limitations of indian seismic design codes for rc buildings

8/13/2019 LIMITATIONS OF INDIAN SEISMIC DESIGN CODES FOR RC BUILDINGS

1/8

1416 14th SEE-2010

1Research Scholar, Department of Earthquake Engineering, IIT Roorkee, e-mail: [email protected] Professor, Department of Earthquake Engineering, IIT Roorkee, e-mail: [email protected] Engineer, NORSAR, P.O. Box 53, 2027 Kjeller, Norway, e-mail: [email protected]

LIMITATIONS OF INDIAN SEISMIC DESIGN

CODES FOR RC BUILDINGS

Vijay Namdev Khose1, Yogendra Singh2and Dominik Lang3

ABSTRACT

Past experience, and traditional design and construction practices have an important role, even in themodern codes; and seismic design codes are no exception. The main objective of design codes is to

provide guidelines to designers and set minimum design criteria. Past earthquakes have demonstrated

that buildings designed by seismic design codes are not always safe against earthquake. Therefore,

it is necessary that codes should be updated time to time. Code updating is done considering current

state of the art, its understanding by design engineers and the construction practices in the country

so that it can be easily implemented on site. Indian seismic design code IS 1893 (Part 1): 2002 and

IS 13920: 1994 are obsolete in many respects, as compared to major national seismic design codes.

This paper identifies the limitations of the present Indian seismic design code and proposes some

topics for updating according to the current state of the art.

INTRODUCTION

During the last few decades, earthquake engineering has undergone significant development. Before

the development of earthquake engineering, very few structures were designed for earthquake

loading. It was observed during past earthquakes that structures designed for some lateral load due to

wind loading or due to some other reason, performed better than structures designed for gravity only.

This experience, initiated design of structures for lateral force equal to a fraction of seismic weight.

Later, with understanding of structural dynamics, concept of period dependent design lateral forces

was developed. With the development of inelastic time history analysis and understanding of seismic

response, it was found that during an earthquake structures are capable of taking many times larger

seismic forces than its design lateral strength, which lead to consideration of ductility in seismic

design of structures. Later equal energy and equal displacement principles were developed and

design base shear on structure is reduced by a force reduction factor to account for the inelasticenergy dissipation in ductile structures. Initially the structures were designed only for force but later

considering the importance of displacement in control of non-structural damage, concept of force

based design with displacement was introduced. With the development and understanding of seismic


2/8

Vijay Namdev Khose, Yogendra Singh and Dominik Lang 1417

behaviour of buildings and other structures, concept of capacity design was introduced, which aims

at avoiding undesirable (brittle) modes of failure by designing a hierarchy of strength of different

components. This is identified as start of performance-based seismic design of structures (Priestley

et al., 2007). In 1990s researchers found that there is need for developing design methodology forachieving multiple performance objectives. Significant efforts have been made and are still being

made in development of Performance Based Design.

Past earthquakes have demonstrated that codes should be updated time to time. Code updating is

done considering current state of the art, its understanding by design engineers and the construction

practices of the country, so that it can be easily implemented on site. Indian seismic design code IS

1893 (Part 1): 2002 and IS 13920: 1994 are obsolete in many respects, as compared to major national

seismic design codes. This paper identifies the limitations of the present Indian seismic design code

and proposes some topics for updating according to the current state of the art.

SITE CLASSIFICATION

Amplification, de-amplification and shape of design response spectrum are governed by soil typeand amplitude of ground motion. Most of the seismic design codes classify the site based on one or

more parameters such as average shear wave velocity,S

V (generally for top 30m soil deposit), SPT

Value, N, unconfined shear strength, low amplitude natural period, etc. IS 1893 classifies soils into

three types, namely, Type I, Type II and Type III, which represent rock or hard soil, medium soil and

soft soil, respectively. However, ASCE 7-05 (2006) classifies soils into mainly five types - A, B, C, D

and E. Type A represents the hard rock and type E represents soft soil. The ASCE 7 classification is

based on average shear wave velocity,S

V , SPT Value, N and unconfined shear strength. However,

IS 1893 classifies soils only on the basis of SPT value, N. Table 1 shows the comparison of IS 1893

soil classification with ASCE 7 based on N values. Corresponding average shear wave velocity for

