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bkkk ...
Joined: 25 Mar 2009 Posts: 188 Location: Tamil Nadu

Posted: Fri Dec 31, 2010 10:09 am Post subject: Upstand beams 


Dear sefians
While designing an up stand beam, where slab joins the beam at beam bottom,can we adopt the effective depth as for a normal beam,what are the factors that are needed to be considered while designing such beams.
Er.Balaji venkateswaran 

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gpsarathyy ...
Joined: 28 Jun 2010 Posts: 483 Location: chennai

Posted: Fri Dec 31, 2010 12:30 pm Post subject: 


Dear Mr. Balaji,
I think you are referring to the inverted Tbeam.
The design is similar ordinary Tbeam, only effective width of Tee beam to be calculated, it depends on the other factors like the length of the span, width of the beam and the c/c to span of the beam.
I hope it clarifies your doubt.
Regards,
G.Parthasarathy M.E. (Structural Engg.,)
Chennai.
Email: gpsarathyy@gmail.com 

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umeshrao ...
Joined: 23 Aug 2010 Posts: 570 Location: Bangalore, India

Posted: Fri Dec 31, 2010 12:48 pm Post subject: Re: Upstand beams 


bkkk wrote:  Dear sefians
While designing an up stand beam, where slab joins the beam at beam bottom,can we adopt the effective depth as for a normal beam,what are the factors that are needed to be considered while designing such beams. Er.Balaji venkateswaran 
Dear Mr. Balaji,
Unless this beam is cantilevered, you can treat this beam as normal beam. If the beam is cantilever, then slab at bottom will be compression face then you may design as T beam.
Regards Umesh Rao 

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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 2212

Posted: Sat Jan 01, 2011 4:32 am Post subject: Upstand beams 


Upstand beam for sagging moments will be a rectangular beam
but for hogging moments will be a T Beam
Further to add that in upstand beam the hanging action of
slab loading shall be accounted by adding additional
area required for direct tension due to hanging
(though small) to the area worked for shear reinf.
best regards
vikramjeet
 ญญ
Posted via Email 

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umeshrao ...
Joined: 23 Aug 2010 Posts: 570 Location: Bangalore, India

Posted: Sat Jan 01, 2011 6:35 am Post subject: Re: Upstand beams 


[quote="vikram.jeet"]Upstand beam for sagging moments will be a rectangular beam but for hogging moments will be a T Beam. Further to add that in upstand beam the hanging action of slab loading shall be accounted by adding additional area required for direct tension due to hanging (though small) to the area worked for shear reinf.
best regards vikramjeet
Dear Vikramjeet,
Very interesting note on shear. Though we have been using this thought for water tank beams/walls, where slab is hung from beam, to restrict tension in stirrups or vertical reinforcement in the wall. But never gave a thought for slab supported at the soffit of beams.
May be,as you rightly said that this has to be considered for design of shear, it is not done for various reasons. Normally loads on slab are small, unlike water tank slab. And hence minimum shear reinforcement takes care.
Thanks for the new thought[ atleast for me.], I will check next time and revert back.
Regards Umesh Rao 

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shirishmehta3 SEFI Regulars
Joined: 21 Jun 2010 Posts: 23

Posted: Sun Jan 02, 2011 8:48 am Post subject: Re: Upstand beams 


vikram.jeet wrote:  Upstand beam for sagging moments will be a rectangular beam
but for hogging moments will be a T Beam
Further to add that in upstand beam the hanging action of
slab loading shall be accounted by adding additional
area required for direct tension due to hanging
(though small) to the area worked for shear reinf.
best regards
vikramjeet
 ญญ
Posted via Email 
Dear Sir
Can you please further clarify regarding this thought on shear??
I want to share my experience I faced once.
I was designing a flat slab structure having spans of 78m. The slab had peripheral inverted beams. I considered the following points while designing these beams for shear.
1) I calculated the shear force in the beam due to load transferred from slab and line loads due to facade. Say for a simply supported beam Vu= wl/2
Now, assuming a cold joint between the slab and the beam as it may be the case because of an upstand, I neglected the contribution of concrete in resisting shear as a weak plane is formed at the slab beam junction. Assuming the stirrups take this entire shear I got the value of Av.
2) As you said, these stirrups are subjected to direct tension in a way as the slab seems to hang from the beam. So I thought of checking whether the stirrup area Av obtained from item 1) stated above is sufficient to carry the 1way shear from the slab. ( i guessed it should b the slab shear and not the beam shear... because the slab shear acts as a uniformly distributed load on the beam which will be transferred to the stirrup by means of direct tension). But to my dismay I found that this method leads to very high shear reinforcement
I also went one step further to ensure that only the 1st outer leg of the stirrup takes this direct tension because it is not clear to me whether the crack would be intercepted by all stirrup legs or not during this transfer of shear by means of direct tension. This further led to an uneconomical design.
I am a bit lost in this method which i used as it was giving very high reinforcement. Although the transfer of forces as explained in both items above seem logically appropriate to me.
It would be indeed appreciable if anybody can comment on the approach used and point out the mistakes in my assumptions.
Regards
Shirish. 

