timber pole bridge analysis

[gbirth8]

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".

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

I say small span as in 22' clear span I am thinking.

are the cross sectional areas of each pole able to be added together during bending analysis? assuming the load is distributed accordingly (which I know is probably not the case as the inner poles would carry more then the 2 outer, but this is for general discussion)

Again this is just for my general knowledge and discussion, however when built these bridges will be used for agricultural purposes so it would be nice to have an idea as to what a producer can traverse this bridge with as they have some heavy equipment sometimes and timber is generally the way to go around here as cost is a huge driver

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

cheers

G

 

[ChorasDen]

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?

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....

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

 

[ChorasDen]

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.

 

[gbirth8]

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?

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

 

[JStephen]

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.

 

[Ron247]

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.

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".

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.

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

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