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Flat slab bridge thickness

Flat slab bridge thickness

Flat slab bridge thickness

(OP)
Good day.
I was told to look into a conceptual design of a simply supported flat slab bridge.
That is, a bridge consisted of normally reinforced slab only, and no girders.
It can not be a portal type frame, it must be simply supported. It can not be prestressed / post tensioned.
The span is 20.3 m (66.6 feet).
How do I estimate a thickness of this slab? Is span divided by 20 a reasonable assumption? But that will be a huge thickness. On the other hand, a smaller thickness will give unacceptable deflection at the mid-span.
I have never worked on a girder-less bridge before. So any suggestion will be helpful.
Thank you

RE: Flat slab bridge thickness

Type of loading might affect it.

And type of design, RC , PT?

If you are a bridge designer you should be able to some quick numbers in about 5 minutes!

L/20 might be too thin for many cases!

RE: Flat slab bridge thickness

There really is no rule of thumb for solid slab bridges of that length. Generally, they're considered uneconomical, both in material costs and labor costs. A voided slab or box girder bridge would be much lighter, much less expensive to build, and probably not any deeper than the solid slab. A CIP concrete T girder bridge would only be slightly deeper.

Just curious, why no girders, prestressing or postensioning?

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

RE: Flat slab bridge thickness

(OP)
@ rapt: Thank you for your reply.

@ BridgeSmith:
Why no girders - local NU girder manufacturer seems to supply span larger than 25 m. This bridge is only 20.3 m



Why no post tensioned voided slab - local jurisdiction suggest solid section for small spans.


RE: Flat slab bridge thickness

Girders don't need to be precast. Form them, conventionally reinforce, and cast in place. A uniform thickness slab spanning 20 metres is wasteful.

RE: Flat slab bridge thickness

(OP)
@ hokie66: Yes you are correct. For 20 m span, ribbed / haunced / T-beam slabs are more appropriate than a uniform thickness slab.

RE: Flat slab bridge thickness

Or put void formers in the slab if you do want to simplify formwork and not use to cast in place beams to reduce self weight.

RE: Flat slab bridge thickness

Keeping void formers in position while placing the concrete is not easy. A way to reduce mass, but not necessarily better labor wise than forming beams. And if you use polystyrene voids, I think it is as expensive as concrete, volume wise.

RE: Flat slab bridge thickness

For bridges in that span range, we've used prestressed decked bulb tees, and steel girders with CIP concrete deck. We've considered prestressed double tees, and if deflection wasn't critical, we'd even consider a prestressed tri-deck superstructure. We wouldn't even consider a slab, or any CIP superstructure, for that span - too much shoring and formwork.

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

RE: Flat slab bridge thickness

(OP)
Indeed, normally reinforced slab wouldn't be a good option for that span range.
It's a road over train track grade separation bridge, so there is train clearance height issue. That's one reason why girders and bulbs were initially not considered.
It's only at conceptual level at this time, and a posttensioned slab might work. Girder options give about 1200 mm of superstructure depth, while normally reinforced and posttensioned slabs give about 1100 mm and 750 mm respectively.

RE: Flat slab bridge thickness

We consider a girder depth to span ratio of .04 to be optimal for composite steel girders, and we routinely go as low as .032 where clearance is limited. With a 200mm CIP concrete deck, and a 900mm girder, for a total superstructure depth of 1100mm, your depth to span ratio would be .044.

I did a 42" precast prestressed decked bulb tee for a simple span of 65'. We had to vary the asphalt overlay thickness from 2" (at centerline) up to 4" (at the abutments) to compensate for the camber, but it worked. If you can accommodate a little bit of a crest vertical curve, you could probably eliminate the overlay.

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

RE: Flat slab bridge thickness

Another issue with an non-prestressed concrete slab is that it will creep something like 2x the elastic deflection.

RE: Flat slab bridge thickness

Creep can be more a problem with prestressed than non-prestressed.

RE: Flat slab bridge thickness

Yes- true, creep happens with prestressed too, but usually hogging up. With non-prestressed the bridge will keep sagging until the total final deflection is about 3x the elastic dead load deflection. This would need to be considered in low headroom situations and could be unsightly, cause ponding, and other problems.

RE: Flat slab bridge thickness

Yes, concrete creeps, but it's fairly predictable. So prestressed girders can be balanced so that the girder starts out with a slight crest and the creep results in the girder being the right shape after a couple of years, and very little additional deflection in the decades that follow.

Postensioned slabs and girders can be adjusted for creep throughout the service life, in order to maintain the desired shape.

Anyway, the OP's query was regarding the use of a CIP slab bridge. I think there have been plenty of reasons given why it would not be an efficient or economical choice for the span proposed, especially given the depth available for the superstructure.

