Load Distribution Question

[lsmfse]

I am designing a bridge on private property that the owner would like to be wood. However, due to the span and clearance limitations, I will be using timber planks over steel stringers. I was comparing load distribution between AASHTO 9.9.3.1 and table 4.6.2.2.2a-1. The 36" spacing or 6x nominal plank thickness limitation bothers me. In the commentary, the code states that the approximate methods of analysis are not valid and two dimensional methods of analysis are recommended.
Now, I'm having difficulties with the "Orthogonal plate theory" method, especially considering a wood plank deck. Can someone shed some light on this?

 

[bridgebuster]

Is this a one lane bridge? If so, LL distribution goes away. Also are you using planks clipped to the stringers or a stress laminated deck? If the former, I doubt you can use orthotropic plate theory..

 

[bridgebuster]

Is this a one lane bridge? If so, LL distribution goes away. Also are you using planks clipped to the stringers or a stress laminated deck? If the former, I doubt you can use orthotropic plate theory.

 

[lsmfse]

Yes, a one lane bridge. I was thinking of using 8x planks. I haven't decided if I would fasten a nailer to the steel stringer or just provide welded steel studs to anchor the planks. So, if a one lane bridge, LL distribution goes away?

 

[bridgebuster]

Technically speaking, even a one lane bridge requires a distribution factor. However, and some will disagree with my approach, the purpose of the distribution factor is to reduce the total live load in the stringer being designed. If you disregard the DF you'll simply end up with a more conservative design.

One thing I should have pointed out earlier: Table 4-etc deals with the DF for the stringer design. Chapter 9 deals with the actual deck design.

What live load are you using? HL-93? If this is access to a private home you could justify using something less, for example the larges anticipated fire truck.

 

[lsmfse]

Yes, this bridge is on private property, but I want to design for HL-93. They have dump trucks, farm equipment, etc and the County wants it rated 20 ton for their fire trucks. I have the deck designed, I was moving on to the stringers. I remember back in my ASD designs, I used S/4 distribution for stringer design. If I disregard the DF, then I end up with the entire wheel load supported by one stringer. In my case, using the current specifications, S/6.7 gives DF at .45. I suppose that I could assume LL resisted by 2 stringers (or DF of .5) and be conservative. Your thoughts?
Thanks!

 

[bridgebuster]

You could use the lever rule also. How long is your span? If it is shear might control. Similarly, minimum depth might control unless the owner isn't overly concerned about deflection.

 

[lsmfse]

The stringer span is 70'. Yes, using the lever rule, based on my geometry, essentially one stringer supports the wheel load.
This will be slightly conservative and there is nothing wrong with that.