# Bridge Building Beam Size?



## DEKE01

I'm building a horse bridge, 16 ft long, 4 ft wide. How do I calculate the required beam size with 3 beams? I can put in as many support posts as needed. I'm thinking of using doubled 2X8s but have no idea if that is too little, overkill, or close enough. I would like the max clear span to be 8ft, but I can use some cross bracing to reach towards the center of the span if needed to prevent flexing.

The max live load I want to build to is 2 fat draft horses and riders, so I'm going with 6000lbs. Add that to the weight of the bridge, and you are looking at another 1000 lbs is my guess. I don't know how to convert that load to PSI which is what the beam calculator websites need. 

Your assistance is greatly appreciated. This bridge will be over a 12 ft drop, so I have to get this right.


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## Bearfootfarm

I'd go no smaller than 2 X 10's with .5" plywood in between attached with bolts and epoxy
It will only get weaker over time, so don't start at the least you can get by with


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## MattB4

To calculate psi loading you simply divide the total weight of the load (bridge material added in also) and area in sqft . So if you have a 4 by 8 span, that is 32sqft by 144 sqin/sqft = 4608sqin. Say your total load of the bridge span is 1000lbs and you say the horse load is 6000lbs=7000lbs 7000lbs/4608sqin = 1.52psi 

One thing to keep in mind that the load is dynamic. So you need a safety margin because of the extra force made from hoofs slamming down.


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## Darren

MattB4 said:


> To calculate psi loading you simply divide the total weight of the load (bridge material added in also) and area in sqft . So if you have a 4 by 8 span, that is 32sqft by 144 sqin/sqft = 4608sqin. Say your total load of the bridge span is 1000lbs and you say the horse load is 6000lbs=7000lbs 7000lbs/4608sqin = 1.52psi
> 
> One thing to keep in mind that the load is dynamic. So you need a safety margin because of the extra force made from hoofs slamming down.


It's more complicated than that. If you visualize using a lever to multiply your force to move something, the spacing of the load does the same thing to your disadvantage. Obviously the stress on the bridge won't be as much when the horses first move onto the bridge compared to being in the middle of the bridge.


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## farmrbrown

The calculations of bearing the load are complicated, that's why they have the engineering tables for you to look at for a reference. Span, type of wood, how it is attached together, etc. all go into a bridge calculation.
Since you're asking about horses and riders and not vehicles, I think the load calculation you need to input is a little easier, but as Bearfoot said, it's better to overbuild than go for the minimum to start with, deterioration over time can't be overlooked.

To get to the number you need, I would start with the square inches of the horses' hooves.
8 hooves (2 horses, carrying riders) at ?sq. inches apiece divided into 6,000 lbs.
If you're not good at figuring areas of circles, it's not that hard, just round DOWN for safety, which makes the pressure go UP.
The center of the hoof to the edge is the radius = R.
Area is 3.14 (Pi) times R squared.

If the hoof is 6 inches across, that's a 3 inch radius.
3.14 x 3 x 3 = 28.26 inches.
8 hooves = 226.08 inches
(double check my math please, my calculator battery is going dead, lol)

This is where I would round down to 200 sq. in. and divide into 6000 lbs.
That gives you 30 psi. 
Assuming you're going not going at a full gallop, the dynamic load should be small, lol.
Add more if after a Thanksgiving meal, lol.

Now, I don't know if the tables you're looking at include the load of the bridge itself, but you get the idea.


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## MattB4

Darren said:


> It's more complicated than that. If you visualize using a lever to multiply your force to move something, the spacing of the load does the same thing to your disadvantage. Obviously the stress on the bridge won't be as much when the horses first move onto the bridge compared to being in the middle of the bridge.


The psi calculation would not change for the overall psi. The material of the bridge must spread the load equally without breaking. This is why designs like arches are helpful when dealing with load distribution. There should be little lever action if the span and materials do not flex.


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## Darren

MattB4 said:


> The psi calculation would not change for the overall psi. The material of the bridge must spread the load equally without breaking. This is why designs like arches are helpful when dealing with load distribution. There should be little lever action if the span and materials do not flex.


He has to take into account the shear forces and the forces caused by the moment which is the force acting through the distance (lever). That's why some bridges have beams that are deeper at one point than another. He'll want to minimize deflection. I'm not sure how horses will react to a bridge that gives a lot beneath them.

