You will have to roll the vehicle out from under the beam while the engine is hanging, most likely several feet in the air if you are working on an F350.


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## jimbom (Sep 16, 2011)

I agree with Dune and Woodgeek and the others.

The grade of your lumber makes a difference.  Southern pine can range from Fb 2800 to 800.  The size, number, and strength of the screws and fasteners make a difference.  Design of the connection makes a difference.  Since your 2 x 8s don't rest on the column, the load has to be transferred by shear through the screws unless the 4 x 6 and 2 x 8s can be made to function as one beam.  The stability of the frame makes a difference.  Centering and spreading the load where it rests on the beam makes a difference.  The rigging makes a difference.

My advice is to try out your lightest lift.  Just lift the load 1" off the ground.  Watch carefully and above all never get under the load or close in such a way the frame can come down on you.  If your lightest load doesn't seem to strain things, then try heavier loads.  Always keeping clear in case of a collapse.  If you can lift twice the load you need in service, then you may consider using the gate frame for lifts that are half your test load.  Always keeping from under the lifts and clear of a frame collapse.  The assembly can pop quickly and launch odd pieces in your direction.


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## woodgeek (Sep 16, 2011)

You could build one and try to break it--hoist your truck completely off the ground--and then declare it safe for half the load.


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## Badfish740 (Sep 16, 2011)

woodgeek said:
			
		

> You could build one and try to break it--hoist your truck completely off the ground--and then declare it safe for half the load.



:lol:  Pretty sure the wife would kill me.  Might be fun though...  



			
				JimboM said:
			
		

> The grade of your lumber makes a difference.  Southern pine can range from Fb 2800 to 800.  The size, number, and strength of the screws and fasteners make a difference.  Design of the connection makes a difference.  Since your 2 x 8s don't rest on the column, the load has to be transferred by shear through the screws unless the 4 x 6 and 2 x 8s can be made to function as one beam.  The stability of the frame makes a difference.  Centering and spreading the load where it rests on the beam makes a difference.  The rigging makes a difference.



I think maybe I wasn't able to clearly articulate how the beam will be constructed before.  It's hard to convey this kind of stuff in words sometimes so I did a few more Sketchup pictures:

Here is phase one:






which is simply setting the 4 x 6 beam/lintel on top of the 4 x 6 posts.  The unsupported span of the beam is 8'-1".  This by itself would probably be relatively strong on its own, but I decided to add the 2 x 8s on either side to stiffen it.  My thinking (and please correct me if I'm wrong here) was that the load would really transfer from the 4 x 6 beam through the posts since it rests directly on top of them, but that I'd likely get some deflection under heavy loads, so it would be a good idea to add stiffeners on either side.  Therefore the screws are not really carrying a lot of the load, at least so assumes me, the amateur engineer :lol:  The 4 x 6 beam will get two 8" Timberlock bolts screwed down through it into the ends of the posts to secure it.  The 2 x 8 sandwich will lock it into place, which is phase two.






I think the end view gives an even better idea of what I'm doing.  I plan on screwing the hell out of the 2 x 8s every 6" or so and what the hell, a good application of construction adhesive would probably be a good idea too.


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## jimbom (Sep 16, 2011)

Every six inches with screws staggered(not in line) would make the single beam you need.  Cross that one off the list.


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## Dune (Sep 16, 2011)

Badfish740 said:
			
		

> woodgeek said:
> 
> 
> 
> ...



If you think you will lift a 7.3 someday, use 1/2" galv bolts every foot staggered instead of screws.  

Additionaly, some kind of diagonal brace from each post to the beam (also bolted) to eliminate wracking and further diminish span is mandatory.

Timber locks into the end grain top of the posts will not be strong enough to prevent wracking.


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## smokinj (Sep 16, 2011)

Im not an engineer but 4x6 that deep into clay I wouldnt be afraid to do what your doing. That with a couple 45's for bracing should do a couple tons!


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## ironpony (Sep 16, 2011)

the general rule of thumb is 1" of header for 1' of span
ex. your span is 8'1" so your header should be 8"s 2-2x8's with 1/2" ply-osb 
this would apply for 100lbs per foot LIVE load
however your 4- 2x8 thick so you are way over that rule
you are in the 400lbs per foot live load range
you should be able to easily lift a ton with that
hope this helps
also a single 2x12 on a 16' span is 100lbs per foot live load (think floor joist)


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## Badfish740 (Sep 16, 2011)

ironpony said:
			
		

> the general rule of thumb is 1" of header for 1' of span
> ex. your span is 8'1" so your header should be 8"s 2-2x8's with 1/2" ply-osb
> this would apply for 100lbs per foot LIVE load
> however your 4- 2x8 thick so you are way over that rule
> ...



Thanks!  I actually got a similar answer over on another forum where engineers and other like minded folk hang out.  I doubt that I would ever try to lift more than 1000lbs-like I said, even if I yanked the engine out of my truck it would be just under 1000.  Looks like I'll be just fine.


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## maplewood (Sep 20, 2011)

woodgeek said:
			
		

> You could build one and try to break it--hoist your truck completely off the ground--and then declare it safe for half the load.



Ha ha ha.
What a theory.
How's the marriage going for ya?  Tested those waters lately?


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## woodgeek (Sep 20, 2011)

Second marriage is going strong.  The first one was poorly designed.


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