Hacker News Reader

Comment by jessaustin | Hacker News Reader

There might well be problems with the supports, but I'm surprised that there are no beams in this design. Steel beams would be the most practical for this size, but I don't see any pre-stressed concrete beams either. Concrete deck is poured in place with non-stressed reinforcement, so it isn't very resistant to bending, and is rarely expected to span such a distance by itself. I know of a shorter one-lane bridge that never has more than one tractor or big truck on it at one time, that nevertheless has a 6" steel beam under the concrete deck under each tire track. Perhaps it was over-engineered, but then again perhaps the bridge in TFA was under-engineered...

View on HN

The larger picture linked in https://news.ycombinator.com/item?id=18020427 pretty clearly shows that it is just precast beams.

None of them failed either, they just shifted off the support.

It looks to me like they are using timbers as beams, simply supported on the steel piers. It's a very common bridge design for forest service roads and other rural routes.

Can you point out where you see timber beams in the photo? I can't even find a place where they would go; to me the deck looks like pour-in-place unreinforced concrete that sheared right off at the piers; the deck is then covered in what looks like gravel (which also seems a bit bizarre), then a wooden guardrail on wooden posts is bolted into the concrete deck.

The bridge looks to be precast concrete box-girders (hollow rectangular beams with plenty of steel reinforcement around the edges). The 'gravel' is the asphalt roadway (often this will be cast-in-place concrete) that's poured on top of the precast girders. The purpose of pouring the asphalt/concrete roadway on top is as a sacrificial wear element to prevent the structural beams being worn down by traffic.

Ah, it looked like timbers on my phone, but on a large screen it does look like precast.

Definitely not unreinforced concrete though, and not poured in place. You can see how its made up of 6 individual beams

https://i.imgur.com/Kt85XFR.jpg

Reply on news.ycombinator.com