Flat slab bridge thickness 1

[tmalik3156]

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

 

[hokie66]

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.

 

[BridgeSmith]

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.

 

[rapt]

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?

 

[BridgeSmith]

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.

 

[hokie66]

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.

 

[BridgeSmith]

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.

 

[BridgeSmith]

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.

 

[hokie66]

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.

 

[BridgeSmith]

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.

 

[hokie66]

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.

 

[bugbus]

Segmental bridges are post-tensioned precast

 

[hokie66]

bugbus,

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

 

[BridgeSmith]

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.

 

[rapt]

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.

 

[BridgeSmith]

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.

 

[BridgeNerdGuy]

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.

 

[BridgeSmith]

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.

 

[BridgeNerdGuy]

@BridgeSmith I agree with your terminology distinction here