Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
(OP)
In honor of a fantastic previous thread: Link
So, I have modeled this condition. See attached RISA-3D file. So, I believe it shows that the footing WILL transfer moment to the soil, and quite easily. So, I will go back to the original question lion asked, and say, can we live with that, or should we adjust the design methods?
Backdrop:
1. Pin-pin wall conditions for RESTRAINED retaining walls is conservative for the WALL rebar. No need to argue this.
2. Pin-pin wall conditions for RESTRAINED retaining walls is NOT conservative for the footings. The additional moment transfers to the footing and increases soil pressure.
3. Many in the previous thread assumed that soil spring stiffness, etc. was not that significant compared to stiffness of the footing/wall.
4. Respectfully, I challenge that assumption and present the attached model as evidence.
5. Intuitively, this also makes sense because to me, the stiffness of soil compared to a footing is still quite large.
6. I open the floor for debate as to what methods to use. It makes a HUGE difference in footing size, which means cost, and it may not have an advantage.
With that said, please no, "WELL, We've always done it that way and there haven't been problems" arguments. It's not that I don't care about that argument, it's just that I want to understand the behavior, not justify the design based on empirical experience (even though it has value).
Thanks to all and I hope for some great discussion.
ALSO, know that RISA-Foundation (Josh Plummer please feel free to comment!), does this method. IE, they transfer footing from the base into the soil and design the footing accordingly, leading to larger footings. Albeit, it doesn't model the soil springs, just assumes the moment transfers. The 3D model that is attached is a plate model has been created to simulate the soil springs.
So, I have modeled this condition. See attached RISA-3D file. So, I believe it shows that the footing WILL transfer moment to the soil, and quite easily. So, I will go back to the original question lion asked, and say, can we live with that, or should we adjust the design methods?
Backdrop:
1. Pin-pin wall conditions for RESTRAINED retaining walls is conservative for the WALL rebar. No need to argue this.
2. Pin-pin wall conditions for RESTRAINED retaining walls is NOT conservative for the footings. The additional moment transfers to the footing and increases soil pressure.
3. Many in the previous thread assumed that soil spring stiffness, etc. was not that significant compared to stiffness of the footing/wall.
4. Respectfully, I challenge that assumption and present the attached model as evidence.
5. Intuitively, this also makes sense because to me, the stiffness of soil compared to a footing is still quite large.
6. I open the floor for debate as to what methods to use. It makes a HUGE difference in footing size, which means cost, and it may not have an advantage.
With that said, please no, "WELL, We've always done it that way and there haven't been problems" arguments. It's not that I don't care about that argument, it's just that I want to understand the behavior, not justify the design based on empirical experience (even though it has value).
Thanks to all and I hope for some great discussion.
ALSO, know that RISA-Foundation (Josh Plummer please feel free to comment!), does this method. IE, they transfer footing from the base into the soil and design the footing accordingly, leading to larger footings. Albeit, it doesn't model the soil springs, just assumes the moment transfers. The 3D model that is attached is a plate model has been created to simulate the soil springs.






RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
Caveat - in my mind the first case may create serviceability issues, perhaps large soil movement, or large crack widths at the interface between wall and footing. I like the idea of considering it pin-fix. Then again I do industrial work so what do I know
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
1) Soil failure? Nope. For the very common case where bearing capacity is governed by settlement, I would argue that the addition of moment to the soil/footing interface would not really alter the average settlement strain beneath the wall.
2) Bending failure in the strip footing? Sure but who cares? I'd expect the footing to yield flexurally and then start to behave like the pin that we were hoping for in the first place.
3) Shear failure in the strip footing? Yeah, you might see some overload relative to the pin-pin case so that's probably legit.
4) Anchorage failure in the strip footing when the wall rebar tries to pull out under moment? That's probably legit too and a bit scary as that could compromise your flexural depth in the footing. One would hope that the axial force in the wall would prevent the moment in the joint from ever inducing significant tension. That's hard to guarantee of course.
I would submit that the modelling assumptions that led to the detailing below (not mine) probably represent a scarier issue.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
Now that is a detail for the ages right there...If there was a such thing as a "structural ambulance chaser" lawyer, they should take stock in the company producing details like that...
Anyways, I think I'll likely split the difference by "allowing" a higher soil bearing pressure to develop based on the fixed end moments and designing the wall rebar for Pin-pin. This will help the footing during construction too when they start to backfill before the slab is placed.
I still think it's an interesting problem. I, like you (KootK), agree that the error in neglecting the moment transfer into the footing is potentially scary.