ASCE 7 classes are also shown. It can be observed that IS 1893 classification is much coarser and

Table 1. Comparison of site classification of IS 1893 with ASCE 7

IS 1893 classification ASCE 7 classification

Site Class N - value Site ClassShear Wave

Velocity (m/s)N - value

Type I

(Rock or Hard

soil)

>30

Type A (Hard Rock) > 1524 NA

Type B (Rock) 762-1524 NA

Type C (Very dense soil

and soft rock)

366-762 >50

Type D (Stiff soil) 183-366 15-50

Type II

(Medium soil)10-30

Type E (Soft clay soil) < 183


3/8

1418 14th SEE-2010

is based on only N values, whereas shear wave velocity is a more direct characteristic parameter for

estimating soil amplification of seismic waves. Further, type I of IS 1893 represents a very broad

range of site classes, ranging from ASCE 7 class A to D. Type III site class of IS 1893 representsvery soft soils (even softer than site class E of ASCE 7).

DESIGN SPECTRUM

Traditionally, acceleration spectrum has been used by codes for design of structures. The seismic

design codes generally specify 5 % damped elastic acceleration spectrum as the design spectrum.

But, displacement is also an important design criteria, particularly for safety of non-structural

Fig. 1: Comparison of normalized response spectra of IS 1893 and ASCE 7 a) ASCE 7 site class A, b) ASCE 7

site class B, c) ASCE 7 site class C, d) ASCE 7 site class D, e) ASCE 7 site class E


4/8


components. Further, the recently developed displacement based seismic design methodology is

also gradually becoming popular. Therefore, some codes provide the information to construct design

displacement spectra also. The following sections examine the design spectra of the Indian code

in comparison with ASCE 7.

Acceleration spectra

All the seismic design codes provide standard shape of design response spectrum based on soil

type, which is scaled to the local hazard, which is expressed in terms of zone factor (Effective Peak

Ground Acceleration, EPGA) or spectral ordinates at short and long periods. IS 1893 specifies three

spectral shapes for three different site classes and scales them by zone factor. On the other hand,

ASCE 7 scales the design spectrum by two spectral ordinates at 0.2 sec and 1.0 sec periods. Soil

amplification, de-amplification and spectral shape are governed by soil type and amplitude of ground

motion. As per IS 1893 soil amplification is considered only in long period range and it is a function

of only soil type. The soil amplification in short period range and effect of amplitude of motion on

soil amplification is well known now, but it is completely ignored by Indian code.Figure 1 shows the comparison of normalized response spectra of IS 1893 and ASCE 7 for

ASCE 7 site classes. To reflect the effect of PGA on soil amplification/de-amplification in case of

ASCE 7, normalized spectra for 0.2g and 0.5g, have been plotted. The value of corner period,L

T ,

is considered as 4 sec in present study, which is the minimum mapped value specified by ASCE 7.

The design spectra for rock site (ASCE 7 site class B) are almost same for both the codes, but there

are considerable differences for other site classes. There is a large variation in spectra for ASCE 7

site class E (soft soil). The maximum difference is 113% at short period and is 92% at 2 sec period.

Soil de-amplification with increasing PGA (0.5 g) in medium and soft soils can be clearly observed

in case of ASCE 7 (Figure 1(c)-(e)). Further, it is interesting to note that IS 1893 spectra are on

non-conservative side, as compared to ASCE 7, for almost all the site classes.

