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Re: Elevated Structural Floor Slab Joint

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Dear Paul,

2 comments on your post

1) In one breath you refer to a) theoritical inflection points as being 
your preference and b) midspan being a point of being a zero shear 
point (again a theoretical point) as existing only in a perfect world. 
We can make our judgements based only on our perception of theoretical 
correct solution. So you are being as theoretical, and as dreaming of a 
perfect world as me.

2)Normally in a continuous structure the midspan moments are 50% of the 
support moments and even as less as 20-25% of the support moments in 
cases of high seismic zones, making them redundant. Which also brings 
up the issue of hinges at midspan for lateral forces, thus reinforcing 
my view that midspan is the ideal location for a construction joint.

Also to add, that in normal RCC structures, the dead loads constitute 
60-70% of the gravity loads, so even if the live loads are placed 
eccentricaly, the zero shear point will remain very near the midspan. 
Also by saying midspan I am simply trying to state that the location of 
the joint should be at zero shear. So if by my analysis my zero shear 
point will be at 0.6L, then I will place my joint at 0.6L. 

Also to address that shear is also a stress, so how can you say that 
only maximum moment location is at maximum stress?

Also to address that maximum failures in RCC structures are due to 
shear failure, and one seldomly comes across failure due to flexure, 
particularly in primarily flexure members.

So my preference is to place the joint a zero shear location rather 
than zero moment location.

Regards

Pankaj Gupta
Structures Online
India





 









----- Original Message -----
From: "Paul Feather" <pfeather(--nospam--at)SE-Solutions.net>
Date: Friday, July 11, 2003 8:36 pm
Subject: Re: Elevated Structural Floor Slab Joint

> I have to come in with Roger on this one.  If there must be a 
> joint, I
> prefer the joint at the theoretical inflection point rather than 
> mid-span.
> (Mild steel slabs).  I agree with what you are saying regarding moment
> transfer, however placing the joint at the point of maximum moment 
> placesthe joint at the location of maximum stress. A properly 
> detailed joint will
> still have the ability to transfer shear across the joint through 
> shearfriction and dowel action or interlock from keyways.  I do 
> not agree that
> only the monolithic concrete can transfer shear.  The concept of 
> zero shear
> at mid-span is only in a perfect world, and for a continuous 
> structure this
> point will vary with the envelope.
> 
> 
> Paul Feather PE, SE
> pfeather(--nospam--at)SE-Solutions.net
> www.SE-Solutions.net
> ----- Original Message ----- 
> From: "Structures Online" <3.sol(--nospam--at)spectranet.com>
> To: <seaint(--nospam--at)seaint.org>
> Sent: Thursday, July 10, 2003 10:07 AM
> Subject: Re: Elevated Structural Floor Slab Joint
> 
> 
> > Dear Roger,
> >
> > Construction joints are points of possible weakness.
> >
> > Consider  2 scenarios
> >
> > 1. In case of weakness at a section where there is moment, the 
> steel will
> > still take the tension, while concrete being in compression will 
> still be
> > able to transfer the compressive forces from one side of the 
> joint to the
> > other, so effectively the joint is still capable of transferring the
> moment.
> > Secondly the moments have a tendency to redistribute too.
> >
> > 2. In case of weakness at a section where there is shear, the 
> joint is in
> > the plane of shear failure, and in a RCC slab, there are no 
> mechanisms> present for transfer of shear from one side of the 
> joint to the other
> except
> > through the monolithicity of concrete. Also shear does not 
> redistribute.>
> > What do others think?
> >
> > Regards
> >
> > Pankaj Gupta
> > Structures Online
> > India
> >
> >
> >
> >
> > ----- Original Message -----
> > From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
> > To: <seaint(--nospam--at)seaint.org>
> > Sent: Thursday, July 10, 2003 9:02 PM
> > Subject: Re: Elevated Structural Floor Slab Joint
> >
> >
> > > I agree and disagree with Pankaj Gupta's post.
> > >
> > > I agree that construction joints should be avoided when possible.
> > >
> > > I disagree with the location of construction joints.  When 
> construction> > joints are necessary, I try to locate them at or 
> near service load
> points
> > of
> > > inflection, where both moment and shear are not maximum or 
> close to
> > maximum.
> > >
> > > A. Roger Turk, P.E.(Structural)
> > > Tucson, Aridzona
> > >
> > > Pankaj Gupta wrote:
> > >
> > > . > Construction joint should be avoided if possible.
> > >
> > > . > If not possible to avoid, then the joint should be at mid 
> span,where
> > > . > shear is zero.
> > >
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