"U" Shaped Abutment Design Questions

[3Fan]

Hi, I am in the process of designing a "U" shaped abutment for a grade separation bike path bridge. The bottom of footing to the top of the walls will be about 25' tall. The distance from the inside face of the wingwalls will only be 12.0' The foundation will be a single mat incorporating the breastwall and both wingwalls. I have a couple design questions that I hope someone can shed some light on for me.

1. How would you calculate the amount of horizontal reinforcing at the corners of the breastwall and wingwall? There will be no joint filler between the wall and breastwall. Peck's book in the abutment section says to use "judgement" and heavy reinforcement. Not sure where to go with that.

2. I am designing the mat foundation by hand and am looking at it as a cantilevered retaining wall footing. How much of the distance between the turned back wings walls should I consider as "heel"? All the way to the back face of the adjacent wing? The more distance I consider, the lower my bearing pressures are, but I would be overlapping wingwall foundations anywhere past half way. Is there a better way to design this?

3. I would assume that I don't have to consider sliding since the horizontal earth pressures are acting in opposite directions and are equal.

Thanks!

 

[wiktor]

1. How would you calculate the amount of horizontal reinforcing at the corners of the breastwall and wingwall? There will be no joint filler between the wall and breastwall. Peck's book in the abutment section says to use "judgement" and heavy reinforcement. Not sure where to go with that.
I will use double of your horizontal rebars, as L shaped bars overlapping horizontal. As far as I can recall, distribution rebars are 25% of main, so 50% would suffice.
2. I am designing the mat foundation by hand and am looking at it as a cantilevered retaining wall footing. How much of the distance between the turned back wings walls should I consider as "heel"? All the way to the back face of the adjacent wing? The more distance I consider, the lower my bearing pressures are, but I would be overlapping wingwall foundations anywhere past half way. Is there a better way to design this?
The right way to design this mat foundation will be to model it as a three way supported slab, with overhangs, if any, loaded with total weight of the abutment and fill in between wingwalls, acting as uniform distributed load. This will give you exact moments in the structure. Slab could be easily modeled as a grill using any software.
3. I would assume that I don't have to consider sliding since the horizontal earth pressures are acting in opposite directions and are equal.
Yes, sliding is not an issue. Also, if you would like to reduce moments in wingwalls, consider providing few struts or tension bars connecting wingwalls together.

 

[3Fan]

Thanks wiktor. We will not have the option to use any grill software for the foundation. Could you guide me somewhere to do a little more investigating about 3 way supported slabs? I recall years ago learning about 1 way and 2 way slabs, but don't remember 3 way slabs.

G