Photo taken by James McCray
BH Photo #239676
Since we are discussing it...
The lattice could support a fair amount of weight. The flat bars are will carry tension well, and there are enough of them it would be noticable. The weak point in that line of thought is the top bar, or "chord". It will be under compression and thus it's is subject to buckling. Resistance to buckling is directly related to the width. That double angle top is not wide enough to resist much buckling for a piece that long.
But let's pretend for a bit that it's built from some amazing material that is impossible to bend (ignore those bent ends) so it can't buckle. So now we have a sturdy trussed girder. But for it to be of any help, the bridge live load has to be transferred from the deck to the girder. That is the job of the floor beam. All of the weight carried by the girder gets to it via the floor beams. Those floor beams appear to be much to small to carry the bridge load.
Therefore, my opinion is the lattice is railing, much more ornimental than structural. It is positioned so it could provide some structural support, but not much.
Sort of unrelated, that lattice looks nearly identical to railing on an 1895 truss I just saw. Photos to be uploaded soon, I hope.
Some of this may be restated, but hopefully I can simplify what the story is.
Here is a bridge with lattice "railings" that that are actually lattice "girders" and this is demonstrated by the floorbeams that are connected to the lattice. http://www.historicbridges.org/michigan/veteranshome/ In contrast, this bridge pictured here on BridgeHunter has longitudinal stringers carrying load and also the railings are not mounted in a way that they could contribute structurally. I realize there is a transverse beam running under the bridge connected to the railings however note that these are far less deep than the longitudinal stringers... not going to happen with a girder bridge. This appears to be merely a way to help hold on the railings, which were mounted on the outside of the bridge to maximize roadway width (and thus reduce cost). The design proved unreliable however since the railings are severely susceptible to damage from floods. We have (or had) many such bridges in Michigan, many with damaged railings from floods.
Although the girder I refer to is more significant, I should emphasize that lattice railing stringer bridges have become quite rare and I consider them significant and well worth posting.
The lattice structure could certainly carry some load in the vertical plane; how much requires a force analysis. The fact that image 16 shows missing diagonals and apparently a repair or assembly joint right where the center floor beam "hangs" from, would make this structure quite weak in that area. The fact that it appears to be simply bolted to the flooring away from a stringer also makes it suspect.
The lattice is a manufactured part, and it's length suggests that it was manufactured to be a part of a bridge. It could possibly have been a load bearing part of a catwalk or ship's gangway, for example. Maybe it was just manufactured to be a guardrail, though.
All parts are structural; not all parts are load bearing.
Because of the compressive forces in the top member and the diagonals and verticals providing strength in the vertical plane, failure of a truss will result in sideways buckling.
Trusses are built for strength in the vertical plane, not so much for horizontal loading. Wider, heavier members would obviously resist horizontal forces better, however.
Andrew, I should have said something about your comment on the lattice railing acting as a beam--you're very correct; heavier definitely does not mean stronger, as proven by most any truss bridge out there--built-up members with lacing, lattice, and battens provide strength with efficient use of material and lightweightness--and they look nice! If it were a structural component, the railing on this bridge could function in vertical flexure to a degree, but most likely not well enough to support the weight of the bridge itself or its intended live loads; side buckling would most likely be an issue. Similarly, if trusses are overloaded (especially pony trusses), this side-buckling usually occurs--the truss normally does not buckle in a vertical motion; it will likely "bow" sideways.
Well, I am a civil engineer with limited structural experience (my experience is all transmission line design), but I do know enough about bridge systems to make the argument that lattice railings are not structural just because they are there. There are some cases where they may contribute to the structural capability of a bridge, but in most cases that I've seen, they do not. Many of the old truss bridges that are in existence have lattice railings, but it is quite obvious that the trusses do the structural supporting while the lattice railings merely keep vehicles from running off of the road or damaging the trusses. I've seen enough of Fmiser's posts to know that he (or she) either is an engineer or knows a lot of info about these structures. Keep the questions coming, though, as it gives everyone (especially me!) opportunity to think a bit deeper about these bridges and to learn more about them.
I don't want to argue, but if they are there then they are structural. And I've never heard that girder bridges can't have stringers, although I would expect them to be of far lesser might than the girders themselves.
But I will give in quite a lot; the stringers underneath this one look much heavier than the so-called floor beams, to which the lattice railing is attached. So you are probably right.
But I would want to do a test, because I think that even a flat lattice like the one shown could support quite a bit of weight if each intersection was welded up. Heavy isn't always strong. The railing would really function as an I beam, granted a rather thin one ... yet the anti-sag strength of a girder/beam is mostly in it's height and then in it's thickness, isn't it?
Do we have a real live engineer here we can ask?
I agree with Matt. The floor beams are not heavy enough to be structural, so even if the lattice could carry a load, it wouldn't be transferred from the deck to the lattice.
Stringer bridge with lattice railing.
A correction, if I may--I believe this is actually a stringer bridge and, while it looks very much like the one shown in Nathan's narrative, it functions a bit differently. By definition, a girder bridge has essentially only two beams for longitudinal support, one on the outside of each side of the bridge, with no intermediate stringers. Girder bridges normally have either transverse floor beams (as shown in the narrative), or alternating diagonals (arranged to function as a stiffening truss), while a stringer bridge has one or more stringers in addition to and parallel to the outer stringers (as shown in pic #6 of this bridge). Also, the lattice railings in the narrative feature heavy angle-steel diagonals, which are capable of supporting heavy loads. The railings' diagonals on this bridge are just flat steel and would have very little structural load carrying ability. These railings only provide some barrier protection to traffic; they don't function as structural supports. The floor beams are usually rigidly attached to the girders of a girder bridge, but a stringer bridge typically lacks transverse floor beams altogether. The transverse beams of this bridge are there only to support the railings' outriggers and do not act as floor beams.
Don, that's what is called a Lattice Girder bridge. You have to look closely to see if the "railing" is attached to the floor beams, not to the outer stringers.
The lattice works just like a girder only it's much lighter. Think of the aluminum struts and stuff inside an airplane, and how they all have holes in them to save weight. The lattice "holes" are the same kind of deal.
Actually, this bridge is almost exactly like the one that Nathan Hoth describes on page 114 of An Introduction To Historic Bridges located at
which is a superb source for learning about bridges, trusses, bridge parts, etc. Come on, nobody in the real world knows what "ashlar" means ... but Nathan does, and now we do too, thanks to him. :-)
A steel stringer bridge with a wood deck and lattice guardrails. I can't imagine that the lattice as a "truss" would carry much load. The center beam is just support to keep the lattice from leaning sideways.
At least that's my guess. Other opinions welcome.
So what is this kind of bridge? I need help. I posted this bridge but gave no definite truss design because I do not know what it is. Please leave comments.