As an aside, and I don't mean this negative, but seriously is there an engineer on this site that could look at that retaining wall detail for more than 5 seconds and not question it? I am open to being the crazy one.
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
I agree that the section is unusual, but there may be reasons for some of the variations from what we consider as the norm. Perhaps a bit of discussion of specifics is in order.
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
I am no RISA expert, used it once, and still relatively new to the engineering field, but I had a few questions about your modeling.
It looks like you have a 4.23" thick footing, with a rigid element that is 6" long that is attached to to a piece of concrete 0.1" thick. You place the soil spring restraints on this 0.1" thick piece of concrete. Why do you not place the soil spring directly on the 4.23" thick footing? Were trying to capture some sort of soil behavior? If I were to model this I would have placed the spring directly against the 4.23" shell element.
Also, I noticed inconsistencies in your soil spring stiffness in certain spots. N754 has a stiffness of 1.563k/in while the typical stiffness is see is 1.823k/in at N893 for example. Was there a reason differing spring stiffness on the interior of the shell elements?
The edge springs seem to make sense with the value either being 1/4 or 1/2 of the total spring stiffness mentioned above.
Thanks,
S&T
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
After all, do we not rely on the presence of the slab to resist sliding, with the presence of this force creating a counter-moment potential?
Mike McCann, PE, SE (WA)
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
Different tributary areas result in different spring stiffness values.
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
Please don't. After all we've been through, I feel as though I now have a vested interest in the success of your fledgling firm. I'd hate to see you reduce your competitiveness unnecessarily. I very much consider this to be one of those "do as the Romans do" situations given that, if you ignore the footing moment:
1) No code official will call you out on it.
2) No peer reviewer will call you out on it.
3) It'll be a bit cheaper to built.
4) It may be a bit cheaper to design.
5) I know you don't want to hear it but the very extensive history of success with the traditional method alleviates all practical concern in my opinion.
I didn't share that one detail because it was technically flawed. If anything, other than the turn of the hooks, it probably has more technical merit than what is typically done. The odd detailing results from considering the shear induced by the back-stay effect assuming both the footing and the SOG to be points of wall lateral restraint. It's pretty tough to argue with that really, especially for deep basements with shoring. And it will inevitably make for a wall shear problem and a demand for reinforcement. And you should never add that reinforcement if you hope to keep that sign outside your door illuminated.
For what it's worth, don't think that FEM has any place in routine foundation design. Other than, perhaps, outputting reactions and then designing the foundations just as we would by hand.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
I caught that aspect, but node N754 and node N893 have the same tributary area. What explains the difference?
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
@kootK. As always, I appreciate the challenge. And it appears I misunderstood your OP, my apologies. With that said, it's clear that not all of the romans use that method anymore, IE RISA as one such example. I'm not saying that their method is 100% correct, but I could make a good argument it better captures the actual behavior of the wall/footing. So, with that said, do you (or anyone) know of any published references that cite your method of ignoring the lateral soil moment for the footing design? (I hope there are some because I do like that method, if I can find a way to justify it other than "as the romans do")
I don't want to over-design things. But, I also don't want to ignore frieze-blocking in wood structures just because over 99% of all wood buildings in my area don't have it. I see it as necessary, and I could make the argument for wall footings too.
Also in regards to that detail you posted:
1. Either the wall or the footing appear overdesigned or underdesigned, respectively.
2. Say you are relying on the SOG for sliding. OK, well that EJ material is going to get nice and squished and push any sealant out. That's all fine, and typically I ignore that effect, but this wall looks to be holding back quite a bit of load, more so than a typical basement retaining wall.
3. The hooks are turned the wrong way. This is a two-step problem. #1, it's an obvious mistake that appears not to have been found if that image is from a complete design. #2, if this easy-to-see mistake wasn't found, it begs the question, what BIG mistakes were overlooked? This is akin to when you talk to another engineer that uses the word "cement" instead of "concrete". If I hear "that cement footing" in a conversation with them, I don't need to hear another word, they aren't a qualified engineer. This isn't as bad, but that detail has blatant issues, the hooks being an obvious one, which would lead me to distrust everything else.
4. The detail shows confined rebar (the pier) in the wall, and yet the footing is that small?
Again, I'm not seeing the whole picture, but those are just a few musings based on what I can see.
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
Thanks for expanding on the questions you had about the section. Not my section, but I will make some guesses.
1. I would say the wall is designed primarily for bending. The footing in this case is probably just a starter for the wall forms, and I am guessing it is founded in competent rock.
2. I don't use joint filler in these situations, just turn up the plastic against the wall to provide some slippage parallel to the wall.
