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Controlling behaviour of structure
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akos
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PostPosted: Thu Nov 14, 2013 11:55 am    Post subject: Re: Controlling behaviour of structure Reply with quote

If you are planning to design it as one way slab, the main reinforcement must be put in shorter dimension as the load path always will take shortest path. Putting min reinforcement in other direction may not be so correct as  it will have higher moments at support compare to one way slab.

Akos



Sudhakaran wrote:
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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shenoy
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PostPosted: Fri Nov 15, 2013 1:34 am    Post subject: Re: Controlling behaviour of structure Reply with quote

Dear Mr. Sudhakaran,

I totally agree with Dr. NS.

There hase been always a myth that slabs be designed as one way or two way based on the dimentions.

One way or two way is decided by the support system, not by the dimensions.

If You Provide  Reinforcement in only One direction and no reinforcement in the other direction, it will try to transfer the load only in the direction of reinforcement.

There will be cracks , all along , at the other supports because it is just PCC, Theoretically. That is why, even in slabs supported on all edges but  designed as oneway  ,  minimum reinforcement should be provided across the other supports too, especially  if the slab is continuous over these supports too.

If the deflection is prevented anywhere, better provide reinforcement.

IF there is NO SUPPORT, there need not be reinforcement across that direction. This is purely One way.

Hence a slab 8 m X 1m may have to be designed oneway across 8 m span if it is supported only at the short edges.

It is not the economy.

Shenoy.
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PostPosted: Fri Nov 15, 2013 2:09 am    Post subject: Re: Controlling behaviour of structure Reply with quote

Dear Shenoy sir,
My question is..
How crack forms, if there is no deflection? (Not real case)

Thanks,

Sudhakaran.

shenoy wrote:
Dear Mr. Sudhakaran,

I totally agree with Dr. NS.

There hase been always a myth that slabs be designed as one way or two way based on the dimentions.

One way or two way is decided by the support system, not by the dimensions.

If You Provide  Reinforcement in only One direction and no reinforcement in the other direction, it will try to transfer the load only in the direction of reinforcement.

There will be cracks , all along , at the other supports because it is just PCC, Theoretically. That is why, even in slabs supported on all edges but  designed as oneway  ,  minimum reinforcement should be provided across the other supports too, especially  if the slab is continuous over these supports too.

If the deflection is prevented anywhere, better provide reinforcement.

IF there is NO SUPPORT, there need not be reinforcement across that direction. This is purely One way.

Hence a slab 8 m X 1m may have to be designed oneway across 8 m span if it is supported only at the short edges.

It is not the economy.

Shenoy.
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Dr. N. Subramanian
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PostPosted: Fri Nov 15, 2013 4:14 am    Post subject: Re: Controlling behaviour of structure Reply with quote

How do you say that there will not be any deflection?

Best wishes,
NS


Sudhakaran wrote:
Dear Shenoy sir,
My question is..
How crack forms, if there is no deflection? (Not real case)

Thanks,

Sudhakaran.

shenoy wrote:
Dear Mr. Sudhakaran,

I totally agree with Dr. NS.

There hase been always a myth that slabs be designed as one way or two way based on the dimentions.

One way or two way is decided by the support system, not by the dimensions.

If You Provide  Reinforcement in only One direction and no reinforcement in the other direction, it will try to transfer the load only in the direction of reinforcement.

There will be cracks , all along , at the other supports because it is just PCC, Theoretically. That is why, even in slabs supported on all edges but  designed as oneway  ,  minimum reinforcement should be provided across the other supports too, especially  if the slab is continuous over these supports too.

If the deflection is prevented anywhere, better provide reinforcement.

IF there is NO SUPPORT, there need not be reinforcement across that direction. This is purely One way.

Hence a slab 8 m X 1m may have to be designed oneway across 8 m span if it is supported only at the short edges.

It is not the economy.

Shenoy.
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PostPosted: Fri Nov 15, 2013 6:57 am    Post subject: Re: Controlling behaviour of structure Reply with quote

Dear sir,

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

In academic point, the deflection can be reduced by increasing stiffness. if sufficient stiffness deflection will be zero or very less.

If design the slab for longer direction as one way the moment and the deflection will be very large used for design, Imagine that the design is carried for 0 deflection along longer direction neglecting the longer supports for spanning in longer direction. Practically it may not be possible.

