Sizing bridge beams

[2manyrocks]

I built a similar bridge 10 foot span, out of two 8x8 hemlock beams and 2x8 hemlock decking about 10 years ago. at the time I had my b7800 and it worked OK but over the years one beam failed due to rot.

I just rebuilt it with steel beams doubled up so i used two steel beams per side so four beams total with the same size 2x8 decking. they are six inch steel beams I think W15 (not sure) I doubled them up to match tire tread widths of all my stuff ranging from ATV to an old manure spreader used as a wood trailer that is wider than the tractor.

I also more than doubled my tractor weight in those ten years. b7800 was approx 3000 with a loader and my L4740 is atleast close to 8000lbs with the loader and filled tires. I get what feels like zero deflection when I cross the steel frame bridge with my L4740.

I hate doing stuff twice. my time is worth something, I vote for steel especially for your longer span.

Traditionally, creek crossings got built from whatever decay resistant timber was locally at hand. Then rot took its toll and the crossing had to be reworked. That is why over time as people around here could afford it, they switched to I beams and concrete for its durability and lack of maintenance.

Whatever an I beam costs today, it will probably cost a lot more 10 - 15 years from now after wood has rotted out and needs to be replaced.

 

[LD1]

I disagree, although I agree with the wet location beam, that's a very good point... the wood grains of any planks are longitudinal, the plywood keep the grains bonded uniformly together, so it doesn't separate under stress.

You are entitled to your opinion. But for the record.... I did not say plywood doesnt add strength.

The fact is, plywood is not considered a structural material for beams and headers. And the fact that I am giving it at least and "equal to" rating for dimensional lumber of equivalent size (IE.....3 pcs oc 1/2" plywood = one piece of 2x dimesional lumber) is MORE than any table, graph, formula, or structural engineer would give.

In the world of sizing beams and headers for construction....the plywood is not accounted for and adds nothing to the equation for most circumstances.

Look at the basement carrier beam in most houses sitting atop the jack posts. Not saying it doesnt exist, but I have NEVER seen plywood laminated in those beams unless it was done by a DIY'er with no engineering knowledge. IF they arent steel, they are 3 or 4 plys of 2x12 or 2x10....or LVL's. But never a mix of dimensional lumber and plywood. Why.....because the plywood doesnt add anything more than dimensional lumber does of equal size.

As I said, the reason it is used on headers is to match wall thicknesses. Nothing more. But with LVL's coming in at 1.75" wide instead of 1.5" like dimensional lumber.....the trend now is to just use a double LVL for a 2x4 wall header

 

[deezler]

I mean, it might be prudent to ignore the plywood or OSB that gets sandwiched with 2-bys for structural calculations, but using real 1/2 or 3/4" plywood obviously adds strength. You could build a glue-lam beam out of ripped plywood strips and it would be strong as hell. Stronger than dimensional? probably not. Maybe a little stiffer but with more brittle fracture.

 

[LD1]

I mean, it might be prudent to ignore the plywood or OSB that gets sandwiched with 2-bys for structural calculations, but using real 1/2 or 3/4" plywood obviously adds strength. You could build a glue-lam beam out of ripped plywood strips and it would be strong as hell. Stronger than dimensional? probably not. Maybe a little stiffer but with more brittle fracture.

Yes, it obviously adds strength.....and I never claimed it didnt.

But I swear, some people thing plywood is like some rare unobtanium level strength of material that when sandwiched in a beam magically makes it indestructible. And thats just simply not the case.

3 layers of 1/2" plywood totaling 1-1/2" thick will do NO MORE than simply adding another 2x dimensional board. Thats the reality.

Like I said, the ONLY reason using the plywood was to get a beam or header to match wall thickness. YES, it obviously adds strength....BUT.....for the engineering calculations that strength is irrelevant and omitted. As the 2 or 3 plies of the dimensional lumber is sufficient for the intended load, and the plywood acts as nothing more than filler/shims. And not to mention the fact that openings over 8' typically require multiple pieces of plywood. That seam in the middle......ZERO strength.

 

[2manyrocks]

From a carpentry perspective, every time one board is nailed to another that will be exposed to the elements, there's an opportunity for water to enter the seam and cause rot. Built up beams protected inside a house are one thing, but another thing when proposed for use in constructing a bridge.

Whatever the span tables say today, I'm not aware of any span tables that will tell us the weight carrying capability of a built up wooden beam exposed to the elements for 5, 6, 7, 10+ years.

 

[LD1]

From a carpentry perspective, every time one board is nailed to another that will be exposed to the elements, there's an opportunity for water to enter the seam and cause rot. Built up beams protected inside a house are one thing, but another thing when proposed for use in constructing a bridge.

