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Re: Counterfort Retaining Walls

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Gary,
 
Consider a cantilevered RCC slab. Till 5' or 6' you can manage in your nominal depths of 5" or 6" for the slab, but beyond that the slab starts getting too deep & so uneconomical (its own cost & in terms of the dead load it keeps adding on the structure as a whole). So we prefer to use beams (at appropriate spacing) to be cantilevered & let the slab span between the beams. The greater depth of the beam help in limiting deflections & taking the cantilever moments, while the slab can be designed as either spanning between the beams, or 3 sides supported or in some cases 4 sides supported (if we choose to provide an edge beam resting on the edges of the cantilevered beams)
 
The Retaining wall is similarly a cantilevered wall, which by functional requirements will be require to be 6" to 10" thick anyway, so will be good enough for around 10-12' of retaining. But for higher walls, the thickness & their foundations will become too uneconomical. So you can provide counterforts (Vertical beams) at appropriate spacing (normally 1 H to 3H depending upon the height) & reduce the wall thickness by designing as a 3 way slab. While the counterfort & its foundation can be designed as a cantilever beam for the lateral load transferred on to it by the wall. This system is certainly much more economical for heights greater than 15'.
 
Thor,
 
The counterfort will help with the foundation base dimensions also, due to the reduced cantilever moment of the wall. While the foundation of the counterfort can be locally designed for greater base dimensions. And in a nutshell, the counterfort system will certainly be more economical than a simple cantilever wall, for heights greater than 15' (It is analogous to the fact that a beam/slab or ribbed slab or a waffle slab system is more economical in comparison to a flat plate system for large span structures)
 
 
Regards
 
Pankaj Gupta
Structures Online
India
 
 
 
 
 
----- Original Message -----
Sent: Thursday, September 29, 2005 5:13 PM
Subject: RE: Counterfort Retaining Walls

What is the advantage of a counterfort wall?  I have
never designed one as a cantilever wall always seems
more economical.
Gary


On 28 Sep 2005 at 9:51, Lutz, James wrote:

> For what it's worth, for a plate subject to hydrostatic loading, free
> at the top and fixed on the sides and base, the maximum moments in
> both directions will be about equal when the width of the plate is
> about 0.7H.  This is not necessarily an optimum counterfort spacing,
> because it doesn't consider the increasing cost of counterforts as the
> spacing is reduced. The load has to be carried somehow, and
> counterforts aren't cheap.
>
>    _____ 
>
> From: S. Gordin [mailto:mailbox(--nospam--at)sgeconsulting.com]
> Sent: Wednesday, September 28, 2005 8:41 AM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: Counterfort Retaining Walls
>
>
> David,
>
> Bowles recommends counterfort spacing of 0.3 to 0.6H; H should be
> between 20 an 25 feet for the wall to be economical. Winterkorn & Fang
> recommend counterfort spacing of 0.33 to 0.5H.
>
> HTH
>
> Steve Gordin SE
> Irvine CA
>
>
> ----- Original Message -----
> From: M.  <mailto:davidfinley(--nospam--at)bizsea.rr.com> David Finley, P.E., P.A.
> To: Seaint(--nospam--at)Seaint.Org <mailto:Seaint(--nospam--at)Seaint.Org>  Sent: Wednesday,
> September 28, 2005 6:49 AM Subject: Counterfort Retaining Walls
>
> Is there a general rule of thumb for the length and spacing of
> counterforts on a counterfort retaining walls?
>
> David Finley
> M. David Finley, P.E., P.A.
> 2086 SW Main Boulevard - Suite 111
> Lake City, FL  32025
> 386-752-6400
>
>
>
>
>
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