Need an engineer-strength of wood beam?

[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.