Whatever the span tables say today, I'm not aware of any span tables that will tell us the weight carrying capability of a built up wooden beam exposed to the elements for 5, 6, 7, 10+ years.

They just require regular inspection and one must understand they arent made to last forever.

After all, treated lumber used to build decks....with joists, posts, and beams use similar calculations for for anticipated loading.

Certainly the lumber after 5, 6, or 10+ years isnt as strong.....but that doesnt necessarily mean failure.

Steel is actually the same way if not maintained and allowed to corrode and rust away.

 

[Sawyer Rob]

SR

 

[/pine]

Not germane to build up beams...but an example of the strength of plywood etc...is in the form of engineered wood joists...i.e., TJIs by Weyerhaeuser...
They basically consist of a piece of plywood or OSB etc. typically ripped to 8", 10" or 12" (sized to match common lumber dimensions) with a solid wood rail at the top and bottom...the load bearing capabilities typically surpass the strength of solid lumber joist etc...

 

[Slackdaddy]

Thanks for all the responses.
The creek bottom is only accessible descending a 6' wide path through the woods down a long "finger" , I am not getting and semi trailer beds down there or other large equipment.

The creek is 6-8 feet wide, typ the water is about 1-2' deep, and the top of the water is down about 2' from the surrounding ground,, so typ from the surrounding ground to the creek bed is 3-4'. Any fall could be easily fatal if you are pinned face down in 16" of water.

Steel beams from suppliers is a hard "no go",, they are outrageously priced. Last project (axle jig),, they wanted $300 for a 5-6' cut off of 8-10" high I beam.
I'll check the scrap yards.

Typ about once a year, the creek will flood in a heavy rain,, but the surrounding area is pool table flat for about 100 yards wide.
So when it does flood it is 6-18" of water across the whole bottom.
That is the reason I want to raise the beams up and sitting on the poured concrete "blocks" that are anchored into the ground.

So if I do go with steel beams,, what size to span 12' (I can find an area that the trench of the creek is 8' or less)
Total weight of CK20 is ~2000#, so 2500# with the tiller.
The tractor would not be a true point load,, but what size beams (2 beams) would handle 2500# point load for 12' span.

 

[bcp]

Whether using solid timber beams or laminated, I would add a weatherproof cover over the beams before adding deck boards. I've seen tar, galvanized steel, aluminum, and synthetic material. Extend it an inch or two past the edge.

Wood will last far longer if the top never gets wet.

Bruce

 

[LD1]

Not germane to build up beams...but an example of the strength of plywood etc...is in the form of engineered wood joists...i.e., TJIs by Weyerhaeuser...
They basically consist of a piece of plywood or OSB etc. typically ripped to 8", 10" or 12" (sized to match common lumber dimensions) with a solid wood rail at the top and bottom...the load bearing capabilities typically surpass the strength of solid lumber joist etc...

I wouldnt use that as an example to try and illustrate the strength of plywood.

A beam of any kind......the key to strength is in the extreme most fibers. The top edge of a beam in compress and the bottom edge is in tension. The more "mass" and material you have at the edges the stronger it is a resisting deflection. And all the material in the middle is just there to keep those extreme edges separated.

Thats why the I-beam is such a popular design. IT concentrates ALOT of material where it matters most.

Same is true for those I-joists like you describe. If you had a I-joist built up with a 2x6 or 2x8 in place of the OSB it would fare equally as well. They use OSB because it is cheaper, and only acting to keep the 2x4's on the top and bottom separated. So its strength isnt critical.

You can also compare floor "trusses". Where the strength is in the top and bottom.....and the middle is mostly just air....lol

 

[LD1]

Thanks for all the responses.
The creek bottom is only accessible descending a 6' wide path through the woods down a long "finger" , I am not getting and semi trailer beds down there or other large equipment.

The creek is 6-8 feet wide, typ the water is about 1-2' deep, and the top of the water is down about 2' from the surrounding ground,, so typ from the surrounding ground to the creek bed is 3-4'. Any fall could be easily fatal if you are pinned face down in 16" of water.

Steel beams from suppliers is a hard "no go",, they are outrageously priced. Last project (axle jig),, they wanted $300 for a 5-6' cut off of 8-10" high I beam.
I'll check the scrap yards.

Typ about once a year, the creek will flood in a heavy rain,, but the surrounding area is pool table flat for about 100 yards wide.
So when it does flood it is 6-18" of water across the whole bottom.
That is the reason I want to raise the beams up and sitting on the poured concrete "blocks" that are anchored into the ground.