DISPLACEMENT RESPONSE SPECTRA

Displacement is now widely recognized (Moehle, 1992; Calvi and Kingsley, 1995; Kowalsky, 1995;

Medhekar and Kennedy, 2000; Xue and Chen, 2003; Priestley et al., 2007) as the most important

design parameter and damage indicator. In the conventional force based design also, interstorey drift

is an important criteria, which may govern the design in many cases. Priestley et al. (2007) have

shown that fundamental periods of even moderately tall buildings lie in the displacement controlled

range of spectra. A comparison of displacement spectra (Figure 2) obtained from the design spectra

of IS 1893 and ASCE 7 show even more remarkable differences, which are not as prominently

visible in case of acceleration spectra. One of the parameters governing the displacement spectrum

is corner period between velocity controlled range and displacement controlled range. The corner

period depends on the source mechanism, magnitude of earthquake and distance (Tole and Faccioli,1999; Bommer and Elnashai, 1999; Faccioli et al., 2004; Akkar and Bommer, 2007) and hence

on the local seismo-tectonic setup. ASCE 7 has provided maps for estimation of corner periods at

different locations in the US where the mapped values vary from 4 sec to 16 sec. IS 1893 specifies

design spectra up to 4 sec period only, without any mention about corner period and displacement

controlled range. Figure 2 shows the displacement spectra for ASCE 7 and IS 1893 for 0.2g PGA.

For ASCE 7, the value of corner period has been considered as 4 sec, to plot the displacement

spectra. The figure shows that IS 1893 yields much lower spectral displacement for almost all the


5/8

1420 14th SEE-2010

site classes and there is an urgent need to review and revise the IS 1893 design spectra to get reliable

estimates of displacement.

MODELLING GUIDELINES

Analysis of structures for seismic actions is a complex. Computer Aided analysis is invariably used

in design offices nowadays, mostly due to availability of a large number of commercial software.

However, reliability of analysis results depends on the modelling of structure. An error in modelling

will end up with erroneous designs. Many practicing designers in India are not adequately aware of

modelling rules, which may result in erroneous models and unsafe design of structures, even if the

Fig. 2: Comparison of Displacement Spectra of IS 1893 and ASCE 7 for 0.2g a) ASCE 7 site class A, b) ASCE 7

site class B, c) ASCE 7 site class C, d) ASCE 7 site class D, e) ASCE 7 site class E


6/8


design codes are faithfully followed. So the codes of practice need to provide adequate guidelines

for proper modelling of structures. Several national codes (EC8, 2004; ACI 318M-08, 2008;

ASCE 41-06, 2006), and document (ATC 40, 1996) have included detailed and updated modelling

guidelines, whereas the Indian code is silent on this issues. Detailed guidelines need to be developedfor modelling of members, joints and boundary conditions and incorporated in the revised Indian

code. As the performance based design is gaining popularity, guidelines also need to be developed

for nonlinear modelling of structures.

DUCTILE DETAILING

Currently, all the seismic codes in the world are based on force-based design methodology. In force-

based design, structures are analysed using elastic models and inelasticity in structures is accounted

for by the force reduction factors, which considers ductility, over strength and other unforeseen

factors which cannot be directly accounted for in modeling. Major contribution to reduction factor

comes from the ductility in structure. Codes classify buildings into various ductility classes and

specify corresponding reduction factors. ASCE 7 classifies RC frame buildings into three ductilityclasses, namely Ordinary Moment Resisting Frame (OMRF), Intermediate Moment Resisting Frames

(IMRF) and Special Moment Resisting Frame (SMRF) and corresponding reduction factors are 3, 5

and 8, respectively. However, IS 1893 classifies RC frame buildings into only two classes, namely

Ordinary Moment Resisting Frame (OMRF) and Special Moment Resisting Frame (SMRF) and

corresponding reduction factors are 3 and 5, respectively. Both codes provide constant reduction

factor for all design periods. However, it is well known (Priestley and Pauley, 1992) that reduction

factors should be different in short period and long period range of design spectrum, as the two

ranges are governed by equal energy and equal displacement principles, respectively.