RE: Flat slab bridge thickness

Bridge,

You are assuming non-bonded posttensioning? Bonded posttensioning can't be adjusted, but you wouldn't want to use non-bonded in a bridge. Too much risk of corrosion.

The main problem with creep in bridge decks has been rideability due to upward creep with time. Ever drive across the Chesapeake Bay Bridge, US 13?

RE: Flat slab bridge thickness

Hokie, I get the impression we may need to define some terms.

Post-tensioning in the vernacular we use, involves pre-casting concrete with ducts for high-strength strands or threaded rods, to be installed and tensioned after the structure is in place. Grouting of the ducts is common, but takes place after the strands or rods are tensioned. As far as I know, this precludes the strands from being bonded. After further reading, it appears I was wrong; the grouted post-tensioned system can be considered bonded.

When the strands, either bonded or unbonded, are tensioned, and then the concrete is cast around them, we call that prestressed concrete.

With that out of the way, yes, long term camber due to concrete creep can be an issue with prestressed girders. I'm not sure about grouted post-tensioned, since I don't know how relaxing the strands after grouting would affect the integrity of the grout. Seems that can and does work, but you're likely correct that this type of system can't be adjusted for the effects of creep. As I said, ungrouted post-tensioned girders could be adjusted pretty much at anytime, and yes, they can and are used in bridge superstructures. With proper sheathing of the strands or rods, and detailing to allow water to escape from the ducts, ungrouted post-tensioned girders can last the service life of the bridge. They can also be replaced if necessary.

The amount of camber can be reduced significantly by increasing the time to release of the prestreessed strands, or increasing the age of the concrete at the time of post-tensioning.

RE: Flat slab bridge thickness

Btw, I haven't been anywhere in the US east of western Kentucky, except for one conference in Miami, and a few other stop-overs in the Miami airport.

RE: Flat slab bridge thickness

The bridge I referenced has pretensioned deck panels which have caused a problem over the years due to upwards creep.

RE: Flat slab bridge thickness

Yeah, pretensioned (AKA prestressed) deck panels would seem to be a bad idea. especially if there's not going to be a variable-thickness overlay that can be easily replaced, such as asphalt.

At least with a simple span prestressed girder bridge, it's one curve over the whole length of the bridge, which can sometimes be made to fit the vertical profile of the roadway, but even if it can't, is less obtrusive than a series of little bumps.

RE: Flat slab bridge thickness

I am sure you know, but for the benefit of other readers:

Prestressed encompasses both pretensioned and post-tensioned.

Post-tensioned encompasses both bonded and unbonded.

RE: Flat slab bridge thickness

To avoid confusion, we (and everyone else around here) just use the vernacular I outlined previously, where prestressed denotes the strands stressed before the concrete is cast around them, and post-tensioned denotes strands or rods tensioned against cured concrete.

RE: Flat slab bridge thickness

BridgeSmith,

Why the need to change the logic?

Pretensioning and PostTensioning are both forms of Prestressing.

Pre means you stress before pouring the concrete, Post means you stress after pouring the concrete. As it has throughout history.

How is that difficult?



RE: Flat slab bridge thickness

Quote:

Pretensioning and PostTensioning are both forms of Prestressing.

Pre means you stress before pouring the concrete, Post means you stress after pouring the concrete.

By your definitions, the terminology we use would be correct, or at least more descriptive and less confusing than lumping post-tensioning in as prestressing.

Anyway, it's just the way the terms have been (throughout recent history, at least) and still are used in this region - we use prestressing and post-tensioning, but avoid the use of the term pretensioning.

For the purposes of this discussion, I can start using "pretensioning" instead of "prestressing" for the precasting method where the strands are stretched, concrete is poured around them, and then the strands are released after the concrete has cured, if that will avoid confusion.

RE: Flat slab bridge thickness

Bridge,

Maybe not much of an issue in the bridge forum, but in the structural other topics forum, there is often confusion.

A poster asks a question about prestressed structures, and we have to define what kind before giving a sensible answer.

Precast prestressed is usually pretensioned, but once in a while is post-tensioned. Like the Florida pedestrian bridge collapse, which gives that method a bad name.

Cast in place prestressed is always post-tensioned, I think. But to discuss it further you need to know if it is unbonded, as usual in North America, or bonded, which is prevalent in many other parts, like Australia.

RE: Flat slab bridge thickness

Quote:

Cast in place prestressed is always post-tensioned, I think.

It would have to be, I would think. Pretensioning strands in the final position of the beam would be a logistical nightmare, and ridiculously expensive. On a really large project, it might be worthwhile to bring the casting bed to the site, to precast the beams near where they will be placed.

