CRSI Cant. Retaining Wall

[PEinVA]

I'm designing a retaining wall for a new loading dock.

I have to minimize my heel to a maximum of 1'-6". I have regular loading on one side and a surcharge on the other.

I'm running through the CRSI design guide for the code that applies to this job (CRSI 2002 Guide, Ch. 14)

I think I follow it pretty well, but the one check that is confusing me, is where we check the final summation of moments.

it states the Factored load moment for the heel must be equal or larger than the difference between factoreed moments of stem, key and toe: Muh>= Mus+Muk-Mut.

I get a large negative value for this. If I take the absolute value it is greater then Muh. However, I'm confused on why I'm not getting equal numbers.

Any insights?

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[msquared48]

The wall is unstable - you need a longer toe.

If the wall is too high with too high a load and the the limiting heel length, you may have to use another form of retention. You may overstress the soil too if the heel is not long enough.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[PEinVA]

Mike,

Thakns. I pass all serviceablitity checks and my safety factor against overturning is > 2.

My bearing stress in my serviceability check are only 600 psf to 200 psf.

It is only a 5' wall.

Typical retaining wall -
heel is below backspan soil
toe is below lower elevation.

Is that correct? I just want to make sure we're discussing the correct things.

I've never designed a retaining wall before, but its hard to believe this isn't working if I pass everything else. If this moment is negative (Moment from the stem - moment from the toe) doesn't that mean I actually have active resisting pressure against the soil?

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[rowingengineer]

have you got your sign convention right? hard to check numbers over the net

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[paddingtongreen]

Being an old dinosaur, questions like this, that indicate that someone is using a formula as a black box instead of finding out why it is the way it is, make me cringe.

It's a simple statement that the sum of the moments must equal zero or the thing starts to rotate. If the stem moment is clockwise, so is the key, that total must be opposed by an equal anti-clockwise moment consisting of the toe and the heel.

Draw a free body diagram of the intersection, with moment directions and magnitudes shown, it should become clear.

A minor complexity is introduced if the key is placed somewhere other than directly under the stem.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[PEinVA]

Yes, I actually set up a spreadsheet, and my numbers match within decimals of the example I ran through. It's just my design is not typical and is actually opposite what you normally see, with my longer side of the footing on the low side of the wall.

Again, my stability checks are fine,(overturning 2.3) and bearing is no where near allowable bearing capacity (20%)

I think I'm ending up with a negative because my toe moment is greater than my stem moment and I am subtracting the toe moment from the stem moment, this is from the reaction from the soil up pressure. So as long as I design my steel in my toe for that moment, I should be ok.





RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[PEinVA]

paddingtongreen,

I understand sum of moments, and I appreciate your input, but what you're saying is just really repeating my question, in a way that seems to make you appear "older and wiser" than someone asking this question.

I state in my first post that I am trying to figure out why my summation of moments is off. Not what is the summation of moments. I agree that equations shouldn't be taken as is and this is why I'm asking the question, and not just putting the steel I got in the other 90% of the beginning of my problem.

Please contribute to the thread instead of trashing.

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[paddingtongreen]

RCraine, sorry, I misinterpreted what you wrote because it was ambiguous. Could you post a sketch? I am having a great deal of difficulty seeing a net uplift on the heel. Thinking in terms of action and reaction, the action is the retained soil, the reaction is the pressure under the toe, so it should not be greater than the action.

Second thought, if the toe was very long, the resultant of the horizontal and vertical loads might come down on the heel side of the center line of the footing.....that might do it. It would mean that the bearing pressure was greatest under the heel and least at the extremity of the toe. It really needs a sketch.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[PEinVA]

I agree a sketch would have been helpful.

Any thoughts are appreciated.



RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[paddingtongreen]

Two things:
The top of the heel is in tension and should have top reinforcing.
Don't be insulted by this, but are you accounting for the overburden above the toe when you calculate the toe moment? the toe has soil pressure above as well as below.

I usually take a diagram like yours and plot the centroids of each load, plot the resultant horizontal and vertical positions and and from where those vectors cross, draw the diagonal resultant to find where it crosses the concrete/soil interface. from that, I calculate the bearing pressure, and then I calculate the moments. I prefer to do that because it provides a picture, you can see stuff happening.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[msquared48]

I don't know what the equations you have say, but my information tells me that the heel and toe moments counteract the overturning moment from the stem.

The sum of the toe and heel (resisting) moments should be a minimum of 1.5X the overturning moment.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[PEinVA]

I do have that. I guess I'm forgetting that from my stability check the overturning ratio is > 2.0, which would mean I would end up with my resisting moments greater then my overturning. The fact that it is negative, just tells me that I have enough in the heel alone to resist the overturning moment.

I just wanted to confirm that.

Thanks and sorry if I sounded like a d!ck.

