Steel plate over timber deck to improve load distribution? 3

[structeng24]

Hello,

I am performing a load rating on a steel beam bridge with transverse timber plank deck. The timber deck is causing the bridge not to rate adequately. I am thinking of ways to temporarily strengthen the deck. I think placing a steel plate (3/4" +/-) over the deck would suffice, but am not finding anything in aashto or elsewhere that indicates how exactly this would distribute the load over the timber deck. The steel plate alone would be adequate to support the loads, although I'm somewhat concerned with the timber underneath crushing. I can assume a 45-degree tire load distribution through the steel plate, although I think that's too conservative.

Any Suggestions?

Thanks,

 

[MiketheEngineer]

It will help to distribute any point loads. If the plank is under 12'' - I usually assume the load will spread over three plank. No reason for this other than an engineering judgement. Other than that - it just adds dead Load to the deck.

 

[structeng24]

Yea, that's my feeling to, although looking for something explaining how exactly its distributed.

 

[PEinc]

I don't see how you can distribute a wheel or tire load to more than one plank unless (1) the tire contacts more than one plank, (2) the planks are bolted together with thru-tie rods, or (3) there is a top layer of planks that are perpendicular to the lower planks.

http://www.PeirceEngineering.com

 

[structeng24]

If a steel plate is over the top, I think it would be similar to (3) a top layer of planks perp. to the lower planks.

 

[PEinc]

Steel plate could be heavy, slippery, hot, and expensive - but it would work.

http://www.PeirceEngineering.com

 

[dhengr]

It seems to me the proper solution depends on the types of loads your dealing with; the tire loads, tire spacing and the tire print dimensions on the road, on each axle; and the axle spacing. Then the questions are..., are the deck planks over stressed, are they not able to distribute the loads sufficiently the other steel structural members, and what does the framing layout look like? What are the weak points, is it specifically the deck planks btwn. two stringers? Many times, as PEinc suggested, the easiest solution is two lines of planking perpendicular to the decking planking, these are the running planks.

 

[msquared48]

The dditional concern here is rot of the planking under the proposed steel plate. The members should be thoroughly examined before placement, and periodically as long as the plate is in place.

Normally, in timber bridges of this type, there is a wear strip of timber planking normal to the structural planking you describe that functions to span out the wheel load too. This planking is periodically replaced as it wears out, saving the timbers below. This same transverse planking does though create the potential for unoobserved rot of the planking below.

To control spippage, although somewhat unorthodox, you could put an additional timber planking wear strip over the steel plate to define the drive surface. Just a thought.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[eea]

We just worked through a short single span township bridge of similar sounding nature that had steel plate decking applied in the past to the worn wheel lanes. Problem was surface rotting of the beams contributing to flexing / prying action of the steel decking (1/2"+?, we scrapped it). Result: numerous lag bolts in tires.

We ended up turnkey replanking the entire timber deck for just under $20,000 using Osmose Railroad Services out of Wisconsin. No gain in the structural capacity on the NBIS superstructure re-rating, but easily bought us another 10 to 20 on the lifespan. We also had a light application of asphalt spray patch applied for traction.

PM me and I'll get you more information if you need / want it. As mentioned, the timber deck is a wear item. I'd underscore that with the client and approach it that way. Depending on what thier replacement horizon is, I'd honestly consider a redecking rather than a patch job if the substructure and other elements are sufficent.