Scientific Journal of Impact Factor (SJIF): 4.72 International Journal of Advance Engineering and Research Development Volume 4, Issue, May-2017 Parametric Study of Beam Slab Raft Foundation Sudhir.D.Ravani 1, Nihil Sorathia 2 e-issn (O): 2348-4470 p-issn (P): 2348-6406 1 Civil Department, Parul institute of engineering & Technology,ravanisudhir@gmail.com 2 Civil Department, parul institute of engineering & Technology, Abstract in last some year, there have been increase in high rise building construction due to lack of space and many amenities at a particular place requirement. The use of raft foundation many possiblity to reduce the chances of differential settlement and increased soil pressure at foundation level to be transmitted with the concept of buoyancy raft. The behavior of structure is depends on soil strata.. The design of raft foundation is many different soil structure interaction problems (SSI). SSI effect depends on many types of factors like that modulus of elasticity, loadings, soil subgrade modulus, thickness of raft, column size of structure. The Parametric Study is done on Various Parameter like Subgrade Modulus, Thickness of raft, Column size and Modulus of Elasticity of raft. The bending moment and shear force, soil pressure are significantly effect by Soil Structure Interaction. The raft behaviour has been checked in this Study. Keywords- Raft Foundation, Soil Structure interaction, Moment, Shear Force, Soil Pressure, Raft Settlement. I. INTRODUCTION In the last few years, due to rapid urbanization and lack of space for horizontal expansion, cities have grown vertically. High rise buildings are increasing at a rapid rate. For foundations of such high rise building, normally raft foundation, pile foundation or piled-raft foundation are used. Design of raft foundation is a soil structure interaction problem and there are many factors affecting raft foundation design. It has been observed that in practice, it has been very much difficult for designer to consider the influence of all parameters that are likely to affect design and generally simplified approach of conventional design is adopted in most of the cases. A raft is a combined footing that covers the entire area beneath a structure and supports all walls and columns. When the allowable soil pressure is low, or building loads are heavy, the use of spread footings would cover more than one half of the area and it may prove more economical to use raft foundations. Raft foundation is also used when the soil mass contains compressible lenses or the soil is sufficiently erratic. So that the differential settlement can be controlled. It is also used where the soil is having low bearing capacity in such cases, the concept of floating raft is applied i.e. The weight of the superstructure is balanced by the weight of the soil removed. Ordinarily, rafts are designed as reinforced concrete flat slabs. If the C.G of loads coincides with the centroid of the raft, the upward load is regarded as a uniform pressure equal to the downward load divided by the area of the raft. The weight of the raft is not considered in the structural design because it is assumed to be carried directly by the subsoil. Since this method does not take into account moments and shear caused by differential settlements, it is customary to reinforce the raft more heavily than required according to analysis. II. LITRATURE REVIEW A. G. S. Kame, S. K. Ukarande, K. Borgaonkar &V. A. Sawant is considered a non-rigid or elastic approach to solve the raft foundation. Finite element analysis was utilized to serve the purpose. So as to take into account the effect of transverse shear force/ stresses. In this paper, the effect of modulus of sub grade reaction, thickness of raft and load pattern was observed and variation in the analysis was observed. The aim of the research was to study the effect of different parameters on the raft foundation with the help of finite element procedure. Following were the parameters which researchers have varied:- 1. Thickness of the raft (0.4m, 0.9m, 1.m) 2. Modulus of subgrade reaction (40000, 100000, 200000, 400000) kn/m3, 3. Column loads of different patterns. They have found that, at lower soil modulus, deflections were increasing with raft thickness and at higher modulus trend is reverse. Positive bending moments were increasing with raft thickness and negative bending moments are reduced with raft thickness. When effect of soil modulus was considered, it was found that positive bending moments are decreasing at higher modulus, and negative bending moments are increasing with soil modulus. @IJAERD-2017, All rights Reserved 408
