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RE: Sewer Aerial Crossing

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Just a couple of thoughts.
 
1. Are you stuck with the profile or can you raise the pipe to keep it out of the water? A bridge with no freeboard is usually not a good idea because you can get unpredictable loads from debris.
 
2. Ductile iron pipe limits you because of the frequent joints. Maybe you could do this with a single span with a larger steel pipe, which has lots of section modulus, or consider a larger, self-supporting steel pipe casing for the sewer line., which could probably be HDPE, eliminating joints and weight, not to mention corrosion problems.You could eliminate the midspan concrete work, and maybe the abutments as well, just terminate the casing in a manhole at each end of the span.
 
3. As an aside, do not rely on the standard asphalt coating on ductile iron pipe for long term corrosion protection. If you are going to have this stuff out in the weather and potentially submerged, you should give it a high performance coating. In order to get a decent bond, make sure the contractor gets the pipe as a special order from the factory without the asphalt coating before having it coated, since it is difficult to remove the asphalt completely and you will have problems with other coatings if you don't start with bare metal.
-----Original Message-----
From: Speck, Todd M [mailto:tmspeck(--nospam--at)pbsj.com]
Sent: Wednesday, August 28, 2002 6:16 AM
To: SEAInt (E-mail)
Subject: Sewer Aerial Crossing

I'm designing a sewer aerial crossing bridge.  The bridge will only support the sewer itself, no vehicular traffic.  It's a 16" ductile iron pipe continuously supported by a cast in place cradle/saddle spanning between single column bents/abutments.  The pipe/bridge at 100 yr occurrence will be completely submerged.  The overall span length is only 60'.  I don't believe expansion/contraction will be an issue.
 
1.  I'd appreciate any words of wisdom with respect to general design issues.  I also have a few specific questions.
 
2.  I'll be specifying stainless steel brackets and bolts to connect pipe to the cradle.  What type of stainless steel should I specify?
 
3.  The soil has a potential vertical rise of 2.5".  The abutment foundation will be drilled shafts socketed into hard limestone and will not move.  What is done to assure the pipe can handle the differential movement at the abutment?
 
4.  Sitting in a cradle with a semi-circle cut out to hold the pipe I can envision water being held in contact with the pipe indefinitely.  Is corrosion of the pipe a concern.
 
Thank you in advance for your comments.

Todd M. Speck, P.E.