Structure Evaluation on Skewed Rigid Frame Bridge

[shanmur]

I am planning my approach to do a structural evaluation for a 35' span rigid frame bridge on a 30 degree skew. The purpose is to obtain a permit for an oversized transport truck (265,000lbs). The issue is that the drawings are incomplete. Only the repair drawings are available, which do not provide any reinforcing details in the deck. We had no success in tracking down the original drawings so I wanted to see if there were any other options I have at this point. I have done a comparative analysis using the design live load from the bridge code in the past for simpler structures but I am concerned about the high skew here. Are there any conservative standard or typical reinforcing details I could use? Also, I don't even know when the original structure was constructed. Any suggestions?

 

[bridgebuster]

Here is an article from an old text book on skewed rigid frame bridges. I rated a bunch but never designed one. I recall for skews - as the old timers told me - the reinforcement was increased by the cosine of the skew.

I'm also going to post a manual from the PCA circa 1940.

 

[bridgebuster]

This is the old PCA Manual. I used this in the BCE days (Before Computer Era or is it Error?).

 

[shanmur]

Thank you for the reference material!

 

[bridgebuster]

you're welcome

 

[ukbridge]

I would be extremely cautious about assessing the deck without knowledge of the reinforcement provided. Also for skew bridges note that the orientation of the reinforcement with respect to the skew of the bridge will also make a massive difference in your calculations (see wood armer and the like for more information). Is it a simple slab bridge, or some kind of filler deck?

On a previous job on assessing a building for capacities some kind of tool was used to get pictures of the reinforcement (im no radiologist, so have no clue of the name sorry!)

I guess if thats not an option Id probably employ some kind of simple grillage or shell analysis to get Mx My Mxy contours with which to determine wood armer moments with which to assess the slab for. You can use these assessment moments to determine what the minimum reinforcement provided would need to be to safely carry the vehicle - provided you know what the yield strength of the reinforcement is!

I dont doubt theres some nice hand calc type method in the back of Reynolds Concrete Designer Handbook somewhere, but for me at least its quicker to whip together some kind of simple FE model to play with if you have previous experience with this type of analysis. The results of your hand calc will probably extemely crude as well with the distribution of the wheel loads and the like as youre basically neglecting any transverse distribution. Dont forget to apply a generous DAF if ones not given in whatever code youre using as its oft forgotten!

Good luck! Pete

 

[bridgebuster]

ukbridge - The tool you're thinking of to locate rebar is called a pachometer. Ground Penetrating Radar can also be used.

 

[Shz713]

Bridge Buster is right, I've seen similar cases.
Check out enclosed paper how they assessed and load rated skewed bridge without knowledge of steel rebar using GPR technology!

Shoot for the Moon, even if U miss, U still land among Stars!

 

[yakpol]

Could you use ultrasound or GPR devices to determine spacing and diameter of reinforcement?

 

[shanmur]

I had looked into the GPR option but it was not entertained due to restriction of resources and time. In the end, I went with the shell analysis to show that resultant moment and shears from the permit vehicle were less than the design traffic load based on the controls I had suggested. Much thanks to all for your very helpful advice!