So if I do go with steel beams,, what size to span 12' (I can find an area that the trench of the creek is 8' or less)
Total weight of CK20 is ~2000#, so 2500# with the tiller.
The tractor would not be a true point load,, but what size beams (2 beams) would handle 2500# point load for 12' span.

ID check with your steel suppliers again just to get a price.

Steel is usually sold loosely by weight. And you getting a 6' piece of 10" I-beam......how heavy was it? IF this was a 35# per ft beam and it weighed ~200lbs......that would be about right.....$1.50/lb

Fortunatally.......you can get by with a much smaller and lighter I-beam. The lightest 4" or even 6" beams would be sufficient for the load on a 12' span. For example, a W6x9 (which means W for wide flange.....6" tall, and 9lbs per ft) would be rated for somewhere around 3500lbs point load at midspan. Giving a pair of beams would be good for ~7k. And you will never concentrate all the weight right at the mid span anyway.....so a good margin of error here.

And 12' of 9#/ft beam is gonna mean each beam only weighs 108lbs.......So check your supplier. You may be able to get a NEW beam long enough to do BOTH for ~$300.

To get similar strength to the above 6" beam.....you'd need to stack up 4 plies of 2x12. And a 2x12x12' is ~$35
So you'd have ~$140 per beam anyway.

And even if you decided to drop back down to 2x8's and layered up 6 plies like your original plan (which wouldn't be as strong as 4 2x12's) Those are about $20/board.....so you'd still have $120/beam.

Regardless.....your decking and your footers/anchor points are gonna be your biggest expense. Whether you spend $250 on a pair beams or $400 on a pair of beams....in the grand scheme of things its a drop in the bucket.

Dont skimp on the beams. Do it right the first time.

As to the beams....look around on marketplace and facebook. Dont even have to be I-beams. You can use channel iron, or like 4x6 tubing, etc. Dont know where you are at or I'd search craigslist and marketplace for you and see if there is anything suitable in your area.

 

[LD1]

These would probably be a W8x15 beam.

I'd rather spend $400 on steel beams and have an overkill bridge than try and save $150 and build with wood beams.

Whats your budget on this bridge? How much have you allotted for decking material. What is the decking material going to be, and how are you doing your footers and anchoring? Cause Honestly I see this as a $1500-$2000 project.....so trying to save $100-$150 of the beams doesnt sound wise.

FYI, a W8x15 beam would be good for ~9000# per beam at a 12' span

 

[2manyrocks]

OP would lose $150 in time wasted having to nail dimension lumber together not to mention having to eventually replace the wood beams after they rot due to being in a moisture laden environment.

 

[/pine]

I wouldnt use that as an example to try and illustrate the strength of plywood.

A beam of any kind......the key to strength is in the extreme most fibers. The top edge of a beam in compress and the bottom edge is in tension. The more "mass" and material you have at the edges the stronger it is a resisting deflection. And all the material in the middle is just there to keep those extreme edges separated.

Thats why the I-beam is such a popular design. IT concentrates ALOT of material where it matters most.

Same is true for those I-joists like you describe. If you had a I-joist built up with a 2x6 or 2x8 in place of the OSB it would fare equally as well. They use OSB because it is cheaper, and only acting to keep the 2x4's on the top and bottom separated. So its strength isnt critical.

You can also compare floor "trusses". Where the strength is in the top and bottom.....and the middle is mostly just air....lol

FYI...It's a perfect example...An engineered joist using plywood/OSB etc...is about 20% stronger (i.e., load bearing ability) than typical framing lumber...

The tensile and compressive strengths are in the top and bottom members but it's the strength of the plywood etc. that prevents deflection...it's not just like "air"...LoL...!

Designing beams etc... is all about loads and resistances and the materials that meet that criteria...

 

[LD1]

FYI...It's a perfect example...An engineered joist using plywood/OSB etc...is about 20% stronger (i.e., load bearing ability) than typical framing lumber...

The tensile and compressive strengths are in the top and bottom members but it's the strength of the plywood etc. that prevents deflection...it's not just like "air"...LoL...!

Designing beams etc... is all about loads and resistances and the materials that meet that criteria...

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.

 

[/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.

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...

 

[LD1]

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...

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.

 

[jonsstihl]

with the price of lumber nowadays you will regret having skimped and having to redo it in 5-10 years will negate any cost savings. I wish I had done steel beams the first time.

hearing you guys mention beams I think mine were w6 x15. they are 6 inches high by 6 inches wide about 3/16 to 1/4 inch thick and are way more than enough for 8000 lbs on a 10 foot span. 14ft span is starting to get wide.

 

[/pine]

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.

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...