A Critical Review of IS:13920IS:13920--20162016 · 2019-05-22 · A Critical Review of...

A A Critical Review Critical Review of of A A Critical Review Critical Review of of IS:13920IS:13920--20162016

Prof. Ashok K. JainProf. Ashok K. JainR tdR td P f IIT P f IIT R kR kRetdRetd. Professor, IIT . Professor, IIT RoorkeeRoorkeeIndirapuramIndirapuram, GZB UP 201014, GZB UP 201014

IntroductionIntroductionIntroductionIntroduction

IS:13920 1993 has been revised and 2016 edition has been released recently after nearly 25 years.

It was under revision for nearly a decade.

It is strange that this code relies heavily on nonlinear static and dynamic analysis in certain cases. Even the final draft 1893-part 1 does not specify how to carryout linear time history analysis in view of certain obvious linear time history analysis in view of certain obvious difficulties.

Therefore, to directly jump to nonlinear static or dynamic analysis is scary.

2I.A.S.E. Conference on Planning & Design of Tall Buildings incl. Earthquake and Wind Effects – Dec 6-7, New Delhi,

Clause 1 1 2Clause 1 1 2Clause 1.1.2Clause 1.1.2

Flat slab structures must have a lateral load resisting system capable of providing similar level of performanceas envisioned in this standard and must be designed for drift compatibility. The adequacy of such designs shall be demonstrated by adequate, appropriate y q , pp pexperimentation and nonlinear dynamic structural analysis.

Nonlinear dynamic Analysis


Comment on Clause 1 1 2Comment on Clause 1 1 2Comment on Clause 1.1.2Comment on Clause 1.1.2

The language of this clause is such that it will be The language of this clause is such that it will be impossible to design a flat slab system in seismic zones III, IV and V for the following reasons:

Nowhere the Code specifies the performance criteria for a flat slab or any other system. IS:13920-2016 is not a Performance based Code. It is a clear Prescriptive Code.Performance based Code. It is a clear Prescriptive Code.

Nowhere the Code specifies how to model degradation in strength and stiffness of various elements, hysteresis models, and carryout a nonlinear analysis – static or dynamic. There are large number of parameters that need to be defined very carefully and also how to interpret the to be defined very carefully and also how to interpret the results of a nonlinear analysis. 4

I.A.S.E. Conference on Planning & Design of Tall Buildings incl. Earthquake and Wind Effects – Dec 6-7, New Delhi,


Th C d d t b d t t d b The Code says adequacy must be demonstrated by adequate, appropriate experimentation and nonlinear dynamic structural analysis. It means experimental evidence is essential. There is no “either” or “or”. Is it a practical clause?

At t th IS 1893 d ft 2016 d t d fi At present even the IS:1893-draft 2016 does not define a time history method for linear dynamic analysis because of various reasons. Then how to carry out a nonlinear dynamic analysis is to expect too much.

Such a matter must not be left to specialist literature l it i li bl t b i d d l d t litig tialone as it is liable to be misused and lead to litigation.



The ratio of actual ultimate strength to the actual 0.2% proof strength shall be at least 1.15.



Please refer to Amendment # 1 dated 2013 to IS:1786-2008 wherein there are three grades of steel: 415, 415D and 415S etc. The S grade steel must have this ratio as 1.25 but the Indian steel mills are unable to achieve it. Clause 6 3 makes use of factor 1 4 = 1 25/0 87 assuming Clause 6.3 makes use of factor 1.4 1.25/0.87 assuming the strength ratio is 1.25. Clause 20.2.2.5 of ACI 318-2014 requires that the strength ratio must be 1.25. In the I di t t thi f t 1 4 ght t b d d t Indian context, this factor 1.4 ought to be reduced to 1.15/0.87 = 1.30.


Clause 5 4Clause 5 4Clause 5.4Clause 5.4

In RC buildings plinth beams (where provided), and staircase beams and slabs framing into the columns shall be included in the analytical model for structural analysis.

Sequential qConstruction


Comment on Clause 5 4Comment on Clause 5 4Comment on Clause 5.4Comment on Clause 5.4

On the face of it, this clause looks correct. If inclined slabs (staircase and ramp) are included in the analysis, it will give a very high lateral thrust in columns as the latter provide lateral restraint. However, if you consider the sequence of construction this restraint does not lead to a sequence of construction, this restraint does not lead to a very large force. Therefore, a stage construction analysis must be done to account for sequence of construction in

h d i t t l fsuch cases and arrive at correct column forces.


Clause 5 5Clause 5 5Clause 5.5Clause 5.5

If a building with any kind of irregularity as listed in IS:1893-1 is adopted, detailed nonlinear analysis shall be

f d t d t t th t th i th t t l performed to demonstrate that there is no threat to loss of life and property.

