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Controlling behaviour of structure
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Sudhakaran
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PostPosted: Sat Nov 16, 2013 3:29 pm    Post subject: Reply with quote

Respected NS sir and Sefians,
<o> </o>
I am accepting all the comments… Thanks.
<o> </o>
Now I am raising  an oppositequestion.
<o> </o>
A slab of 2m X 5m has to be designed. As shown in figure.
<o> </o>
Long sides are supported on beams.
Short sides are supported on walls.
<o> </o>
<if>1)      <endif>Generallywe will design the slab purely as one way slab for 2m span(+support width).
<if>2)      <endif>Thenwe will design the beams..

Our assumption is the load from the slab is equallytranmitted to the long beams(to wall nil)
<o> </o>
I think all of you are designed these independently  (monolithic behavier is not considered).
<o> </o>
While construction these both (slab & beam) will becaste monolithic.
<o> </o>
Assumingthe beam deflects by 20mm. Then what will happen.<o></o>
<o> </o>
Theslab will also deflect by 20mm or near value(since it is monolithic).
<o> </o>
If any statement is wrong please correct.
<o> </o>
In this condition, My question is.
<o> </o>
<if>1)      <endif>Isany load comes over wall?
<if>2)      <endif>Ifcrack formed in the slab, Which will be the direction of crack x or y?

<o> </o>
<o>Thanks,</o>
<o>
</o>
<o>Sudhakaran.</o>
<o> </o>

<o> </o>

<o> </o>



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Dr. N. Subramanian
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Joined: 21 Feb 2008
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Location: Gaithersburg, MD, U.S.A.

PostPosted: Mon Nov 18, 2013 6:37 pm    Post subject: Reply with quote

Dear Er Sudhakaran,

I am enclosing a figure to show the difference in load distribution between one-way and two-way slabs.

The deflection in the case of one-way slabs will look like a potion of cylinder where as in the case of two-way slabs it may look like a shallow saucer.

I like the reply of my friend Er. S.P. Srinivasan

Best wishes,
NS
Sudhakaran wrote:
Respected NS sir and Sefians,
<o> </o>
I am accepting all the comments… Thanks.
<o> </o>
Now I am raising  an oppositequestion.
<o> </o>
A slab of 2m X 5m has to be designed. As shown in figure.
<o> </o>
Long sides are supported on beams.
Short sides are supported on walls.
<o> </o>
<if>1)      <endif>Generallywe will design the slab purely as one way slab for 2m span(+support width).
<if>2)      <endif>Thenwe will design the beams..

Our assumption is the load from the slab is equallytranmitted to the long beams(to wall nil)
<o> </o>
I think all of you are designed these independently  (monolithic behavier is not considered).
<o> </o>
While construction these both (slab & beam) will becaste monolithic.
<o> </o>
Assumingthe beam deflects by 20mm. Then what will happen.<o></o>
<o> </o>
Theslab will also deflect by 20mm or near value(since it is monolithic).
<o> </o>
If any statement is wrong please correct.
<o> </o>
In this condition, My question is.
<o> </o>
<if>1)      <endif>Isany load comes over wall?
<if>2)      <endif>Ifcrack formed in the slab, Which will be the direction of crack x or y?

<o> </o>
<o>Thanks,</o>
<o>
</o>
<o>Sudhakaran.</o>
<o> </o>

<o> </o>

<o> </o>



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Fig 10.1 One way & Two-way slabs.tif
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bansalmunish
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Joined: 08 Oct 2011
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PostPosted: Wed Nov 20, 2013 12:12 am    Post subject: Reply with quote

Dear All,

I have read all the comments by eminent persons in this forum and most of them have said that the behaviour of the structure has to be taken according to the geometry of the structure and it cannot be controlled by the designer.

Also, since I had pointed out earlier, this is an academic discussion only, I do not want to apply it practically, so we shall not discuss about economy here.

In my view, the designer should be able to control the behaviour of the structure irrespective of its geometry as per my below explanation :-

(i) When we are designing 3 m x 5 m slab as one way in 5 m direction, obviously the thickness and reinforcement in 5 m direction shall be sufficient to control the bending moments and deflection. Then practically, why will the slab fail ? The slab shall first try to behave as two way but when it will find less reinforcement than required in short direction, then it will crack (i dont know what will be the crack width) in short direction and start behaving as one way in long direction.


(ii) Similarly, when we are designing a circular tank with wall as cantilever, obviously the thickness and vertical reinforcement shall be sufficient for a cantilever wall. In a cantilever wall the horizontal reinforcement is minimum but in a circular wall horizontal rings have to be designed for hoop tension. But when we shall design the circular wall as cantilever and put minimum horizontal reinforcement (rings), then the structure will first try to resist the water pressure through hoop tension. But when it will find that the reinforcement in horizontal direction is less, then it will crack (i  dont know what will be the crack width) in horizontal direction and the start behaving vertically, where sufficient reinforcement and thickness is available.



In both the cases above, although i do not know the crack width, but I think it will be within permissible limits. Please correct me if I am wrong in my above explanation.
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vsmkumar
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PostPosted: Fri Nov 22, 2013 11:03 am    Post subject: Reply with quote

Dear Bansalmunishji,


1)You said

"the designer should be able to control the behaviour of the structure irrespective of its geometry  
"


Science is nothing but discovery of laws of nature, Structural engineering is not an exception.

If you load slender column with l/d > 60 it will buckle no engineer can prevent it,
geometry is law of nature.

similarly if length/width >3 it behaves as one way slab no engineer can prevent it ,it is law of nature.