For shear he'll figure the cross section of the beams to calculate the area through which the force acts. Depending on the wood he'll not have a homogeneous substance like steel. The other issue is the wood will want to twist under the loads. Obviously it will support more weight without deflection that the other. 

While he's figuring it as a horse bridge, what are the chances someone will try to run over it with a truck in the future?


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## MattB4

I would agree that steel beams would be a better building material than wood. If using wood I would not use 2x material nailed or bolted together for beams. Those should be solid wood. 

Postulating someone driving on the bridge as a reason to change how it is designed is a bit outside the scope of the subject. Any bridge design can be overloaded. Indeed all too many decks collapse every year from bad design/age and improper loading.


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## Fishindude

How far of a span?


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## DEKE01

OK, thanks for the input from all and to address some concerns...

The bridge is only accessible by foot - human or horse, as it spans between two hills designed to create a challenging horse climb. It is also only 4 ft wide, so no concerns of loads heavier than specified. 

Dead load - weight of bridge, I'm guessing 1000 lbs so I'm rounding up to 2000 lbs as a safety factor

Live load - usually only one horse and rider, but since 2 horses could fit, I'm allowing for 2 of the biggest, fattest horses and riders - 6000 lbs. So there is plenty of safety factor built in as the live load will usually be <1500 lbs

I found what appears to be an excellent website calculator, after lots of searching with the wrong words...https://courses.cit.cornell.edu/arch264/calculators/example8.1/

Size of bridge - 16 ft x 4 ft or 64 sq ft

Live load = 6000 lbs / 64 = ~94 PSF. But I just had a thought, what if a bunch of people got up there to pose for a picture? Interwebs tells me the avg adult American is well under 200 lbs but I don't believe it, so I went with 200. 36 people (22 inches shoulder width, 4 people deep = 36 people) squeezing onto the bridge results is a live load of 7200 lbs or 7200/ 64 = ~113 PSF. 

Dead load = 2000 / 64 = ~32 PSF ...note I rounded up in both cases for more safety

Spans - I had planned on 4 supports, the beams will sit on a pier/post driven into the hill and then two telephone poles 5 ft from each end, leaving a center span of 6 ft. Under the center span, I will put triangular trusses to spread the live load further and take the bounce out of the bridge, so I have reduced the span to 5 ft. 

putting all this into the calculator...

span = 5 ft
beam spacing = 16 in
load of least duration = live load
wet service conditions = yes
species = southern pine PT
live load = 113
dead load = 32
grade = #2
live load deflection = 360 - I don't know what this means and went with the default and that is the same default on several other calculators
total load deflection = 240 - the default and same comments

The lightest choice - 2x6. I have 2x8x16s in stock, so using those results in more safety factor. 

Does anyone see anything wrong with my method?


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## DEKE01

MattB4 said:


> I would agree that steel beams would be a better building material than wood. If using wood I would not use 2x material nailed or bolted together for beams. Those should be solid wood.


I didn't want to use steel because it makes more cost and work to apply wooden decking. 

As a point of reference, 2 boards glued and bolted together, with the rings facing opposite directions is a stronger beam than 1 solid timber of similar size. So two 2x4s nailed together is stronger than one 4x4.


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## Fishindude

I'd use heavy timbers or steel beams and span the entire 16'.

Demolition contractors and bridge contractors frequently have some used steel beams lying around that can be had cheap.


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## KFhunter

check local steel building manufacturers they often have cut I beams they'll sell off and they'd know the load limits for a 16 foot span, just add more spans until you have more than enough. I'd think 2 or possibly 3 of those I beams would hold your horses and could be had pretty reasonable. Steel prices are very low right now. 

Uniform load limit for a standard 8x4 I beam 16' long is like 7800 lbs for the thin stuff and 11,800lbs for the thick stuff for one beam. Increase the beam size and the loads go way up. 


I'd make concrete footings on each side and slide the I beams across then build wood on top to your specifications. Easy to replace wood decking as needed.


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## flood222

I just built a bridge and understand what you are going thru. Finding out how to build something correctly can drive a person nuts. 