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[PEinVA]

My calcs show that based on my dimensions I have almost even pressure under the footing. Very low only ~800psf (i've changed the dims a bit from what is shown)so even with any pressure from above, it is well below bearing limits.

It just through me off that it was a negative value, but I guess that is due to the odd shape of this wall, and not having a lot of experience with cant. retaining walls.

I did the diagram when it didn't make sense, but I just wanted to run it by the forum because I don't have much experience with these as stated above, thanks.



RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[paddingtongreen]

The shape isn't that odd. But do you agree that the top of the heel is the tension side? I ask because I don't know what sign convention you are using, so I don't have a context for "negative".

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[PEinVA]

Yes, but barely. I do agree that the heel has greater pressure from above, then below. Therefore it is a cant. for supporting the weight (backspan soil basically) above it.

Well for me, the shape is odd, because of my limited exposure to these types of walls. Its reversed for me so its made me extra cautious.

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[doka1]

RCraine, try to visualize the failure mode for the footing. Then you would realize that it needs the top reinforcement. Typical cant. retaining walls to retaing 4' as in a loading dock are usually 4 feet wide, and have a 2' heal, 1' thick wall, and 1' toe.

While we are on the subject, how crucial do people feel about the dowels going underneath the continuous running bars in the footing? Usually the dowels create the transverse bars in the bottom mat, should these bars always be below the running bars?

 

[SpecialEddie]

Hi RCraine,

I quickly breezed through the previous responses, and it sounds like you have everything covered. I just want to point out a couple of things I noticed from your sketch. I do understand that this is a rough sketch and your original post was focused on trying to get the summation of moments about the stem to work, so you may be aware of what I am going to say but left them out for the sake of clarity. If that is the case I apologize, but I will point them out anyway because you will not believe the outrageous things I have seen in my 25 years of practice (which I know is nothing compared to most who post regularly here).

1. You will need top steel on the heel side of your wall to counteract the induced clockwise moment acting on the heel. Even if you run the numbers and the moment is so small that no steel is required or the induced stress in the tension face is less than the product of the rupture modulus and the section modulus, it would be good practice to include it.

2. You probably need to hook the steel at the bottom of the footing on the heel side because 24 inches (12"stem + 15"heel -3" clear) is not enough distance to develop a #7 bar or larger. A #6 bar requires about 20 inches, and a #5 about 17 or 18 inches. I think it would be prudent to hook those bars regardless of size.

3. For a 12 inch concrete wall, you should have 2 curtains of rebar. You should have a curtain of T&S steel on the exposed face of the wall.

Once again, I'm sure you are aware of this so apologies if you simply did not sketch it for clarity or brevity. Just trying to help.

 

[SpecialEddie]

SEISMIC DESIGN...

I forgot to mention, just in case you have not accounted for it. IBC requires seismic design of cantilevered earth retaining walls. IBC 03 Section 1622.1.2 requires seismic analysis of retaining walls. You will need to find k_h (use Mononobe-Okabe equations, or better yet have the geotech engineer provide relevant information). In my area, we assume that k_h = S_ds / 2.5. IBC 06 (ASCE7-05, 15.6.1) requires seismic analysis for retaining walls in Seismic Design Categories D,E, or F. I'm not sure about IBC 09 -- we are not that advanced in my area.

Once again, I realize your original post was regarding summation of moments. But since you also mention in a subsequent post that you've never designed a retaining wall before, I just thought I would mention it.

I know that many building department jurisdictions are ignoring this requirement and feel that the 1.5 safety factors are sufficient, so you might want to clarify with your local jurisdiction.

Good luck.

 

[TXStructural]

As for bar location, the critical feature is that the dowels be sufficiently anchored, and in a shallow footing, the way to develop the bars is to have them hook around transverse reinforcing. Your sketch misses the diagonal dowels (at about 45 degree angle from bottom of heel to front face of wall, crossing the wall/footing interface at the same cover at the steel in the sketch. [Refer to Figure 14-1 (2008 ed.)]

If you are using T&S in the exposed face, CRSI Design Handbook Figure 14-3 (2008 ed.) shows 2/3 of horizontal steel on the exposed face, 1/3 on the retained face. Horizontal reinforcement ratio should be at least 0.002. Vertical on the exposed face #4@18" for walls less than 14 ft tall. As a rule, all bars not more than 18" from adjacent bars in the same face, same direction.

For reference only - assuming soil at 115 pcf, angle of internal friction of 35 degrees, and sliding coeff of 0.55.
Using the CRSI Retaining Wall program f'c=3 ksi, fy=60 ksi. Overall base/footing width of 6 ft, 1.5 ft heel.
This yields an overturning factor of 9, sliding factor about 3.5
Your reinforcing would be something on the order of #4@9" for verts, dowels, diagonal dowels, bottom as you show, AND top in the base. 8#4 longitudinal in the base, #5@12" long. in the wall.