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FINITE ELEMENT ANALYSIS OF 99.60 M HIGH ROLLER COMPACTED CONCRETE _RCC_ GRAVITY DAM - SPECIAL EMPHASIS ON DYNAMIC ANALYSIS

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FINITE ELEMENT ANALYSIS OF 99.60 M HIGH ROLLER COMPACTED CONCRETE _RCC_ GRAVITY DAM - SPECIAL EMPHASIS ON DYNAMIC ANALYSIS Powered By Docstoc
					  INTERNATIONAL JOURNAL OF CIVIL ENGINEERING AND
                              TECHNOLOGY (IJCIET)
   International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),
   ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME
ISSN 0976 – 6308 (Print)
ISSN 0976 – 6316(Online)
Volume 3, Issue 2, July- December (2012), pp. 387-391
                                                                              IJCIET
© IAEME: www.iaeme.com/ijciet.asp
Journal Impact Factor (2012): 3.1861 (Calculated by GISI)                   © IAEME
www.jifactor.com



         FINITE ELEMENT ANALYSIS OF 99.60 M HIGH ROLLER
        COMPACTED CONCRETE (RCC) GRAVITY DAM - SPECIAL
                 EMPHASIS ON DYNAMIC ANALYSIS
                                    KARIM M PATHAN
                                Consulting Structural Engineer,
                   Kasheef and Associates, Aurangabad -05, Maharashtra, India
                 E-mail: kmpathan@rediffmail.com, karimpathan.abd@gmail.com

   ABSTRACT

           Dams are supposed to be very important structures as they play a vital role in the
   economical and social development of the area in which being constructed as well many a
   times useful in Hydro power generation. The effect of failure of dam structures can be
   imagined by studding the area covered by the dam on downstream side. Though, there are
   very few cases of failure of dams, every designer shall think of zero probability of failure of
   such structure. The Dam discussed here is Roller Compacted Concrete (RCC) Dam being
   constructed over Vaitarna River Near Mumbai, India. The Non overflow section has 99.60 m
   height which is very close to 100 m for which dynamic Analysis is desirable as per Indian
   Code of practice. Most of the organizations analyze the dams by elastic method which gives
   very rough results. The tolerance can be accepted in most of the cases as the factor of safety
   used in dam design is 4 for concrete dams. Here Finite Element Approach is used to analyze
   the dam which is proved to be the realistic for such structures. A comparison is done between
   the equivalent static approach of seismic analysis with dynamic analysis by using time
   history. The dynamic behavior is studied by using Time History of actual earthquake of
   koyna ( Maharashtra , India ) and the proportioning is done to satisfy the stress limits.

   Keywords: Finite Element Analysis, Dynamic Analysis, RCC, Gravity Dam, Stress Contours

   1. INTRODUCTION

           Gravity dams are very popular in these days due to ease to construction and
   availability of machineries like concrete pumps and ready mix plants of huge capacity. The
   Vaitarna dam described here is of RCC type. The dam body is designed to withstand the
   water pressure, uplift forces and silt load, if any, along with the weight of the dam. As per the
   Indian Standard Code of practice, dynamic analysis shall be done for dams with height
   greater than 100m. The height of Vaitarna dam is very close to this value, hence dynamic
   analysis is done by using time history of Koyna Earthquake of 10th December 1967 with PGA
   of 0.613g. The effects of equivalent static analysis and dynamic analysis are then compared.

                                                 387
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),
ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME

2. SALIENT FEATURES OF THE RCC DAM

       The dam is having foundation R.L.187.80m and T.B.L at R.L.287.40m. The Full
Reservoir level is at 285.00. Top width is kept as 8.0m whereas bottom width is 91.31m.
Upstream slope is 0.15:1.0 starting from top and downstream slope is 0.75:1.0 which starts
from R.L.278.97m. The Roller Compacted Concrete used is of G-75 Grade. The dam body
and surrounding rock is divided into 84 finite elements each.(See fig.1). The sizes of the
elements near the point of interest is kept smaller compared to the other elements.




        Y
        Z X
                                                                         Load 8




                          Fig.1: Finite Element Model of the Dam

3. FINITE ELEMENT MODELING OF THE DAM

        The dam body is modeled in STAADpro using the SOLID isoparametric finite
elements with eight nodes. Each node has three translational degrees of freedom. The
stiffness matrix of the solid element is evaluated by numerical integration with eight Gauss –
Legendre points.
        The dam is analyzed for several basic loads and load combinations possibly met with
during its service. These are enlisted in table 1 below. The stresses induced are checked for
all the combinations and the dimensions are so framed that the factor of safety mentioned
above is maintained.