3. The hooks should turn in. Drafting error, should have been caught in check. I agree that errors like this send up red flags.
4. I didn't take those ties to be confinement reinforcement for a monolithic pier, but rather shear reinforcement for a wall of constant thickness. Maybe the detail 4 would clarify it. Footing, see 1 above.
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
- A crack opening on the soil side between the footing and wall. This is a serviceability issue.
- Exceeding the allowable bearing capacity of the soil at the toe of the footing. There are 2 thoughts here:
1. The potential for soil punching shear failure - not an issue since any local punching shear failure of the soil will release the moment imposed on the footing and redistribute the bearing stresses. Further, any minor rotation will actually increase the bearing area of the footing (3D element).
2. Local (lateral) soil shear failure - I would look to dismiss any thoughts on soil shear failure through the retained side since this shear plane, due to depth, should far exceed what was considered by the Geotech and acts directly against the retained soil. For the toe side shear, any slipping would relieve some of the soil stress at the toe and consequently relieve the moment imposed on the stem wall.
For simple basement type walls that are single story, I have no qualms with modeling the loads with a full pin-pin type analysis. I argue the crack problem away by detailing proper drainage at the base of the wall. Usually though, the retaining wall is not a simple single story retaining wall and I find myself using whatever methods I can to reduce or eliminate the significant lateral soil load from the structure's upper diaphragm(s). This includes looking at quasi-cantilevered to basement type options.
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
No way, no how am I willing to consider a software package "one of the Romans". Software developers are generally limited to presenting methods that are on the conservative / un-aggressive side. And that, for obvious liability reasons. Just look at the debacle we've gotten ourselves into with those "consider inflection points as bracing" check boxes.
Not so sure. I'd say that the lateral restraint / back-stay effect that informed the wonky detail that I posted is an even more plausible "accurate behavior" story. And if you try to pull that shear reinforcing business on any non-skyscraper building, you can expect a swift beating from your clients and contractors.
I'll try to sift through some of my home references over the weekend. I suspect that most references don't explicitly deal with this because the situation is thought simple enough that the design of the wall and the footing can be considered independently. For now, I'll propose this:
1) See the blurb from ACI314 below. It's not quite explicit in endorsing the pin-pin method but, in my opinion, is headed in the same direction.
2) Detailing varies but the most common version in practice seems to be interior face wall reinforcing and matching, interior face footing dowels. Exactly the opposite of what you'd do if you were wanting to transfer moment through the joint. I take that as evidence that the majority of Romans are doing pin-pin.
3) You rarely see top steel in the strip footings as you might expect if you were planning on transferring moment to the footing (non-eccentric footing). Again, I take this as an indicator of what the Romans have been up to.
Regarding your four questions on the wonky detail:
1) Hokie's response is correct.
2) It's less about whether you can rely on the SOG to take load but, rather, whether you can rely on the SOG NOT to take load when it would be undesirable for it to do so. Here, if the SOG takes load, then you set up the back-stay effect that I mentioned and create a monster shear demand between the SOG and the footing.
3) Since the wall is not designed to transfer moment to the footing, I don't see the hook direction as a serious mechanics problem. I just don't like it because it appears that there isn't physical space for the hooks and they're not shown at the bottom of the footing as I'd expect. Interestingly, this detail appears on a drawing stamped by one of the most famous living structural engineers out there. Seriously, like Fazlur Khan famous. You've read some of her papers and drooled over some of her projects, I guarantee it. Granted, the detail was surely created by one of her lackeys.
4) Hokie's response is correct.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
any response to:
I am genuinely intrested as to why you chose to model this in the manner that you did.
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
maybe Josh Plummer, RISA guy on this forum, can comment on why they do it that way?
@Sticks - reason for the 0.1" thick concrete is to model the thickness of the footing and transfer the horizontal load out at the BASE of the footing instead of the CENTROID of the footing. The reason it's thin is because I didn't want it to impact the vertical reactions much. Model likely isn't perfect, but I tried to get something accurate.
The reason for the 4.23" thickness was taking into account cracked section properties.
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
Whoa now cowbowy... I didn't slam RISA or their product in any way, shape, or form. I simply expressed my opinion that the methods presented in software and the methods used by practicing engineers are not always the same and, thus, I don't consider any software package to represent "a Roman" in the sense that we've been discussing it here. And I'd be quite surprised if Josh doesn't agree with me in this regard. Software automates calculation, it doesn't apply engineering judgement. Or at least it shouldn't. What we're talking about here is, decidedly, judgment in my opinion.
