Nathan is absolutely correct.
That’s why when you look at the drawing from the "generic" swing bridge I posted yesterday you will notice the entire end of the truss is highlighted meaning all of those are tension members.
If you have studied engineering you will know that you can not have every member in tension like that at one time. The reason all of them are highlighted is due to the reversal of forces. When the bridge is closed and under traffic the bridge acts as a continuous truss supported at both ends and in the center. Under these conditions the bottom chord will take tension loading for a set distance. When the bridge is open it acts like a continuous truss that is cantilevered from the center without end support. Under these conditions the bottom chord will take no tension loading and only compression.
All of these movable bridges are pretty amazing machines. The ability to balance such a tremendous amount of weight and have it glide in and out of place is pretty remarkable, especially considering the technology in which these structures were designed and built with.
Swing bridges are supported at only the pier when open, but supported at the ends when closed (note that swing bridges have end wedges that lock the bridge in place and allow load transfer to abutments). As such, some forces may change and even reverse in a swing bridge when open or closed. This is likely why swing bridges have odd configurations that are difficult to classify.
I agree with these statements also; with the entire span essentially supported at the center pier, this span basically functions as an upside-down Pratt. When viewed upside down, the diagonals actually slope inward and are under tension, true to Pratt truss design. Interesting!
This is a very common truss arrangement for a center pivot swing bridge. The forces in the members are as described by Fmiser. Since it is continuous over the center pier one must remember the forces essentially act in reverse over the pier, with tension in the top chord and compression in the bottom chord.
I attached a photo of a “generic” swing bridge (without any particular name) showing the general forces. The solid black members are the ones that are generally in tension.
So as Fmiser stated the truss acts more like a Pratt than anything else, though I do not necessarily agree with calling it a Pratt. Swing bridges are sort of a unique beast since they not only carry static loads (traffic, self-weight ect.) but also dynamic loading when opened.
I make no guarantees about the accuracy of my sketch. Bird's Eye view (as well as my vision) seems to be a little skewed anyway, so what appears to be vertical might not quite be.
I see what you say about the diagonals all being under tension, and that appears to be correct when the bridge is swung open parallel with the river, but then it is(hopefully) only carrying it's own weight plus a bird or two.
When closed,and carrying a train, I assume the piers also carry some load, in which case I see what you are saying about the center of the bridge being two verticals from the outer piers where the diagonals oppose each other. The Pratt type section then lies between the two piers on either side.
Interesting too that the inside two piers appear to be positioned at the center of a panel instead of under a vertical member, but that might be a perception thing.
I'm going to have to do a little research on swing bridges.
> Don wrote:
> Is this correctly described as a Warren truss?
If your sketch is accurate, it is not a Warren.
It does kinda look like a Howe - but that is misleading because it is a center-supported swing rather than an end-supported fixed span. As drawn, the diagonals are under tension load thus making it a Pratt. Think of it as a two span bridge joined in the middle at the swing bearing. Viewed that way, the center of each span is not the truss center. The truss center is two verticals from the abutments. Then the diagonals slope down toward the "center" just as a Pratt should.
However, if what is shown as verticals are not vertical but actually slope, then Warren would probably be correct.
That's my opinion! *smiles*
Oh, yeah. Forgot to mention that I looked at it in Bing Bird's Eye to make my nice sketch. You might want to look at that, too.
Okay. While looking at a bridge downstream that lies on the Jefferson /Hardin line, I got curious about this bridge.
I added that it crosses the Bullitt / Hardin line.
Is this correctly described as a Warren truss? It looks unusual to me. It is a swing span, but it looks like two identical trapezoidal trusses joined at the pivot. Mostly similar to a Howe truss, with a little pratt at the ends, and the center joining structure is similar to Baltimore.
I have rendered it in MS Paint for your viewing pleasure.
found removal photos of the bridge. here is a link waiting for a response for permission to use them on the sight and other info such as build date and builder of the old bridge.