pinned base in a concrete wall - is it really achievable?
i have a somewhat theoretical question regarding concrete walls as they are a reasonably popular topic of discussion on this forum.
i, as many others, often design concrete foundation walls as pinned-pinned when there is a first floor diaphragm to tie into. i understand people have been doing this forever and i am not questioning the practice. my question is this: is a pinned base really able to be achieved in any typical detailing that anyone does?
i am thinking that any rebar tying the wall to the footing is providing some moment capacity. i understand the pinned-pinned case is the worst case for the wall, so that is being conservative is you get some end moment capacity out of the foundation to footing connection. assuming a pinned base is unconservative for the footing since there will be some moment at the base. even if the steel yields and a "plastic hinge" forms at the base it doesn't lose its moment capacity.
also, there must be some steel at the interface of the footing/wall since they are usually seperate pours and the steel is need for shear.
any opinions on this would be greatly appreciated.
the term "yielding" that you have used is associated with large rotation at that loaction, which dissipates the energy. on top of that the amount of reinforcement that are usually provided at the joints is quite small than the amount that will require the joint to act as fixed. in other words the small amount of moment that may generate due to the placement of the minimum reinforcements at the joint should not change the design assumptions (pinned-pinned).
shin-
i understand that a large rotation is required. i also understand that this is not overly relevant for the wall since the most conservative case for the wall is pinned-pinned (assuming that you are placing the rebar in the center of the wall).
that being said, the rebar you provide at teh interface will provide some moment capacity. why wouldn't you take that phimn at ultimate and dump that into the footing? i realize that this phimn will likely be smaller than that for the wall at midspan, but is a moment nonetheless and should probably be accounted for.
i have never put any numbers to this, so it is possible that it is very small and negligible, but given the way moments affect footing design, shouldn't it at least be checked?
just thinking out loud here, but if you have
#6@12" for the wall reinforcing (not unreasonably small) and
#5@18" for the dowels (not unreasonably high), phimn at the base is about half of that for the wall section. that is not insignificant in my estimation.
also, given that the wall will act as fixed at the base until the reinforcement yields, it will attract moment. is that a fair statement?
i think you're missing a big source of flexibility at the wall base: the footing.
i've never modeled it, but i suspect that the footing will rotate to relieve most of the moment in most cases. it would be pretty easy to test this. create a model of the wall and footing in your favorite program. use the subgrade modulus to determine spring stiffnesses to model the soil below the footing. run it and see how much moment is generated at the base.
granted, there are exceptions such as pile supported grade beam as the base or a strip footing on solid rock.
211828
or a foundation wall to a rigid mat. zero rotation of mat.
connect2, i agree. i'm sure there are lots of examples.
most walls are supported on narrow strip footings supported on plain ole soil. those are the cases i was typing about.
271828 and connect2-
please excuse me if i am missing something that is staring me in the face.
if you have a narrow strip footing on plain soil and the footing rotates, how do you automatically assume pinned? the rotation of the footing may cause the bearing pressure to be exceeded, correct? the rotation is caused by the moment at the base of the wall, correct? i am not understanding why that amount of moment can be just neglected.
if you have a rigid mat this is less of an issue because the moment thrown in by one section of wall footing will likely be much less significant with respect to the capacity of the footing than for an isolated strip noted above. that being said, the lack of rotation of the mat doesn't mean zero moment being transferred from the footing to the mat, does it?
all i am saying is this. we obviously have our choice when we begin the design process of designing as a fixed base or not. if we arbitrarily decide to design as not fixed does not necessarily mean it will behave that way. i am trying to understand how you neglect the moment that will be transferred. as i stated in the second post, the base connection will have some moment capacity, that is not debateable in my opinion unless someone can impart something to me that i haven't learned yet (i know there is a ton i haven't and i am always looking to learn, so please pass the wisdom along!). that moment capacity does not go away when the rebar yields and the walls gains curvature (a plastic hinge forms). that plastic hinge maintains that moment capacity and i do not see how it does not get transferred into the footing.
"if you have a narrow strip footing on plain soil and the footing rotates, how do you automatically assume pinned? "
design simplification. because the moment will probably be small and, like you typed earlier, it's on the safe side to assume it's zero.
"the rotation of the footing may cause the bearing pressure to be exceeded, correct?"
seems unlikely to me because the rotation angle will be extremely small.
"the rotation is caused by the moment at the base of the wall, correct?"
not really. imagine the end of a simply supported beam. there's no moment there, but it certainly has some rotation due to curvature along the beam's length.
""the rotation is caused by the moment at the base of the wall, correct?"
not really. imagine the end of a simply supported beam. there's no moment there, but it certainly has some rotation due to curvature along the beam's length."
the simply supported beam has no end restraint, a wall with reinforcement does have end restraint.
""if you have a narrow strip footing on plain soil and the footing rotates, how do you automatically assume pinned? "
design simplification. because the moment will probably be small and, like you typed earlier, it's on the safe side to assume it's zero."
it is only conservative for the design of the wall. it is unconservative for the design of the footing.
- i understand there is a lot of design simplification involved in this. what i am trying to understand is whether someone has run any numbers at any point to determine if it is actually negligible. maybe i should make the question a little more clear, also. while i am talking about the wall/footing assembly, i am more concerned with the moment at the base of the wall going into the footing, not the wall design.
i've never heard of anybody running such a number. maybe you can be the first and report back the result!
structuraleit
need to really hone in on soil spring constants. if the soil deflects enough to create an active pressure on the main vertical span of the wall, it will certainly deflect enough below the basement slab where the passive pressure exists.
if your wall has enough moment capacity to span vertically 10' or 12' to resist the active pressure moment, it will likely have enough reinforcement carried through to the footing to resist the passive pressure moment.
btw, footing failure is relative to the amount of deflection one can sustain. the building isn't going to crack in half if you exceed the bearing capacity on the toe of a strip footing.
2009-09-15, 12:34 PM
pinned base in a concrete wall - is it really achievable
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