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20320140505010

  1. 1. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online), Volume 5, Issue 5, May (2014), pp. 97-104 © IAEME 97 EVALUATION OF SOIL-STRUCTURE INTERACTION GUIDELINES IN INDIAN SEISMIC CODES Ravi Kant Mittal1 , Vaibhav Gajinkar2 1 Assistant Professor, Department of Civil Engineering, Birla Institute of Technology and Science, Pilani – 333031. India 2 Master Student (Structural Engineering), Department of Civil Engineering, Birla Institute of Technology and Science, Pilani – 333031, India. ABSTRACT According to new guidelines from Indian standards on Earthquake resistant design of structures (IS1893-Part1 and 4), seismic soil-structure interaction (SSI) should be considered if structure is resting other than rock or rock like material having SPT value less than 50. In the present study adequacy of codal provisions related to SSI, given in 1893- part 4 (2005) are evaluated. A parametric study is carried out on a 150 m tall RCC chimney considering the effect of seismic zone on structural response of chimney by incorporating strain dependent shear modulus. The analysis and results show that the time period increases with increase in soil flexibility, while design base shear and design bending moment decreases with increase in soil flexibility. Effect of seismic intensity on shear modulus of soil is considered and consequently the effect on the response of the structure is determined. The reduction in the shear modulus of soil could tend to affect the response as compared to the fixed base by a huge amount in zones of high seismic intensity. Indian code should include provisions to account for this reduction in shear modulus depending upon seismic zone. Keywords: Seismic Soil-structure interactions (SSI), seismic zone, strain dependent shear modulus, Indian Seismic Code 1. INTRODUCTION Conventional structural design suggested in various codes neglect the SSI effects for light structures resting on rock or stiff soil. Considering soil-structure interaction makes a structure more flexible and thus, increasing the natural period of the structure compared to the corresponding rigidly supported structure. Moreover, considering the SSI effect increases the effective damping ratio of the system. The smooth idealization of design spectrum suggests smaller seismic response with the increased natural periods and effective damping ratio due to SSI. With this assumption, it was INTERNATIONAL JOURNAL OF CIVIL ENGINEERING AND TECHNOLOGY (IJCIET) ISSN 0976 – 6308 (Print) ISSN 0976 – 6316(Online) Volume 5, Issue 5, May (2014), pp. 97-104 © IAEME: www.iaeme.com/ijciet.asp Journal Impact Factor (2014): 7.9290 (Calculated by GISI) www.jifactor.com IJCIET ©IAEME
  2. 2. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online), Volume 5, Issue 5, May (2014), pp. 97-104 © IAEME 98 traditionally been considered that SSI can conveniently be neglected for conservative design. This conservative simplification is valid for certain class of structures and soil conditions, such as light structures in relatively stiff soil. However, most of cases structure is not light and resting on soil which is not stiff or rock. In fact, the SSI can have a detrimental effect on the structural response, and neglecting SSI in the analysis may lead to unsafe design for both the superstructure and the foundation. Effects of SSI, plays an important role for heavy structures resting on relatively soft soils for example nuclear power plants, high-rise buildings, chimneys, elevated-highways on soft soil, structures with massive or deep-seated foundations, such as bridge piers, offshore caissons, and silos. ([1] to [10]) 2. SSI PROVISIONS IN INDIAN SEISMIC CODES According to new guidelines from Indian standards on Earthquake resistant design of structures (IS1893-Part1 [1] and IS1893-Part 4[2]), soil structure interaction (SSI) should be considered if structure is resting other than rock or rock like material having SPT value less than 50. First time detailed formulation how to include soil structure interaction for chimneys and stack like structures is given in [2] and described below. 