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Controlling behaviour of structure
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bansalmunish
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PostPosted: Wed Nov 13, 2013 9:29 am    Post subject: Controlling behaviour of structure Reply with quote

Dear All,

This is an academic question, I do not want to apply it practically.

I want to know that if there is a two-way slab of size 3 m x 5 m and I design it as one way (spanning in 5 m direction and not 3 m direction), is it OK ? In this case main reinforcement shall be along 5 m span and only minimum reinforcement shall be put along 3 m span.

Will the deflections and crack width pattern will be in tune with the proposed behaviour of the slab as one way ?

Basically, i want to ask that whether the behaviour of any structure is fixed depending upon the geometry of the structure or does it behave as per the designer ?

In the same respect, we design a circular wall (of a water tank) through H2/Dt method. If we design the same wall as cantilever (limiting crack width to 0.2 mm) with minimum horizontal reinforcement, will the wall start behaving as rectangular instead of circular ? Will the actual crack widths in both vertical and horizontal direction be within the designed limit of 0.2 mm ?
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shk
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PostPosted: Wed Nov 13, 2013 12:25 pm    Post subject: Reply with quote

In my opinion, if the slab of 3m x 5m is supported along all four sides, you can't design it as one way slab. If designed as one way slab, deflections and crack width patterns will not match to that of one way slab. Basically you need to focus on structural analysis results; deflections and bending moments in the mid region and close to the supporting walls or beams and then figure out where and which direction the slab requires reinforcement. Just a note, you can not call this as one way slab unless it is supported only on two opposite edges.
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Sudhakaran
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PostPosted: Wed Nov 13, 2013 2:04 pm    Post subject: Re: Controlling behaviour of structure Reply with quote

Dear Bansalmunish,

Why not?
It will be Ok since the design will be with more thickness and steel along longer direction.But it will be least economical. You just think with a slab of size 1.5mx8m, How will be the design? in your way that is with 8m span and 1.5 m span.

A designer should also go for economy.

Regards,

Sudhakaran.




bansalmunish wrote:
Dear All,

This is an academic question, I do not want to apply it practically.

I want to know that if there is a two-way slab of size 3 m x 5 m and I design it as one way (spanning in 5 m direction and not 3 m direction), is it OK ? In this case main reinforcement shall be along 5 m span and only minimum reinforcement shall be put along 3 m span.

Will the deflections and crack width pattern will be in tune with the proposed behaviour of the slab as one way ?

Basically, i want to ask that whether the behaviour of any structure is fixed depending upon the geometry of the structure or does it behave as per the designer ?

In the same respect, we design a circular wall (of a water tank) through H2/Dt method. If we design the same wall as cantilever (limiting crack width to 0.2 mm) with minimum horizontal reinforcement, will the wall start behaving as rectangular instead of circular ? Will the actual crack widths in both vertical and horizontal direction be within the designed limit of 0.2 mm ?
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Dr. N. Subramanian
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PostPosted: Wed Nov 13, 2013 2:44 pm    Post subject: Re: Controlling behaviour of structure Reply with quote

Dear Er. Bansal,

As rightly pointed out by Shk, you can not consider it one way, and spanning in the long direction, unless there are supports only in the short direction. If it is supported on all edges, it will behave as two way slab and the yield lines will form as a two way slab. As the detailing is different in both cases, cracks will form, if you detail it as a one way slab. That is the reason that we stress understanding of the behavior of any system is more important than the actual analysis and design.

As Er Sudhakaran has pointed out it is uneconomical to design it as one way slab spanning in the long direction. The behavior is fixed based on the geometry and NOT as imagined by the designer!

What is H^2/Dt method? Note that the behavior of circular and rectangular tank walls are completely different. In circular tanks the reinforcement required May be less due to the fact that there will be hoop tension, where as in rectangular walls, bending May be predominant. Hence we need to detail as per the behavior only! Otherwise, cracks are bound to occur. The crack width will depend on several parameters.


Best wishes,
NS
bansalmunish wrote:
Dear All,

This is an academic question, I do not want to apply it practically.

I want to know that if there is a two-way slab of size 3 m x 5 m and I design it as one way (spanning in 5 m direction and not 3 m direction), is it OK ? In this case main reinforcement shall be along 5 m span and only minimum reinforcement shall be put along 3 m span.