Nonlinear analysisFor which irregularity ?

itt d t permitted or not permitted


Comment on Clause 5 5Comment on Clause 5 5Comment on Clause 5.5Comment on Clause 5.5

It is a very interesting clause. I have carried out extensive nonlinear analysis of residential buildings with different types of irregularities in plan and elevation. A nonlinear pushover static analysis up to 5% lateral drift is not likely to exceed I-O or L-S states except for a soft storeyto exceed I O or L S states except for a soft storeyirregularity. Thus, this clause becomes a very good escape route. It should also be noted that IS:1893-2016 draft does

t i li l i not require any nonlinear analysis.



This Clause deals with the width of a beam with respect to that of the column for the purpose of providing shear reinforcement in beams.



Printing error

This clause is adopted from ACI 318 In Fig 1A the This clause is adopted from ACI 318. In Fig. 1A, the direction of analysis should have been shown as in the original ACI figure.

Printing ErrorPrinting Error



In the calculation of design shear force capacity of RC beams, contribution of the following shall not be considered:

(a) bent-up bars

(b) inclined links, and

(c) concrete in RC section



In the previous edition (1993), only the first two points were included. Now a third point has been incorporated, that is, concrete.

It is based on the assumption that the entire concrete has k d d l f i i f H th cracked under reversal of seismic force. However, the

same philosophy is not used for computing the flexure resistance of the section. This clause has been adopted from the ACI 318 with modifications.


Comment on Clause 6 3 4 Comment on Clause 6 3 4 –– contcontComment on Clause 6.3.4 Comment on Clause 6.3.4 –– cont.cont.

The ACI 318-2008, clause 21.6.5.2 — Transverse reinforcement,

Transverse reinforcement over the lengths lo, identified in 21.6.4.1, shall be proportioned to resist shear assuming Vc = 0 when both (a) and (b) occur:

(a) The earthquake induced shear force calculated in accordance with (a) The earthquake-induced shear force, calculated in accordance with 21.6.5.1, represents one-half or more of the maximum required shear strength within lo;

(b) The factored axial compressive force Pu including earthquake effects is (b) The factored axial compressive force, Pu, including earthquake effects is less than Ag fc’/20.

The ACI 318-2014 has added a commentary to this clause:

“H hi h f i l i i li i h ld b “However, this stratagem chosen for its relative simplicity should not be interpreted to mean that no concrete is required to resist shear. On the contrary it must be argued that the concrete core resists all the shear with the shear reinforcement confining and strengthening the concrete.”the shear reinforcement confining and strengthening the concrete.


Comment on Clause 6 3 4 Comment on Clause 6 3 4 –– contcontComment on Clause 6.3.4 Comment on Clause 6.3.4 –– cont.cont.

These two critical conditions have been omitted in the Indian code without any logic.

Moreover, the new commentary in ACI 318-2014 gives the importance of shear strength of concrete.

The Indian Clause is unnecessary being overconservative.



The minimum dimension of a column shall not be less than 20 db (Fig. 7). b

Based on Cl 18 8 2 3 Based on Cl 18 8 2 3 Based on Cl. 18.8.2.3 Based on Cl. 18.8.2.3 ofof

ACI 318 ACI 318 -- 20142014


Clause 18 8 2 3 ACI 318-2014Clause 18.8.2.3 ACI 318-2014

Where longitudinal beam bars extend through a beamextend through a beam--column jointcolumn joint, the column dimension parallel to the beam reinforcement shall be at least 20 times the largest longitudinal beam bar.

Th t th t it i li bl t t i ht The commentary says that it is applicable to straight beam bars which may slip within the beam-column joint under severe moment reversals during the formation of adjacent beam hinging.

I.A.S.E. Conference on Planning & Design of Tall Buildings incl. Earthquake and Wind Effects – Dec 6-7, New Delhi,19

Comment on Clause 7 1 1 Comment on Clause 7 1 1 Comment on Clause 7.1.1 Comment on Clause 7.1.1

Printing error

The term dimension is misleading Is it width or depth or The term dimension is misleading. Is it width or depth or both? Moreover, Fig 7 shows 15 db instead of 20 db. It is a typo error.

Printing ErrorPrinting Error



The cross-section aspect ratio (that is, ratio of smaller dimension to larger dimension) shall not be less than 0.45. Vertical members of RC buildings whose cross-section aspect ratio is less than 0.4 shall be designed as per requirements of 9requirements of 9.



Very Confusing


Comment on Clause 7 2 1 contComment on Clause 7 2 1 contComment on Clause 7.2.1 cont.Comment on Clause 7.2.1 cont.


Comment on Clause Comment on Clause 7 2 1 cont7 2 1 contComment on Clause Comment on Clause 7.2.1 cont.7.2.1 cont.

(5) The rules of (4) and (5) of this sub clause are waived at the top level of multi-storey buildings.

Thus factor 1.3 has been included to take care of soft storey mechanism.

Moreover, Clause (5) is missing in IS:13920.


Clause 8Clause 8Clause 8Clause 8

SPECIAL CONFINEMENT REINFORCEMENTSPECIAL CONFINEMENT REINFORCEMENT

The requirements of this section shall be met with in beams and columns, unless a larger amount of transverse , greinforcement is required from shear strength considerations given in 6.3.3 for beams and 7.5 for columnscolumns.