Volume 4, Issue, May-2017, e-issn: 2348-4470, print-issn: 2348-6406 B. R. Ziaie- Moayed and M. Janbaz (2009) Studied In their study they had modelled the foundation on clay soil with Finite Element software to investigate the validation of Terzaghi s Formula and the effect of different parameters on the modulus of sub grade reaction. This modulus of sub grade reaction depends on some parameters like, 1.) Size effect of foundation, 2.) Effect of foundation s shape, 3.) Effect of Depth of foundation, 4.) Effect of rigidity of foundation, The figure below shows the soil system for which modulus of sub grade reaction. The vertical settlement for each analysis was obtained and the load- settlement graph were plotted, then the secant modulus of subgrade reaction of each graph was determined. Dishing effect of flexible foundation was also discussed. Figure1. Soil model in plaxis The value of K obtained according to the Terzaghi s equation is first calculated by Plate Load test. Then this value is modified for the width, depth, shape. Also the research has shown the relation on the basis of which we can analyze a foundation on the basis of the conventional design procedure or as a beam on elastic foundation. The effects of different parameters are studied and following are the results: modulus of sub grade decreases as the side dimension of footing increases. The modulus of sub grade reaction is directly influenced by the consistency of clayey soil. If the consistency of soil decreases the modulus value decreases. K value increases with depth of embedment. C. D. Daniel Thangaraj and K. Ilamparuthi (2010) have tried to show the interaction among superstructure, substructure and soil system and compare it with no interaction analysis. The analysis of superstructure without modeling foundation and conversely analyzing the foundation system without considering the rigidity of the superstructure may mislead the estimation of the force, bending moment and the settlement etc. In interaction analysis, all the three components namely soil, raft and superstructure are analyzed as a single compatible unit and compared with non-interaction analysis. A detailed parametric study was conducted by varying the relative stiffness of superstructure, ksb and the raft, krs. The relative stiffness Ksb and Krs are determined based on the recommendation of Brown et al. (1986). The influence of these two parameters on the forces and moments in superstructure and the raft were studied. Analyses are carried out for the following ranges of values: Ksb =1 to 100 and krs =0.001 to 0.01 The interaction analysis showed less total and differential settlements than the non-interaction analysis did. Between the two parameters, krs and ksb, ksb has a significant influence on both the settlements indicating that the modulus of the soil plays major role in the performance of the raft. Also in the analysis, the moments in the raft, the end span is increases with the increase in ksb value whereas krs alters the moment below the interior columns as well as the span moment in the interior panels of the raft. D. Mohamed Saad Eldin & Arafa El-Helloty (2014) Studied done for behavior of raft foundation with opening of raft foundation with opening. The finite element method is carried out with help of software Plaxis 3D.The @IJAERD-2017, All rights Reserved 409