2.1 Fixed Base Time Period As per the codal provision, the time period of stack like structures when fixed at base shall be calculated using the formula ܶ ൌ ‫ܥ‬்ට ௐு ா೎஺௚ (1) where CT= coefficient depending upon the slenderness ratio of the structure given in Table 6 of IS 1893 (Part 4), W = total weight of the structure including weight of lining and contents above the base, H = height of structure above the base, Ec= modulus of elasticity of material of the structural shell, A = area of cross-section at the base of the structural shell, For circular sections, A = 2πrt, where r is the mean radius of structural shell and t its thickness, and g = acceleration due to gravity. 2.2 Horizontal Seismic Coefficient Using the period T, the horizontal seismic coefficient Ah shall be obtained from the spectrum given in IS 1893(Part 1). The design horizontal seismic coefficient for design basis earthquake (DBE) shall be determined by the following expression adopted in [1]. ‫ܣ‬௛ ൌ ௓ ଶ ூ ோ ௌೌ ௚ (2) Z = zone factor. This is in accordance with Table 2 of 1S 1893 (PartI), I = importance factor as given in Table 8 of IS 1893(Part 4), R = response reduction factor as given in Table9 of [2], and Sa/ g = spectral acceleration coefficient for rock and soil sites as given in Fig. 1 of [1]. 2.3 Design Shear force and Moment Either simplified method (that is, equivalent static lateral force method) or the dynamic response spectrum modal analysis method is recommended for calculating the seismic forces
  3. 3. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online), Volume 5, Issue 5, May (2014), pp. 97-104 © IAEME 99 developed in such structures. Site spectra compatible time history analysis may also be carried out instead of response spectrum analysis. 2.4 Simplified Method (Equivalent Static Lateral Force Method) The simplified method can be used for ordinary stack-like structures. The design shear force, V and design bending moment, M, for such structures at a distance X from the top, shall be calculated by the following formulae: a. ܸ ൌ ‫ܥ‬௩‫ܣ‬௛ܹ௧‫ܦ‬௩ (3) b. ‫ܯ‬ ൌ ‫ܣ‬௛ܹ௧ĥ‫ܦ‬௠ (4) Cv = coefficient of shear force depending on slenderness ratio k given in Table 6 of IS 1893(Part 4), Ah, = design horizontal seismic coefficient, Wt= total weight of structure including weight of lining and contents above the base, h = height of centre of gravity of structure above base, and Dv, Dm= distribution factors for shear and moment respectively at a distance X from the top as given in Table 10 of IS 1893(Part 4). The expression for the distribution factors for moment and shear along the height is given in Table 11 of IS 1893(Part 4) for use in computer programme. 2.5 Effective time period due to SSI effect In general form, the effective fundamental period of a structure as modified by the soil has been given Velestos and Meek, 1974, ASCE 7([11], [12]) as Ť ൌ ܶඨ1 ൅ ௄ೣ ൬1 ൅ ௄ೣ ĥ మ ௄ഇ ൰ (5) T = the fundamental period of the structure for fixed base case, = the stiffness of the structure when fixed at the base, defined by the following: ൌ ସగమௐ ௚்మ (6) ĥ = height of centre of gravity of structureabove base, Lateral stiffness (Kx) and rocking stiffness(Kθ) of the foundation given by Richart et al., 1970[13] is adopted in [2] ‫ܭ‬௫ ൌ ଷଶሺଵିఔሻீ௥బ ሺ଻ି଼ఔሻ (7) ‫ܭ‬ఏ ൌ ଼ீ௥బ య ଷሺଵିఔሻ (8) G = shear modulus of soil, ‫ܩ‬ ൌ ߩܸ௦ ଶ ρ = unit weight of soil, Vs = shear wave velocity of the medium, υ = Poisson’s ratio of soil, ro = radius of circular raft foundation. For rectangular foundation effective radius ‫ݎ‬଴ ൌ ට ௔௕ గ may be taken, where a and b are the dimension of the rectangular foundation.