Further, ductility in RC structures is achieved through proper detailing of reinforcement.

Ductile behaviour of RC buildings is ensured through capacity design, providing special confining

reinforcement in potential plastic hinge regions, providing adequate anchorage in joints and byadequate joint shear strength. For ductile detailing of RC frame buildings ASCE 7 refers to ACI

318 and IS 1893 refers to IS 13920 for specified ductility classes. As per both codes, in case of

OMRF buildings, earthquake forces are considered in design but no special provision is made for

ductility. Ductile detailing provisions for IMRF of ASCE 7 and SMRF of IS 1893 are identical and

justify the use of same response reduction factor for the two cases. In both the cases, ductility of

RC frame buildings is enhanced through capacity shear design of columns and beams, and special

confinement reinforcement (closely spaced hoops) in potential plastic hinge zone of columns and

beams. In case of SMRF of ASCE 7 two additional provisions as compared to IMRF are made:

(i) strong column weak beam design, (ii) design of beam-column joints to avoid shear failure. The

current Indian detailing code IS 13920 completely ignores these two aspects and need to be revised

to yield satisfactory designs for high seismicity areas.

DRIFT CONTROL

Performance of structural as well as non-structural components of a building is controlled by inter-

storey drift. Inter-storey drift also controls the P- effects. Therefore, it is also one of the most

important design parameters in all the seismic design codes, and governs the member sizes in many

cases, particularly in tall buildings. As per ASCE 7, elastic displacement at floor level is calculated

and amplified by deflection amplification factor provided for different types of buildings. IS 1983


7/8

1422 14th SEE-2010

provides the drift control limits directly on the elastic displacement at the design load, without any

amplification for ductility demand.

ASCE 7 limits the total storey drift within 1.5-2.5% depending on the occupancy category

for multistory RC frame buildings. IS 1983 limits the interstorey drift to 0.4% at design loadlevel, which renders it dependent on the ductility class of the building (Haldar and Singh, 2009).

Considering the ductility factor approximately equal to the response reduction factor, the effective

limits at ultimate drifts are 1.2% and 2% for OMRF and SMRF, respectively. This leads to some

discrepancies (Haldar and Singh, 2009) in the design.

Consideration of effective stiffness of RC members is obviously the most crucial step in estimation

of building deformations and inter-storey drifts. Major national design codes such as ACI 318, EC8,

NZS 3101 (2006) have fairly detailed provisions for effective stiffness of RC members. In addition

to the code provisions, several proposals for effective stiffness of RC members under seismic loads

are available and vary significantly (Kumar and Singh, 2010). Indian code is, however, silent on

this crucial issue.

CONCLUSIONS

Various limitations of the Indian seismic design and detailing codes, IS 1893 and IS 13920 as

compared to the current state of the art have been identified. A comparative study of various

provisions related to site classification, design response spectrum, modelling guidelines, drift control

criteria and ductile detailing has been made with ASCE 7. The Indian site classification is based

on single parameter, i.e. SPT value. However, a more direct characterization can be made using

shear wave velocity. Type I soil of IS 1893 is a very broad class covering from Classes A to D of

ASCE 7. The Indian code specifies design spectrum up to 4 sec period only, without any mention

about the displacement controlled range of the spectrum, whereas the design period of medium rise

and high rise buildings may be longer than 4 sec. In Indian code, soil amplification in short period

range and de-amplification for higher PGA. The ignorance amplification in short period results inhighly non-conservative design spectra. Although, it is well known that reliability of seismic design

depends to a large extent on the accuracy of the modeling, the Indian code does not provide modeling

guidelines, leaving it to the skills of individual designers. Ductile detailing provisions of the current

Indian code are obsolete and important issues like strong column and weak beam and joint shear

design are ignored. Code limits the interstorey drift to 0.4% at design load level, which renders it

dependent on the ductility class of the building. Further, the code does not provide any guidelines

about effective stiffness of RC members, rendering the check on interstorey drift, meaningless.