Will the deflections and crack width pattern will be in tune with the proposed behaviour of the slab as one way ?

Basically, i want to ask that whether the behaviour of any structure is fixed depending upon the geometry of the structure or does it behave as per the designer ?

In the same respect, we design a circular wall (of a water tank) through H2/Dt method. If we design the same wall as cantilever (limiting crack width to 0.2 mm) with minimum horizontal reinforcement, will the wall start behaving as rectangular instead of circular ? Will the actual crack widths in both vertical and horizontal direction be within the designed limit of 0.2 mm ?
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Sudhakaran
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PostPosted: Thu Nov 14, 2013 1:54 am    Post subject: Re: Controlling behaviour of structure Reply with quote

Respected Dr. N Subramanian sir,

How crack forms?
Asper our theory there will  be crack...

I hope practically it will not..

For developing crack there should be some deflection. If deflection is controlled in design(sufficient steel and thickness) how it cracks?

Thank you

Sudhakaran.

Dr. N. Subramanian wrote:
Dear Er. Bansal,

As rightly pointed out by Shk, you can not consider it one way, and spanning in the long direction, unless there are supports only in the short direction. If it is supported on all edges, it will behave as two way slab and the yield lines will form as a two way slab. As the detailing is different in both cases, cracks will form, if you detail it as a one way slab. That is the reason that we stress understanding of the behavior of any system is more important than the actual analysis and design.

As Er Sudhakaran has pointed out it is uneconomical to design it as one way slab spanning in the long direction. The behavior is fixed based on the geometry and NOT as imagined by the designer!

What is H^2/Dt method? Note that the behavior of circular and rectangular tank walls are completely different. In circular tanks the reinforcement required May be less due to the fact that there will be hoop tension, where as in rectangular walls, bending May be predominant. Hence we need to detail as per the behavior only! Otherwise, cracks are bound to occur. The crack width will depend on several parameters.


Best wishes,
NS
bansalmunish wrote:
Dear All,

This is an academic question, I do not want to apply it practically.

I want to know that if there is a two-way slab of size 3 m x 5 m and I design it as one way (spanning in 5 m direction and not 3 m direction), is it OK ? In this case main reinforcement shall be along 5 m span and only minimum reinforcement shall be put along 3 m span.

Will the deflections and crack width pattern will be in tune with the proposed behaviour of the slab as one way ?

Basically, i want to ask that whether the behaviour of any structure is fixed depending upon the geometry of the structure or does it behave as per the designer ?

In the same respect, we design a circular wall (of a water tank) through H2/Dt method. If we design the same wall as cantilever (limiting crack width to 0.2 mm) with minimum horizontal reinforcement, will the wall start behaving as rectangular instead of circular ? Will the actual crack widths in both vertical and horizontal direction be within the designed limit of 0.2 mm ?
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PostPosted: Thu Nov 14, 2013 2:33 am    Post subject: Behaviour of Twoway and oneway slab Reply with quote

Practically for small spans like toilet slab, kitchen room  short span less than 3.0m, even though the slab has to be designed as Twoway, some Engineers cast oneway slab taking  care negative BM steel at support in the other longer span. But in this case, the slab is spanning in long direction with 5m , definitely nobody will execute like this, as slab thickness will be abnormally more lead to uneconomical in design. I have seen in the field some mason have cast  slab wrongly by tieing  rods long span main rod at bottom, there will be excessive deflection and leads to crack in long standing. Always slab is costly, as far as  possible thickness of slab should be minimum from practical point of view to reduce dead load on column and foundation.
V.M.RAJAN.
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Dr. N. Subramanian
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PostPosted: Thu Nov 14, 2013 2:37 am    Post subject: Re: Controlling behaviour of structure Reply with quote

Dear Er Sudhakaran,

If designing and detailing is done as one way slab, there will not be required steel in the other direction, and hence there will be cracks.

Best wishes
NS
Sudhakaran wrote:
Respected Dr. N Subramanian sir,

How crack forms?
Asper our theory there will  be crack...

I hope practically it will not..

For developing crack there should be some deflection. If deflection is controlled in design(sufficient steel and thickness) how it cracks?

Thank you

Sudhakaran.

