I have seen a particular style of timber truss bridge in a few photographs from the 1870-1880 era. But, I have never seen any reference to this specific design execution in bridge books or articles, other than a reference to the generic concept called an arched timber truss bridge, or perhaps a Whipple bowstring truss. Given the fact that the following photographs show nearly identical bridges of this type, I wonder if this specific execution was a standard product, and maybe had a name. Does anybody have any information on it?
What I would really like to find is engineering drawings if they exist. Notice how the arches are built up of timber plank laminations, and there are more laminations near the center that make the arch beam thicker there. The design sure seems like a monument to carpentry. However, this was probably in the era before wood preservative, and with all of those water-trapping mortise joints, the lifespan must have been alarmingly short.
First of three M&StL RR bridges over Minnesota River at Carver, MN:
The 1st one for sure - and possibly the 2nd - look like center pivot swing bridges to me. Right there that makes them unusual because of the center-only support in the open position, and the correspondingly rare and difficult cantilever loading - though symmetrical - which results from that. Then, even more unusual because they are wood. However, the angle of both of the photos is so shallow I’d have a tough time recognizing the truss pattern without a lot of study. Fortunately, for a center-pivot there’s not too many generally-accepted designs in wood, so that range of possibilities is small.
The 3rd photo is a good side-on view to enable ID of the truss type. I have a good ASCE reference book on early American wood bridges that might be helpful, but it’s in storage right now. I’ll see if I can make digging it out a comparative priority, and get back to you on this.
The bridges in the first and second photo, and the M&StL bridge in Excelsior that I mentioned, were all center-pivot swing bridges. The water is too high to tell with the NP bridge in the third photo. The side truss braces in all of them are “X” design. The first photo shows a bit of the top structure, which appears to also be made of “X” truss timbers. I would like to see exactly how they built those timber crosses, and how they connected them to the rest of the structure.
The swing bridge in the first photo was replaced by a steel swing bridge that was nearly identical to two other steel swing bridges just downstream from this one. Those three steel swing bridges had a taller “A” frame at the center, and a through-truss on either side. Diagonal straps from the “A” frame held up the ends of the two through-trusses. This structure addressed the issue you mentioned regarding the need to support the ends of the swing span when it was open.
Ahh - discerning questions, these are. How many others have looked at these photos, but did not see what you have ? Anyway, here’s what I can add:
The normal configuration / location of tension and compression members is reversed for these cantilevered bridges - at least for when they’re open - because then the bottom chords would be in compression and the arched top chords would be in tension. That tension force would of course be at its largest over the single center support - and since wood is poor in resisting tension - that’s why those top chords are so large in the middle.
When the bridges are closed (so trains can cross them), there should then be 3 points of support - the center pivot, and both ends. In that position, each half of the bridge should be preferably designed and constructed to be a span that is simply-supported at only its 2 ends = the outer end, and at the center support. Then, they would work like 2 regular through truss bridges that are just placed end-to-end - the bottom chords will now be in tension, and the top chords will be in compression. Interestingly, that would normally mean very little compression in the top members where they cross over the center support - in non-movable multiple span truss bridges, there very often are no compression members spanning that upper gap between 2 sucessive spans. Here, however, there are sizable members already in place, so the designer may have been trying to use them as a continuous structure across the center pivot. [Don’t worry about that if you don’t understand it - pretty much only structural engineers would, and that remark is mainly to let them know that I saw and noted that aspect of this structure - let’s see if anybody else has thoughts on it.]
The hard task for the engineer is to choose a design and size the members to be able to meet the requirements
There are quite a few online references to the Whipple bowstring truss. But as I understand it, Squire Whipple invented the design and introduced it quite early (pre-1850) executed in iron construction, and targeted it for railroad applications. However, each of the arch truss bridges that I have linked above are all-timber construction, and yet they date from considerably later than Whipple’s iron bowstring truss development.
