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

Transcription

1 A Critical Review of IS: Prof. Ashok K. Jain Retd. Professor, IIT Roorkee Indirapuram, GZB UP

2 Introduction IS: 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 difficulties. Therefore, to directly jump to nonlinear static or dynamic analysis is scary. 2

3 Clause Flat slab structures must have a lateral load resisting system capable of providing similar level of performance as envisioned in this standard and must be designed for drift compatibility. The adequacy of such designs shall be demonstrated by adequate, appropriate p experimentation and nonlinear dynamic structural analysis. Nonlinear dynamic Analysis 3

4 Comment on Clause 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: is not a 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 results of a nonlinear analysis. 4

5 Comment on Clause The Code says adequacy must be demonstrated t d 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 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 alone as it is liable to be misused and lead to litigation. 5

6 Clause The ratio of actual ultimate strength to the actual 0.2% proof strength shall be at least

7 Comment on Clause Please refer to Amendment # 1 dated 2013 to IS: 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 the strength ratio is Clause of ACI requires that the strength ratio must be In the Indian context, t this factor 1.4 ought to be reduced d to 1.15/0.87 =

8 Clause 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 Construction 8

9 Comment 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 very large force. Therefore, a stage construction analysis must be done to account for sequence of construction in such cases and arrive at correct column forces. 9

10 Clause If a building with any kind of irregularity as listed in IS: is adopted, detailed nonlinear analysis shall be performed to demonstrate t that t there is no threat t to loss of life and property. Nonlinear analysis For which irregularity? permitted or not permitted 10

11 Comment on Clause 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 storey irregularity. Thus, this clause becomes a very good escape route. It should also be noted that IS: draft does not require any nonlinear analysis. 11

12 Clause 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. 12

13 Comment on Clause Printing error 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 Error 13

14 Clause 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 14

15 Comment on Clause 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 cracked under reversal of seismic i 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. 15

16 Comment on Clause cont. The ACI , clause Transverse reinforcement, Transverse reinforcement over the lengths lo, identified in , 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 , represents one-half or more of the maximum required shear strength within lo; (b) The factored axial compressive force, Pu, including earthquake effects is less than Ag fc /20. The ACI has added a commentary to this clause: However, this stratagem chosen for its relative simplicity i 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. 16

17 Comment on Clause cont. These two critical conditions have been omitted in the Indian code without any logic. Moreover, the new commentary in ACI gives the importance of shear strength of concrete. The Indian Clause is unnecessary being overconservative. 17

18 Clause The minimum dimension of a column shall not be less than 20 d b (Fig. 7). b Based on Cl of ACI

19 Clause ACI Where longitudinal beam bars extend through a beam- column joint, the column dimension parallel to the beam reinforcement shall be at least 20 times the largest longitudinal beam bar. The commentary says that t 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. 19

20 Comment on Clause Printing error The term dimension is misleading. Is it width or depth or both? Moreover, Fig 7 shows 15 d b instead of 20 d b. It is a typo error. Printing Error 20

21 Clause The cross-section aspect ratio (that is, ratio of smaller dimension to larger dimension) shall not be less than Vertical members of RC buildings whose cross-section aspect ratio is less than 0.4 shall be designed as per requirements of 9. 21

22 Comment on Clause Very Confusing 22

23 Clause

24 Comment on Clause

25 Comment on Clause cont. 25

26 Comment on Clause 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:

27 Clause 8 SPECIAL CONFINEMENT REINFORCEMENT The requirements of this section shall be met with in beams and columns,, unless a larger amount of transverse reinforcement is required from shear strength considerations given in for beams and 7.5 for columns. 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, 27

28 Comment on Clause 8 This clause should have been numbered as 7.6 (corresponding to 7.4 of IS: ) 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 already been covered in 6.3 and Fig

29 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. 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 and Paste 29

30 Clause Width of beam column joint It is a repetition of Clause and Fig. 7. It is again a poor copy and paste. Poor Copy and Paste 30

31 Clause (a) The minimum thickness of special shear walls has been fixed at 150 mm. Earlier it was preferably not less than 150 mm. Comment 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 too conservative without any rational. 31

32 Clause

33 Comment on Clause

34 Comment on Clause cont. Poor Copy and Paste 34

35 BACKGROUND HISTORY OF CONFINEMENT STEEL IN RC COLUMNS 35

36 BACKGROUND HISTORY OF CONFINEMENT STEEL IN RC COLUMNS 36

37 BACKGROUND HISTORY OF CONFINEMENT STEEL IN RC COLUMNS 37

38 Comment on Clause 8 38

39 Concluding 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. The significant points that need urgent attention of the Code are as follows: 39

40 Concluding Remarks cont. There are a number of typo errors Clause 6.1.4, 6.3.4, 7.1.1, 7.1.2, 8.1, (Table 1). Drafting has been poor at some places such as Clause 7.1.2, 8, and Table 1. The Code has referred to the use of experimentation and nonlinear dynamic analysis at Clause and nonlinear analysis at Clause 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 interpretation of the results. 40

41 Concluding 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 slight oversight may lead to absurd results and their interpretation. It can lead to unnecessary confusion and litigation. 41

42 Concluding Remarks Once the IS: 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. It is an unnecessary conservatism. Moreover, there is no experimental research in India to justify retaining the old conservative clause. 42

43 A big Thank You Time for Q & A 43