Dr. N. Subramanian wrote:
Dear Er. Bansal,

As rightly pointed out by Shk, you can not consider it one way, and spanning in the long direction, unless there are supports only in the short direction. If it is supported on all edges, it will behave as two way slab and the yield lines will form as a two way slab. As the detailing is different in both cases, cracks will form, if you detail it as a one way slab. That is the reason that we stress understanding of the behavior of any system is more important than the actual analysis and design.

As Er Sudhakaran has pointed out it is uneconomical to design it as one way slab spanning in the long direction. The behavior is fixed based on the geometry and NOT as imagined by the designer!

What is H^2/Dt method? Note that the behavior of circular and rectangular tank walls are completely different. In circular tanks the reinforcement required May be less due to the fact that there will be hoop tension, where as in rectangular walls, bending May be predominant. Hence we need to detail as per the behavior only! Otherwise, cracks are bound to occur. The crack width will depend on several parameters.


Best wishes,
NS
bansalmunish wrote:
Dear All,

This is an academic question, I do not want to apply it practically.

I want to know that if there is a two-way slab of size 3 m x 5 m and I design it as one way (spanning in 5 m direction and not 3 m direction), is it OK ? In this case main reinforcement shall be along 5 m span and only minimum reinforcement shall be put along 3 m span.

Will the deflections and crack width pattern will be in tune with the proposed behaviour of the slab as one way ?

Basically, i want to ask that whether the behaviour of any structure is fixed depending upon the geometry of the structure or does it behave as per the designer ?

In the same respect, we design a circular wall (of a water tank) through H2/Dt method. If we design the same wall as cantilever (limiting crack width to 0.2 mm) with minimum horizontal reinforcement, will the wall start behaving as rectangular instead of circular ? Will the actual crack widths in both vertical and horizontal direction be within the designed limit of 0.2 mm ?


Last edited by Dr. N. Subramanian on Thu Nov 14, 2013 2:09 pm; edited 1 time in total
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vsmkumar
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PostPosted: Thu Nov 14, 2013 5:12 am    Post subject: Reply with quote

In one way slab we take unit width let us say 1m width and calculate BM and other structural actions as required.

We are taking some unit  width is not just for calculation, it means we are idealising the structural element (slab  in our case) by some unit width as if members of this unit width when placed side by side will deflect in unison (i.e. there is compatibility) without causing any relative displacement leading to distortion.

This is actual behavior in oneway slab which is supported along longitudinal edges(i.e. the slab can be designed as flanks of some constant width placed side by side).

Coming to current case
assume 0.6m width and 5m length planks placed side by side forming 3mx 5m floor.
when it is subjected to UDL ,it is obvious that all planks behave similarly and will hav max deflection at center.

Now if you observe end flanks along its edge parallel to support  at ends deflection will be zero and at center deflection will be maximum so it will seperate from support .
But if end edge supported on 5m edge the edge plank will be subjected to torsion and it will topple.

So if we design 3x 5 m slab as slab spanning along 5m then the edges are subjected to 2 structural action which we are neglecting and not designing.
1) torsion and
2) Compatibility at edge
and the slab will fail at designed load.
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ykalamkar
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PostPosted: Thu Nov 14, 2013 8:02 am    Post subject: Reply with quote

Dear all,
I think, for design purpose, if I am assuming some thing and designing and detailing in the same way, it should be ok.
When I am providing 3x5 m slab, supported on all edges but still designing as one way along long span, first it will behave as two way slab. But as steel in shorter direction is less than the required, it will get cracked. Thickness of crack may be less hence it may not be noticed also, but it will be cracked. Redistribution of forces would take place at this time and then it will start the behaving as oneway. The long beam would be susceptible in such case. It may deflect and help the slab to behave oneway. I think slab will not fail, but  the other elements specially adjacent slab if any and long beams supporting the slab will be more critical.
Now if we can allow the cracks in our designed structure, then we can design slab in such way otherwise not!
Experts may correct my opinion if wrong...
Yogesh Kalamkar
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vsmkumar
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PostPosted: Thu Nov 14, 2013 9:32 am    Post subject: Reply with quote

Dear Yogesh Kalamkar
,

The long beams are very stiff compared to slab, so the path of least resistance is cracking of beam along long edge instead of the long beam being deflected along with slab.

When we design the slab for given load then the crack along long edge will be beyond limit and the slab considered as failed (failure does't mean collapse of slab).

With regards,
manikumar
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