I don't think posttensioning CIP concrete is hardly ever done, either. The placement of the reinforcing and ducts for a beam or slab that is to be post-tensioned is precision work that I don't think I'd ever have confidence could be done correctly in the field.

RE: Flat slab bridge thickness

Anyway, back to the subject. I just found out today that we're developing a bridge superstructure configuration utilizing precast, pretensioned solid slabs (4' wide sections) for single span bridges up to 60'. I'm looking at the details of the 50' span right now, and the slabs are 16" deep.

RE: Flat slab bridge thickness

Bridge,

It is post-tensioned precast which is rarely done. Post-tensioned CIP is typical, all over the world, for building structures. The location of ducts is not too difficult, although weaving them in slabs as is the norm in bonded work requires a bit of practice.

RE: Flat slab bridge thickness

I didn't know that, hokie. I thought post-tensioned precast was the preferred method, at least in the US. I've never seen CIP post-tensioned used for bridge construction; maybe it's used in other regions, but not in mine. I don't really keep track of construction methods in the building industry.

RE: Flat slab bridge thickness

Until the Florida overpass disaster, I didn't know that post-tensioning in precast was a thing. Now, the post-tensioned precast was not really the issue, but the concrete truss/frame was.

Yes, bridges and building structures are very different critters. By the way, I think 16" is much too thin for a 50' span. Trampoline comes to mind.

RE: Flat slab bridge thickness

Segmental bridges are post-tensioned precast

RE: Flat slab bridge thickness

bugbus,

Yes, they are. Forgot about them. And I frequently go over, under, or near the Gateway Bridge in Brisbane.

RE: Flat slab bridge thickness

Quote:

By the way, I think 16" is much too thin for a 50' span. Trampoline comes to mind.

I guess we'll find out, but the The pretensioned concrete manufacturers we're working with believe they can make it adequately stiff. The moment of inertia of slab superstructure is actually fairly close to the CIP RC Tee beam superstructure it's replacing. The beams will be connected with continuous shear keys, so we'll get some participation from the whole width of the slab. Our live load deflection criteria is quite a bit more stringent than for buildings (L/800, or L/1000 w/ pedestrians).

AASHTO did make those deflection limits optional, since it's actually vertical acceleration that produces discomfort for users. Many long-span bridges exceed the deflection limits, but the acceleration is acceptably low.

Anyway, that's barely below the depth to span ratio allowed by the AASHTO spec for steel girders (0.027), which are typically spaced 7' to 9' apart. Granted, in order to meet the deflection limits, girders that shallow typically need to be composite with the concrete deck.

RE: Flat slab bridge thickness

Yes lots of post tensioned precast done in bridges and wharf/pontoon structures.

I would have trouble making 400mm deep 15m simply supported work for deflection with building loads. Going to be interesting for bridges.

RE: Flat slab bridge thickness

I was slightly misinformed as to the depth of the pretensioned superstructure slabs we're planning on using. For the 50' span, we're proposing to use a 18" slab for a depth to span ratio of 0.030, which is in line with the minimum depth to span ratios used by several other states.

It appears that for spans over about 40', most states are using voided slabs, similar to hollow core sections, rather than solid. Our head honchos want to avoid the issues with future deck repairs and possible water accumulation that may arise with the use of voided slabs, so we're going to only use solid slab sections, with spans up to 57'.

Some states are using voided slab or box girder sections for spans up to 70', so the 20m span the OP is investigating isn't as strange as I thought. It doesn't look like anyone is doing CIP slab bridges of that span length, though.

RE: Flat slab bridge thickness

I am commenting for the sake of collecting opinions and empirical evidence for future readers regarding the post-tensioning discussion in the middle of this thread.

1. CIP Post tension multi-cell boxes are quite common in some markets. Caltrans uses this structure a lot and is a great resource if you get the pleasure of designing one. CIP on falsework, PT in the field, remove falsework. A great structure type for tight horizonal curves.
2. Post-tensioning prestressed beams to complete long spans is a useful technique. Prestressed elements erected on a combination of permanent and temporary supports, cast a closure pour, PT in the field, remove temporary towers. Yay, span is now longer than what could be shipped as a single beam.
3. I've also seen PT performed in the casting yard for curved tub girders. In lieu of chorded spans this allowed for a curved CIP section to be cast, lifted out of the bed, PT in the yard, then shipped and erected like any other prestressed beam.

RE: Flat slab bridge thickness

Thanks for the additonal info BridgeNerdGuy. I'll only take issue with one minor terminology usage - a girder cast in a casting bed is not "CIP", which is short for "cast in place", as in it's cast in it's final and permanent location and position. Anything cast and then moved into it's final position would be precast.

RE: Flat slab bridge thickness

@BridgeSmith I agree with your terminology distinction here

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