Need an engineer-strength of wood beam?

[IHDiesel73L]

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. 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 :shocked: Any ideas?

 

[Chef Pa]

Maybe this will help.

First would be diy engineering:

CECalc.com - Structural Wood Calculations

A reference table (pdf)

http://www.awc.org/pdf/wsdd/c2b.pdf

Greg

 

[PapaPerk]

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. 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 :shocked: Any ideas?

First of all you should not use screws. You should use galvanized carriage bolts. :thumbsup:

Here's how I would make the beam... I would use two 2"x12".... and place an 8" section of the 4x6 post every couple of feet. The beam would be attached to either side of the main post with the carriage bolts. This will sandwich the main posts and beam together making a very strong structure.

The other issue you have to work out is attaching the load to the beam. The attachment must distribute the load evenly to the beam. I recommend some sort of steel plate that attaches with multiple carriage bolts.

The biggest issue I see with the set-up is side loading on the beam... not a direct vertical pull on the beam... this could cause the whole outfit to lean... so firmly mounted posts are critical... also small kickers or braces maybe needed to support the posts in this situation. An ideal design would use four posts forming an A-frame on each end. This would be very strong! But it all depends on how much loading you are carrying.

Good luck!

 

[Pipestone Kid]

The design you described (4x6 with a 2x8 on each side will hold everything you described. (listed) However, if you are a little unsure, put a strip of sheet metal on each side of the 4x6 before you add the 2x8's. When the sheet metal is held firm by the lumber, it cannot buckle and you have, in effect, created an I beam. (Two I beams to be more exact). You could lift your whole truck if you wanted.

 

[biggerten]

I agree with PapaPerk, side loading needs to be a concern, if the weight starts shifting it'll either want go over or want 'rack' - somw gussets would help with the racking, and I like the A-frame idea as well. Also, sandwiching the sheet metal is a good idea, you could sandwich plywood instead of the sheet metal, too. Also, something to keep the beam from 'rolling' off of the posts.

 

[Brad_Blazer]

Uh, the strength in an I-beam is not in the web, it's in the top and bottom flanges.

It looks like the described beam is good for about 1 ton using Fb=1000psi and derating the distributed load by a factor of 5/16 for a point load in the center. A lot depends on the lumber grade. Make sure the top sheds water so it doesn't deteriorate. The strength is in the top and bottom fibers of the wood so don't cut any notches, especially near the center where the bending stress is greatest.

The top beam will also tie the posts together to resist any fence tension and the cantilever weight of the gate.

 

[bironacad]

Maybe this will help.

First would be diy engineering:

CECalc.com - Structural Wood Calculations

A reference table (pdf)

http://www.awc.org/pdf/wsdd/c2b.pdf

Greg

I like how everyone is leaving this alone, lol,:thumbsup:

 

[IHDiesel73L]

I like how everyone is leaving this alone, lol,:thumbsup:

I looked at it but unfortunately I don't possess the math skills to utilize the information. I'm sure I could do the calculations if I knew what numbers went where.

Uh, the strength in an I-beam is not in the web, it's in the top and bottom flanges.

It looks like the described beam is good for about 1 ton using Fb=1000psi and derating the distributed load by a factor of 5/16 for a point load in the center. A lot depends on the lumber grade. Make sure the top sheds water so it doesn't deteriorate. The strength is in the top and bottom fibers of the wood so don't cut any notches, especially near the center where the bending stress is greatest.

The top beam will also tie the posts together to resist any fence tension and the cantilever weight of the gate.

Thanks! It sounds like you know how to utilize the information bironicad referenced below. I looked at those tables but I don't how to plug in what numbers where in order to come to the conclusion you did. Would you mind showing the calculation that led you to that number? I'm not sure of the grade of the lumber, I just know that it's Southern Yellow Pine. Would the grade be stamped right on it? I could always call the lumberyard and ask I suppose. I will be locating the load point in the center in the form of a large eye bolt of the appropriate working load drilled through the beam and secured at the top with a nut and a very large washer. As for the load rating itself I doubt I would ever try to lift more that 1000lbs (if that even), its just nice to know how overbuilt it is :laughing:

 

[flusher]

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. 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 :shocked: Any ideas?

Take three 2x8s the length you need to span the distance between your posts and use them to fabricate a glulam/nail-lam beam. Use exterior grade wood glue and a lot of C-clamps to glue the 2x8s together and then use 16p cold dipped galvanized nails to nail them together.

 

[Pipestone Kid]

Uh, the strength in an I-beam is not in the web, it's in the top and bottom flanges.

It looks like the described beam is good for about 1 ton using Fb=1000psi and derating the distributed load by a factor of 5/16 for a point load in the center. A lot depends on the lumber grade. Make sure the top sheds water so it doesn't deteriorate. The strength is in the top and bottom fibers of the wood so don't cut any notches, especially near the center where the bending stress is greatest.

The top beam will also tie the posts together to resist any fence tension and the cantilever weight of the gate.

If the strength of an I beam is not in the web, why does a 12 inch beam have a greater load capacity than an 8inch? Try taking the web out and see how much it holds.

 

[MotorSeven]

IHD...your fine with what you designed...build it!

 

[IHDiesel73L]

IHD...your fine with what you designed...build it!

Well, I went ahead and did just that!

