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Camber
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vikram.jeet
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PostPosted: Fri Jun 08, 2012 6:47 am    Post subject: Camber Reply with quote

Camber :

Providing Camber in beams is an age old construction practice to negate
deflection of beam. It is applicable to RCC beams as well as steel lattice girders
i.e. truss type. However providing camber to steel beams is difficult.

Camber is attempted to negate DL deflections.

Provision of camber may induce little horizontal reactive force at supports
but generally it is negligible and not accounted in calcs in routine designs


best regds

vikramjeet



Respected Members I have a doubt related to camber, so kindly through some light on this. 1- Can we use camber to Bye pass the deflection limit of IS:456. 2- Deflection which we have to compare with the given limit in IS 456 should be from where? Initial CIS position or CAMBERED position ( Refer Attached figure). 3- If we study language of SP -24 (handbook of IS 456) and IS 1343 it is some how diplomatic . As per my understanding, I conclude something in attached figure related to RCC and prestress structure. Kindly comment on this.4. From my side rule of RCC beam will also be applicable to Steel Girders.With Regards Jitendra Sharma
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P.K.Mallick
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PostPosted: Sat Jun 09, 2012 9:03 am    Post subject: Reply with quote

There are stipulations and guidelines in various handbook to limit the camber so that horizontal thrust at the support is minimal.
CPWD Specifications 2009 says the following regarding Camber:

Suitable camber shall be provided in horizontal members of structure,especially in cantilever span to counter act the effect of deflection.The form work shall be so assembled as to provide for camber.The camber for beams and slab shall be 4mm per meter (1 to 250) or as directed by the Engineer-in-charge ,so as to offset the subsequent deflection. For cantilevers the camber at free end shall be 1/50th of projected length or as directed by the Engineer-in-charge.

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Jeet_mbm
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PostPosted: Tue Jun 12, 2012 5:38 pm    Post subject: Camber in Beams Reply with quote

Respected Sirs;

     Sorry to say but still I not find answer of my basis question. Sir I appreciate you observation that provision of camber in RCC beam is older practice but I want to know basic concept.
Sir I request you to have a re-look on my basis post :: in which I summarized my understanding in form of Diagram

http://www.sefindia.org/forum/viewtopic.php?p=48220#48220

In simple word my doubt is ::


Say structural deflection is = 100 mm
       Codal permissible limit = 60 mm (say)
       provided Camber         = 70 mm


Now one can say Deflection is = 100-70 = 30 < 60 (codal requirement)
  So Hence codal requirement of deflection is fulfilled....!!!!


From my side this solution is not acceptable. What your views ????


With Regards


Jitendra Sharma
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Dr. N. Subramanian
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PostPosted: Tue Jun 12, 2012 7:00 pm    Post subject: Re: Camber in Beams Reply with quote

Dear Er Jeet,

Camber is provided so that deflection will not be seen. It is not an antidote to deflection!

Best wishes
NS
Jeet_mbm wrote:
Respected Sirs;

     Sorry to say but still I not find answer of my basis question. Sir I appreciate you observation that provision of camber in RCC beam is older practice but I want to know basic concept.
Sir I request you to have a re-look on my basis post :: in which I summarized my understanding in form of Diagram

http://www.sefindia.org/forum/viewtopic.php?p=48220#48220

In simple word my doubt is ::


Say structural deflection is = 100 mm
       Codal permissible limit = 60 mm (say)
       provided Camber         = 70 mm


Now one can say Deflection is = 100-70 = 30 < 60 (codal requirement)
  So Hence codal requirement of deflection is fulfilled....!!!!


From my side this solution is not acceptable. What your views ????


With Regards


Jitendra Sharma
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Jeet_mbm
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PostPosted: Wed Jun 13, 2012 11:29 am    Post subject: Re Camber in Beam Reply with quote

Thank you Dr. N Subramanian Sir, for giving valuable guideline.


