wooden bridge beam size

[LD1]

I know this much: a square beam will sag quite a bit, even under just its own weight.

NOT TRUE.

The figures I was comming up with, with a point load of ~4200lbs in the middle, that was all using L/240 deflection.

In short, what that means is the 4200lb load centered over a 10' span with an 8x8 hemlock beam will only deflect (sag) the beam 1/2".

And I have a 6x6 beam over a kids playset spanning 12', and it doesnt even sag under its own weight.

 

[LD1]

NOT TRUE.

The figures I was comming up with, with a point load of ~4200lbs in the middle, that was all using L/240 deflection.

In short, what that means is the 4200lb load centered over a 10' span with an 8x8 hemlock beam will only deflect (sag) the beam 1/2".

And I have a 6x6 beam over a kids playset spanning 12', and it doesnt even sag under its own weight.

Also note that just because it is square, doesnt make it sag more.

The depth determins "sag under its own weight". NOT width. So what I am saying is, an 8x8 isnt going to sag anymore than a 2x8, 3x8, 4x8, or 6x8 of the same species spanning the same distance. Becasue comparing a 4x8 and 8x8, sure the 8x8 doubles the weight, but it doubles the strength too.

Think about it this way, you have a 2x4 spanning a given distance. It sags some under its own weight. You put another 2x4 Right beside it and laminate it to the existing 2x4. Thus making a roughly square beam of 3.5" or so. Does that magically make it sag anymore????

 

[forgeblast]

I have eastern hemlock as decking for a bridge and its holding up well. I also have it as siding on my barn.
http://www.fpl.fs.fed.us/documnts/usda/amwood/239hemlo.pdf

 

[Army grunt]

Divide the span in 1/2,Simply support a 4x4 pt an set it in a cutoff 55 gal barrel in the stream bed.You need to rember the day the hem hits the ground it starts rotting, or gets eaten by bugs. set it on a pt. pad.
Army Grunt

 

[LD1]

I still vote to go steel. It may actually be cheaper than the hemlock.

Without having a chance to throw some #'s at it, so I am only guessing at the moment, but a steel beam in the 15lb per foot range sould suffice.

That is only 300lbs of steel beam. (steal is usually priced by the lb)

And beam is usually 0.50-0.70 per lb. So you are only looking @ $150-$200 to have a steel bridge.

I may run some actual #'s when I get a chance

 

[Fixemall]

I had a similar dilemma with a 12' span, we simply searched out some railroad switch ties and took four of them and used threaded rod to bolt them together, creating two heavy side beams. We decked it with cutoffs/rejects from construction jobs and I have crossed it several times with machines weighing in excess of 17,000lbs. Total cost was time and about $50 in threaded rod.

 

[LD1]

I had some time to run some deflection #'s.

The formula for steel I-beam deflection is

d=(F x S^3)/(48EI)

Where:
d=deflection in inches
F=Force applied in pounds
S=span in inches
48=constant
E= modulus of elasticity (for steel, this is 30,000,000psi)
I=Moment of inertia (I^4). Can be found online and is different for every beam.

Deflection limits for i-beams sould use L/360. So a span of 120"/360 would give you an allowable deflection of .333"

So reverse figuring to solve for I we get the following(assuming 4300lb point load becasue that is how strong you 8x8 woudl be):

(4800 x 120^3)/(48 x 30,000,000 x I)=0.333

5.76/I=0.333

result, I>17.28 (I must be greater than or equal to 17.28 to have the same strength of the 8x8 hemlock).

So now we can look at some beam charts to do some shopping:thumbsup:

I like this website for that Structural Steel W Flange Section Properties Moment of Inertia, Steel Beam Size,Cross Section Area - Engineers Edge

So it looks like a W8x10 beam is the lightest beam that would meet the criteria with an I4 in the xx axis of 30.8 That is a 8" beam that weighs 10lb per foot. So only 200lbs of steel:thumbsup: Probabally will only cost you $150 at the most. And with the I4@30.8, would actually hold a point load in the middle of over 8500lbs. So a good bit of saftery factor as well.

BTW, how much are these 8x8 hemlock beams going to cost you???

 

[jonsstihl]

sorry haven't had much time lately but thanks for all the input

the beams are 36 dollars each so they are significantly cheaper than steel.

The real issue is time. I only have one more week off work so this needs to come together quickly.

I'm planning on making concrete bases similar to the blocks in parking lots. to set the beams on and then building up gravel ramps to match the deck height.

I just need to figure out the appropriate track.

I agree that steel would be a better long term solution but more $$ and more complicated to attach a wooden deck to the steel beam.

 

[MossflowerWoods]

I had some time to run some deflection #'s.

The formula for steel I-beam deflection is

d=(F x S^3)/(48EI)

Where:
d=deflection in inches
F=Force applied in pounds
S=span in inches
48=constant
E= modulus of elasticity (for steel, this is 30,000,000psi)
I=Moment of inertia (I^4). Can be found online and is different for every beam.

Deflection limits for i-beams sould use L/360. So a span of 120"/360 would give you an allowable deflection of .333"

So reverse figuring to solve for I we get the following(assuming 4300lb point load becasue that is how strong you 8x8 woudl be):

(4800 x 120^3)/(48 x 30,000,000 x I)=0.333

5.76/I=0.333

result, I>17.28 (I must be greater than or equal to 17.28 to have the same strength of the 8x8 hemlock).

So now we can look at some beam charts to do some shopping:thumbsup:

I like this website for that Structural Steel W Flange Section Properties Moment of Inertia, Steel Beam Size,Cross Section Area - Engineers Edge

So it looks like a W8x10 beam is the lightest beam that would meet the criteria with an I4 in the xx axis of 30.8 That is a 8" beam that weighs 10lb per foot. So only 200lbs of steel:thumbsup: Probabally will only cost you $150 at the most. And with the I4@30.8, would actually hold a point load in the middle of over 8500lbs. So a good bit of saftery factor as well.

DAYUM! I learn SO MUCH on TBN... :thumbsup::thumbsup:

I've been lurking in this thread because probably next summer I need to build a tractor/horse bridge over a small stream that only runs ~11 months of the year and feeds the branch which is my northern property boundary.

I had no idea I could get steel by the lb like that, and no idea how to estimate the right size etc....

You TBN fellers ROCK, and LD1 you are one smart hombre!
Good to have you on my side!

Be well,
David

 

[LD1]

I had no idea I could get steel by the lb like that, and no idea how to estimate the right size etc....


Be well,
David

Well, I am unsure what you mean by "get steel by the lb like that"

You can get steel however you want. Typical lengths, like 20' may be a little cheaper than shorter parts because the supplier doesnt have to cut it, which may add to the cost.

But I dont think they will actually tell you a price per/lb. They will usually quote you by the foot for a given beam + whatever cut fee.

But I assure you, the root of the price is based on weight. And it is usually in the 0.50-0.70 per lb for steel. Wether it be angle, channel, tube, whatever.

I was really just trying to point out the weights of the beams. Obviously a lighter beam is going to be cheaper. So, sometimes going taller and ligher will be just as strong.

And...Thanks for the kind words