  4. 4. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online), Volume 5, Issue 5, May (2014), pp. 97-104 © IAEME 100 3. DEPENDENCE OF THE SHEAR MODULUS ON THE STRAIN LEVEL Low strain dynamic shear modulus measured by in situ tests corresponds to shear strain amplitude of less than 0.001 %, represented as Gmax. However shearing strains due to earthquakes range from 0.01 to 0.5 % and may reduce Gmax by 0.9 to 0.2 Gmax. As the shear strain of the soil increases during seismic events, the shear modulus decreases. The shear modulus is used in calculating stiffness values for footings. The difference between the small-strain values of Vs, such as those measured by in situ tests, and the values compatible with the strain levels induced by the design earthquake shall be taken into account in all calculations involving dynamic soil properties under stable conditions. There are no provisions in the Indian code to account for the reduced G value. According to Eurocode 8-part5, 2004 [5], if the ground acceleration ratio is equal to or greater than 0.1 g, (i.e. equal to or greater than 0.98 m/s2 ), the reduction factors for Vs and G value may be applied as given in table 1. The same reduction factors were used in the current analysis for comparison. Table 1: Average soil damping factors and average reduction factors (± one standard deviation) for shear wave velocity Vs and shear modulus G within 20 m depth. (Vsmax=average Vs value at small strain (<10-5 ), not exceeding 300 m/s. Gmax=average shear modulus at small strain) Ground acceleration ratio, α Damping factor ࢂ࢙ ࢂࡿ࢓ࢇ࢞ ࡳ ࡳ࢓ࢇ࢞ 0.1 0.03 0.9 (±0.07) 0.8 (±0.10) 0.2 0.06 0.7(±0.15) 0.5 (±0.20) 0.3 0.10 0.6 (±0.15) 0.35 (±0.20) 4. PROBLEM STATEMENT Understanding the importance of effect of soil structure interaction on the seismic response of tall slender structures, in this study attention is focused on evaluating the seismic response of tall chimney considering the effect of soils with different shear velocity ranging from 150 m/s to 1200 m/s (soft soil to hard strata respectively.) and comparing the results obtained with those from fixed base assumption. For analysis a chimney 150m high with raft diameter of 18m and area of shell 8.5m2 was considered. The time period of vibration for the structure fixed at base and the flexible base period were computed separately. The equivalent lateral force static method (clause 17.1, IS 1893-part 4) of analysis was used to compute the design shear force and design bending moment. The properties of different types of soils that were used in study are represented in table 2. The table was extracted from the Mehta and Gandhi, 2008 [14] and sample calculation is given in Appendix A Table 2: Properties of soil Velocity of Shear waves, Vs (m/s) Soil type Density, ρ (kN/m3 ) Poisson’s ratio, υ Shear modulus, G (kN/m2 ) Elastic modulus (kN/m2 ) 150 Soft Soil 16 0.49 36700 14.95 x 104 300 Stiff Soil 20 0.45 183500 25.836 x 105 600 Dense Soil 22.4 0.35 322000 50.53 x 107 1200 Rock 25.6 0.30 3758900 30.42 x 107
  5. 5. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online), Volume 5, Issue 5, May (2014), pp. 97-104 © IAEME 101 5. RESULTS AND DISCUSSION MS Excel spreadsheet was developed to analyse for different responses using Equivalent Static Lateral Force Method for tall chimneys. The following tables 3 and 4 enable us to view the results of analysis for different responses and understand the importance of Soil Structure Interaction (SSI) effects on seismic response of tall chimneys Table 3: % change in base forces with consideration of flexible foundation with respect to fixed foundation for zone IV Type of soil Shear wave velocit y, Vs (m/s) Density,ρ (kN/m³) Poiss on's ratio, ν Without SSI (Fixed base) With SSI (Flexible base) % reducti on in forces Time period, T (sec) Base shear force, V (kN) Base moment, M (kN-m) Time period, T (sec) Base shear force, V (kN) Base moment, M (kN-m) Soft soil 150 16 0.49 1.122 4191.2 213402.4 2.548 1846.49 94016.8 55.94 Stiff soil 300 20 0.45 1.122 3413.2 173788.8 1.545 2479.52 126248.4 27.36 Dense soil 600 22.4 0.35 1.122 2509.7 127785.8 1.248 2257.69 114953.8 10.04 Rock 1200 25.6 0.3 1.122 2509.7 127785.8 1.153 2442.85 124381.3 2.66 Table 4: % change in base forces with consideration of flexible base and reduced G value with respect to fixed foundation Zone Zone factor, Z Without SSI (Fixed base) b With SSI and reduced G valueb % reduction in forces Time period, T (sec) Base shear force, V (kN) Base moment, M (kN-m) Time period, T (sec) Base shear force, V (kN) Base moment, M (kN-m) II 0.10 1.122 1422.172 72412.005 1.634 977.062 49748.58 31.30 III 0.16 1.122 2275.475 115859.208 1.755 1455.703 74119.29 36.03 IV 0.24 1.122 3413.212 173788.813 1.955 1959.574 99774.65 42.59 V 0.36 1.122 5119.818 260683.219 2.117 2714.673 138221.13 46.98 b The values were determined for stiff soil having Vs= 300 m/s, ρ= 20 kN/m³ and ν= 0.45 1. It is observed from Table 3, that the time period for chimney is more for flexible soil. The time period goes on decreasing as the soil goes on getting stiffer. 2. Time period goes on decreasing as the shear velocity increases i.e. for stiffer soils with higher shear velocity the time period values approach nearer to that obtained by fixed base assumption. Hence for shear velocity in excess of 600m/s soil flexibility can be ignored and base can be treated as fixed. 3. Due to SSI effect, the design shear force and bending moment was found to reduce by a greater percentage in case of soft soil when compared to fixed base condition. However for stiffer soils this reduction was very less. 4. Maximum Shear Modulus corresponds to low strain dynamic shear modulus measured at shear strain amplitude of less than 0.001 %. However shearing strains due to earthquakes range from 0.01 to 0.5 % and may reduce Gmax by 0.9 to 0.2 Gmax. Computation with reduced G value leads to further reduction in the shear force and bending moment values as shown in table 4. The reduction is very high in zones of high seismic intensity.