8.1(b) (1) ¼ of minimum member dimension of the beam or column,,

8.1(b) (2) 6 times diameter of the smallest longitudinal bars,


Comment on Clause 8Comment on Clause 8Comment on Clause 8Comment on Clause 8

This clause should have been numbered as 7.6 (corresponding to 7.4 of IS:13920-1993) instead of 8. This would have avoided the confusion whether this clause is applicable to columns alone or both beams and columns.

The special confinement reinforcement in beams has alreadyThe special confinement reinforcement in beams has alreadybeen covered in 6.3 and Fig. 6.


Comment on Clause 8 contComment on Clause 8 contComment on Clause 8 cont.Comment on Clause 8 cont.

Clause 6.3.5(b) says “ 8 times the diameter of the smallest longitudinal bar”. Apparently, there is a confusion. Is it 6 times or 8 times in beams?

Clause 8 should refer only to columns and not beams. Al Fi 12 d l 8 1( ) d ( ) f l t Also, Fig. 12 and clause 8.1(a) and (c) refer only to columns.

It is a clear case of poor copy and paste Poor Copy It is a clear case of poor copy and paste. Poor Copy and Paste



Width of beam column joint

It is a repetition of Clause 7.1.1 and Fig. 7.

It is again a poor copy and paste.Poor Copy and Pasteand Paste



(a) The minimum thickness of special shear walls has been fixed at 150 mm.

E li it f bl t l th 150 Earlier it was preferably not less than 150 mm.

Comment

Th i tl ti l b hi d ki it There is apparently no rational behind making it a minimum value of 150 mm.

It is killing freedom to the designer and making the code It is killing freedom to the designer and making the code too conservative without any rational.


Comment on Clause Comment on Clause 10 1 6 cont10 1 6 contComment on Clause Comment on Clause 10.1.6 cont.10.1.6 cont.

Poor Copy d P tand Paste


BACKGROUND HISTORY OF BACKGROUND HISTORY OF BACKGROUND HISTORY OF BACKGROUND HISTORY OF CONFINEMENT CONFINEMENT STEEL IN RC COLUMNSSTEEL IN RC COLUMNS


BACKGROUND HISTORY OF BACKGROUND HISTORY OF BACKGROUND HISTORY OF BACKGROUND HISTORY OF CONFINEMENT STEEL IN RC COLUMNSCONFINEMENT STEEL IN RC COLUMNS


Comment on Clause 8 Comment on Clause 8 Comment on Clause 8 Comment on Clause 8

3838


Concluding RemarksConcluding RemarksConcluding RemarksConcluding Remarks

This Code was under revision for more than a decade. A number of drafts were circulated. The final Code has included a number of explanations and a few new clauses pertaining to flexure and shear strength of beam-column joints joints.

The significant points that need urgent attention of the Code are as follows:


Concluding Concluding Remarks contRemarks contConcluding Concluding Remarks cont.Remarks cont.There are a number of typo errors – Clause 6.1.4, 6.3.4, 7.1.1, 7.1.2, 8.1, 10.1.6 (Table 1). 7.1.1, 7.1.2, 8.1, 10.1.6 (Table 1).

Drafting has been poor at some places such as Clause 7.1.2, 8, 9.1.3 and Table 1.

The Code has referred to the use of experimentation and nonlinear dynamic analysis at Clause 1.1.2 and nonlinear analysis at Clause 5 5 With the easy availability of very analysis at Clause 5.5. With the easy availability of very powerful software with nonlinear static and dynamic analysis capability, there is an urgent need to emphasize that the designer may undertake such an analysis only if they have sufficient understanding, exposure and experience in carrying out a nonlinear analysis and experience in carrying out a nonlinear analysis and interpretation of the results. 40


Concluding Concluding Remarks contRemarks contConcluding Concluding Remarks cont.Remarks cont.

The Code does not give any guidance on how to model degradation in strength and stiffness of structural elements, carryout nonlinear analysis, and interpret the results.

A li ht i ht l d t b d lt d th i A slight oversight may lead to absurd results and their interpretation.

It can lead to unnecessary confusion and litigationIt can lead to unnecessary confusion and litigation.


Concluding RemarksConcluding RemarksConcluding RemarksConcluding Remarks

Once the IS:13920-2016 has adopted several figures and clauses from ACI 318 including the confinement steel for circular columns, there is no justification for not updating the expression for confinement steel in rectangular columns which is 3 times of that in the ACI 318 columns which is 3 times of that in the ACI 318.

It is an unnecessary conservatism.

Moreover there is no experimental research in India to Moreover, there is no experimental research in India to justify retaining the old conservative clause.


A big Thank YouA big Thank YouA big Thank YouA big Thank You

Time for Q & ATime for Q & A


A Critical Review of IS:13920IS:13920--20162016 · 2019-05-22 · A Critical Review of...

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