Cantilevered RC Retaining Wall Design
Cantilevered RC Retaining Wall Design
(OP)
I have a few questions regarding RC Cantilevered RW's. Essentially I am looking for what is typical (i.e. conservative) and what is according to code.
Footing design and load factors.
Eccentricity - Should the loads be factored as follows: 1.6 x lateral pressures + 1.2 x resisting dead loads (soil and DL surcharge) + 0.9 x passive pressures (is 0.9 correct?) + ? x Vertical components of soil pressure. Then analyze the footing to determine a new eccentricity and thus a new bearing pressures? Or Should you use the same eccentricity as found when using 'service' loads and checking for stability? Then factor up the bearing pressure x 1.6 for toe design, then factor the soil weight and surcharge for the heel design? Once again is one 'technically correct' vs what is typical or conservative?
Vertical component of soil pressure.
I am trying to get a better understanding of when vertical components are typically used.
Wall stability - can be included. Does drainage aggregate impact this decision?
Bearing pressure - seems that using pv would decrease the eccentricity but may increase the bearing pressure.
Footing design - I'm not sure how you would factor this. I suppose since it is a resisting load it could be multiplied by 0.9 however I'm not sure if that makes the calculation more or less accurate/conservative.
Hooked dowels into the footing
Does the hooked bar embedment / development length need to be followed to for cantilever footings?
Meyerhoff pressure distribution
I have heard that the Meyerhoff pressure distribution may me more realistic. Does any one use this? Can it be used? Essentially it is a uniform (not triangular) pressure distribution over an effective width of footing B'= L-2*e.
Where B'= effective width, L=width of footing, e= eccentricity
Footing design and load factors.
Eccentricity - Should the loads be factored as follows: 1.6 x lateral pressures + 1.2 x resisting dead loads (soil and DL surcharge) + 0.9 x passive pressures (is 0.9 correct?) + ? x Vertical components of soil pressure. Then analyze the footing to determine a new eccentricity and thus a new bearing pressures? Or Should you use the same eccentricity as found when using 'service' loads and checking for stability? Then factor up the bearing pressure x 1.6 for toe design, then factor the soil weight and surcharge for the heel design? Once again is one 'technically correct' vs what is typical or conservative?
Vertical component of soil pressure.
I am trying to get a better understanding of when vertical components are typically used.
Wall stability - can be included. Does drainage aggregate impact this decision?
Bearing pressure - seems that using pv would decrease the eccentricity but may increase the bearing pressure.
Footing design - I'm not sure how you would factor this. I suppose since it is a resisting load it could be multiplied by 0.9 however I'm not sure if that makes the calculation more or less accurate/conservative.
Hooked dowels into the footing
Does the hooked bar embedment / development length need to be followed to for cantilever footings?
Meyerhoff pressure distribution
I have heard that the Meyerhoff pressure distribution may me more realistic. Does any one use this? Can it be used? Essentially it is a uniform (not triangular) pressure distribution over an effective width of footing B'= L-2*e.
Where B'= effective width, L=width of footing, e= eccentricity





RE: Cantilevered RC Retaining Wall Design
EIT
www.HowToEngineer.com
RE: Cantilevered RC Retaining Wall Design
RE: Cantilevered RC Retaining Wall Design
I'm trying to create a spreadsheet that will allow me to use different options.
EIT
www.HowToEngineer.com
RE: Cantilevered RC Retaining Wall Design
RE: Cantilevered RC Retaining Wall Design
Then you can use a copy of this same beam worksheet three other times to design the reinforcement for Heel, then for Toe, and then for Keyway, if applicable.
RE: Cantilevered RC Retaining Wall Design
FE - Thanks, I always appreciate your advice. I have set up the spreadsheet very similar to how you suggest and that seems to be working well. However do you perform a separate analysis for bearing pressure using factored loads? Or do you "factor up" the bearing pressure?
If factoring the loads then finding a new bearing pressure is the "code correct" way to do this, what factor should be applied to passive pressure? 0.9 seems logical.
The stem or wall is pretty straight forward in that you will have 1.6 x the lateral earth pressure from soil or surcharge.
The footing is my issue right now. Currently I have what I believe is too many options. I have all the load factor combinations, then I have a section to over ride these, and then I even have the option to just apply a single factor to the bearing pressure for the toe design and then you can even neglect the soil over the toe. For the heel you have all the same options except you can neglect the bearing pressure.
EIT
www.HowToEngineer.com
RE: Cantilevered RC Retaining Wall Design
As hokie66 mentioned, to get you factor of safety for overturning and sliding, just use the unfactored earth pressures. For Stem design, you will need to get your Vu and Mu (factored) and then also compare it to PhiVc and PhiMn. I don't count on the soil cover above the toe for resisting calculations. i also discount any cohesion in the Actie backfill.
Go to this website and watch the Retaining Wall video and Download the Powerpoint notes.
http://anc-tbquimby01.uaa.alaska.edu/courses/ce433...
Then get from your library (as opposed to purchasing)Ferguson, Breen and Jirsa's "Reinforced Concrete Fundamentals", 1988 (5th edition) to get the structural design details. To do the shear key design at top of footing + all the geotechnical aspects, get from your library "Foundation Design" by Cernica, 1995. The online notes + these two references will answer all of your analysis and design questions. All of these suggestions are incorporated in this program: http://www.soilstructure.com/cantileverretainingwa...
Then comes the detailing. For this you can buy "Standard Cantilever Retaining Walls", 1976 by Newman.
For footing design- I find it easier to think of it as heel slab and toe slab. See Dr. Quimby's video link above. Don't forget to throw in T & S reinforcement.
One thing that many U.S. textbooks don't cover but is covered by F.D.C. Henry's 1986 "The Design and Construction of Engineering Foundations" are two items. First is the deflection of the stem. Most often, the stem does not deflect enough to be in the Active state, so use a K value between Active and At Rest state. Second, to avoid differential settlement, the toe bearing pressure must not be more than 3 times the heel bearing pressure. Given the fact the most retwall footings are 4 to 7 ft or so, why would you have for instance 1950 psf bearing pressure on the toe and then only 75 psf on heel?
One final thing, in the corners of retaining walls, the earth pressure is really close to At rest State, so try to decrease stem reinforcement spacing accordingly. It is very enjoyable learning experience & you will see it is really involved subject. Good luck and keep us posted.