Sizing bridge beams

[LD1]

You make my point by missing the point...!
You were talking about built up headers and beams...I remarked about the strength of structural plywood etc...and used TJIs as an example...you said they used OSB because it was cheap...That is not the primary reason...the main reason is it is stronger than typical lumber..and much less (thinner) material is required to achieve the same strength...to use regular lumber it would have to be structural grade lumber with very minimal knots etc. (clear) it the strength of plywood etc. that makes it viable...

On beams etc...to add strength to a built up beam using plywood "flitch" members...requires engineered fastening along with effective adhesive...not as strong as a beam with steel flitch plates but significantly stronger than just 2x's nailed together...

That is NOT where the strength comes from.

Again, you can achieve the same strength with a floor "truss", which DOES use dimensional lumber as well as alot of "air" between the top and bottom members.

You can take a 4-ply built up header.......with OSB or plywood in between each of the boards. This header beam will be 7.5" thick...(4 dimensional boards at 1.5" and 3 plys of 1/2" plywood).

You can simply OMIT the 3 plys of plywood and opt for FIVE plys of dimensional lumber instead to have the EXACT SAME 7.5" thick header/beam.

Now in reality the strength will be little to no different. But in terms of engineering and structural calculations......the 5-ply of dimensional lumber will be stronger and rated to carry MORE.

You can harp about the merits of plywood, grain structure, no knots, etc etc all you want. But it doesnt change reality.

You could build your engineered I-joists and use a 1x8 for the web (which is 3/4" thick) or you can use 3/4" thick OSB......
As long as the flanges are the same size and the joist is the same depth.....it wont have an effect on its load carrying stength. Therefore......OSB is no better than dimensional lumber, and is indeed used because it is the cheapest solution.....NOT because it is the strongest.

 

[MAX-24-Dean]

You make my point by missing the point...!
You were talking about built up headers and beams...I remarked about the strength of structural plywood etc...and used TJIs as an example...you said they used OSB because it was cheap...That is not the primary reason...the main reason is it is stronger than typical lumber..and much less (thinner) material is required to achieve the same strength...to use regular lumber it would have to be structural grade lumber with very minimal knots etc. (clear) it the strength of plywood etc. that makes it viable...

On beams etc...to add strength to a built up beam using plywood "flitch" members...requires engineered fastening along with effective adhesive...not as strong as a beam with steel flitch plates but significantly stronger than just 2x's nailed together...

And using structural grade lumber adds to the cost.

 

[MAX-24-Dean]

Not sure what point you are trying to make or argue here anymore?

The discussion was about built up headers and beams. Some people swear sandwiching plywood inbetween makes the beam ALOT stronger, which is false.

Then the "Structual" strength of plywood was called into question....

Now we are talking about OSB and debating about I-joists and where how they get their inherent strength?

Seem to be talking in circles here. Please point out anything I said that was incorrect.

FYI...The plywood/osb web and the adhesive used to secure the bottom member (dado) greatly enhances the tensile strength of the bottom member...this allows lengths of joists (spans) to far surpass solid lumber...like 50++ feet...even considering that the top and bottom member lengths are limited to typical lumber lengths and are finger jointed together to achieve whatever lengths and prevents them from being pulled apart from the tension...

When members I percieve as having a good knowlegde base go back and forth like this is when I learn the most.
Thank you,
Dean

 

[/pine]

That is NOT where the strength comes from.

Again, you can achieve the same strength with a floor "truss", which DOES use dimensional lumber as well as alot of "air" between the top and bottom members.

You can take a 4-ply built up header.......with OSB or plywood in between each of the boards. This header beam will be 7.5" thick...(4 dimensional boards at 1.5" and 3 plys of 1/2" plywood).

You can simply OMIT the 3 plys of plywood and opt for FIVE plys of dimensional lumber instead to have the EXACT SAME 7.5" thick header/beam.

Now in reality the strength will be little to no different. But in terms of engineering and structural calculations......the 5-ply of dimensional lumber will be stronger and rated to carry MORE.

You can harp about the merits of plywood, grain structure, no knots, etc etc all you want. But it doesnt change reality.

You could build your engineered I-joists and use a 1x8 for the web (which is 3/4" thick) or you can use 3/4" thick OSB......
As long as the flanges are the same size and the joist is the same depth.....it wont have an effect on its load carrying stength. Therefore......OSB is no better than dimensional lumber, and is indeed used because it is the cheapest solution.....NOT because it is the strongest.

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...!

 

[deezler]

I decree, Pine wins, build the bridge out of plywood. Wait, this thread is about a bridge, right?

 

[/pine]

I NEVER said its just like air.

OSB is used in i-joists because its cheaper than dimensional lumber. Not because it increases the strength.

A typical 10" I-joist would have a ~7" piece of osb for the web and a 2x3 or a 2x4 for both top and bottom members.

If you replaced that piece of OSB with a dimensional board......1x8 or 2x8.....and still had the 2x3 or 2x4 on top and bottom.....it would be just as strong if not stronger.

The OSB's only job is to keep the top and bottom members separated. Beyond that it isnt adding strength. Hence the analogy of floor trusses. Rather than a piece of OSB to keep the top and bottom members separate.....they use webbing. (mostly air aside from webbing). AS long as the webbing is strong enough to keep the top and bottom members separated....thats all it needs to do because the strength is in the flanges.

The reason the I-joists are stronger than a dimensional board of the same depth is NOT......NOT......because of the OSB/plywood. It is because it concentrates mass at the edges where compression and tension happens

You can take a steel I-beam and make the web thicker and it does little to add to the strength of the beam. Add mass to the flanges though....and the beam becomes MUCH MUCH stronger.

So you obviously think that the compressive forces at the top of a joist and the tension at the bottom of said joist only exist in the depth of the top and bottom members i.e., 1-1/2 inches (at the top and bottom) since you say that the plywood or OSB isn't adding strength and is only there to keep the top and bottom members separated...

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...!

 

[Rockbadchild]

Yes well not sure of as many as you want, but that's pretty much what I learn in structural ... a downward load on a 4x8 beam will result in the bottom two inch's take care of the tension (minus force) the top two inch's take care of compression (positive force) the rest of the beam body is only there so the two doesn't separate basically taken no load it is neutral force... so you can remove it all and replace it with anything capable of doing only that, keeping together the two side in place, that's where the I shape beam come from ...

 

[/pine]

Yes well not sure of as many as you want, but that's pretty much what I learn in structural ... a downward load on a 4x8 beam will result in the bottom two inch's take care of the tension (minus force) the top two inch's take care of compression (positive force) the rest of the beam body is only there so the two doesn't separate basically taken no load it is neutral force... so you can remove it all and replace it with anything capable of doing only that, keeping together the two side in place, that's where the I shape beam come from ...

The compressive and tensile forces are not constant along the tops and bottoms especially the bottoms near the bearing points...the top depends on the design live load...

 

[dodge man]

Keep in mind going beyond theory, the beam wants to maybe twist and also bend sideways. The web helps with this.

 

[Rockbadchild]

The compressive and tensile forces are not constant along the tops and bottoms especially the bottoms near the bearing points...the top depends on the design live load...

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.