With Regards

Jitendra
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kunalkansara
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PostPosted: Wed Jun 13, 2012 2:19 pm    Post subject: Reply with quote

Dear Mr Jitendra Sharma

As Dr Subramanian and Mr Vikramjeet have pointed out, the camber is provided to nullify primarily a small fraction of the total maximum deflection, not the entire or a sizeable fraction of the total deflection. Self-weight, for example, is permanent and relatively more confidently known type of load. Therefore, the deflection under it also can be predicted relatively more confidently. If we can nullify this deflection through a camber, it will give better serviceability conditions and appearance for the users and for the service-lines. However, if you try extending this practical concept to the superimposed live loads, for example, you might end up with trouble since these loads vary differently and so do the deflections due to these loads. So if you provide a camber under the extreme value of such loads (that too factored), your camber will be actually more deflected than the deflection due to the actual load, leaving your floor with a residual upward deflection!! So you just canít have (100 Ė 70) leaving 30 mm residual deflection which is lesser than the permissible limit of 60 mm. I hope you are able to visualise the implication of the point that I am trying to put across. Also, having excessive camber in any case would potentially magnify the secondary effects which are generally ignored and absorbed within the safety pockets. That is the reason behind why there are upper-bound restrictions are set on the camber.

I hope this will be of your use.

Best wishes,

Kunal Kansara
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bijay sarkar
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PostPosted: Wed Jun 13, 2012 3:20 pm    Post subject: Reply with quote

Yes...i still have not come across such cambering provided in RCC structures.

These are provided in steel structures from some serviceability point of view such as in truss of roof to maintain a predefined clear height at all time over cranes in industrial type structures.


regards,

bijay sarkar
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P.K.Mallick
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PostPosted: Wed Jun 13, 2012 5:57 pm    Post subject: Reply with quote

The clause no 23.2 of IS:456-2000 is about Control of Deflection.It states that the deflection of a structure or part thereof shall not adversely affect the appearance or efficiency of the structure or finishes or partitions.The deflection shall generally limited to the following:
a) The final deflection due to all ---------------------------------------span/250.
b) The deflection including ------------------------------20 mm whichever is less.

If the first condition of total deflection is not satisfied ,it can be made to satisfy by provision of initial upward deflection or camber.

However ,if second condition is not satisfied ,appropriate measures are required to be adopted to reduce deflection.

In other words only short term deflection is controlled by camber.Long term deflection is not controlled by camber because camber will leave a upward curve in the horizontal member which will take long time to get compensated by deflection due to creep and shrinkage.

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Jeet_mbm
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PostPosted: Thu Jun 14, 2012 8:29 am    Post subject: Re Camber in Beam Reply with quote

Respected Members

        I am totally agreed with you all. But I had a discussion with one of structural designer, who is advocating about codal language and when I looked into this matter in detail, I found following statements.

As per Code:

Expanatory handbookSP-24-(1978) :
22.2 a - The final deflection (including the effects of temperature,creep and shrinkage) measured from the as-cast level of floor and roof........... should not normally exceed span/250. This limitation is based on crack limitation with which the code is very much concerned and to avoid psychological upsetting ofthe occupants or affect the appearance of structure.

IS-456:2000 Cl 23.2 andIS-1343-1980 Cl-19.3.1:
a) --The final deflection due to all load including theeffects of temperature, creep and shrinkage and measured from the as-cast level of the supports of floor and roof........... should not normally exceed span/250.


My Submission:
(Assuming the case in which camber is within permissible maxlimit for camber, and value is not too much)

In case of camber in RCC member ĖDue to cambering there is no stresses get developed (as  initial casting of shutters is in cambered profile) and in service condition as load act the structural deflection would be same as of un-cambered beam. Shown in figure -1 attached. So widening of crack would same as case of un-cambered beam

In case of Prestress beam camber is provided somehow due to external axial compressive force.  So when load act ----for the part of deflection equal to camber there would be no crack(in this range initial stress compressive stress get nullify) .  The deflection in excess to camber (deflection-camber) would only be responsible for crack width. Thatís why I am saying the permissible limit should be compared with net deflection (Deflection-Camber) in pre-stressing case.

            Our distinguished SEFI member Mr Mallick Ji also put a very important point on this about --- comparing the two limitations of code with proper consideration of duration of load which is responsible for deflection.  Which is very relevant but I failed to get this statement in anywhere. Thank Mallick Ji for sharing this.

At last I once again need a nod from all respected members on my and Mallick jiís conclusion.


With Regards

Jitendra Sharma



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P.K.Mallick
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PostPosted: Thu Jun 14, 2012 2:07 pm    Post subject: Reply with quote

From the upward curve level (due to camber)when the beam comes down to perfectly horizontal portion,no cracks develop on the bottom fibre of the beam rather bottom gets subjected to compression due to unyielding support. This compressive force helps in controlling the crack when the beam further deflect from horizontal position to downward.


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