huangyhg

cantilever retaining wall or not?
ok, hopefully i can explain this correctly. i have a situation where i have a medical facility that has a 14'-0" high walk-out basement. approx. 150'x60'. cmu/cast in place(haven't decided on cmu vs. poured) on 3 sides. actually the wall only wraps with cmu for 40' on each side. the floor is precast concrete plank.
i'm trying to avoid providing a cantilever retaining wall along the back side. not that i have a problem with it but will this truely act as a cantilever retaining wall with such a rigid diaphragm? would i also need to reinforce along the inside face in this case? i'm also pressing the envelope on designing the wall as simply supported and taking my floor diaphragm load out to the side walls. would it be reasonable to design the wall being fixed at the base and pinned at the floor to ease the reaction going into the floor diaphragm?
thanks for the help
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ars001:
working on a very similiar case right now.
i designed my back basement wall as a 'propped-cantilever' wall with a fixxed based, and a pinned top. this reduced the reaction to my 2nd floor diaphragm significantly. i then designed the floor diaphraph to take the reactions out to the side walls. needed to add rebars in the floor topping ( in this case 3" composite deck with 3" of concrete fill ) to provide adequate tensile capacity to the diaphragm 'beam'.
hope this helps.
lkjh345,
thanks for the quick response. thats probably the route i'm going to go. i shouldn't have a problem transfering my diaphragm load into my shear wall. i can do this with dowels from the precast into the wall. my tension chord for my diaphragm 'beam' shouldn't be an issue since i have a continuous beam line supporting the precast at the a walkout.
how did you reinforce your back wall. being fixed on one end and pinned on another did you provide reinforcing on both faces of your wall?
yes. i am using reinforcing on both faces. due to some architectural precast panels coming part way down the wall, and the need to provide a ledge for them to sit on, my wall is 14" thick at the base (8" thick above the precast ledge), so, in my case, the reinforcing in not all that bad.
if you pin the top of the retaining wall, you're just designing a wall as you would any other with lateral soil pressure on it. when you go to a supported wall instead of a cantilever your diaphragm at the top of the wall has to take the reaction of the wall. if you use a cantilever retaining wall then youhave no reaction and hence no imposed force on the diaphragm. you would, though, have to design the diaphragm and lateral system to take the imposed deformation of the top of the wall.
i'm not sure how pinning the top reduces the reactions at the diaphragm, as lkjh mentioned, since without the top support you have no top reaction hence no imposed force.
"i'm not sure how pinning the top reduces the reactions at the diaphragm, as lkjh mentioned, since without the top support you have no top reaction hence no imposed force"
i took it to mean reduced relative to a wall pinned top and bottom.
apsix:
correct. if you fix the base of the wall, the reaction at the top of the wall (to the 2nd floor diaphragm) will be less than if you use a wall pinned at the top and bottom.
in my case, i have a long, fairly shallow diaphragm to work with becasue of a two story entry lobby in the front of the building, so i am trying to reduce the load on the diaphragm as much as possible.
i misread it then, thanks.
it is hard geotechnically to justify that the base is truly fixed. soil has to yield a bit before it takes up load, and this can reduce the effectiveness of the cantilever.
in this type of situation i would usually treat it as pinned top and bottom in order to get the top reaction (truth is it is somewhere in between).
even if you assumed pinned top and bottom, it is often of benefit to put some cantilever action into the base so that the shores might be removed earlier to facilitate construction.
dont forget that if you prop the top of the retaining wall then you must use 'at rest' soil pressure (ko) as opposed to the 'active' soil pressure (ka) that you use for a cantilever wall.
i agree with csd72 in that it's hard to justify a truly fixed base. it's well known that a cantilevered retaining wall rotates and has lateral deflection at the top, as shown in crsi.
i think the best solution for situations like this (questions about the wall interacting with diaphragm & other basement walls acting like shearwalls, etc.) is to design and detail the wall as a cantilever.
assuming it's backfilled before the slab's connected, it will have already deflected so won't load the diaphragm.
if it's backfilled after the slab is in place, then you know it's ok because the cantilevered wall would've handled the load by itself. surely it's ok with the diaphragm & other basement walls helping (adding them could make it fail somehow?!). think lower bound theorem like used for steel connections. these are not brittle limit states, so the same theory applies.
another advantage is that you don't really have to worry about things like when the slabs are in place like you do for a pinned-pinned basement wall. for example, i've had basement walls that were backfilled before the basement slab-on-grade was placed & cured. probably really lucky not to have a sliding bottom of the wall. there's also no issue of bracing the top of the wall, construction sequence, etc.
i've done this probably a half-dozen times and haven't had any trouble. it was the most common practice at my old firm.
oh, i forgot one thing. when i do what i propose in my previous post, i reinforce the inside face also because there might be tension on that face depending on when the framed slab is placed.

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