Precast/Prestressed Concrete Box Girders

[GalileoG]

I am currently working on the design of a precast/prestressed concrete box girder (a bunch of box girders laid side by side,) each girder has one square opening at the center (about 70% of the total x-sectional area) and spans approximately 10 meters.

I have never designed a box girder before, let alone one for a bridge! I am designing it as per AASHTO 1998. I am curious, is there anything out of the ordinary that I should pay attention for that someone whose been accustomed to building design would miss? Any special considerations?

Are there any guides out there that can put me in the right direction?

We're also looking to purchase a software that will aid us in designs like this one. We've been using Concise Beam for our precast/prestressed beams but that software does not have the bridge code. Is there a software out there ($500-$1000 range) that we should be eyeing for this application?

Thanks,

Clansman

"If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[GalileoG]

Attached is a picture that best describes my concrete girders.

Clansman

"If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[JackTrades]

Your picture indicates a RR bridge unless you were not intending that as part of the description. If so, you should get a copy of the AREMA Code.

 

[GalileoG]

My bridge will not support a RR, just trucks. The picture is only to illustrate structural system of the deck (found it online.)

Clansman

"If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[kslee1000]

I would watch out for joint design (girder-girder interface, temperature effects, uneven deflections), as well as the deck to girder connections (I guess the two will form a single unit through reinforcing and concrete).

 

[hokie66]

I think this type of precast plank is more like a commodity than something you design each time. If you are working as a consultant, go to the precasters in your area for available sections. If you are working for a precaster, it sounds like you have a steep learning curve. I don't mean to disparage the importance of knowing how to design these elements, but it has been done before. Sorry I don't have references, but like you, I have been mostly involved with buildings.

 

[kslee1000]

hokie is correct in pointing out that the precasters can provide valuable infos. However, it usually stops short. A lot of times you are only getting a standard cross section with basic design informations, especially for DOT jobs I was aware of. Time may have changed, I wondering how much changes have occurred in the practices. (One may risk been accused of favoring one precaster over the other) But DOT has a lot of standard details, try to understand and stick to as close as possible.

 

[Qshake]

You can get the PCA design books for less than $1000 ( or you could). Software to check out would be CONSPAN. It is very flexible as it handles I-girders, box girders and slabs.

Why are you using 1998 AASHTO? Should be using the latest LRFD code?

Also some DOT's like Illinois DOT has charts for the design of their box girders.



Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[GalileoG]

Thanks all.

I was able to find some great standard details on Oregon's DOT.

I was able to download the trial license software for Conspan and browse through it. It seems perfect for my application, definitely something we will consider (still waiting to recieve a quote from them.)

Clansman

"If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[kslee1000]

Good luck from there. Keep us posted.

 

[miecz]

I always cringe when I hear that someone is going to design something they've never designed before and then asks for a software package. That said, try the following link

http://www.pci.org/publications/bridge/

There is a lot of theory available for download together with design examples. Then, try the Pennsylvania DOT for thier PS3 software.

 

[COEngineeer]

Or you can buy the culvert/bridge from the company I work for and we will design it

Never, but never question engineer's judgement

 

[kslee1000]

Which one is free - design, or software?

 

[WpgKarl]

PCI has a Bridge Design Manual. Around here, a std. box girder is 4ft wide by varying depth.

Box girders are erected side by side and have a continuous shear key along the top that gets filled with NS-grout. Then they are laterally post-tensioned together with a couple of 0.5" dia. strands in PT ducts running thru intermediate diaphragms. This enables load sharing between box girders.

Make sure to talk to local precasters on details and std construction practices. Watch out for skew of bridge girders - eventually you reach a point where girders are skewed too much to PT together laterally, PCI Bridge Manual has something on that.

 

[hokie66]

I think the lateral stressing is more commonly done with Macalloy bars or similar rather than strands. But that may vary from one DOT to another.

 

[graybeach]

The geometry can be complicated especially if your bridge is on a skew with a longitudinal slope. Be sure to consider carefully. Screwing that up can necessitate that the concrete topping be very thick in some places to make the roadway surface right. This could affect the live load capacity. Also check creep. The bridge will tend to hog up over time, which can be an issue if you are doing stage construction.

 

[enginerding]

You will also want to be sure that there are at least two layers of reinforcement in the bottom. Because you won't be able to inspect what is going on inside the girder, you won't be able to see a problem until the lower mat rusts and causes the concrete to crack or spall. To get enough rust on the reinforcement to crack or spall the concrete means that you will have loss of section on an incredibly small diameter strand. Significant loss of capacity for the beam.

Having two layers of strands means that the beam will still have some strength even when the bottom strands rust and spall. This makes it possible to notice the problem in time to fix it before the whole thing comes down.