Whipple’s bowstring truss and the ones I have linked are identical in structural principles except for one thing. With the Whipple truss, the arched top chord continues its curved arch right down to the deck chord. With the ones that I linked, the each end of the arched top chords terminate at a vertical member looking to be maybe six-eight feet high. So, it is like a Whipple truss with the ends truncated.
Interestingly, these early center-pivot swing bridges were opened and closed by hand.
Although a lot of the focus on Whipple’s invention and work was in iron, I think his real contribution was instead the introduction of the use of mathematical analysis of the bridge structure, so as to be able to predict and estimate the loads in each member with a much higher degree of certainty than had previously been the practice. It just happened that he preferred to be working in iron, not wood - but his method of analysis is equally valid for any material. Most likely those western railroads compared the cost of iron vs. wood for each of those bridge designs - and wood came out the winner - most likely due to its being available locally, fabricated by a lot of comparatively unskilled labor, needing only basic tools and equipment, etc. I don’t think it’s coincidental that all 3 of those bridges from about the same time are wood - the same market forces would have been in effect for all of them, and that’s why they all came out the same way. I believe that iron was still pretty expensive back then, and those bridges would have used a lot of it. Also, iron is brittle and - like wood - is not real good or reliable in tension either (unlike steel, which is), so using iron would not have been a huge advantage in reducing the size or number of the structural members. Also, compression members may have to be of a certain minimum geometric size to avoid buckling under load, and in those conditions they are usually not fully loaded, so again the potential advantage of iron could not be fully utilized.
I’m not surprised by the truncated ends of the Whipple truss photos that you linked. It has to do with carrying the shear (= weight load) through the end of the bridge so that the load can get down into the foundation. In brief and very generally, the shear capacity is related to the height of the bridge right there. The Whipple bridge patent drawing
We may be looking at a Howe truss that’s been modified to have a curved top chord - at least in the 3rd (NP) photo of your original post - as opposed to the tied-arch of the Whipple bowstring truss design. See the discussion / explanation on the “Bridge Basics - A Spotter’s Guide to Bridge Design” page at the “Bridges & Tunnels of Allegheny County and Pittsburgh, PA” website at:
The Whipples are about 2/3 of the way down, and the Howes trusses are about 4/5 of the way down. The key point is that this guide says that the Whipple’s tension members extended across 2 panels - not just 1 as with most trusses - although that’s not supported by the Whipple patent drawing at the Black River Canal websitre that you linked.
A bit off the topic of wooden swing bridges, but… the two bridges over the Trent Canal in Peterborough, Ontario are fascinating; I’ll try to find my pictures. They’re the same – a pair of Warren trusses with a centre pivot support joining them. Being Warrens, the lines of action are very easy to visualise. One of them is still in use on the Canadian Pacific line to Havelock, and takes full modern loads. The other is out of use (except by the occasional undergrad) but by the same token is easy to get to and study. Both are hand swung and a little unusual in that the centre pivot is on land on one side of the canal, and one of the trusses swings over dry land; the other goes over the canal. To swing the bridge you walk to the centre pivot structure, climb up and turn a very large crank mechanism – does wonders for your upper body strength.
One thing to note about railway swing bridges is that when open, they are cantilevers – but they only have to handle the dead weight of the bridge itself. Closed they are simply supported at both ends.
Peterborough is also of note because of the lift lock on the Trent waterway – worth the trip, if you like fascinating engineering contraptions!
You mentioned the hand crank used to open the two swing bridges. The fact that some swing bridges were opened and closed manually, and the mechanism needed to accomplish this, is one of the most fascinating aspects of these type of bridges. I would guess that there might have been quite a variety of
I have some random thoughts. Some of these points you have already made, so this is kind of a summary.
The pivoting truss, when open, is only directly supported in the center, so it needs structure to support the cantilevered load of the two unsupported ends. When closed, the structure is directly supported on three points, so the closed bridge could fundamentally be two independent trusses. One truss would span from the first support point to the second support point, and the other truss would span from