  6. 6. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online), Volume 5, Issue 5, May (2014), pp. 97-104 © IAEME 102 6. CONCLUSION When a chimney was analyzed using equivalent static lateral force method for different types of soils considering flexible base and fixed base, the obtained results showed the importance of soil structure interaction effects. As the shear strain of the soil increases during seismic events, the shear modulus decreases. This could tend to increase or decrease the response as compared to the fixed base by a huge amount in zones of high seismic intensity. There are no provisions in the Indian code to account for the reduced G value. It is thus concluded that seismic response of tall chimneys is influenced greatly by soil supporting its base and nature of earthquake excitations striking the base. Ignoring any one of them, can significantly affect the performance of chimney during earthquake and lead to devastating effects. There is need to revise Indian code by including guidelines on reduction factor used to assess the effect of reduced G value on response of the structure as mentioned in Euro code EN 1998-5:2004 and ASCE 7. REFERENCES [1] IS: 1893 (Part 1), Criteria for Earthquake Resistant Design of Structures (Part 1) General Provisions and Buildings, India: Bureau of Indian Standards, 2002 [2] IS: 1893 (Part 4), Criteria for Earthquake Resistant Design of Structures (Part 4) Industrial Stack-like Structures, India: Bureau of Indian Standards, 2005 [3] ATC-40, Seismic evaluation and retrofit of concrete buildings, Applied Technology Council, 1996. [4] FEMA-450, "NEHRP recommended provisions for seismic regulations for new buildings and other structures," ed. Washington, D.C.: Building Seismic Safety Council, 2003. [5] Eurocode 8- Part-5, EN 1998-5, Design of structures for earthquake resistance-Part 5: Foundations, retaining structures and geotechnical aspects, European Standard, 2004 [6] J. P. Wolf, Dynamic Soil-Structure Interaction. Prentice-Hall, Inc., Englewood Cliffs, New Jersey, 1985 [7] S. L. Kramer, Geotechnical Earthquake Engineering: Pearson Education India, 1996, 294- 305. [8] G. Mylonakis and G. Gazetas, Seismic soil-structure interaction: beneficial or detrimental?, Journal of Earthquake Engineering, 4(3), 2000, pp. 277-301. [9] G. Jie, M. Preising, and B. Jeremic, Benefits and detriments of soil foundation structure interaction, Proceedings of Sessions of Geo-Denver, 2007. [10] Wikipedia http://en.wikipedia.org/wiki/Soil_structure_interaction [11] A. S. Veletsos and J. W. Meek, "Dynamic behaviour of building-foundation systems," Earthquake Engineering and Structural Dynamics, vol. 3, pp. 121-138, 1974. [12] ASCE 7, Minimum Design Loads for Buildings and Other Structures, USA: American Society of Civil Engineers, 2010, 199-202 [13] E. Jr., Richart, J. R. Jr. Hall, and R. D. Woods, Vibration of soils and foundations, Prentice- Hall, 1970 [14] D. Mehta, and N. J. Gandhi, Time response study of tall chimneys, under the effect of Soil Structure Interaction and long period earthquake impulse. In: Proceedings of the 14th World Conference on Earthquake Engineering, Beijing, China, 2008
