Steel bridge beam cover plate economics 2

[Enginerdad]

I'm designing a bridge superstructure and the client wants to evaluate a "rolled section only" option as well as a "smaller rolled section with bottom cover plate" option. I've determined that by using a cover plate, we can reduce the weight of the rolled beam by 69 plf, but the cover plate is 31 plf, so a net savings of 38 plf. It's my judgement that the savings in steel weight isn't enough to justify the additional fabrication costs of adding a cover plate, but I don't have a good reference to back that up. Does anybody have guidance on the cost of fabrication relative to steel weight for this type of value engineering? If it matters at all the combined length of all beams is 456 ft.

 

[SlideRuleEra]

I suppose you will have to determine the actual costs... but for a minute look at the calculation:

Added cost for "large" beam instead of "small" beam + cover plate is only the material cost for 38 plf.

Cost of "small" beam with cover plate is the total of:

1) Material cost of "small" beam and cover plate.​

2) Fabrication of "small" beam to add cover plate is based on the total weigh of "small" beam + cover plate... several hundred pounds plf , I expect.​

Fabrication cost will be a few times material cost (per lb.). But, say, fabrication cost equals material cost... do the math.

Is "large" beam material cost of 38 plf less than "small" beam/cover plate material cost plus fabrication cost of several hundred plf? ... absolutely yes.

 

[BridgeSmith]

First, let's take a step back. Based on what you've presented, I suspect there may be better options than either of these 2. How many spans? What is the length and width of the bridge?

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Enginerdad]

I suspect there may be better options than either of these 2

There certainly are better options, but the client (DOT) has limited us pretty severely. The bridge is single-span, 55.5' span and 44' curb-to-curb. I wanted to do NEXT Beams, which are pretty popular around me, but the client has a policy that strongly dislikes prestressed bridge beams due to their limited ability to be repaired following a vehicle collision. They're also not interested in anything newer or innovative. They also preclude precast decks for whatever reason. So that basically leaves us with conventional rolled steel beams with CIP deck, which they strongly favor generally. Oh, also they want it built, including new substructure, in a single construction season while maintaining traffic using three construction stages. And we have about 9 weeks to go from zero to 90% design. Welcome to my reality.

If you have any other suggestions, I'd still be happy to hear them however. We're too far into this project and time is too tight to make a change now, but I'm always looking for other options for future projects. There's the Valmont U-Beam system that I think would be a perfect fit for this structure, but we don't have time to get the necessary approvals from the client on our condensed timeline.

https://www.valmontstructures.com/products-solutions/traffic-mass-transit/bridge-systems

 

[BridgeSmith]

I assume the height you have available for the superstructure is severely limited. Our typical superstructure type for something like this would be welded plate girders (typically 6 girders for that width), primarily because the dead load deflection for shallow rolled beam girders would be too much to mitigate through thickening the deck (we limit the increase to 1") and cambering rolled beams is not cost effective when compared to plate girders.

That gives us full flexibility to use whatever flange sizes are needed, and precise control over the depth of the girder, to maximize the available space.

Anyway, back to the original question. I can see the cover plate option being economical only if you can limit it to half of less of the span, which should be doable. The biggest issue with that would probably be staying under the fatigue stress limit for the base metal at the transverse welds at the ends of the cover plate. This can be mitigated somewhat by tapering down the width of the cover plate at the ends. See AASHTO section 6.6. I assume it's going to be composite, so that will help with the applied fatigue stress.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[BridgeSmith]

Is there an existing bridge at this location that can be used while you build half (in your case, it would likely need to be more than half, to maintain 2 way traffic) of the new bridge beside it? That what we generally do in cases like that - maintain traffic on the existing bridge while we build half of the new bridge, then transfer traffic to stage 1 of the new bridge, tear down the old bridge, and build the 2nd half of the new bridge, minus about a 4' section of the deck that's left for a closure pour after both sides have deflected to their final position.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[bugbus]

It's probably a similar cost overall. Is the welding likely to be done manually or automated?

 

[Enginerdad]

Around me plate girders generally don't start being the more economical choice until around 80-90 feet. The fabrication cost controls until then. And yes, there's an existing bridge to maintain alternating one-way traffic while we build the rest.

 

[BridgeSmith]

Rolled beams are more economical for us as long as the we can meet the required vertical profile by varying the thickness of the deck, which usually only works for short spans with lightweight and deep rolled beams. Where vertical clearance is tight, and we have to use shallower and heavier sections, the spans where we could use a rolled section get ridiculously small. How are you compensating for the dead load deflection of the girders due to the weight of the slab? Or are you going to allow the vertical profile to be a sag vertical curve?

