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Clarification concerning IS 13920 : 2016
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hsrai
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Joined: 26 Jan 2003
Posts: 155
Location: Ludhiana, Punjab, India

PostPosted: Sun Apr 30, 2017 4:15 am    Post subject: Reply with quote

B.V.Harsoda wrote:
Refer Original Draft code by GSDMA at below link
http://www.iitk.ac.in/nicee/IITK-GSDMA/EQ11.pdf

In GSDMA  draft Cl. 9  is for shear walls.
hsrai wrote:
On page 7, clause 7.1.2 states that columns having aspect ratio < .4, are required to be designed as per clause 9, however clause 9 is for Joints.

That implies that columns having aspect ratio < .4 or .45 needed to be designed as per clauses of 10 (for Special Shear Wall).

However, clause 10 are applicable to aspect ration <= 0.25.

That mean a column of size 300 x 800 mm (aspect ration of 0.375) is supposed to be designed by clause 10, which will be applicable if ist size is 300 x 1200 mm.

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hsrai
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Joined: 26 Jan 2003
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PostPosted: Sun Apr 30, 2017 9:46 am    Post subject: Reply with quote

Quote:
Clause 7.2 entitled "Relative Strengths of Beams and Columns at a Joint"'s sub-clause 7.2.1 says that the sum of nominal design strength of columns meeting at that joint (with nominal strength calculated for the factored axial load in the direction of the lateral force under consideration so as to give least column nominal design strength) along each principal plane shall be at least 1.4 times the sum of nominal design strength of beams meeting at that joint in the same plane.

The text written in parenthesis is not clear. I am interpreting it as:

Calculate nominal flexural strength under factored axial load.

Then it says "in the direction of the lateral force under consideration ". Now if I consider this part of text with "axial force", then it does not make any sense. As "Lateral force" considered is due to Earthquake, and is acting in the horizontal direction. Axial force in column shall be vertical. So this axial force can't be in the direction of lateral force.

If I consider same text with "nominal design (flexural) strength of columns", then it will looks like:

Calculate nominal design (flexural) strength (along with factored axial load) in the direction of "lateral load" (which is horizontal).

The above statement in simple words says:

Find bending strength of column in X or Z direction (as the case may be; taking Y axis as vertical), which in case of lateral force in X direction (causing bending in XY plane and moment about Z axis) mean:

Find bending strength of column in X direction.

Which I my opinion is confusing and is not clear, though I understand the intention of the clause. But IS code must make all efforts to make things non-ambiguous, and nothing should be left to the imagination of readers.

I hope I am able to create a confusion .

I suggest that the wording of this needed to be rephrased.

What do you think?

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H.S.Rai
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hsrai
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PostPosted: Sun Apr 30, 2017 10:00 am    Post subject: Reply with quote

Quote:
7.2.1 says that the sum of nominal design strength of columns meeting at that joint (with nominal strength calculated for the factored axial load in the direction of the lateral force under consideration so as to give least column nominal design strength) ...

Further sub-sub clause 7.2.1.1 says that:

Quote:
the design moments of resistance of a beam shall be estimated based on the principles of mechanics and the limiting strain states of the limit state design method enunciated in IS 456. The design moment of resistance of a column shall be estimated as in case of beams corresponding to zero axial force on the design P-M interaction diagram.

Find Moment of Resistance (MoR) of beam using basic principles.

Then it suggests to find MoR for Columns, as in case of beams (where P-M diagram can't be used), using P-M diagram with axial force zero; while 7.2.1 says "nominal strength calculated for the factored axial load".

Now there looks contradictions.

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hsrai
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Joined: 26 Jan 2003
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Location: Ludhiana, Punjab, India

PostPosted: Tue May 02, 2017 2:06 am    Post subject: Reply with quote

Quote:
Clause 7.2 entitled "Relative Strengths of Beams and Columns at a Joint"'s sub-clause 7.2.1 says that the sum of nominal design strength of columns meeting at that joint (with nominal strength calculated for the factored axial load in the direction of the lateral force under consideration so as to give least column nominal design strength) along each principal plane shall be at least 1.4 times the sum of nominal design strength of beams meeting at that joint in the same plane.

This check is not required for joints at roof level, for building of storeys >= 5.

Any justification / logic?

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Rajesh Kaundal
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Joined: 25 Sep 2009
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PostPosted: Sun Jun 23, 2019 7:58 am    Post subject: Reply with quote

There seems to be no logic....
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hemalmistry
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PostPosted: Sun Jun 23, 2019 1:30 pm    Post subject: Clarification concerning IS 13920 : 2016 Reply with quote

Requirement of strong column weak beam (SCWB) is to reduce the possibility of hinge formation in columns that are considered as part of the LLRS. If columns are not stronger than beams framing into a joint, there is increased possibility of inelastic action. In the worst case of weak columns, flexural hinges can form at both ends of  all columns in a given story, resulting in a column failure mechanism that can lead to collapse. Joints with discontinuous columns above the joint, such as roof-level joints, are excluded from SCWB check if the column axial load is low (<0.08*fck*Ag), because special moment frame columns with low axial stress are inherently ductile and column yielding (hinge formation) at such levels is unlikely to create a column failure mechanism that can lead to collapse. 


Requirement of indian code of SCWB check at roof level for buildings less than 4 storey is conservative. According to other international codes SCWB check is not required at roof level except for single storey building.


Regards
Hemal Mistry

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