Design of Huge RCB End Section

[OSUCivlEng]

One of my coworkers dropped a Plan & Elevation on my desk of an RCB with some very custom end sections. I haven't ever seen anything like this before, but essentially the downstream end section has an 11' drop from the barrel to the apron, and the apron flowline is 3' or 4' below the existing channel flowline. The RCB barrel is 8' tall, so the wing will be about 20' tall at the barrel and slope down to about 5' or 6' at the end.

I'm sure terms vary depending on where you live, but what I call an end section is the wing walls and apron. Usually the apron runs the full length of the wings (and in between) and also acts as the footing for the wings.

I plan to analyze the the wings and apron as a U shaped concrete frame in the same way I would analyze an RCB as a rigid frame. This image from MODOT shows how I would apply the loads to an RCB.

http://epg.modot.org/index.php?title=Image:751.8.2.5.1.jpg

My concern is how to apply the distributed load acting upwards against the bottom of the apron. Obviously I won't have any live load and will have the self weight of the wings and apron for DC2. I am not sure how to define the load applied to the bottom of the apron due to the earth pressure on the wings. I do it all the time for retaining walls, but this thing is double sided and that is throwing me off.

 

[BridgeEI]

The only upward force on the apron would be the self weight of the apron and wings divided by the apron length. You won't apply vertical earth pressure to the apron, only horizontal to the wings unless the apron is going to be covered but that would probably be unconservative.

The ODOT standards were designed as an L shaped retaining wall and the toe length is iterated to satisfy a bearing pressure of 2ksf.

 

[TehMightyEngineer]

Whenever I've had to look at large box culvert wing walls with large aprons and/or skews I've always modeled the walls in a FEA program with the apron supported on springs with a suitable subgrade modulus. This seems to capture the effects I would expect. For what I recall, when I last had to do one of these I enveloped the design with FEA as described above and additionally looking at the wing walls as a heel-less retaining walls similar to how BridgeEI described it.

If you're doing this as precast then you have even more weirdness as you'll likely have a shiplap joint in the apron which you could argue will transfer shear across the joint for one apron/wing wall but will not transfer shear for the other side.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[OSUCivlEng]

Thanks Bridge EI. I was always curious how the ODOT standards were designed with respect to the apron acting as a footing. I take it you work in Oklahoma as well. I wish OK had a simple bridge manual that contained design assumptions, guidelines and policies like this.

 

[BridgeEI]

Yea I work in Oklahoma. I doubt they ever get a manual, it's just one of those things you figure out when you've been around the block like I have...

I like TehMightyEngineers's approach and have been wanting to do it so I can reduce a lot of reinf steel in those things. I'm just not fully proficient in being able to do the FEA. I've also spent the time and have a library that has reinforcing and concrete details for walls up to 20+ feet. I got extremely tired of designing and detailing them so now my techs know to go find the library.