Volume 4, Issue, May-2017, e-issn: 2348-4470, print-issn: 2348-6406 study is to know about what effect produce when raft is have opening and bending moment and shear force behavior is analysed. The various parameter used are different types of soil and settlement in raft. They have taken three type of soil which are hard soil, medium soil and soft soil. The analysis of raft and analysis of raft with opening is done. The settlement in both raft is checked. They have found that with opening in raft the moment increasing with different types of soil. The opening have largest effect on corner side the moment have larger than simple raft. They have also taken in different types of soil and found that loose soil has maximum effect of soil the settlement and moment are increase in the raft with soft soil and have opening. While dense soil has less effect on settlement and moment. III. MODELLING AND STRUCTURAL IDEALIZATION Model created in CSI ETAB 201 and Dead load and Live load taken and the load on that column reaction taken for raft. In Etab 201 12 story Building has been created.that reaction taken into CSI SAFE v12 as input file. This reaction is Total load for Raft foundation and manually total load calculation is made and basic geometry has been selected and geometry has created in CSI SAFE v12 Software. The basic parameter which is required that are assign to raft and analysis of raft is performed and various parameters are checked and after that parameter like modulus of subgrade, thickness, column size and Modulus of elasticity changed. The results of bending moment, shear force, deflection, soil pressure and settlement are considered. For present study of mat response a typical 12 storey frame building is modeled and Basic raft size is 14m x 14 m. This reference model has four bays in both directions. The variations of parameters of these models are given below in Table 1 Table 1.Variation of parameters of models Variations of parameters of these models Parameters Ranges of variations Soil subgrade modulus(kn/m 3 ) 26000,30000,34000 Raft thickness(mm) 400,40,00 Modulus of elasticity(n/mm 2 ) M2,M30,M3 Column Size(mm) 300 x 600, 300 x 70, 300 x 82 Figure2.Basic geometry of beam slab raft @IJAERD-2017, All rights Reserved 410
Volume 4, Issue, May-2017, e-issn: 2348-4470, print-issn: 2348-6406 IV. RESULTS AND DISCUSSIONS 4.1. Variation in Soil Subgrade Modulus Modulus of subgrade is important parameter which affects the raft. Initially SBC is assumed 200 KN/m2 from that Modulus of subgrade is find by J Bowels formula. First modulus of subgrade k=26000 KN/m3 taken and after that modulus of subgrade reaction value has been increase to k=30000 KN/m3, k=34000 KN/m3. As shown in below table2.as Soil subgrade modulus value k increase the Positive moment Increase and negative moment Decrease. Because Soil stiffness get increase. Also figure 3. Plotted. As the Soil subgrade modulus increase the shear force get decrease. Table 2.Variation in Soil Subgrade Modulus with Moment Soil Subgrade Modulus (KN/m3) K=26000 K=30000 K=34000 Distance(m) 0 8.7883 8.8461 8.8923 1 4.679 4.9262 46.1394 2.0-116.039-114.3806-112.8742 2.9-107.389-10.118-103.1242 3.8 39.6093 40.081 40.4807 4 1.7211 2.2144 2.62.0-71.117-69.987-69.0339.9-66.723-6.9982-6.21 6.8 47.8994 48.003 48.0481 7 60.3166 60.4466 60.061 8.0-9.6781-9.128-8.816 8.9-77.239-7.9776-74.987 9.8 41.4274 41.8269 42.14 10 1.9638 2.4604 2.8713 11.0-98.713-96.749-94.977 11.9-121.820-120.0083-118.373 12.8 33.9132 34.2122 34.4738 13 48.1399 48.4199 48.661 14 0 0 0 Figure 3. Variation in Soil Subgrade Modulus with shear force @IJAERD-2017, All rights Reserved 411
Volume 4, Issue, May-2017, e-issn: 2348-4470, print-issn: 2348-6406 Modulus of subgrade reaction is increase when Soil pressure is getting increased as shown in figure 4.and Modulus of subgrade reaction is increase when Deflection is getting decreased as shown in figure. Figur4. Soil pressure vs modulus of subgrade Figur. Deflection vs modulus of subgrade 4.2. Variation in Raft thickness Thickness is the most important parameter which also effect the economy. First thickness of raft is taken as 400mm, after that thickness is increase to 400mm, 40mm&00mm. As shown in table 3. Raft thickness increased when Negative and positive both moment Decrease. And also shown in Figure 6. Shear force are also get decrease as the thickness of raft is increased. Increase in Modulus of subgrade reaction Soil pressure is getting increased as shown in figure 7. Increase the thickness of raft when deflection in getting increased as shown in figure 8. @IJAERD-2017, All rights Reserved 412