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Enginerdad]

We compensate for dead load deflection by cambering the beams. I'm confused as to why you would ever vary the thickness of your deck. Do you not have haunches that you can vary to get your final elevations?

 

[BridgeSmith]

We generally don't do haunches; not sure why. I would speculate there are construction issues with doing so. Apparently your state DOT doesn't do that either; probably for the same reasons.

The fabricators in our area tell us that heat cambering requires alot of highly skilled labor and is energy intensive, so it's more expensive than fabricating plate girders. Nobody in the area has the equipment to do cold cambering 'the hard way' for rolled girders in the sizes we would need.

In our area, plate girders are not that much more than rolled beams, anyway; maybe $0.50/lb. The flange to web welds are done with an almost fully automated process, and the rest of the fabrication (adding connection plates, stiffeners, shear studs, etc.) is pretty much the same for either one, with the notable exception of fitting bearing stiffeners, which is far more work for WF section.

We're also somewhat constrained by our policies that require a minimum 1 1/2" bottom flange over roadways. There's less damage and they're easier to fix when they get hit by overheight loads, which happens more often than you'd think.

We do have alot of concrete slab bridge replacements coming up, so we're looking for options for shallow superstructures. We've been looking at the U-beams also.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Enginerdad]

We generally don't do haunches; not sure why

This is fascinating to me. I've never even considered the concept of not having haunches on beam-slab bridges.

Apparently your state DOT doesn't do that either; probably for the same reasons

I'm not exactly sure what you mean by that. We use haunches in all beam-slab bridges, minimum 2" thick for this DOT. We camber the beams for the anticipated permanent dead load deflection so that they don't sag, but since both predicting deflection and fabricating camber are more art than science, we have the haunches to allow for fine adjustment. This way we have a uniform thickness deck everywhere with proper finished grades.

We're also somewhat constrained by our policies that require a minimum 1 1/2" bottom flange over roadways.

This is also novel to me. I guess I understand the concept, it just seems a bit excessive. Even if the collisions happen "more often than [I'd] think", what percentage of all bridges over roads is it really? 1%? Maybe less? Requiring such a thick flange on thousands of bridges on the chance that 10 of them get hit during their service life seems like a losing bet, no? I'm not asking you to defend your state DOT's practice, just pointing out that I have a very different perspective with my experience being where it is. I work primarily in CT, MA, and RI. Would you be willing to share where your DOT is located? I'm very curious.

As always, I appreciate the responses. It's always useful to have a wider understanding of practices and perspectives.

 

[BridgeSmith]

We use haunches in all beam-slab bridges, minimum 2" thick for this DOT. We camber the beams for the anticipated permanent dead load deflection so that they don't sag, but since both predicting deflection and fabricating camber are more art than science, we have the haunches to allow for fine adjustment. This way we have a uniform thickness deck everywhere with proper finished grades.

I get the concept of using haunches to get the right finished grade and uniform deck thickness. What I'm puzzled by is why you'd add the expense of cambering the girders, if you're going to have a haunch, anyway.

Apparently, we're not as picky about the "fine adjustment". We just have them cut the web of the plate girders to compensate for the calculated DL deflections, and set the bottom of slab either at the top of the top flange (for composite) or 3/4" below the top of the flange (non-composite). I've only seem haunches on one widening we did where we needed to change the superelevation.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Enginerdad]

The camber is to prevent the beam from having a visible sag under permanent dead load. If we used a straight beam, it would be sagging a few inches at the center by the time it was built, even if you leveled the deck nicely with the haunches. The other thing is that we don't want the haunches to get too high. Above a few inches they need reinforcement which is costly to add.

The article below from Modern Steel Construction says that cold cambering costs $30-$75 (in 2006 dollars, $43-$110 now) per beam. Heat cambering is 5x-10x that cost, so let's say $430-$1100 worst case scenario. If we assume a typical cross section, maybe 8 beams, that's $880-$8,800 for the whole bridge. Cold cambering is essentially free on the scale of a $1 million-plus bridge, and even heat cambering is almost negligible.

Specifying Camber, Modern Steel Construction, July 2006

 

[BridgeSmith]

I think the cold cambering cost is most likely for the 22' to 24' beams they can do in one shot in the press. For the length of beams we use, we're probably in that "run up to five times" category. There's also this from the article:

Most fabrication shops surveyed had machines that
could handle up to 27”-deep beams. For deeper beams, contact
your fabricator, as each shop has its own maximum and minimum
sizes that it accommodates.

I don't think the shops in our region even have the equipment to ones that big, much less the 30" to 36" ones we most commonly use for spans that would require us to camber. The 24" to 27" sections are only used for really short spans, where the DL deflection can be mitigated by varying the slab thickness.

Rod Smith, P.E., The artist formerly known as HotRod10