AASHTO non-composite steel shear design 1

[Enginerdad]

Hello all,
I've been tasked with designing a temporary bridge superstructure to consist 1" thick steel road plates on top of 12" deep timber crane mats. The two materials are not connected in any way, so no composite action. This design is for a state highway, so all calculations have to be in accordance with the AASHTO LRFD Bridge Design Specifications. My question is 2 part:

1. Does anyone have any guidance on how to distribute the shear force at the ends of the superstructure between the two materials? I know how to do it for the moments at midspan (based on the relative E*I's of each member/material). I feel like it should be distributed based on the relative cross-sectional areas of each member (i.e. the 1" plate is 1/13 of the total thickness, so it should resist 1/13 of the total shear). Or perhaps is should be based on A*E to account for the different stiffnesses of the two materials? I can't find any references to back up either of those theories.

2.I can't seem to find a section in AASHTO that defines the shear resistance of non-composite steel plate. I assume it would be the same as described in AISC, basically 0.6*Fy*b*d, but I need to show a code reference in my calculations. Can anyone point me to a part of AASHTO that talks about this? If not, I'm just going to reference the AISC section, since anything not covered in AASHTO can be gathered from external sources.

Thanks for any insight you can provide.

 

[Lomarandil]

I'm looking at a note in a calc instead of AASHTO itself, but the shear check for a steel non-composite girder is in [6.10.3.3-1]. It may be general enough to apply to a plate in weak axis shear as well, I'm not sure off the top of my head. It's certainly very similar to AISC, except I think AASHTO uses the more precise 0.58 instead of 0.6

Regarding the shear distribution, AE would be my gut instinct, but I'll wait to hear what some of the wiser minds have to say.

 

[KootK]

1. Does anyone have any guidance on how to distribute the shear force at the ends of the superstructure between the two materials? I know how to do it for the moments at midspan (based on the relative E*I's of each member/material). I feel like it should be distributed based on the relative cross-sectional areas of each member (i.e. the 1" plate is 1/13 of the total thickness, so it should resist 1/13 of the total shear). Or perhaps is should be based on A*E to account for the different stiffnesses of the two materials? I can't find any references to back up either of those theories.

I think that it should be based on EI. And, given that EI timber is going to be 100+ times EI steel, I'd just ignore the steel and put all of the load on the timber. I'd expect the still to simply be for durability and load distribution.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BridgeEI]

Your steel plate is sitting on top of the timber beam right? So if you designed the steel plate to take care of the shear, that shear still has to transfer down to the timber. Follow the load path in my mind and design the timber for the full force.

 

[KootK]

Another way to think of it:

1) if all bending is in the timber, then so is all the horizontal shear.
2) since horizontal and vertical shear are complementary, if all horizontal shear is in the timber then all of the vertical shear is in the timber too.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Lomarandil]

Right, of course! If the steel plate is sitting on the mats, which are just sitting on support elements themselves, there would be no load path for the steel plate to carry shear. My apologies.

 

[Enginerdad]

Another way to think of it:

1) if all bending is in the timber, then so is all the horizontal shear.
2) since horizontal and vertical shear are complementary, if all horizontal shear is in the timber then all of the vertical shear is in the timber too.

Thanks guys. KootK, this is definitely how I would normally do this type of structure, just applying all the load to the wood. However, the Contractor wanted to use these timber crane mats he already owned, and my design was failing by a small margin, so I was trying to neck it down to make it work. However, after some fiddling with loads and distributions, I was able to make it work without relying on the steel for shear or moment.