Optimum span length 2

[tmalik3156]

Good day all.
We are doing a very preliminary concept design of a 50 m long bridge. We are considering a two-span option, and a three-span option.
I am wondering if anyone can provide reference to equations that relate cost of bridges to span length for steel girders and concrete girders. I understand that there are lots of variable to consider. But just for preliminary estimate, I am looking for cost (C) as a function of span length (L). If you know any such equations, kindly share.
Thank you

 

[BridgeNerdGuy]

Multiple span bridges with continuous concrete girders is not a straightforward design. If both steel and concrete are available options, generally steel girders would be preferred.

I said (or meant to convey) that making prestressed concrete girders continuous for live load requires a more involved design process, and a significant amount of extra care and attention to proper construction, to function properly.

Interesting. I think these comments are regionally motivated or influenced. Simple span made continuous for live load is a bread and butter preferred structure type for highway bridges in my market. The design tasks aren't too hard either, anyone capable of designing a steel bridge can easily handle the details and calcs for simple made continuous concrete spans; there are a few interesting calcs to do but it's primarily a detailing exercise. Steel is hardly ever economical in our region, especially with a relatively short bridge like this.

- The 0.7-0.85 end span to internal span ratio in 3 span structures is always a decent starting point.
- 50m is well within precast girder simple span feasibility depending on structure depth but local shipping length & weight limits would need considered.
- Short end spans in continuous structures can result in uplift at the bearings which needs considered
- Simple spans resulting in joints at the piers should definitely be avoided

 

[BridgeSmith]

The design tasks aren't too hard either, anyone capable of designing a steel bridge can easily handle the details and calcs for simple made continuous concrete spans; there are a few interesting calcs to do but it's primarily a detailing exercise.

Interesting. Maybe it's a matter of familiarity and experience. I find the design effort considerably more for prestressed concrete superstructures, and we don't even do the actual design of the prestressing. Steel girders are our bread and butter, so I find them easy. Our biggest difficulty with prestressed girders is handling the camber, especially on the decked bulb tees.

 

[OSUCivlEng]

I knew Texas, like many other states, has a significant inventory of multiple simple span bridges, but I didn't think anyone was still building new ones. I suppose in areas that don't use salt on the roadways, leaky intermediate joints would be less detrimental.

We've rehabbed some bridges that had double-bearing piers and deck joints, and replaced portions of the deck to with link slabs, to eliminate the joints. As far as I know, all of those have been fairly recent, so we have yet to see how they hold up.

Texas builds quite a few simple span bridges with prestressed concrete girders as does Oklahoma. Best I can tell, TXDOT does not like continuous for live load designs or link slabs. Oklahoma doesn't really like continuous for live load either, but does use link slabs.

I have also designed a simple span plate girder up to 200' in length as well. I wouldn't recommend doing that a lot, but that was what the client wanted to do.

 

[bridgebuster]

OSU - out of curiosity, why the objection to live load continuity? is it a settlement concern? Back in the 80's I worked on a multibridge NJDOT project. They wanted simple spans - their philosophy: simple bridges, simple problems; continuous bridges, continuous problems - however, since it was Federally funded FHWA insisted on continuity. Since the Feds were writing the check, they won.

Link slabs have become very popular in NY for bridge rehab projects. Most of the multispan bridges built in NY during the late 50's to early 70's were mostly simple spans.

 

[BridgeSmith]

I don't think anyone objects much to having continuity for live load. It seems it's avoided due to the difficulty in effectively providing the continuity in a way that's durable, while accommodating the rotation due to the long-term camber.

 

[BridgeNerdGuy]

AASHTO [5.12.3.3.4] allows for the calculation of restraint moments (caused by deck shrinkage and restrained girder rotations) to be waived when a positive moment connection per [5.12.3.3.9] is provided (usually by simply extending prestressing strands into diaphragm) and requiring the girders to be 90 days old at the time the deck is poured. Colorado even shortens this 90 days to 60 in the state Bridge design manual.

 

[BridgeSmith]

It makes sense that if the girders are 90 days old before placing the deck, there wouldn't be much additional rotation, or shrinkage for that matter. So yeah, that would make the detailing much easier. Around here, it seems the precasters and contractors are too impatient for that, though. I'd suspect CDOT gets alot of push-back even on the 60 days.

 

[OSUCivlEng]

OSU - out of curiosity, why the objection to live load continuity? is it a settlement concern? Back in the 80's I worked on a multibridge NJDOT project. They wanted simple spans - their philosophy: simple bridges, simple problems; continuous bridges, continuous problems - however, since it was Federally funded FHWA insisted on continuity. Since the Feds were writing the check, they won.

This explains the TXDOT philosophy

https://texashistory.unt.edu/ark:/67531/metapth637392/m1/12/

They love their precast deck panels and that could cause problems with negative moment over the piers. They also say any perceived savings in structure cost or structure depth has never been realized.

For quite a while ODOT was focused on building the most economical prestressed concrete bridge possible, hence mostly simple spans. They do use a lot of link slabs and integral abutments.

This article from Modern Steel makes the case that simple span with link slabs may be cheaper than continuous girders.

https://www.aisc.org/globalassets/modern-steel/archives/2014/09/2014v09_piece.pdf

 

[BridgeSmith]

This article from Modern Steel makes the case that simple span with link slabs may be cheaper than continuous girders.

In some situations, particularly where construction time is short, span by span construction with prefabricated superstructure elements can be the preferred option. We aren't usually in that much of a time crunch for our bridges.

We do alot of staged construction (build half of the new bridge next to the existing, demo the old, and build the other half), where detours would be difficult or expensive.

 

[bridgebuster]

Thanks for the report OSU. Perhaps I misread some of the earlier posts as I was thinking continuous for DL & LL. I don't disagree with the TxDOT findings. While I haven't done much prestressed design, I've done a handful of prestressed bridges continuous for live load. Yet, I remain somewhat skeptical about the effect of continuity. However, eliminating joints, at least in the northeast, is a good thing.