                                             388
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),
ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME

         The base of the dam is to rest on rock and the extra excavation is to be filled with
concrete of same strength, the foundation rock of approximately equal to the height of dam is
modeled around and below the foundation level.
         The Young’s modulus for concrete is used as 2.26 x 104 N/mm2 and density 26.27
      3
kN/m . For the foundation rock these properties were 1.0 x 104 N/mm2 and 28.8 kN/m3
respectively. Poisons ratio for concrete is 0.17 whereas for rock it is 0.16. Damping fraction
is assumed as 0.1, 0.2 and 0.3 for first three modes.
             Table 1: Basic Loads and Load combinations used during the analysis
 Sr. Basic Loads                                                  Load Combinations
 No
 1      Self Weight ( Wg )
 2       Hydrostatic pressure on upstream face ( Wp )
 3       Uplift pressure ( Uw)
 4       Silt Loads ( Sw)
 5       Equivalent Static Analysis for Earthquake Loads
         ( ESA )
 6       Hydrodynamic Effects due to sloshing ( Hd )
 7       Time History loads ( Wt )
 8                                                                      ( Wg+Wp+Sw+Uw+ESA+Hd)
 9                                                                      ( Wg+Wp+Sw+Uw+Wt+Hd)

4. DYNAMIC RESPONSE OF THE DAM

        Time history load is applied to the Finite Element model of the Dam by standard
software, STAADpro, with 500 pairs of time and acceleration of the Koyna Earthquake in
one of the horizontal direction. The finally reduced dimensions give maximum horizontal
displacement as 70.47 mm. The modes of vibration are shown in fig.2. Time period for the
first mode is found to be 0.5498s and frequency as 1.819 Hz.
        The stress variation on the finally proposed section is studied for all load
combinations. The axial stress contours are shown in fig.3. Maximum normal stress induced
is 4.15 N/mm2 for the case with time history loading.




 Y                                                             Y
 Z   X                                                              X
                                 Load 6 : Mode Shape 1             Z                Load 6 : Mode Shape 2




                            Fig.2: Modes of Vibration for the Dam body

                                                         389
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),
ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME




   Y                                                   Y
   Z   X                                               Z   X
                                        Load 7                                         Load 8




                        Fig.3: Stress Contours for the Dam Cross Section

5. RESULTS AND COMPARISON

        Maximum normal stress in material is observed due to the time history load case
which is 4.15 N/mm2, whereas for Equivalent static analysis using Response Spectra Method
described in I.S.1893 gives maximum normal stress as 3.44 N/mm2. Thus dynamic analysis
governs. The tension induced in the body of dam in case of Time History loading is observed
as 2.8 N/mm2 whereas in case of Equivalent static analysis the tension is 1.16 N/mm2. The
effect of soil structure interaction is to reduce the net tension in the body of the dam. The
deflection of the dam in case of time history loading is nearly three times of deflection due to
equivalent static analysis.

6. CONCLUSION

        Dams being very important structure shall be designed with very great accuracy.
Finite Element method shall be preferred over the conventional elastic methods. The great
advantage is that, the stress variation through the whole body can be studied carefully and the
slopes can be designed according to the stress pattern. The points where slope changes, are
points of stress concentration. Such points can be observed carefully and taken care to avoid
over stressing. Moreever, the stress concentration near the gallery can be studied. As far as
dynamic analysis is considered, the stresses are more as compared to the equivalent static
analysis, hence shall be preferred while designing the important structures like dams.

REFERENCES

       1. R. W. Clough, “The Finite Element Method in Plane Stress Analysis,” Proceedings of
          2nd ASCE Conference on Electronic Computation, Pittsburgh, PA, September 8–9,
          1960.
       2. R. W. Clough, “The Finite Element Method after Twenty-Five Years: A Personal
          View,” Computers and Structures, Vol. 12, 1980, pp. 361–370.
       3. O. C. Zienkiewicz and Y. K. Cheung, “Finite Elements in the Solution of Field
          Problems,” Engineer, Vol. 220, 1965, pp. 507–510.
       4. Edword L Wilson, “Three Dimensional Static and Dynamic Analysis of Structures”,
          Computers and Structures, Berkeley, California. ( Third Edition)



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International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),
ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME

   5. Roman Lewandowski, “Dynamic Analysis of Structures with Multiple Tuned mass
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   7. I.S.1893-1984, “Criteria for Earthquake Resistant Design of Structures”. B.I.S. New
       Delhi.
   8. I.S.6512-1984, “Criteria for Design of Solid Gravity Dams”. B.I.S. New Delhi.
   9. Raju Sathish Kumar, Janardhana Maganti and Darga Kumar Nandyala, “Rice Husk
       Ash Stabilized Compressed Earth Block-A Sustainable Construction Building
       Material – A Review” International Journal of Civil Engineering & Technology
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   10. Vidula S. Sohoni, and Dr.M.R.Shiyekar, “Concrete–Steel Composite Beams Of A
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   11. Dr. Shanthappa B. C., Dr. Prahallada. M. C. and Dr. Prakash. K. B., “Effect Of
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       Engineering & Technology (IJCIET), Volume2, Issue1, 2011, pp. 17 - 24, Published
       by IAEME
   12. Dr. Prahallada. M. C., Dr. Shanthappa B. C., and Dr. Prakash. K. B., “Effect Of
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       (IJCIET), Volume2, Issue1, 2011, pp. 25 - 34, Published by IAEME




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