Sizing bridge beams

[/pine]

mmm, action reaction... the only time the forces from the top vs the bottom would be different in a cross-sectional slice of a beam is the result of a failure of the beam. yes the load varies along the beam, but the forces would be equal in each cross-section.

You're saying the forces are the same at the center of the span as they are say at 25% from a bearing point??

 

[Rockbadchild]

You're saying the forces are the same at the center of the span as they are say at 25% from a bearing point??

Rockbadchild said:
mmm, action reaction... the only time the forces from the top vs the bottom would be different in a cross-sectional slice of a beam is the result of a failure of the beam. yes the load varies along the beam, but the forces would be equal in each cross-section.


no you miss understood me .. I know you know this it's just a communication issue and with LDI it's a technicality issue... as far as calculation goes I know he is right but practically I would be curious to see it... and I think we have the same opinion in that regard ...

I am saying that the compressive and tensile forces are always the same in each cross-section slice of the beam... action = reaction ... but yes these forces do change as you go along the beam but in each cross-section if you have a +5 compressive force on top as a result you will have a -5 tensile force as a result at the bottom.

 

[/pine]

Rockbadchild said:
mmm, action reaction... the only time the forces from the top vs the bottom would be different in a cross-sectional slice of a beam is the result of a failure of the beam. yes the load varies along the beam, but the forces would be equal in each cross-section.


no you miss understood me .. I know you know this it's just a communication issue and with LDI it's a technicality issue... as far as calculation goes I know he is right but practically I would be curious to see it... and I think we have the same opinion in that regard ...

I am saying that the compressive and tensile forces are always the same in each cross-section slice of the beam... action = reaction ... but yes these forces do change as you go along the beam but in each cross-section if you have a +5 compressive force on top as a result you will have a -5 tensile force as a result at the bottom.

Yes I see now what you were saying...cross sections but it's not germane to the point I was making...and BTW...LD1 is wrong about a few things...

 

[LD1]

Yes I see now what you were saying...cross sections but it's not germane to the point I was making...and BTW...LD1 is wrong about a few things...

Please enlighten us.

 

[LD1]

And basically what you're saying is that with a solid member (2x10 etc) joist or beam...as long as you leave that 1-1/2" of material at the top and bottom you can drill as many and as big of holes as you want...NOT...!

You mean similar to this?

 

[LD1]

You are making assumptions making believe it's "reality"...It ain't...geeze...!
LoL...FYI they use plywood or OSB because it's strength allows a much thinner piece of material which is lighter (and cheaper) to be used...many engineered joist employ only 1/2" ply...

Also if you take any typical building lumber say a 2x10 and then take two pieces of 3/4" structural grade plywood and laminate them together (so they are the same dimension as the 2x10)...and do structural tests on the two members the laminated plywood one would far surpass the solid lumber one...on everything but being able to make toothpicks from what was left after the testing...!

What assumptions am I making?

You seem hell-bent to think that OSB or plywood is some super strong material.

When used correctly, like in I-joists, YES it can make a strong member. But you think too highly of the OSB and what it does. Again, it keeps the flanges separated. Same thing can be accomplished with webbing forming a truss and the use of NO osb or plywood.

But this post is about using a built-up beam/header for a given span.

I challenge you to find me an engineering spec, or span table, or calculation that shows a built up beam with plywood sandwiched is rated higher than the same width out of dimensional lumber only. IE: my earlier example of 4 2x boards with 3 pieces of 1/2" plywood sandwiched vs just using an extra 2x dimensional board in lieu of the 1-1/2" worth of plywood.

 

[/pine]

You mean similar to this?
View attachment 778878

That is an engineered floor truss...where the web members are structural and are charged with distributing the load...
They offer a more rugged floor but are heavy and expensive...

Again...it is the strength of plywood or OSB that makes I-joists a light, viable alternative...otherwise they could use something like luan...LoL

 

[/pine]

What assumptions am I making?

You seem hell-bent to think that OSB or plywood is some super strong material.

When used correctly, like in I-joists, YES it can make a strong member. But you think too highly of the OSB and what it does. Again, it keeps the flanges separated. Same thing can be accomplished with webbing forming a truss and the use of NO osb or plywood.

But this post is about using a built-up beam/header for a given span.

I challenge you to find me an engineering spec, or span table, or calculation that shows a built up beam with plywood sandwiched is rated higher than the same width out of dimensional lumber only. IE: my earlier example of 4 2x boards with 3 pieces of 1/2" plywood sandwiched vs just using an extra 2x dimensional board in lieu of the 1-1/2" worth of plywood.

Find your own proof...you are the one making assumptions...
A TJI is at least 20% stronger than a solid lumber joist... with way less mass...you do the math...

 

[LD1]

Find your own proof...you are the one making assumptions...
A TJI is at least 20% stronger than a solid lumber joist... with way less mass...you do the math...

There is no "finding my own proof".

Because it doesn't exist.

YOU are the one that believes plywood is stronger. I'm simply asking you to show proof.

Saying an I-joist is stronger than a dimensional board is indeed factual. Because the strength isn't because plywood is used for the web and you are changing the structural shape vs comparing apples to apples is size.

Furthermore, I gave you an example of achieving the same or greater strength WITHOUT plywood but via floor trusses.

And you have yet to cite anything I said that is inaccurate.....yet you make the claim in an attempt to discredit what I have said

 

[Slackdaddy]

Steel it is! I will check out the scrap yards to see what is available.
Next I will try to locate an area of the creek that is narrowest. Keeping my footers/concrete blocks 2' from the bank, I am going to end up with a 11' - 14' span.

Is there a good online calc for "point loads" of the different sizes of steel beams?