Vu for timber stringer

[lsmfse]

Eq. 4.6.2.2.2a-1 requires that for undistributed wheel loads, one line of wheels is assumed to be carried by one bending member. If I have a condition where stringer spacing is 12" o.c., it doesn't make any sense that the load couldn't be shared by 2 bending members when calculating VsubLU. The bridge is a timber stringer with transeverse timber planks.

 

[lsmfse]

Let me expand on the question above. Basically, I read AASHTO that wheel loads are distributed over 20" wide strip for transverse timber planks. If my stringers are spaced at 12" o.c., is there rational that the assumption of one bending member carry one wheel load valid? I can understand if stringers are spaced 24" or greater it would be prudent to assume one stringer to support the wheel load.
See attached.

 

[ishvaaag]

It seems to me you have a very valid argument. For what AASHTO is saying with distribution of loads to the substructure of the transverse timberplanks can be taken at 20" whatever structural is kept within such width *** could *** be thought apt to resist the load for it *** would *** be getting there already distributed. That's the positive look.

The negative one. It seems AASHTO is saying doesn't want in place any bending member not able to carry at least an undistributed line of wheels. Hmmm. This sets a minimum for such "bending member" leading to big sizes, then the same for transverse.

Now ... I READ (sometimes do) we are talikng of Shear.
Maximum shear wheel loads near supports have almost NIL lateral distribution. How more when the bending members may be thought unable to pass shear ones to others, such in stringers. So, well, this puts the size of the overall stringer if of constant section in the need to meet such sehar when the loads are at maximum, at ends. After this thought, I would either meet the requirement or reshape the structure.