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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 2212

Posted: Mon Jan 03, 2011 5:06 am Post subject: Upstand beams 


Dear Er Sirish,
Your problem is in two parts
i) Transfer of shear from slab to upstand beam
ii) Hanging action of slab on upstand beam
i) Transfer of shear from slab to upstand beam
(Cold joint is is rarely in practice since slab is cast with beams),
However taking to consideration of cold joint between slab and beam , the shear
between slab and beam will be transferred through the slab reinf bars through
dowel action and slab reinf entering the beam needs to be enhanced at
beam junction. Preferably Bottom bars shall be added with Ld embedment on
either side (and top bars as per original design for partial fixity w/o enhancement)
ii) Hanging action of slab on upstand beam
This would require accountal in shear reinf(i.e. stirrups) being provided in beam
Asv = area required for stirrups for spacing =S
due to SF =( w+w')L/2 where L is span of beam
w is the (self wt +any partition/facade load over beam) per m
and w' is load from slab per m
Addtional area in length of one meter from hanging action A1=[ w'/perm.stress]
perm stress in direct tension, probably this would be a small addtional since
Additional area required for spacing S cm= A1*S/100
For sake of simplicity , only working stress is used but problem can be
solved in limit design with appropriate LF and allowable stresses
best regards
vikramjeet
Dear Sir
Can you please further clarify regarding this thought on shear??
I want to share my experience I faced once.
I was designing a flat slab structure having spans of 78m. The slab had peripheral inverted beams. I considered the following points while designing these beams for shear.
1) I calculated the shear force in the beam due to load transferred from slab and line loads due to facade. Say for a simply supported beam Vu= wl/2
Now, assuming a cold joint between the slab and the beam as it may be the case because of an upstand, I neglected the contribution of concrete in resisting shear as a weak plane is formed at the slab beam junction. Assuming the stirrups take this entire shear I got the value of Av.
2) As you said, these stirrups are subjected to direct tension in a way as the slab seems to hang from the beam. So I thought of checking whether the stirrup area Av obtained from item 1) stated above is sufficient to carry the 1way shear from the slab. ( i guessed it should b the slab shear and not the beam shear... because the slab shear acts as a uniformly distributed load on the beam which will be transferred to the stirrup by means of direct tension). But to my dismay I found that this method leads to very high shear reinforcement
I also went one step further to ensure that only the 1st outer leg of the stirrup takes this direct tension because it is not clear to me whether the crack would be intercepted by all stirrup legs or not during this transfer of shear by means of direct tension. This further led to an uneconomical design.
I am a bit lost in this method which i used as it was giving very high reinforcement. Although the transfer of forces as explained in both items above seem logically appropriate to me.
It would be indeed appreciable if anybody can comment on the approach used and point out the mistakes in my assumptions.
Regards
Shirish.
 ญญ
Posted via Email 

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bkkk ...
Joined: 25 Mar 2009 Posts: 188 Location: Tamil Nadu

Posted: Mon Jan 03, 2011 7:42 am Post subject: Up stand beams reg 


Thank you Prtahsarathy sir, Umesh rao sir,vikramjeet sir and shirish sir for fruitful discussions and clarifications
Er.Balaji venkateswaran 

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umeshrao ...
Joined: 23 Aug 2010 Posts: 570 Location: Bangalore, India

Posted: Mon Jan 03, 2011 8:51 am Post subject: Re: Upstand beams 


[quote="shirishmehta3"] vikram.jeet wrote:  Upstand beam for
Dear Sir
Dear Shirish,
Can you please further clarify regarding this thought on shear??
I want to share my experience I faced once.
I was designing a flat slab structure having spans of 78m. The slab had peripheral inverted beams. I considered the following points while designing these beams for shear.
1) I calculated the shear force in the beam due to load transferred from slab and line loads due to facade. Say for a simply supported beam Vu= wl/2
[ I do hope you added equivalent shear due to torsion to vertical shear.]
Now, assuming a cold joint between the slab and the beam as it may be the case because of an upstand, I neglected the contribution of concrete in resisting shear as a weak plane is formed at the slab beam junction. Assuming the stirrups take this entire shear I got the value of Av.
[ A weak plane assumed can only be for inplane shear and hence stirrups will act as shear heads and take across shear and not as tension.This shear is moment in the beam divided by lever arm of beam. This shear will be carried by all legs of stirrups.]
2) As you said, these stirrups are subjected to direct tension in a way as the slab seems to hang from the beam. So I thought of checking whether the stirrup area Av obtained from item 1) stated above is sufficient to carry the 1way shear from the slab. ( i guessed it should b the slab shear and not the beam shear... because the slab shear acts as a uniformly distributed load on the beam which will be transferred to the stirrup by means of direct tension). But to my dismay I found that this method leads to very high shear reinforcement
[To V_{s }add tension caused by hanging of slab and then work back A_{sv} and S_{v}.]
I also went one step further to ensure that only the 1st outer leg of the stirrup takes this direct tension because it is not clear to me whether the crack would be intercepted by all stirrup legs or not during this transfer of shear by means of direct tension. This further led to an uneconomical design.
I am a bit lost in this method which i used as it was giving very high reinforcement. Although the transfer of forces as explained in both items above seem logically appropriate to me.
It would be indeed appreciable if anybody can comment on the approach used and point out the mistakes in my assumptions.
Regards
Shirish. 
Regards Umesh Rao 

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thirumalaichettiar Silver Sponsor
Joined: 26 Jan 2003 Posts: 3459

Posted: Sun Sep 17, 2017 4:57 am Post subject: 


UPSTAND BEAM
What id the EFFECTIVE FLANGE WIDTH for an up stand (inverted beam)? Is any guided value in IS 4562000? Hope that there is no such guidance in IS code 456.
Please inform.
Er.T.Rangarajan 

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