timber pole bridge analysis

How will the poles be graded, i.e. how will you establish properties? I think you may get into a situation depending on span and loading where lateral torsional is a concern, although the weak-axis stiffness is going to be quite large for a pole such as this.

I would not go with the approach of determining a 'max load' as you've stated, particularly for new construction. Instead, for first steps, I would focus on determining the properties of wood members in question (ensure you account for reductions for treatment and wet use), this will likely require a third party agency to perform the grading. Next, determine what design loading you'll apply to the structure (vehicular, pedestrian, built-up snow, etc) using the applicable load combinations per chapter 2 of ASCE 7, and then any environmental loading from wind, water, etc.

How do you anchor the poles to the abutment, and how do you isolate the poles from soil to help protect from decay?

gbirth8 said:

I have attached a sketch of what I am thinking about building...

...Again this is just for my general knowledge and discussion, however when built these bridges will be used for agricultural purposes....

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well which is it, general knowledge, or a direct application?

To address the load sharing concept, without doing some calcs and understand what load is to be applied, I cannot easily answer. Let's work backwards, how do you propose the load sharing to occur, how would you justify it and/or confirm it, and how might deficiencies arise? Considering the scope of loading applied (tractors, trailers filled with hay, etc), typical nominal wood connections may not be sufficient to achieve load sharing.

Have you drawn a free body diagram yet? It'd be best to start there, include it here on your post, and we can dig more deeply into an approach.

ChorasDen said:

How will the poles be graded, i.e. how will you establish properties? I think you may get into a situation depending on span and loading where lateral torsional is a concern, although the weak-axis stiffness is going to be quite large for a pole such as this.

I would not go with the approach of determining a 'max load' as you've stated, particularly for new construction. Instead, for first steps, I would focus on determining the properties of wood members in question (ensure you account for reductions for treatment and we use), this will likely require a third party agency to perform the grading. Next, determine what design loading you'll apply to the structure (vehicular, pedestrian, built-up snow, etc) using the applicable load combinations per chapter 2 of ASCE 7, and then any environmental loading from wind, water, etc.

How do you anchor the poles to the abutment, and how do you isolate the poles from soil to help protect from decay?


well which is it, general knowledge, or a direct application?

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Ok well to start it off it is i guess both general knowledge and direct application as I have seen this bridge already made along a local snowmobile trail, which sparked my curiosity as to the capacity of it as it has to support at least a trail groomer, which can be quite stout I am unsure of what groomer that particular association uses

the poles i have found out there are class 1 SYP - .60 CCA treated direct from manufacturer - not sure on grading as i have not yet delved that far into it however for the basis of this thread i think it can be safe to utilize the properties in the NDS supplement table 6A or 6B as the properties do not change for southern pine between ASTM D25 and ASTM D3200 (6A and 6B respectively)

The poles lets assume are not anchored directly to the abutment but rather held in place by both a "face board" on the ends of the poles and the decking material itself. There is also 6x6 blocking in between each pole to help prevent rotation

The poles in theory will never be in direct contact with the soil

The poles I seen on the existing bridge were about 2' OC giving a deck width of 12'

as for a load lets assume a typical tractor, say 2 5000 pound axles spaced 10' apart

Load sharing is the big question, in my mind the load would be distributed to the poles through the decking material, although I do not believe it would be even across all of them. Decking material is attached to each pole utilizing 2 - 4" structural screws such as a GRK

Thanks for the reply

It would simplify the analysis if the small ends all pointed the same way.
If these are new creosoted poles, that helps. If you're just cutting timber for it, deterioration may be a major concern. Or if they are used telephone poles, getting properties may be a problem.

As far as a general discussion, load sharing is highly dependent on decking material and Ix of the beams. Your alternating tapers makes Ix non-uniform at any location except midspan. The wheels matter a lot on the decking. Tire pressure, hard wheel versus pneumatic matter. Having the decking break can add a side-load to the beam as the wheel drops between the beams.

Many people approach a design differently. For me, you currently do not have enough preliminary loading information to check anything. You need to get away from terms like "trail groomer" and "stout". I have no idea what a trail groomer is load wise, and if I did, I imagine there are several models and sizes. Even saying two 5k axles that are 10' apart is not enough unless that truly is the design vehicle.

