Why are bridges like these stable? http://www.fhwa.dot.gov/bridge/prefab/images/psbs047.gif
Wound’t the under-truss structure add a lot of extra weight to bring the bridge down?
The truss does add weight, which is why the bridge has those solid concrete supports on each end. The truss also adds resistance to bending. Without it, the bridge deck would fold in the middle. There is no big difference in having the truss above or below the deck.
An undertruss doesn’t add any more weight than an above-deck truss would. As AdamF points out, the truss (taken as a whole) doesn’t care whether the load it bears (the cars and the roadway) are applied high up or low down; it only cares how much weight is being applied, and how far that weight is from the nearest support columns (in this case the ends of the bridge).
In one sense it does care: when it’s above the roadway, its main structural member (the horizontal beam furthest from the roadway) is under compression; below the roadway, it’s under tension. The loading of the minor beams will be different in the two configurations, as well.
Wouldn’t an under-truss bear all the weight above it, while a truss above the road doesn’t bear any?
Why is it part of the bridge, if not to bear weight?
A truss either above or below the bridge will bear the weight of the bridge by stiffening the roadway against bending.
Engineering is not my strong point, but I find the linked picture interesting. The image is small and grainy, but it seems that on the far end of the bridge there’s a concrete slab supporting the triangular section of the truss. There doesn’t seem to be a matching hunk of concrete on the near end, but possibly the bushes are hiding a support pillar of some sort.
My instincts (often untrustworthy) tell me that not having a support at that point -where the diagonal meets the horizontal bottom-- would make for a dramatically weaker span. How wrong are my instincts in this case?
To get our terms straight, it’s a “through truss” when it’s above the road, and “deck truss” when it’s below the road.
The purpose of the bottom structures is to keep the top structures that the road rests on from bending, and the bottom structures are held in place by the cross members. There’s no significant downward force at this point that would need supporting.
Here’s a better view of the bridge clearly showing no support
Sometimes a truss bridge will survive the failure of a piece away from the road, as one completely failed on the Oakland Bay bridge. A truss bridge will not survive a failure of a piece adjacent to the road, as happened to the I-35W bridge.
With authority born of 15 minutes of googling, it seems to me that there are multiple terms in play – although possible not in the U.S.
I introduce that quibble only because the links next to the photo in the OP lead to an English steel construction company that uses the term “Under Truss”.
And the (English) Manual Of Bridge Engineering uses the term “underslung truss”.
Irondequoit Bay Bridge (deck truss bridge) in Western NY.
I drive on this bridge quite a bit. It’s similar in design to the I-35W bridge that collapsed. It is currently undergoing repairs in response to the I-35W disaster.
Does an undertruss bridge have any advantages compared to an overtruss bridge? Does it cost less or weigh less? Is it smaller, easier to construct, easier to maintain?
I don’t believe there was a strong advantage to one type over the other, which is why both types lasted in tandem until it became mostly uneconomical to build either type. The I-35W bridge was a nightmare to inspect and maintain, but it was because there were no shoulders so they had to close down traffic lanes (and pissed off drivers would throw stuff at the inspectors) as well as hollow structural members being ideal place for pigeons to nest, rather than it being a deck truss as opposed to a through truss. Maybe in the old days deck trusses were harder to inspect, but now you can just park a boom truck and lower someone down.
I can see the advantage of an undertruss if the truss was connected to pillars and slabs and thus transferring the weight of the bridge to those pylons.
But for those that don’t do that and are solely to stiffen the bridge, it seems to me to be a bad idea because you now have a bridge with a lower underpass. The day that there is a freak mega-flood will find the flood waters and stuff floating in it slamming into that undertruss and pull the bridge apart even if it were never to get as high as the bridge itself.
You could make it lighter by being under the bridge by using different materials. As Xema mentioned the main structural member in the truss below the bridge is in tension. You could use steel cable for this and save weight I’d assume whereas the main member above the bridge needs compressive strength.
I opened up the question in a roads forum, so maybe an actual engineer will answer. But one thing to point out is that a through truss naturally lends itself to a covered bridge. With the overhead structure it’s easy to throw a roof on to protect the wood bridge from damage.
As far as freak floods- some areas simply never get them, or the bridge is so high that’s not a consideration. The water never got close to the I-35W bridge deck truss.
But a side note, the “Swinging Bridge” at Jay Cooke State park is a suspension bridge so there’s nothing below the deck, but water got to it twice anyway. It was completely destroyed in the early 1950s, and then heavily damaged last year.
Thanks for this. 