Need help applying tire load to timber deck

[figurnitout]

I am checking the adequacy of 12" x 12" timber crane mats being used as the deck for a temporary construction work bridge. I am not sure how to apply the tire loads to the deck.

This is my first post, so I will attempt to attach a sketch.

The tire load is 12,000 lbs (concrete truck). The tire type is 445/65/22.5. The width of the tire is 16". I'm trying to figure out to apply it to the deck. The 12x12's are transverse to the tire load. My question is:

1. What is the length of tire in contact with the deck? Will the tire load be carried by only one 12" timber? Or is the length of tire greater than 12" and be carried by two 12" timbers?

Thanks!

 

[kelowna]

I do not know about the american code. The canadian code has a footprint of .25 by .25 meters for a truck's single tyre. In your particular example, my approach would be to assume the whole load to be taken by one piece of timber, It seems like the sensible thing to do.

I actually have a temporary bridge similar to what you show, but it has a top layer of timber planks running perpendicular to the 12" ones, about 4" thick, I guess they distribute the load to a greater area below.

Are you designing the bridge only for concrete trucks or is that your highest load? The funny part of temporary bridges is that once they are there, people will use them, so make sure you got the highest possible load crossing it and put the load rate of the bridge in a very visible place

Good luck

 

[figurnitout]

Thanks a lot! That's a huge help.

Actually, I designed the bridge to handle a Link-Belt 218 crawler crane. The sideframes are centered over each girder pair (so there is(was) only compression on the timbers). I wasn't worried about the timbers until the contractor mentioned to me that he wanted to use the bridge to deliver concrete to the job!

The 4" runners make a lot of sense - to help distribute the load to two 12" timbers. I was hoping to spare the contractor the expense of placing the runners, but it may be unavoidable.

Thanks again for your help.

 

[bridgebuster]

Per AASHTO 3.30, for HS Loading use a tire contact area of 10" in the direction of traffic and a width of 20", but, for other tire sizes it's up to the engineer.

AASHTO doesn't require nailers but if you do, thenyou can distribute the load over a longer length in the direction of traffic.

 

[PEinc]

Individual timbers in a 4' or 5' wide deck mat are often (usually?) thru-bolted together which will helps distribute the load to adjacent timbers.

Heavy trucks usually have double tires and concrete trucks have extra wide tires. Therefore, it is not unreasonable to distribute the "tire" (half axle?) load to at least 2 timbers.

http://www.PeirceEngineering.com

 

[figurnitout]

That's a great point. If the timbers are bolted together, the mat would act as a unit distributing the half axle (tire) load over two 12" timbers.

Thanks for the help!

 

[lsmfse]

The attached paper may clarify this for you.

 

[figurnitout]

The paper was outstanding. Thank You!

I have another question if you can help me...

For checking bending stresses, I would typically compare the actual bending stress, fb=M/S to the allowable, Fb. However, in the literature that I was given from the material supplier (attached), Fb is not given. Is there a way to apply the allowables that are given to evaluate bending?

Also attached are strength properties from Chapter 4 of the Wood Handbook (Forest Service). Again, Fb is not given, but other Static Bending properties are. Is there a way to use these properties to check the timbers in bending?

Or am I missing something?

Thanks for your help.

 

[figurnitout]

Here's the other attachment

 

[lsmfse]

What wood material are your timbers made from? Attached is a sample page from the 2005 NDS. Note that you still need to apply all the adjustment factors to Fb

 

[figurnitout]

The timbers are made primarily from oak. I am familiar with NDS, but didn't trust it for this application because the numbers differ greatly between NDS and what the supplier says the mats are made of (attached).

For instance, the values in NDS for shear parallel to the grain for oak is around 150 psi. In the attached the value is between 1,200 and 2,000 psi.

The compression parallel to the grain in NDS is around 600-700. The attached shows 6,000 psi +/-.

Any thoughts or suggestions?

Thanks Again

 

[lsmfse]

Are there footnotes to this table? Are these ultimate values? Do you have AASHTO? The attached page shows values given in AASHTO LFRD bridge specifications. Unfortunately, I don't have the table for hardwoods.