Cantilevered RC Retaining Wall Design 3

[RFreund]

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

EIT

http://www.HowToEngineer.com

 

[RFreund]

I should clarify - when I say code, I am referring to IBC / ACI 318. But I would be open to other references as well.

EIT

http://www.HowToEngineer.com

 

[hokie66]

For stability, i.e. overturning and sliding, I just keep everything as service loads and check against an acceptable factor of safety. The factor of safety sometimes varies, depending on how critical the installation is, based on judgment. Then for the concrete design, I tend to factor the loads, but even then, I sometimes us working stress design.

 

[RFreund]

Hokie - Thanks for the insight. I will more into the IBC requirements. I guess I'm really trying to determine how passive pressure and vertical components are typically used and what is code correct vs typical/conservative.

I'm trying to create a spreadsheet that will allow me to use different options.

EIT

http://www.HowToEngineer.com

 

[TXStructural]

The CRSI Design Handbook steps through this design. The only deviation from this you might need is to apply pressure to the back side of the key. A change in 2009 to IBC chapter 18 added this to the procedure, although many designers find it irrational. When I get the update to the CRSI design chapter completed, there will be discussion on this and tables will be modified to comply with the current IBC.

 

[FixedEarth]

Think of the stem wall as a concrete beam that is fixed at the top of the footing. This "beam" can be loaded with earth pressure, groundwater, Heel surcharge, and seismic pressure, if applicable. See attached spreadsheet printout. I assumed up to 4 soil layers in mine (at very early stage now). Then you can use the earth pressure equation from excel chart and get your shear, moment, etc. by integration. You can then assign all the ACI 318 and 530 Code factors on separate cells, so Building code changes will not cause much disruption every 2 years. For external surcharges such as strip, line and point loads, get their respective resultant and its location from top of the wall and apply it to your cantilever beam worksheet as a point load. You can further develop the shear and moment capacity curves of the stem and instantly see where you are.

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.

 

[RFreund]

TX - Thanks. We really should have a copy of that publication at the office. I'm actually really surprised we don't. As for the Key, I can see both sides but I tend to lean toward the "i think it may be a bit irrational". Does the CRSI address vertical components and/or passive resistance. If so this may be what I'm looking for. Currently I have many good texts but the problem is, is that the text does not want to be code specific (which I get) but they usually say "conservatively neglect this or Conservative factor that" which is fine, but I'd like to know what is "code correct" as well. Actually I'd really like to know what is really happening, what is conservative and what is code correct...baby steps though.

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

http://www.HowToEngineer.com

 

[FixedEarth]

Anytime Kid. Your spreadsheet will also help you with the design of cantilever sheet piles, soldier beams and timber retaining walls.

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/Quimby/ce43310.htm

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/cantileverretainingwall.html

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.