Volume 4, Issue, May-2017, e-issn: 2348-4470, print-issn: 2348-6406 Table 3.Variation in raft thickness with Moment Raft Thickness(mm) 400mm 40mm 00mm Distance(m) 0 8.7883 8.420 8.1164 1 4.679 4.6786 4.6048 2.0-116.039-129.8188-141.069 2.9-107.389-118.48-128.4462 3.8 39.6093 40.7067 41.2732 4 1.7211 2.1417 2.06.0-71.117-81.6773-91.293.9-66.723-72.622-79.1812 6.8 47.8994 0.3166 1.8632 7 60.3166 62.022 62.888 8.0-9.6781-68.436-76.9877 8.9-77.239-8.2703-93.3476 9.8 41.4274 42.7419 43.4472 10 1.9638 2.366 2.278 11.0-98.713-112.3234-123.842 11.9-121.820-133.146-143.1968 12.8 33.9132 34.0796 34.1436 13 48.1399 47.7683 47.374 14 0 0 0 Figure 6. Variation in raft thickness with shear force @IJAERD-2017, All rights Reserved 413
Volume 4, Issue, May-2017, e-issn: 2348-4470, print-issn: 2348-6406 Figure 7.Soil pressure vs Raft Thickness Figure 8.Deflection vs Raft Thickness 4.3. Variation in Column size Column size is parameter which affect the moment value in the Raft. Initially column size is 300mmx600mm after increasing column size 300mmx70mm and 300mmx82mm. Column size increase when Negative and positive bending moment get decrease as shown in table 4.and column size and Column size affect Shear force in raft column size increase Shear force is getting decrease negative shear force get sudden decrease but positive shear force is decrease very marginally as shown in figure 9. @IJAERD-2017, All rights Reserved 414
Shear Force(KN) International Journal of Advance Engineering and Research Development (IJAERD) Volume 4, Issue, May-2017, e-issn: 2348-4470, print-issn: 2348-6406 Table 4.Variation in column size with Moment Column size(mm) 300mm x 600mm 300mm x 70mm 300mm x 82mm Distance (m) 0 8.7883 6.624 4.1779 1 4.679 36.7827 32.092 2.0-116.039-116.7349-107.3906 2.9-107.389-109.321-106.6279 3.8 39.6093 32.0612 24.739 4 1.7211 41.9024 33.4208.0-71.117-72.241-67.813.9-66.723-68.972-70.1977 6.8 47.8994 38.4271 29.487 7 60.3166 48.72 38.141 8.0-9.6781-60.7623-6.8732 8.9-77.239-79.62-79.7916 9.8 41.4274 33.681 26.1993 10 1.9638 42.0267 33.327 11.0-98.713-99.6411-92.1096 11.9-121.820-123.9868-118.492 12.8 33.9132 26.9017 22.1261 13 48.1399 37.9423 31.4004 14 0 0 0 100 0 0-0 -100-10 -200-20 -300-30 0 1 2.0 Variation in Column Size(mm) 2.9 3.8 4.0.9 6.8 7 8.0 8.9 9.8 10 11. 0 11. 9 12. 8 13 14 C-300X600-21. 33. -30-47 43. 39. -30-40 2. 66. -27-32 8. 72. -27-1 32. 0 C-300X70-21 44. 34. -31-33 29. 40. -30-27 37 66. -28-21 42. 73. -27-38 17. 0 C-300X82-21 40. 3. -31-2 26 41. -30-19 33. 66. -28-13 38. 71. -27-30 12. 0 Figure 10. Variation in column size with shear force @IJAERD-2017, All rights Reserved 41
Shear Force(KN) International Journal of Advance Engineering and Research Development (IJAERD) Volume 4, Issue, May-2017, e-issn: 2348-4470, print-issn: 2348-6406 4.4. Variation in modulus of elasticity Modulus of Elasticity is Varies to M2, M30 and M3 and various results are plotted. Modulus of elasticity is increased when decrease in positive moment & increase in negative moments, but that variation is not more but modulus elasticity is effect some points. Modulus of elasticity is increased when increase in shear force at some point but same at other points. Therefore shear force also not much affected by modulus of elasticity. Table.Variation in Modulus of elasticity with Moment Modulus of elasticity(n/mm2) M2 M30 M3 Distance (m) 0 8.7883 6.624 4.1779 1 4.679 36.7827 32.092 2.0-116.039-116.7349-107.3906 2.9-107.389-109.321-106.6279 3.8 39.6093 32.0612 24.739 4 1.7211 41.9024 33.4208.0-71.117-72.241-67.813.9-66.723-68.972-70.1977 6.8 47.8994 38.4271 29.487 7 60.3166 48.72 38.141 8.0-9.6781-60.7623-6.8732 8.9-77.239-79.62-79.7916 9.8 41.4274 33.681 26.1993 10 1.9638 42.0267 33.327 11.0-98.713-99.6411-92.1096 11.9-121.820-123.9868-118.492 12.8 33.9132 26.9017 22.1261 13 48.1399 37.9423 31.4004 14 0 0 0 100 0 0-0 -100-10 -200-20 -300-30 Variation in Modulus of Elasticity 0 1 2.0 2.9 3.8 4.0.9 6.8 7 8.0 8.9 9.8 10 11.0 @IJAERD-2017, All rights Reserved 416 11.9 12.8 13 14 M2-214.1 33.2-309 -47 43.6 39.8-303 -40 2.3 66.1-276 -32 8. 72.9-272 -1 32.7 0 M30-211 6.9 3.9-308 -46 43.8 39.7-303 -40 2.2 66. -276-32 8.1 70.7-27 -2 31.3 0 M3-210 7.6 36.8-308 -46 43.7 39.3-304 -40 2.2 67-276 -32 8.2 70.2-277 -3 30.8 0 Figure 11.Variation in Modulus of elasticity with shear force
