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Location of Construction joints in continuous RCC Slab.
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sidhudaljeet
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PostPosted: Wed Mar 19, 2008 5:20 pm    Post subject: Location of Construction joints in continuous RCC Slab. Reply with quote

Dear Friends,It is general practice to Locate the construction joints in continuous RCC slabs at L/4 from the support. That is tentative point of contra-flexure with uniform loaded slab. But one of my senior colleague is advocating the theory that the construction joint should be located where Shear force is Zero that is middle of slab. As per the M.N. Bussell there is hardly any effect of construction joint on strength of member if the joint is constructed with fairly good precautions.You r Valuable opinion please.Daljeet Sidhu

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thirumalaichettiar
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PostPosted: Wed Mar 19, 2008 6:51 pm    Post subject: Location of Construction joints in continuous RCC Slab. Reply with quote

Yes, the senior engineers point in providing the construction joint for slab and beasms at the point where the shear force is zero is absolutely correct.

sidhudaljeet[AT]yah... wrote:
Dear Friends,It is general practice to Locate the construction joints in continuous RCC slabs at L/4 from the support.  That is tentative point of contra-flexure with uniform loaded slab.  But one of my senior colleague is advocating the theory that the construction joint should be located where Shear force is Zero  that is middle of  slab. As per the M.N. Bussell there is hardly any effect of construction joint on strength of member if the joint is constructed with fairly good precautions.You r Valuable opinion please.Daljeet Sidhu

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With Beat regards
T.Rangarajan

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sahilmujawar
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PostPosted: Wed Jul 04, 2012 3:06 am    Post subject: Reply with quote

Why is it so?What is concept ?

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Sahil Mujawar
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kishorpss
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PostPosted: Wed Jul 04, 2012 5:53 am    Post subject: Reply with quote

T.Rangarajan Sir,

The point of Zero Shear is the point of Maximum Bending moment. Pl clarify theory behind the point of construction joints.

Brgds
KISHORE
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B.V.Harsoda
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PostPosted: Wed Jul 04, 2012 3:49 pm    Post subject: Reply with quote

This topic is already discussed  at below link:-

http://www.sefindia.org/forum/viewtopic.php?t=11574
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badamsundararao
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PostPosted: Wed Jul 04, 2012 4:24 pm    Post subject: CONSTRUCTION JOINT IN SLABS Reply with quote

Dear friend,

      To my knowledge the construction joint should be at a place where the bending moment becomes zero ie., at the Point of Contraflexure and  in continuous slabs it will be at 0.30 l to 0.33l. This is to my memory .

Badam Sundara Rao.
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bijay sarkar
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PostPosted: Wed Jul 04, 2012 5:59 pm    Post subject: Reply with quote

I am trying to explain. Other opinions may be there.

For Concrete :
Theory says, it is to be at middle. because, shear acts in vertical plane of the section whereas moment is a combination of two equal & opposite horizontal forces.

At middle, shear is about nill but moment is maximum. We design concrete section as cracked in tension and steel bars take the whole tension. By keeping a construction joint at mid, design concept does not suffer. Hence by theory, we want a weak vertical plane at the middle, whenever required.

But theory is full of assumptions. Concrete is not a perfect homogeneous material and also, may not obey the man made other assumptions. Force reversal may also happen.

Thereby, human psychology further works to put this weaker joint at a suitable location where combination of shear & moment are minimum and the joint is normally provided at 1/3 rd distance from support.

regards

bijay sarkar
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thirumalaichettiar
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PostPosted: Thu Jul 05, 2012 12:15 am    Post subject: Reply with quote

Er.Kishore,

You can browse the link below for more explanations apart from other's explanation.


ttp://www.sefindia.org/forum/viewtopic.php?p=49140#49140

I have put this important explantion with references under the  Experts Dedicated forum so that this can easily be viewed at any time.

T.RangaRajan.
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vikram.jeet
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PostPosted: Thu Jul 05, 2012 5:50 am    Post subject: Location of Construction joints in continuous RCC Slab. Reply with quote

Agreegate interlocking is main concern at CJ and section is somewhat weak  
in respect to aggregate locking at CJ .Regarding bending moment at CJ, there
seems no problem since compression side(above NA) bear against each  
other whereas tension is taken by reinforcement fully bonded on both sides.
Main concern is regarding shear transfer which rely on aggregate locking
(apart from dowel action by reinf) . Hence it is preferred to have CJ at location
of practically Zero Shear . However in practicality , CJ is attempted at  
location of less shear as well as less BM.

In a continuous slab , the position of max/min BM and max/min shear are as under - -
Support
Max -ve moment - - - at support edge edge
Max shear force - - -at support edge edge
Hence CJ is NEVER desirable at support edge

Mid span
Max +ve moment - - - at around mid span  
Zero shear force - - -at around mid span  
Hence CJ can be provided but since it is max span mo location , it  
is generally discouraged there though SF is zero/min .

1/3rd to 1/4th span
Bending moment - - -is about zero(pt of contra-f) if continuous and about 75% if SS
Shear force - - -about Half
CJ is generally provided as a compromise location with less SF and less BM  

best regds

vikramjeet




T.Rangarajan Sir, The point of Zero Shear is the point of Maximum Bending moment. Pl clarify theory behind the point of construction joints.BrgdsKISHORE
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kishorpss
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PostPosted: Sat Jul 07, 2012 7:04 am    Post subject: Reply with quote

Vikramjit Sir,
Thanks for your posting. Since last 25 years after starting carrier as a structural designer as well as a construction engineer I was following the principle for providing of CJ at a compromise location with less SF and less BM i.e. 1/3rd to 1/4th distance from support.

With regards

KISHORE
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