2-day National workshop on National Building Code of India 2016 & Revised Seismic Codes

Transcription

1 2-day National workshop on National Building Code of India 2016 & Revised Seismic Codes Structural design - Interpretation and use of some of the clauses in the codes in NBC Sumantra Sengupta Chief Manager (Design) STUP Consultants Pvt. Ltd.

2 Concept of Limit State Design

3 Design of a structure Aim of structural design Structure should fulfill its intended purpose during its intended lifetime with 1) Adequate safety 2) Adequate Serviceability 3) Economy Definition of Adequacy?

4 Working stress Working stress method Considers elastic stress-strain behaviour with safety margin in terms of permissible stress Service load behaviour (Characteristic strength) 0.33fck 0.55fy.0004

5 Characteristic Strength of concrete Frequency Grade of concrete is the characteristic compressive strength measured in 150mm cubes at 28 days expressed in MPa Characteristic strength of concrete is the strength below which not more than 5% of test results are expected to fall

6 Characteristic Strength of steel Characteristic strength of steel is specified minimum yield stress or 0.2% proof stress

7 Characteristic Load Characteristic load is the load which has a 5% chance of exceedance in the life span of the structure

8 Characteristic Load

9 Deficiency of Working stress Method RC sections behave in-elastically at higher load Transient load like Seismic load will call for inelastic behaviour of concrete section to assess the ductility of concrete section in post elastic range Degree of uncertainties in different load can not be ascertained. Thus in WSM exact load factor (Ultimate/ Service load) is unknown Stress-strain relation is time dependent and hence modulous of elasticity varies. The concept of modulour ratio in WSM thus may be replaced if actual stress-strain relation can be considered

10 Ultimate Strength design Ultimate strength design is based on ultimate strength at ultimate load (1950) Partial safety factor for material strength - This accounts for under strength ie possible shortfall in the computed nominal resistance owing to uncertainties related to material strengths, dimension, theoretical assumptions etc. Load factors are intended to ensure adequate safety against an increase in service load beyond load specified in design so that failure is extremely unlikely Ultimate strength design reserves strength resulting from a more efficient distribution of stress allowed by inelastic strain

11 Ultimate Strength method and working stress method If sections are proportioned by ultimate strength only, there is danger that although load factors are adequate, cracking and deflection at service load is excessive Crack may be excessive if steel stress is high or reinforcements are not properly distributed. Deflection may be critical if section is shallow Crack/ deflection is checked by elastic theory as their limit dictated by functional requirement keeps stress-strain range within elastic limit working stress method Thus design ideally should combine the best features of ultimate strength and working stress design

12 Probabilistic reliability Method Probabilistic concept (1960) of design was based on theory that various uncertainties in design could be handled more rationality by probability theory Risk involved in design was quantified in terms of probability of failure reliability based method Basic uncertainty factors: 1) Load 2) Material property 3) Dimension Other uncertainty factors: 1) Assumptions in analysis & Design 2) Construction method 3) Workmanship 4) Quality control 5) Intended service life 6) Possible future change of use 7) Frequency of loading etc

13 Strength design reliability model S = Load effect R = Ultimate resistance f S (S) = Probability density function of random variable S f R (R) = Probability density function of random variable R P f = Probability of failure Reliability = 1 - P f Design resistance (f.r n ) >= Design Load effect (g.s n )

14 Probabilistic reliability Method Probabilistic reliability method was simplified to deterministic form involving multiple partial factor of safety instead of probability of failure - LSM Code recommended partial factor of safety values are primarily based on experience, tradition and engineering judgment Attempts are made to specify more rational reliability based safety factors

15 Partial safety factors for material strength

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17 Limit State Method encompassing all design features in simplified way In LSM both ultimate strength approach and serviceability criteria approach by working stress method is covered thus leaving the necessity of keeping working stress method exclusively in the codes redundant In 1964, European Concrete Committee produced first recommendation of LSM for international code of practice for RCC design Amongst the most important Limit States recommended in this code was 1) strength at ultimate load, 2) Deflection at service load and 3) Crack width at service load

18 Limit State Method Definition A limit state is a state of impending failure, beyond which a structure ceases to perform its intended function satisfactorily, in terms of either safety or serviceability, ie it either collapse or becomes unserviceable. The objective of limit state design is to ensure that the probability of any limit state being reached is acceptably low. This is made possible by specifying appropriate multiple safety factors for each limit state.

19 Load Resistance Factor Design (LRFD) Design resistance (f.r n ) >= Design Load effect (g.s n ) R n = Nominal or Characteristic values of resistance S n = Nominal or Characteristic values of Load f = Resistance factor g = Load factor S n < R n /( g/f) Safety concept of WSM R n > ( g/f) S n Safety concept of ULM

20 Various Limit States covered in NBC Concrete structure Limit State of Collapse: in Flexure in Compression in Shear in Torsion Limit State of Serviceability: Deflection Cracking

21 Various Limit States covered in NBC Steel structure Limit State of Collapse: Loss of equilibrium Loss of stability Failure by excessive deformation Fracture due to fatigue Brittle fracture Limit State of Serviceability: Deformation and Deflection Vibration Crack due to fatigue Corrosion, Durability Fire

22 Seismic Design Concept

23 Concept of Seismic Design IS 1893 (Part 1) : 2002

24 Concept of Seismic Design IS 1893 (Part 4) : 2015

25 Stress-strain relation of Concrete 0.33fck.004

26 Stress-strain relation of reinforcement steel Stress-strain curve of Reinforcement steel as per Burns and Siess [19] (1962)

27 Displacement (m) Response reduction factor and its application f w u w 0.04 Time step non-linear method with dt = sec; Ry = 8, Max disp = m; Time period = 0.5 sec; zai = Time (sec)

28 Effect of confining reinforcement on Stress-strain relation of concrete

29 Stress in concrete (MPa) Effect of confining reinforcement on Stress-strain relation of concrete Stress-Strain relationship of concrete against different transverse steel (for M27.6 & sh/b = 0.5) % 20 2% % 0.25% 0.5% 1% Strain in concrete

30 Moment (tm) Effect of confining reinforcement on Curvature ductility 3 x 105 M vs Phi curve against varying transverse steel ratio for circular hollow section Allowable ultimate curvature.0027 to.0047 Mu % 2.05% 0.1% 0.2% 0.4% 1.5 My = 1.68E5 tm Yield curvature = 1.95E-4 Do = 16m; Di = 14m Main steel = 1% P/A/fck = 0.05 Confining width = 0.85m Spacing of stirrup = 150 mm Phi x 10-3 Moment vs curvature curves for a 16m dia hollow RCC section

31 Concept of modes of vibration

32 Concept of response spectrum

33 Displacement (m) Modal combination practice m; T = 0.47 sec.0567m; T = 0.5 sec m; T = 2 sec Total response = 0.17m Absolute sum = m SRSS = m 0.05 CQC = m Time (sec) RHA of SDF system in El Centro 1940 EQ

34 Multidirectional seismic force combination IS 1893 (Part 4) : 2015

35 Sumantra Sengupta Chief Manager (Design) STUP Consultants Pvt. Ltd.