Therefore, there is an urgent need to revise the relevant provisions of the Indian code in the light of

the new research and the state of the art in the other major national codes.

REFERENCES

1. ACI 318M-08, 2008. Building code requirements for structural concrete and commentary,American

Concrete Institute (ACI), USA.

2. Akkar, S. and Bommer, J. J., 2007. Prediction of elastic displacement response spectra in Europe and the

Middle East,Earthquake Engineering and Structural Dynamics, 36, 1275-1301.

3. ASCE 41-06, 2006. Seismic rehabilitation of existing buildings, American Society of Civil Engineers

(ASCE), Reston, Virginia.

4. ASCE 7-05, 2006. Minimum design loads for buildings and other structures, American Society of Civil

Engineers (ASCE), Reston, Virginia.


8/8


5. ATC 40, 1996. Seismic Evaluation and Retrofit of Existing Concrete Buildings, Applied Technology

Council, Redwood City, California 94065.

6. Bommer, J. J., Elnashai, A. S., 1999. Displacement spectra for seismic design,Journal of Earthquake

Engineering,3(1), 1-32.

7. Calvi, G. M., and Kingsley, G. R., 1995. Displacement-based seismic design of multi-degree-of-freedom

bridge structures.Earthquake Engineering & Structural Dynamics,24(9), 1247-1266.

8. Eurocode 8 (EC8), 2004. BS EN 1998-1:Design of structures for earthquake resistance- Part 1: General

rules, seismic actions and rules for buildings, British Standard (BS), UK.

9. Faccioli, E., Paolucci, R., and Rey, J., 2004. Displacement spectra for long periods,Earthquake Spectra,

20(2), 347-376.

10. Haldar, P. and Singh, Y., 2009. Seismic performance and vulnerability of Indian code designed RC frame

buildings,ISET Journal of Earthquake Technology, 46(1), Paper No. 502.

11. IS 13920, 1993.Ductile detailing of reinforced concrete structures subjected to seismic forces-code of

practice, Bureau of Indian Standards (BIS),,New Delhi 110002.

12. IS 1893 (Part 1), 2002. Criteria for earthquake resistant design of structures Part 1 General provisions

and buildings, Bureau of Indian Standards (BIS), New Delhi 110002.

13. Kowalsky, M. J., Priestley, M. J. N. and MacRae, G. A., 1995. Displacement-based design of RC bridgecolumns in seismic regions,Earthquake Engineering & Structural Dynamics, 24 (12), 1623-1643.

14. Kumar, R. and Singh, Y., 2010. Stiffness of reinforced concrete frame members for seismic analysis,ACI

Journal of Structural Engineering, to appear in - 107(5).

15. Medhekar, M. S. and Kennedy, D. J. L., 2000. Displacement-based seismic design of buildings-theory,

Engineering Structures, 22(3), 201-209.

16. Moehle, J. P., 1992. Displacement-based design of RC structures subjected to earthquakes,Earthquake

Spectra, 8(3), 403-428.

17. NZS 3101, 2006. Concrete structures standard, Part-1 The Design of Concrete Structures and Part-2

Commentary on Design of Concrete Structures, New Zealand Standard (NZS), Wellington 6020.

18. Pauley, T. and Priestley, M.J.N., 1992. Seismic design of reinforced concrete and masonry buildings,

Wiley, New York.

19. Priestley, M.J.N., Calvi, G.M., Kowlsky, M.J., (2007).Displacement-Based Seismic Design of Structures,

IUSS Press, Pavia, Italy.20. Tole, S. V. and Faccioli, E., 1999. Displacement design spectra,Journal of Earthquake Engineering, 3(1),

107-125.

21. Xue, Q., and Chen, C. C., 2003. Performance-based seismic design of structures: A direct displacement-

based approach.Engineering Structures,25(14), 1803-1813.

limitations of indian seismic design codes for rc buildings

Documents