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ZIA UD DIN

Conceal Beam

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My friends, How to decide the width of the conceal beam . Which formula is used for it ?Secondly, How to make sure that conceal beam width is sufficient for controlling deflection in it ? Thirdly, if there is any conceal beam supporting a wall on slab. As this conceal beam will only support weight of the wall or also some portion of the slab as well as described by the area distribution method for others beams( L,T Beams) ? Friends I need your replies.

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On 11/3/2016 at 4:27 AM, ZIA UD DIN said:

Thirdly, if there is any conceal beam supporting a wall on slab; As this conceal beam will only support weight of the wall or also some portion of the slab as well as described by the area distribution method for others beams

On 11/3/2016 at 4:27 AM, ZIA UD DIN said:

My friends, How to decide the width of the conceal beam

 

Some portion of slab is used by many engineers to take into the account the effect of line load on the slab. Many call it a beam, because the reinforcement arrangement resembles like a beam. But it cannot technically be called a beam; beam is a member that attracts force by virtue of its stiffness. Since, the thickness of so called beam is equal to that a slab, it cannot attract forces.

It is a simplified way of taking into the account the effect of line load on slab; by arranging reinforcement in this way, one assumes that the effect of line load will be limited within the width of so called beam. So the width of the beam will be decided by the design moment.

The beam will be only designed for the load of the supported wall, or the load of supported wall + the load transferred by the wall from upper levels. The so called beam cannot take any load from adjacent slab area, as it does not have the stiffness to attract any load. So tributary area thing is not applicable to that member.

If you understand what is written above, you should conclude that there is no need to check the deflection.

In many cases, this way of locally reinforcing the slab ( so called concealed beam) will not be able to support the load transferred from above levels, unless you increase the thickness of slab.

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If beam is stiffer than slab, the slab will bend down more and beam will bend less. In this case slab load will be transferred to beam and beam will be supporting load of wall as well as load of slab on the base of tributary area. But since CONCEALED beam is not thick enough to be stiff enough so it will not support slab.  It will just act as part of slab because it has same thickness as slab (please note that stiffness is mainly dependent on cross section and affected very little by reinforcement). If you think wall load can not be born by slab alone, instead of providing concealed beam, either go for inverted beam or convince architect for suspended beam.

 

@baz Brother I m confused whether providing reinforcement below wall in slab is beneficial by any mean (regarding strength or serviceability)?

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On 2016-11-25 at 7:06 PM, Engr Waqas said:

If beam is stiffer than slab, the slab will bend down more and beam will bend less. In this case slab load will be transferred to beam and beam will be supporting load of wall as well as load of slab on the base of tributary area. But since CONCEALED beam is not thick enough to be stiff enough so it will not support slab.  It will just act as part of slab because it has same thickness as slab (please note that stiffness is mainly dependent on cross section and affected very little by reinforcement). If you think wall load can not be born by slab alone, instead of providing concealed beam, either go for inverted beam or convince architect for suspended beam.

 

@baz Brother I m confused whether providing reinforcement below wall in slab is beneficial by any mean (regarding strength or serviceability)?

There might be some affect on deflection due to additional reinfo.

Thanks.

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On 11/25/2016 at 7:06 PM, Engr Waqas said:

I m confused whether providing reinforcement below wall in slab is beneficial by any mean (regarding strength or serviceability)?

AFAIK, provision of additional reinforcement in the slab portion below the  wall, will at least reduce the long term deflection to certain extent.

Regards.

Edited by engruzair

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    • By Engr Waqas
      Assalam o alaikum,
      Dear seniors,
      Recently i had two design a concealed beam for a case where, A slab of Lounge of dimensions 18 ft x 28 ft is to be designed. The slab is resting on walls o all 4 sides. I broke the span of slab in between and converted it into two panels of 14 ft x18 ft with a beam spaning in 18 ft direction. Slab is of 5 inch thickness and 4 inch thickness finishing. So Total debth of concealed beam i can provide is 9 inch ( 5 inch + 4 inch). Total factored load that comes out to be acting at slab area is nearly 0.22 ksf. I designed this concealed beam with dimensions 24 in width and 9 inch depth. The steel i provided is 10 # 8 US bars at bottom and 6 # 8 US bars at top of beam through out. The design philosofy i followed is that i designed for moment wl2/8 for 9 inch depth and steel required came out 8 # 8 bars. a secion of 24 x 9 in2 can take just 4 #8 bars as singly reinforced section ( i.e. p.max). I had to provided 8 # 8 bars so I provided 10 # 8 bars at bottom and 6 # 8 bars at top ( to balance extra steel provided in bottom so that compressive strength of section equally get increased as tensile strength is increased in excess of rho max) and designed ad doubly reinforced beam. The stirups i provided is #4 @ 6" c/c 2 legged...... Can any one of you confirm me this design?
       
      Thanx.