What are you trying to do to me? Get me kicked off of the RISA Christmas card mailing list?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
I don't want to be convinced of anything other than best practices. Facts are stubborn things. It just happens that art isn't, and there is an art to structural engineering. I just want to challenge the norm of Pin-pin for a minute to make sure it's an acceptable method. I could argue it either way. Nothing in this thread has convinced me that pin-pin is the best method, and nothing has convinced me that it's not acceptable. I just did a model that appears to indicate that the ACTUAL behavior is likely closer to pin-fixed. Everyone else has suggested things that intuitively make sense, like "the soil will give some and release moment", etc. but my model appears to indicate otherwise.
As for Falzur Kahn, I don't know of them, but I may have seen their work without knowing who they are. I do know that that detail is not impressive to me. I also know that a good and thorough engineer should have caught that detail. I would also note that any half decent architect can cover up a multitude of structural errors with some nice finishes. I say that to say that just because it's a "nice building" doesn't mean it's designed correctly.
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
I still feel as though the point that I was trying to make with the wonky detail is slipping under the radar. I'll take another stab at it:
If there's a slab on grade and it's not specifically designed to NOT provide lateral restraint to the wall, then pin-fixed is not much closer to the actual behavior than pin-pin. The actual behavior may well be closer to one of the models shown below. And that would lead to back-stay shear that you'd need to reinforce for. So, in summary, my point is that the "actual behavior" path may lead you:
1) somewhere pretty undesirable in terms of your competitiveness;
2) somewhere not necessarily in line with the RISA results anyhow and;
3) somewhere where you won't find many Romans around to keep you company.
In my opinion, the RISA results are just one alternative among several, and not just pin-pin.
Are you interested in any in-print stuff that suggests pin-pin? I'm happy to go digging in my collection but I don't want to expend the effort if you're only interested in the theoretical aspect of this. Even if I can find published examples of pin-pin, I very much doubt that they'll go into the kind of theoretical detail that we've been discussing here.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
I'll start out by saying that I have no opinion on the "right" way to model basement walls. All connections are somewhere between pinned and fixed. But, we will typically use our engineering judgment to say that it's one or the other. So, as long at the foundation details back up your assumptions / judgment you should be fine.
As to reason why the RISAFoundation retaining walls work the way do, I can only point towards a natural "evolution" of the program. When we first added retaining walls they were purely stand-alone cantilever walls. Our users liked them, but then immediately requested that we add on option to pin them at the top where they might connect to a floor slab or diaphragm or such. So, we added the pinned top - bottom fixed option. The natural next step from a pure cantilever wall.
Now, we've gotten some requests for pinned top - pinned bottom walls. That doesn't seem like an unreasonable option to me. It's not the type of structure I've worked on before. But, the industrial world (where I spent most of my design life) is often different from commercial and residential buildings.
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
@Canwest: try this in your head (or in SAP) and let me know if it doesn't check out:
1) Start with a 10' beam fixed at the left and pinned at the right.
2) Apply a uniform load of any intensity.
3) Instead of conventional fixity at the left end, extend the beam 6" to the left and provide another pinned support there. All three supports are now pinned.
4) Review your moment and shear diagrams. The original span diagrams should remain essentially unchanged. Shear should skyrocket over the 6" extension.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
Sure, but don't spend more than 5 mins on it. A suggestion of a good foundation reference book would likely suffice. The references I have to date have sufficed to date, but the current project I'm working on requires a bit more involvement.
Thanks to all
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
Haha, no big surprise there. If anything, this thread has convinced me that we don't really know the true behavior, but be consistent in your design assumptions.
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
To an extent, you can think of an analogous cantilever having a cant span matching the distance to the inflection point in the real span. You'd need the fictitious cantilever distance to be several multiples of the backspan before you'd see critical shears in the back span. In your example that ratio appears to be only about two.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
FWIW, the first example had a 5:1 ratio. I've bimped it to 50:1, and the shear is now around 23 kips for the same load. As the pin approaches the fixed support this value will eventually reach the ~27 kip that we saw when the pin wasn't there.
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
Fazlur Khan <> Firm. Fazlur Khan = Man. A structural engineer not being familiar with Khan is a bit like a lumberjack not being familiar with Bunyan. If you're a reader, I'd recommend picking up his autobiography, written by his daughter who is also a structural engineer (Link). It's probably the most inspiring trade publication that I've ever read and is sprinkled with all manner of technical goodies. For anyone even remotely interested in structural engineering, it reads like blueberry waffles eat: fast and joyously.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Retaining Wall Footings (For the Fourth!) - Fixed or Pinned Base