What is the max. curvature that post tentioning can be done? 2

[yjung]

I have a curved bridge for the cost analysis.
The radius is 167 ft with 4 span bridge.
Each span is about 80 ft.

My question is; can i use a post tension box girder with this curvature or not? Anyone knows where i can find the limit of post tensioning for the horizontally curved bridges?

My second question is; what is max. overhang length and min. overhang length if i want to use the straight girders for this bridge? and where i can find any good information about steel, AASHTO girder, CIP box etc.

Thank you

 

[Qshake]

I am not aware of any governing criteria for horizontal curvature on post-tensioned bridges. A radius of 167' is a very sharp highway curve. I assume this is part of a ramp or something similar.

The obvious concern with the horizontal curvature is the horizontal force imposed on the girders web and the additional loss of prestress force due to friction on the vertical plane in addition to the more typical loss on the horizontal plane. All this translates to large webs and high jacking forces.

Slab on curved girder bridges can be cost effective here too, especially since the spans are small. At 80 feet or so I believe you'd be looking at 36" standard beams or some 42" plate girders. Either way, both have been used countless times in these applications. There is a new AASHTO curved girder guide specification out that you'll have to be aware of.

As for the overhang on curved bridges, I have always strived to keep the overhang on slab and girder bridges to around 3' or 4' for tangent structures. I do the same for curved bridges with tangent girders.

I would look into the curved steel girder bridge. My experience urges me to say it will be cheapest in this situation but I'm not familiar with the site. Curved steel girders are always a lot of fun to design and are very aesthetically pleasing.

By the way, the new AASHTO guide spec on curved girders has several great examples though very lengthy.

Good Luck.



Regards,
Qshake

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

 

[yjung]

Thanks Qshake!!!

Yes, this bridge is for a lane freeway ramp.
Is the max overhang 7 ft and min 6 inch overhang reasonable, if i really want because of the bridge curvature?

I am in AZ, people told me that the curved structural steel cost is very high, when compared to concrete cost.

Qshake, where is your work place? are you still working for any firm? I wish i could work for you.

 

[VoyageofDiscovery]

yjung,

To answer your first question. I can't see it being a problem just thicken the webs.

To answer your second question. In such cases, transverse post-tension your slab.

Regards

VOD

 

[Qshake]

Yjung, thanks for the kind words.

7 feet is not out of the question although aesthetically it may look odd unless its a constant amount.

I can imagine that steel is expensive in Arizona...I should have mentioned something about the availability and usage in various areas. A lot of states and other agencies will use concrete over steel due to the availability, construction practices etc. Some of those that I've encountered are Florida, Texas, and California. Many others will go either way.

Good luck.

Regards,
Qshake

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