CIP vs Precast at water crossing? 1

[SoeSoe]

The water slow moving and is 4 ft deep and the soffit is 20 ft up from the stream bed at the center pier. The proposed bridge is 220 ft long and 2 spans.

1) When it goes to bid, how much will the costs of erecting the falsework play in the cost? It seems like at water crossings, that this would be when the CIP comes in higher than the precast.

2) Reviewing the book "Formwork for Concrete" they show a picture of some wide flange beams cast into the pierwall to support the superstructure falsework. After these beams are cut off flush with the pierwall, won't they rust and eventually cause problems?

3) Has anyone at the get go provided two sets of bridge plans for bidding and let the contractor decide? What types of problems arise by letting the contractor submit an alternative design? What types of provisions should be included to address this?

 

[Qshake]

1. Generally speaking, CIP is always more costly than setting precast units, if for no other reason than the labor cost for erecting and wrecking the formwork. Back in say the 50s, 60s and to some extent, the 70s labor was cheap compared to structural elements. Nowadays it is the other way around. And at 110' spans these will be ideal for precast prestressed AASHTO Type 4 Girders (something around 4.5' deep).

2. In most cases, formwork is not allowed to be part of the finished structure (there are exceptions for mammoth structures). Typically with a pier wall in the center, the beams will be held tightly against the pier but supported on steel beams that may set directly on the foundation below (just to take advantage of the footing as a support shelf). If the substructure is comprised of multiple columns then the contractor will use friction collars to hold the supporting beams in place. This is the most widely used method in the Midwest to set formwork beams.

3. Providing two sets of bid documents has been around for many, many years and in most cases it is favored when there is some ambiguity in determining what the best choice of construction will be steel or concrete. In those cases, there is usually a big difference in substructure due to the max span lengths for each material. Otherwise clients expect the engineer to decide what can be built more economically. I don't think in your case you'll do any good with two bid sets. For me the choice is obvious...precast.

Don't be afraid to talk to contractors especially if you talk with more than one.

Regards,
Qshake

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[Dinosaur]

I must be missing something. What sort of CIP structure are you proposing? It appears obvious the superstructure will be P/S beams composite with a deck slab unless there are architectural concerns. What am I missing?

 

[BradPL]

I'm a little lost too. Are you referring to a cast-in-place superstructure such as a T-Beam or are you referring to deck construction?

I didn't know anyone was still doing CIP with mild steel except for the smallest of stream crossings. Prestressed members are standard fare 'round here.

 

[SoeSoe]

Qshake,
I've seen a lot of CIP Prestressed boxes built in the 80's and 90's over waterways and highways. Caltrans shows the CIP box as cheaper than the prestressed.

http://www.dot.ca.gov/hq/esc/estimates/

I would think the box would also have certain advantages for skewed and curved crossings. So far as aesthetics, I think the box is as ugly as the 'I'. Ofcourse everyone is entitled to his opinions and prejudice, I think the only nice looking concrete bridges are slabs, arches, and parabolic girders.

I agree, it seems to me the precast would be so much simpler and easier to construct.

 

[Zambo]

As stated above I am sure there would have to be another reason to go for CIP (and this would almost certainly be post-tensioned not reinforced concrete)rather then cost alone. These reasons could be conncted with architecture or poor access to the site for plant to erect prestressed beams. But assuming that there are no particular reasons why not then prestressed beams with a deck slab has to be the way.

 

[Qshake]

SoeSoe,

I don't disagree with your experience. I myself have seen ample CIP structures. There are advantages to using them in certain situations. But for what you've described, I can't see the CIP being very economical compared to Precast.

For curved applications CIP is common where Steel isn't the material of choice (examples would be CA and FL).

For long span bridges CIP sometimes competes very well with precast units. Especially where there is little room to have a precast yard on site.

I've no doubt that there are many other reasons also.

Regards,
Qshake

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