Truss Bridge Stringer Secondary Stresses

[mchen96]

Hello,

I'm currently designing a truss bridge that consists on longitudinal stringers connected to transverse floorbeams, which are subsequently attached to the truss nodes. I have a feeling I'm overthinking the issue, but since Google hasn't given me a concrete answer, I'd like some confirmation to my current approach.

The intended action of the stringers is to transfer the loads from the bridge deck to the floorbeams in bending. That being said, since the stringers are approximately at the same level as the tension chord, they experience a large tension stress due to the bending of the overall truss, particularly the stringers near the center of the bridge.

Do you:
a) Design the stringers (and connections) just for bending, assuming the connections (double angle shear connection) between the floorbeam and the stringers will have enough "give" to prevent the stringers from carrying high tension loads.
b) Use slotted holes on the connection to allow some displacement in the connection. (If so, would you consider the floorbeam as braced by the stringer during the deck casting?)
c) Design the stringer connections for both bending and for the secondary tension stresses caused by the bending of the overall truss.

While fatigue isn't a concern for this particular bridge (close to no traffic, almost no truck traffic), if a or b, where would you fit fatigue on all of this?

Here's an image I found online that illustrates the issue.

 

[BridgeSmith]

With a typical truss bridge, where the floorbeams are not braced laterally by a diaphragm (the deck), the small amount of axial deformation of the tension chord, combined with the low lateral stiffness of the floorbeams, keeps the axial forces in the stringers fairly small. Quantifying them with any accuracy without a FEA would seem to be a daunting task. If the floorbeams are locked into the deck, it gets very messy, with potentially high lateral shear and bending of the floor beams as the deck attempts to carry a significant portion of the load that would normally be carried by the tension chord.

This is usually avoided by having the stringers continuous over the floorbeams and not restrained against longitudinal movement.

The last time we were tasked with one of those, we opted for a standard steel girder bridge with faux (non-structural) trusses sitting along side.

 

[mchen96]

We would usually go for steel girders as well, but there is very little clearance for this particular bridge, which is why a through truss bridge is the only viable choice. For the same reasons (low clearance), having a single continuous stringer is not possible in this scenario, which is why the stringer is being coped and attached flush with the floorbeams.

And yes, it does get rather messy if restrained (and more so if the deck is made composite with the floorbeams). If done, the stringers do attempt to carry a significant portion of the load, which lead to a whole set of very big connections.

 

[Lomarandil]

My experience is also that stringers will take a noticeable amount of bottom chord tension if allowed by the connections. Accordingly, I nearly always saw the stringers with one end slotted (bridges designed in the 30s to 50s).

If needed, you can always put in a slip critical bolt or two for floorbeam bracing during construction, then replace it later. Or a temporary brace to burn away.

If your bridge has any grade, don't forget that the floorbeams will need weak axis bracing for that reason as well. Adjustable with a turnbuckle if possible. (SFOBB designers forgot that and it caused them a world of trouble).

----
The name is a long story -- just call me Lo.

 

[rb1957]

are the stringers continuous or pieces 1 truss frame bay long, connected together at the frames ?

another day in paradise, or is paradise one day closer ?

 

[BridgeSmith]

As I see it, you basically have 3 options (there are numerous permutations of each of course):

1) Fully integrated, composite structure. The deck, stringers and floorbeams are a rigid system, rigidly connected to the truss. This essentially makes the deck the bottom chord of the truss. It is likely the simplest to analyze and detail, although the connections will need to be fairly large and robust between the deck and the truss. The standoff distance from the edge of the deck to the truss would have to be kept to a minimum.

2) Elastic system, along the lines of the system shown in your link. Everything is bolted together with typical connections. The standoff distance may have to be fairly large to allow the floorbeams to flex laterally, while remaining elastic while accommodating the deflections of the truss. The analysis for this option is likely to be fairly rigorous; best accomplished with a detailed FEM. The detailing of the connections is fairly straightforward, however.

3) Isolated structure. The floorbeams are isolated from the truss using either hangers or bearings. If bearings are used, it will be tricky to avoid having some torsion introduced into the truss by eccentricity of the support. If seismic is involved, this option becomes more attractive. Detailing can become fairly complex, but the analysis is fairly simple. The truss likely would be more robust for this option than the others.

Much of the decision depends on your background, and the tools and resources you have available. I hope this helps.

 

[mchen96]

@rb1957
They will be pieces 1 truss frame bay long, connected to each floorbeam by a double angle shear connector.

@Lomarandil
I do believe having the stringers with one end slotted and using slip critical bolts whose slip resistance is enough to brace the floorbeam but slips when the tension load rises may be the simplest solution, unless there are any downsides I am not currently thinking of. The bridge has a negligible grade.

@HotRod10
For this bridge in particular, option 2 is probably not viable. The bridge in question is a pony bridge (no top bracing). Since the bridge depends on U-frame action to prevent the buckling of the compression chord, large standoff distances decrease the rigidity of the U-frame. For the same reason, isolating the floorbeams from the truss is not viable, as they must be rigidly connected.

That being said, I'm exploring the possibility of somehow releasing some of the tensile stress in the stringers. My first choice would be to slot the stringer connection on one end to allow some slip; however, that would mean the stringers would no longer brace the floorbeam. Whilst non ideal, a heavier floorbeam could potentially solve the issue.

 

[BridgeSmith]

I would be concerned about how to release the deck from the floorbeams without creating a serviceability issue for the concrete at the interface with the floorbeam, if there is continued movement between the floorbeam and the deck soffit under live load. You might consider embedding steel plates in the deck at the floorbeam locations for the top flanges of floorbeams to rub against instead of the concrete. Then again, I could be anticipating an issue that won't actually be an issue.

 

[BridgeSmith]

Sounds like a fun challenge, although I'd be out of my comfort zone on something like this. We'd be pressing the road designers to provide us with more grade separation so we'd have room for girders.

 

[canwesteng]

I would feel fine ignoring tension in the stringers. Tension has a stiffening effect, so no concern for buckling. The system as described just needs enough ductility to redistribute the loads, so my only real concern is failure of the connections or tension getting transferred to the deck. Not sure if this helps you though.