2) You asked why the slab or tank will fail,
  please note that here failure does't mean collapse of structure.
  here failure means the response parameters of the structure going beyond acceptable limits.
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Dr. N. Subramanian
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PostPosted: Fri Nov 22, 2013 3:57 pm    Post subject: Reply with quote

Dear All,

I like this posting by Er Manikumar. As expressed by someone earlier there should be a like button as in Facebook. There is only a button for Thankful for the topic.

Best wishes,
NS
manikumar wrote:
Dear Bansalmunishji,


1)You said

"the designer should be able to control the behaviour of the structure irrespective of its geometry  
"


Science is nothing but discovery of laws of nature, Structural engineering is not an exception.

If you load slender column with l/d > 60 it will buckle no engineer can prevent it,
geometry is law of nature.

similarly if length/width >3 it behaves as one way slab no engineer can prevent it ,it is law of nature.


2) You asked why the slab or tank will fail,
  please note that here failure does't mean collapse of structure.
  here failure means the response parameters of the structure going beyond acceptable limits.
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Sudhakaran
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Joined: 16 Jul 2013
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PostPosted: Sun Nov 24, 2013 7:02 am    Post subject: Reply with quote

Dear all,

I agree with manikumar and Subramanian sir,

But this type query will arise in different situations-Controlling behavior.

In case of circular water tank, we design the side wall for hoop tension and cantilever action. Why these are doing, If the design for hoop tension is OK, how cantilever action?



Similarly in Buttress or counter fort retaining wall, slab  designed for one way slab and cantilever action,
If design for one way slab is Ok, How cantilever action?

In these cases,

If the cantilever action takes place, It will be prevented by the One way slab design or hoop design, if the design are safe.



Regards,

Sudhakaran.

  



manikumar wrote:
Dear Bansalmunishji,


1)You said

"the designer should be able to control the behaviour of the structure irrespective of its geometry  
"


Science is nothing but discovery of laws of nature, Structural engineering is not an exception.

If you load slender column with l/d > 60 it will buckle no engineer can prevent it,
geometry is law of nature.

similarly if length/width >3 it behaves as one way slab no engineer can prevent it ,it is law of nature.


2) You asked why the slab or tank will fail,
  please note that here failure does't mean collapse of structure.
  here failure means the response parameters of the structure going beyond acceptable limits.
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vsmkumar
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PostPosted: Sun Nov 24, 2013 9:55 am    Post subject: Reply with quote

Respected Dr. N. Subramanian sir,
Thank you for your encouraging words of appreciation .
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vsmkumar
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PostPosted: Sun Nov 24, 2013 10:17 am    Post subject: Reply with quote

Dear Sudhakaran ji,

The cylindrical wall of water tank must be attached to base slab for stability and water tight ness.

Here attaching means fixing the cylindrical wall with base.
But the maximum hoop tension in cylindrical wall occurs at the bottom which leads to maximum radial deflection, but by providing fixity we are making the radial deflection zero.

In addition to deflection we are making curvature( in vertical plane oriented radially) also zero because of fixity(the base slab generally designed to have sufficient rigidity to exert fixity on cylindrical wall).

So here we are making two physically measurable quantities at the joint zero. They are
1.Radial deflection
2.curvature
for this to happen the base slab must exert corresponding forces on the cylindrical wall
They are
1. Horizontal shear force exerted by base slab on the wall acting radially.
2. Moment exerted by base slab on the cylindrical wall acting about circular joint in clock wise direction if we see along clock wise direction (wrt top view).


So we need to provide vertical reinforcement at joint to resist moment and shear as described above.



The same explanation can also be used to describe counter fort wall joint.

with regards,

Manikumar
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Sudhakaran
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PostPosted: Sun Nov 24, 2013 3:41 pm    Post subject: Reply with quote

Dear Manikumar sir,

Thank you, explanation are very helpful for understanding..

In water tank the bottom joint is necessary.

Why the same joint is in retaining wall..

Regards,

Sudhakaran.


manikumar wrote:
Dear Sudhakaran ji,

The cylindrical wall of water tank must be attached to base slab for stability and water tight ness.

Here attaching means fixing the cylindrical wall with base.
But the maximum hoop tension in cylindrical wall occurs at the bottom which leads to maximum radial deflection, but by providing fixity we are making the radial deflection zero.

In addition to deflection we are making curvature( in vertical plane oriented radially) also zero because of fixity(the base slab generally designed to have sufficient rigidity to exert fixity on cylindrical wall).

So here we are making two physically measurable quantities at the joint zero. They are
1.Radial deflection
2.curvature
for this to happen the base slab must exert corresponding forces on the cylindrical wall
They are
1. Horizontal shear force exerted by base slab on the wall acting radially.
2. Moment exerted by base slab on the cylindrical wall acting about circular joint in clock wise direction if we see along clock wise direction (wrt top view).


So we need to provide vertical reinforcement at joint to resist moment and shear as described above.



The same explanation can also be used to describe counter fort wall joint.

with regards,

Manikumar
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vsmkumar
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PostPosted: Mon Nov 25, 2013 7:04 am    Post subject: Reply with quote

Dear Sudhakaranji,

From the structural point of view: The fixity at base alters Geometry of vertical wall between counterforts, as a result the vertical wall behaviour changes from one way to two way action which is more economical.

From serviceability point of view: if we dont provide fixity, then water and mud passes  to other side creating dirty area.
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