It ended up being a kind of "half A-frame" with diagonal supports on one side, which should be plenty. I don't think I'll be pushing the envelope with it really, but it should be good and strong for what I need. Next comes finishing the concrete, putting up the fence, and bringing in gravel/crusher run to extend the driveway into the backyard.

 

[bironacad]

I see you put diagonal bracing that was one of my concerns when you first started to talk about lifting heavy. Did you lag the 2x6 to the 6x6? should be fine for hanging deer in any case. I reference the engineering numbers from someone a previous post not to say that it is good but for the average guy like myself it is out of my scope. The larger the span between the posts the less the 2x6 will carry, two 2x6 a 2' in length can hold a quite a bit when on edge make it 24' and I would not walk on it, lol. So without being there you will have to use good judgment as to the loading of the beam.

 

[LD1]

If the strength of an I beam is not in the web, why does a 12 inch beam have a greater load capacity than an 8inch? Try taking the web out and see how much it holds.

A 12" beam can support more than an 8" because the flanges are farther appart.

Part of the calculation to figure the strength of a beam is the distance the farthest fibers are from the x-axis. IE: how far the flanges are from the middle of the web. And also the mass @ that distance.

The web is ONLY their to hold the flanges appart. The mass of the flanges and the distance from center is what determines strength. The web isnt under a lot of stress. But the top flange is under a lot of compression and the bottom is under a lot of tension.

I'll give you two examples to prove this point. Take a w-type I beam that is roughly square. Like a W8x31 beam. It is exactally 8"x8". The web is 0.288" and the flanges are 0.433". Why is it not stronger laying on its side???? It would have 2 8" verticals @ .433 thick vs only one @.288 standing up??? And I assure you, it is a LOT stouter standing up.

Another example, look at beams of the same depth. Why does the strength increase dramatically when the flanges increase in width and thickness, yet the web is still the same size????

 

[PapaPerk]

Well, I went ahead and did just that!

It ended up being a kind of "half A-frame" with diagonal supports on one side, which should be plenty. I don't think I'll be pushing the envelope with it really, but it should be good and strong for what I need. Next comes finishing the concrete, putting up the fence, and bringing in gravel/crusher run to extend the driveway into the backyard.

No carriage bolts.

 

[slowzuki]

You can take a bunch of the web out and change very little in the performance. The web does a few things, keeps the flanges spaced apart, keeps them from twisting or upsetting, and transmits shear forces. The shear forces are much less than the tension and compression loads in the flanges so it can be smaller.

Its why steel I beams are used instead of solid steel bar. Also why trusses bar joists are used vs solid pieces. Skim out the unneeded bits leaving whats used.

If the strength of an I beam is not in the web, why does a 12 inch beam have a greater load capacity than an 8inch? Try taking the web out and see how much it holds.

 

[IHDiesel73L]

No carriage bolts.

I really didn't seen the need for them. The "inner beam" (4 x 6) sits directly on top of the posts and is secured in place with two lag bolts through the top down into the posts at each end. The 2 x 8 "sandwich boards" on either side of the 4 x 6 were first screwed with 3" deck screws in a staggered pattern every 4". After that, 6" Timberlocks were sunk every 12" from each side. If the beam isn't fully tied together after all of that, I don't know what it would take.

 

[rambler]

Deck screws are very brittle and will not hold a shear load. I'm not a fan of that part of your design at all. The one lag screw down from the top pins it a bit, but does not tie in with a triangle design, so it does not add any ridgidity to your design. As weight & side-loads pile on, things want to twist, bend, shift to one side or another. It is then when just a little bit stronger fastener or 2-3 fasteners instead of one big one will prove to keep things squared up.

Drilling the hole in the center of the 4x6 beam weakens it a bit in the worst possible place.

Actually you built it pretty good, other than the deck screws, they are sub-standard.

I'm happy to see those angle braces, I was thinking the weakest part was the verticals, with no support they will want to bow out. Wood isn't perfect, it's the knots and bad grain you need to watch for.

--->Paul

 

[IHDiesel73L]

Deck screws are very brittle and will not hold a shear load.

I have to admit that I still don't see how the screws are really carrying much of the load at all. Again, the 2 x 8s fastened to either side of the 4 x 6 were really intended to stiffen it-not really carry the load. The load will be carried by the 4 x 6 which sits directly on top of the posts. Also, what about the 1/4" thick stainless steel lag fasteners that were placed every six inches?

I'm not a fan of that part of your design at all. The one lag screw down from the top pins it a bit, but does not tie in with a triangle design, so it does not add any ridgidity to your design. As weight & side-loads pile on, things want to twist, bend, shift to one side or another. It is then when just a little bit stronger fastener or 2-3 fasteners instead of one big one will prove to keep things squared up.

This is why I used 2 x 8s which lock the 4 x 6 in place on the tops of the beam. I am also going to add braces at 45 degree angles from post to beam at either end which should help with rigidity.

Drilling the hole in the center of the 4x6 beam weakens it a bit in the worst possible place.

I haven't done this yet-what would be the best way to attach a block and tackle/come-a-long/chain hoist, etc... I have some heavy (10,000+lb) nylon lifting straps with loops at either end, should I just throw one of those over the top?

I'm happy to see those angle braces, I was thinking the weakest part was the verticals, with no support they will want to bow out. Wood isn't perfect, it's the knots and bad grain you need to watch for.

They were essential and provided a great deal of stiffness.