That is quite a bit more load than I had to deal with as the bridge I built was for a few humans to cross 80ft. Horses!? Oh man, that would add a whole new dimension to it. I wish I had more useful information for you. Keep at it! It sounds like a fun project.


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## KFhunter

flood222 said:


> I just built a bridge and understand what you are going thru. Finding out how to build something correctly can drive a person nuts.
> 
> That is quite a bit more load than I had to deal with as the bridge I built was for a few humans to cross 80ft. Horses!? Oh man, that would add a whole new dimension to it. I wish I had more useful information for you. Keep at it! It sounds like a fun project.


nice work, that looks like it would hold a whole team of horses if it were wide enough. 

I've been looking at building a bridge myself to access an island when it's high water, but even low water the area I need to cross stays muddy. I want to drive an 8000lb tractor across though.
I've been kicking around getting an old railroad flatbed and laying that across, but they're an eyesore so I'll make something nice to look at like you've done.


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## DEKE01

flood222 said:


> I just built a bridge and understand what you are going thru. Finding out how to build something correctly can drive a person nuts.
> 
> That is quite a bit more load than I had to deal with as the bridge I built was for a few humans to cross 80ft. Horses!? Oh man, that would add a whole new dimension to it. I wish I had more useful information for you. Keep at it! It sounds like a fun project.


Very impressive. Your beams look small for horses, but your posts and cross bracing look like they could support an army.

did you add side rails?


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## flood222

The beams are single 2x8s on each side. If they were doubled up it would probably be ok with horses, but I really have no idea. I just built it based upon what I felt was "good enough". When you guys talk about thousands of pounds in horses and tractors it scares me. 

This bridge is 20 foot tall at the center. Side loading becomes an issue at that height especially with more weight! 

I will add side rails after I move some larger items across it to a cabin I am building on the other side.


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## TennHalfBack

Deke - Looks like you are using the wrong beam tables. It's not correct to spread the weight of the horses over the entire area of the span. OK on the dead load, but the horse is more like a point load (or concentrated load in some manuals) which usually give higher stresses.

I went to the web site you referenced and could not get it to do a concentrated load scenario. I found one here http://www.totalconstructionhelp.com/beams.html (I don't know how to make a link.) but it is kind of involved. I came up with (5) 2X12's needed to support a 6000 lb load centered on an 8 foot span. (Wood with a max bending stress of 1000 psi.) 

The dynamics of a moving load also need to be factored in, as well. I doubt you'll find a "load table" for that. You will probably have to wing that based on how docile you horses are.

Bob


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## DEKE01

THB - thanks for the input. I think both sites are giving me the same answer. On my link, if you look at load type b, that represents the concentrated load. 

If I put in an 8 ft span, 12 inch spacing or 5 beams on a 4 ft wide bridge, 6000 lb concentrated live load on an 8ft x 4ft span (188 psf), 2000 lb dead load over a 16ft x 4 ft bridge (32psf), my site gives me 5 2x12s, same as yours. 

But you did make me rethink everything and rework my numbers, which was good. I was dividing the weight of 2 horses and riders over the entire 16 ft which was wrong. I need to put the entire weight of 1 horse and rider at the center of a span and I need to allow that it is not a docile horse because it is probable that a untrained horse on a bridge could spook and do all sorts of wild motions. Actually, the purpose of this structure is to train horses, so we'll always be starting with potential problem beasts. 

So I have limited my spans to 5 ft, and doubled the weight of the horse and rider which gives me a 300psf live load in a 4ft x 5ft span, and still results in 2x8 beams. I'll also add trusses under the beams to transfer loads back to the posts in both length and width directions. So if the entire weight of the horse is momentarily on the outer edge of the bridge, the outside beam will be supported against downward and sideways stresses. The trusses will effectively shorten the spans to 2.5ft, which my site says would allow for 2x4 beams, but I'll still use the 2x8s

If anyone sees a flaw in my logic, please yell, because this stuff is all new to me.


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## Fishindude

Tenhalfback is correct. Designing a structure to support horses involves "point loads" rather than uniform load. I would think you would have to assume that at any one time the entire weight of the horse could be on two hooves.

Engineering like this can all be figured out be reading, using the proper load tables, etc. if you have a basic understanding, but it is a bit too complicated to explain on this forum. You really need actual dimensions, sketches, photos, etc.


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