Assume that the deflection asper the above design is 0, when it is supported on shorter supports to span along longer span.

In this situation if we introduce the other support ie, longer and allow to span both way. Then how the slab deflects?

Sir, Please correct me if my thinks are wrong.

Thank you,

Sudhakaran.


Last edited by Sudhakaran on Fri Nov 15, 2013 7:36 am; edited 4 times in total
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vsmkumar
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PostPosted: Fri Nov 15, 2013 7:10 am    Post subject: Reply with quote

Dear Bansal munish,

To explain the behaviour of 2-way slab i used fixed width planks as analogy .
Similarly
for circular tanks if you want to design the wall as cantilever fixed at base(Bottom of circular wall) then the behavior of circular tank can be explained by analogy of   fixed width planks fixed at bottom (base).

assume that the circular wall of tank made with 1m wide .3mm deep and 5m length(here it is height) planks(RCC) placed side by side all around periphery.
These planks are designed for hydrostatic load acting on them and vertical reinforce ment placed to counter the BM (catilever) at base.

Then you can easily visualise that as the planks deflect radially outwards a gap will be created between adjacent planks which is maximum at top.

When we cast the circular wall with out any vertical joint then it resists the gap (which was described above ) formation leading to hoop tension.

So hoop tension is inevitable and you must design for it and provide circular(horizontal )reinforcement.

The inference is that primary design for circular tanks is by H2/DT  method to account for hoop tension.
And vertical reinforcement must be provided to make joint with base slab.

With regards,
Manikumar
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vsmkumar
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PostPosted: Fri Nov 15, 2013 10:13 am    Post subject: Reply with quote

Dear Sudhakaran,

you cannot  make deflection at center of long span zero.

What you can do is reduce deflection at center by providing more reinforcement.
If want to reduce deflection further by providing more reinforcement  you won't achieve desired results, for that you need to increase depth.

once you increase depth of slab , self weight increases and deflection increases.

So practically and theoretically it is not possible to have zero or "structurally  negligible" deflection in our case.

You may think that by some means you very achieved very small deflection at center of long span which is adjacent to long edge, but that small deflection induces compatibility moment at long edge which we ignored in design and leads to cracking.

With regards,

Manikumar
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shk
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PostPosted: Sat Nov 16, 2013 7:16 am    Post subject: Reply with quote

Dear Mr. Sudhakaran,

Let's say we have done some magic and achieved zero deflections along the longer span. In that case, whether the slab is supported along the longer edge or not, it doesn't make any difference to the slab as the slab is already stable sitting on supports along the shorter edges. Then why do we call that a "support" along the longer edge and how can we say the slab is spanning in two directions? I wonder, if you are able to design the slab with zero deflections then whether the slab requires any main reinforcement at all?

Shekhar
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spsvasan
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PostPosted: Sat Nov 16, 2013 10:22 am    Post subject: Re: Controlling behaviour of structure Reply with quote

Dear Er.Bansalmunish

To a certain extent the structure forgives us our follies and tries to adjust itself to our whims and fancies. As far as the equilibrium conditions (Sigma V = 0; Sigma H = 0; and Sigma M = 0) are satisfied and if the strength of each section exceeds the force at that section, the structure adjusts itself to our assumptions and survives.

This may be the reason why the columns designed only for axial load survive (until an earthquake strikes!).

Similarly, the 3m x 5m slab designed as a oneway slab of 5m span and provided with main rods in the 5m direction may be able to carry its full load, even though it may crack badly. In my opinion, the slab will first try to transmit the load along the short direction, will crack and then transmit the load along the long direction.

Regards
S.P.Srinivasan



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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sknsrinivasan
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PostPosted: Sat Nov 16, 2013 12:05 pm    Post subject: Reply with quote

Dear all
  I  would like to mention one example here. Consider 10mX2mX0.02m  (thickness is very less) Glass or plywood. How we will hold it, If we hold it in shorter side what will happen? or If we hold it longer span what will happen?
  If we hold it in short side how the Glass or plywood will Bend?  Inthis case slab will also bend like that. Better to create two support along the long side  unless otherwise we have to design the slab as cantilever slab

Thanks
N.Srinivasan
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