Timber Bridge Lateral Bracing

[ataman]

Hi,

I have a short timber bridge which is 10'0" long x 8'-0" wide. The architect wants a concrete abutment at one end and diagonal braces from the bottom of the concrete abutment to support the other end of the bridge. We would like to keep the bridge independent from the existing structure.

I am concerned about the lateral stability of the bridge since in effect I have a cantilevered diaphragm. Are there any limitations to this? And what rule of thumb would you use to calculate the lateral load since it is pretty much an open light bridge.

Thanks

 

[AlmostPE]

Well, the bridge is 8 foot wide so you should be able to design it to take lateral no problem. How high is this bridge? I wonder how you do the transition from the bridge to the structure. You need to make sure there is plenty of gap. If the edge of the cantilevered bridge trusses touches the structure, it will put out load almost like simply supported beam.

 

[bridgebuster]

You have to provide at least two sets of horizontal cross bracing. One in the plane of the diagonal braces and one below the superstructure. You might need vertical cross bracing near the abutment.

 

[ataman]

The bridge is about 10 feet off the ground. I've designed it pretty stiff and left a 1/2" gap at the end. I'm hoping that I don't have to put in any bracing but to use the timber deck as a diaphragm. Worse case scenario, I'm thinking off adding angles on each side of the bridge at the structure location just so I don't have a cantilevered diaphragm. The angles will be connected to the structure and would allow the bridge to deflect but not move from side to side.

Any comments?

 

[AlmostPE]

sounds like a good idea. You have to make sure the bridge is really stiff and wont budge due to win. Or you will get your self a repeated impact load on to the angles that might damage the structure.

 

[gwynn]

What is the decking? I'm thinking planks only, which will not work as a diaphragm.

It could be designed as a diaphragm, but you would need to go through the same design requirements as per a shear wall. After including the restraint needed for wind loads on the beams, it's probably easiest to follow bridgebusters suggestions.

 

[ataman]

Hi Gwynn,

Why can't I use 1x6 T&G planks as a diaphragm. What's the difference between that at plywood other than the spacing of the joints (ie every four or 8 feet for plywood and every 6" for the decking)

For the load to transfer across the joints I've always looked at it as the load getting into the nails, then the rafters, back through the nails across the joint and then back into the diaphragm.

Am I missing something

 

[ataman]

Hi Gwynn,

Why can't I use 1x6 T&G planks as a diaphragm. What's the difference between that at plywood other than the spacing of the joints (ie every four or 8 feet for plywood and every 6" for the decking)

For the load to transfer across the joints I've always looked at it as the load getting into the nails, then the rafters, back through the nails across the joint and then back into the diaphragm.

Am I missing something?

 

[gwynn]

The difference is the moment arm between connecters. For a 1x6 plank you have at most 5" between nails in the same member. In a sheet of plywood or OSB you have much further between the edge nails. Without this moment developing you have no restraint on lateral movement of the bridge, both beams will just deflect the same amount.

Any significant lateral loads on the bridge could fail the connections (likely pullout of the nails). This is the reason most areas have banned the use of shiplap sheathing in shear walls and require OSB, plywood, etc. instead. Even if it does not fail the connections, you will likely have large lateral deflections.