Moving vehicle parameters matter a lot. Weight, axle spacing, transverse wheel spacing, speed, braking, front loader that is loaded versus front loader that is empty, are just examples. Tractor, tractor pulling a wagon are yet another example.

gbirth8 said:

Hello all,

New to the timber world and I am basically self taught in everything, but i am curious in how you would go about analyzing a small span bridge with timber poles for a "max load".

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Do you mean you are new to timber, or new to structural analysis? The design steps are still, create a structural concept, apply loads, analyze and lastly do a material check. Analysis is where this load sharing is determined, and at that stage, the computations do not know it is timber, just E, Ix, Area, Moments, Shears and axial loads. It is during the final material check that you consult the timber code and get into Fb, Fc etc.

gbirth8 said:

Again this is just a general question on how you would approach the analysis for the system with multiple members acting together

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Sounds like you do intend to build this bridge. Way too much investigation not to.

If you want this bridge to transport anything other than a snowmobile or similar then you really need to stack these pokes tight side by side and secured them to prevent spreading.

That 2 x10 deck board is far too flimsy to transmit load so at best any wheel load is going to be taken by two poles max as there is no definitive idea what the wheel spacing would be.

Consider shear as well as bending if you get say rear axle and a trailer axle or simply two axles on your bridge at the same time but close to the ends of the poles.

In reality timber will give you warming noises. Don't ignore them or any splits or damage.

Don't expect more than 10 years service.

Load sharing is dependent on the stiffness ratio of the member transferring the load (decking) compared to the member carrying the load (pole). If they have similar stiffnesses, you can probably make an assumption that the decking can spread load over a certain area, I'd typically consider what the deflection of the decking is relative to the deflection of the carrying member(s) for a given load scenario to determine if load sharing is appropriate. As Littleinch hinted at, a flat 2x10 is going to deflect signficantly more than a timber pole for a given load scenario, so load sharing is likely not substantial.

gbirth8 said:

the poles i have found out there are class 1 SYP - .60 CCA treated direct from manufacturer

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Be sure you account for reductions in design properties for the treatment of CCA, you may need to get confirmation from the treater for these design adjustments. See reference 30 in the NDS for additional info.

gbirth8 said:

The poles lets assume are not anchored directly to the abutment but rather held in place by both a "face board" on the ends of the poles and the decking material itself.

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What happens if a tractor or truck is driving overtop at 20mph and hits the brakes? Poles bridges don't work that well when one end is in the river below.

gbirth8 said:

There is also 6x6 blocking in between each pole to help prevent rotation

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How often, how are they connected, where does the load go from the 6x6: what's the load path? Try drawing a FBD, post it here, and all these things will make more sense.

gbirth8 said:

The poles in theory will never be in direct contact with the soil

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Maintenance of the bridge structure is by who? Consider realistic conditions for a bridge, if you have agriculture vehicles using the bridge, you can expect a lot of dirt, animal feces, and biomass to accumulate across the structure for any given year. 0.60 CCA may not be sufficient.

Ok, let's take a step back guys. This can be done, and is done, all the time in rural, agricultural, and forest service scenarios. And they last longer than 10 years.

If you're getting timber poles of the type that are commonly used for power distribution (as OP mentions, Class 1 SYP), those have known grading and strength properties that can be applied here. The taper isn't really that big of an issue -- a little nuance, but not bad.

2x10 timber decking doesn't do a great job distributing load by itself. But with a nice earth or gravel wearing surface above, it can work surprisingly well for moderate wheel loads. Or use the longitudinal running boards as shown, aligned above poles, to guide heavier vehicles to align where you want them.

6x6 blocking adjacent to a steel threaded through-rod can work great to hold the poles in position relative to each other.

A "face board" against an approach element or even well compacted roadbase on each end can restrain longitudinal movement under braking loads. (Though I'd still make sure that there's a good bit of pole beyond the abutment face to allow movement over time).


Again, to my fellow ET peanut gallery. If you don't know, it's OK to say so and move on to another topic.