  7. 7. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online), Volume 5, Issue 5, May (2014), pp. 97-104 © IAEME 103 APPENDIX A Structure details: Height of the RCC chimney, H= 150 m Diameter of raft = 18 m Area of the shell, A = 8.5 m2 Moment of inertia, Ib = 92.5 m4 Assume structure is located in zone IV Soil properties: Unit weight of soil, ρ = 20 kN/m3 ,Poisson's ratio, υ = 0.45 Shear wave velocity, Vs = 300 m/s, Dynamic shear modulus of soil, ‫=ܩ‬ 183486.24 kN/m2 Shell material (concrete) properties: Modulus of elasticity of concrete, Ec = 3.12 x 108 kN/m2 Unit weight of concrete, γc = 25 kN/m3 Without considering SSI effect (Fixed base condition) Weight of chimney ܹ ൌ ‫ߛܪܣ‬௖ 31875 kN Height of centre of gravity of structure above base ĥ ൌ ‫ܪ‬ 2ൗ 75 m Radius of gyration of the chimney ‫ݎ‬௘ ൌ ට‫ܫ‬௕ ‫ܣ‬ൗ 3.299 m Slenderness ratio k = H/re 45.47 From table 6 of IS 1893 (Part 4) CT 82.8 CV 1.473 As per IS 1893 (Part 4), time period of a fixed base chimney is given by ܶ ൌ ‫ܥ‬்ඨ ܹ‫ܪ‬ ‫ܧ‬௖‫݃ܣ‬ 1.122 sec Considering medium soil, (Clause 6.4.5 of IS 1893- part 1) ܵ௔ ݃ ൌ 1.36 ܶ 1.2116 Zone factor (Table 2 of IS 1893- part 1) Z 0.24 Importance factor (Table 8 of IS 1893- part 4) I 1.5 Response reduction factor (Table 9 of IS1893-part 4) R 3 Horizontal siesmic coefficient ‫ܣ‬௛ ൌ ܼ 2 ‫ܫ‬ ܴ ܵ௔ ݃ 0.0727 At base of the chimney From table 10 of IS 1893 (Part 4) DV 1 Dm 1 Design shear force ܸ ൌ ‫ܥ‬௩‫ܣ‬௛ܹ௧‫ܦ‬௩ 3413.212 kN Design bending moment ‫ܯ‬ ൌ ‫ܣ‬௛ܹ௧ĥ‫ܦ‬௠ 173788.81 kNm
  8. 8. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online), Volume 5, Issue 5, May (2014), pp. 97-104 © IAEME 104 Considering SSI effect and Gmax value Radius of raft r0 9 m The fixed base stiffness of the chimney ൌ 4ߨଶ ܹ ݃ܶଶ 101807.863 kN/m Horizontal soil stiffness ‫ܭ‬௫ ൌ 32ሺ1 െ ߥሻ‫ݎܩ‬଴ ሺ7 െ 8ߥሻ 8548300.054 kN/m Rocking soil stiffness ‫ܭ‬ఏ ൌ 8‫ݎܩ‬଴ ଷ 3ሺ1 െ ߥሻ 648540450.4 kN/m Effective time period Ť ൌ ܶඨ1 ൅ ‫ܭ‬௫ ൭1 ൅ ‫ܭ‬௫ ĥ ଶ ‫ܭ‬ఏ ൱ 1.545 sec For medium soil ܵ௔ ݃ ൌ 1.36 ܶ 0.8802 Horizontal siesmic coefficient Ah 0.0528 Design shear force ܸ ൌ ‫ܥ‬௩‫ܣ‬௛ܹ௧‫ܦ‬௩ 2479.520 kN Design bending moment ‫ܯ‬ ൌ ‫ܣ‬௛ܹ௧ĥ‫ܦ‬௠ 126248.474 kNm Percentage reduction in shear 27.36 % Considering soil effect with reduced G value Ground acceleration ratio (for zone IV) α 0.24 As per Euro code 8- part 5 (table 1) average reduction factor ‫ܩ‬ ‫ܩ‬௠௔௫ 0.44 Thus reduced dynamic shear modulus of soil ‫ܩ‬ ൌ ߙ‫ܩ‬௠௔௫ 80733.945 kN/m2 Horizontal soil stiffness ‫ܭ‬௫ ൌ 32ሺ1 െ ߥሻ‫ݎܩ‬଴ ሺ7 െ 8ߥሻ 3761252.024 kN/m Rocking soil stiffness ‫ܭ‬ఏ ൌ 8‫ݎܩ‬଴ ଷ 3ሺ1 െ ߥሻ 285357798.2 kN/m Effective time period Ť ൌ ܶඨ1 ൅ ‫ܭ‬௫ ൭1 ൅ ‫ܭ‬௫ ĥ ଶ ‫ܭ‬ఏ ൱ 1.955 sec For medium soil ܵ௔ ݃ ൌ 1.36 ܶ 0.6956 Horizontal siesmic coefficient Ah 0.0417 Design shear force ܸ ൌ ‫ܥ‬௩‫ܣ‬௛ܹ௧‫ܦ‬௩ 1959.574 kN Design bending moment ‫ܯ‬ ൌ ‫ܣ‬௛ܹ௧ĥ‫ܦ‬௠ 99774.65 kNm Percentage reduction in shear 42.59 %

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