Volume 4, Issue, May-2017, e-issn: 2348-4470, print-issn: 2348-6406 V. CONCLUSIONS In the design of raft foundation, Soil structure interaction should be given importance, as adopt rigid method which is not true in all cases, as it does not take soil properties into account and also gives heavier design, which also affects cost. In case study, flexible methods have been carried out. In flexible method, modeling has been carried out in CSI SAFE V12 software. Modulus of subgrade reaction has been assumed based on the range specified in J. Bowles. In parametric study, flexible method has been carried out. The flexible method has been carried out by Winkler method. In Winkler method, variation in thickness of raft and modulus of subgrade reaction,variation in Modulus of Elasticity of raft is carried out and change in moments and Shear force soil pressure have been observed. 1. By changing the modulus of subgrade reaction, it has been found that with the increase in k there is Decrease in negative and positive moments with the increase in modulus of subgrade reaction, there is a reduction in moments. Also by changing Soil subgrade Shear force are getting decreases but not much. With increase in Soil subgrade pressure is increase and Deflection Decrease. 2. By changing the thickness of raft, it has been found that with the increase in thickness there is decrease in negative moments and positive both moments. Also decrease in shear force by increase thickness. Change in thickness does not affect overall pressure but pressure below column has been change the pressure below column decrease as increase in thickness. Increasing thickness of raft deflection in getting increase. Also Soil pressure is decrease by increase the Slab Thickness. 3. By changing the modulus of elasticity, it has been found that at lower modulus of elasticity there is not major increase in moments or shear force. But at very high modulus of elasticity can make difference. Increase in modulus of elasticity Deflection decrease. 4. By changing the column size, It has been found that at increase the column size When Negative and positive bending moment get decrease. Column size affect Shear force in raft column size increase Shear force is getting decrease negative shear force get sudden decrease but positive shear force is decrease very marginally. And that Increase in Column Size Deflection is getting decreased. And Increase in Column Size Soil pressure is getting increased REFERENCES [1] G. S. Kame, S. K. Ukarande,K. Borgaonkar, V. A. Sawant A Parametric Study on Raft Foundation International Association for Computer Methods and Advances in Geomechanics (IACMAG) 2008. [2] R. Ziaie- Moayed and M. Janbaz. Effective Parameters on Modulus of Sub grade Reaction in Clayey Soils, (2009), Journal of Applied Sciences 9 (22): page no. 4006-4012. [3] Dr. D. Daniel Thangaraj, Dr. K. Ilamparuthi, Parametric Study on the Performance of Raft Foundation with Interaction of Frame (2010), EJGE, Vol. 1 page- 861-878 [4] Mohamed Saad Eldin, Arafa El-Helloty, Effect of Opening on Behavior of Raft Foundations Resting on Different Types of Sand Soil International Journal of Computer Applications Volume 94 No.7, May 2014. [] Imam, Md. Hasan. Influence of structural and soil parameters on Mat deflection International journal of civil and structural engineering volume 2, no 1(2011) [6] Bowles, Analytical and Computer Method in Foundation Engineering, Mc Graw Hill Book Co., New York,1996. [7] Sharat Chandra Gupta Raft Foundations Design and Analysis with a Practical Approach,Book New Age International Publishers. (2006). @IJAERD-2017, All rights Reserved 417