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FOUNDATION ENGINEERING



ANALYSIS, DESIGN AND DETAILING OF RAFT FOUNDATION



Faculty Name Dr. Paresh Shah



By Soumyakanti Dhavala PT402016



INTRODUCTION A raft is a thick reinforced concrete slab which supports all the load bearing walls and column loads of a structure or a large portion of a structure. A raft is required when the loads are very high and the soil is very weak or highly compressible. A raft is more economical than individual when the total base area required for individual footing exceeds about one-half of the area covered by the structure. When is raft foundation preferred? A raft foundation is preferred to individual footings when the soil mass has very erratic properties and contains lenses of compressible soils. In such a case it would be difficult to control the differential settlements if individual footings are provided. The raft spans over weal patches of the soil and thus the differential settlements are considerably reduced Like all other Shallow foundations, a raft must be safe against shear failure and the settlements should be within the allowable limits. As the width of a raft is very large, the pressure bulb is quite deep . Thus the loose soil pockets under a raft maybe more evenly distributed. This results in a smaller differential settlement than individual footings. As rafts are generally at some depth below the ground surface, a large volume of soil is activated and therefore the net pressure on the soil is considerably reduced. An advantage of this reduction in the pressure can be taken while designing a raft. COMMON TYPES OF RAFT FOUNDATIONS Flat plate type: In this type of mat Foundation format of uniform thickness is provided. This type is most suitable when the column loads are relatively light and the spacing of columns is relatively small and uniform. Flat plate thickened under columns: When the column load are heavy, this type is more suitable than the flat plate type . A portion of slab under the column is thickened to provide enough thickness for negative bending moment and diagonal share. Sometimes, instead of thickening the slab, a pedestal is provided under each column above the slab to increase the thickness Beam and slab construction: In this type of construction, the beam runs in two perpendicular directions and a slab is provided between the beams. The columns are located at the intersection of beams. This type is suitable when the bending stresses are high because of large column spacing and unequal column loads. Box structures: In this type of raft foundation, a box structure is provided in which the basement walls act as stiffeners for the raft. Boxes may be made of cellular construction rigid frames consisting of slabs and basement walls this type of raft Foundation can resist very high bending stresses. Mat place on piles: The raft foundation is supported on piles in this type of construction. This type of raft is used where the soil is highly compressible and water table is high. This method of construction reduces the settlement and also controls buoyancy.



ANALYSIS, DESIGN AND DETIALING OF RAFT FOUNDATION Name of Project :



Design Example 18.5, Page 561, Reinf concrete A.K.Jain



Given Data: 1 Grade of Steel



Fe



415



2 Grade of Concrete



M



15



3 Size of Column



(X x Y)



300



4 Safe Bearing Capacity of Soil



65.00



mm



(Along X-direction)



20



mm



6 Diameter of Bars



(Along Y-direction)



20



mm



2



Y



3



300



mm



KN/m2



5 Diameter of Bars



1



X



4



C



C



B



B X



A



A O



1



2



3



4



Details of Columns Considering column A-1 as origin (O)



Column



Moment about X-axis Moment about Y-axis



X-cord Y-cord (in m) (in m) A-1 0 0 A-2 7 0 A-3 14 0 A-4 21 0 B-1 0 6 B-2 7 6 B-3 14 6 B-4 21 6 C-1 0 12 C-2 7 12 C-3 14 12 C-4 21 12 Cantilever length from centre line of columns Left 0.3 m Top Right 0.3 m Bottom



Load (KN) 550 1200 1200 550 600 2000 2000 1200 500 1500 1500 500



Mx clockwise +ve My Anticlock -ve



Moment My Moment Mx (KN-m) (KN-m) 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.3 0.3



m m



To Run the Program Press < Ctrl+w >



Total Vetical Column Load



P=



13300



KN



Eccentricity along x-direction Taking moment of column forces about the grid 1-1 x = 10.974 m ex = 10.974 10.5 = 0.474 m Eccentricity along y-direction Taking moment of column forces about the grid A-A y = 6.226 m ey = 6.226 6 = 0.226 m Ix



Iy



A



=



21.6



X



=



3600.68



=



12.6



12.6



3



21.6



3



12 m4 X



=



12 m4 10581.58



= =



12.6 X 21.6 2 m 272.16



Mxx =



P.ey



=



3000.00



KNm



Myy =



P.ex



=



6300.00



KNm



P/A =



KN/m2



48.87



Soil pressure at different points is as follows s



=



P A



Myy



+



Iy



.x



+



Mxx Ix



.y



Corner C-4 sC-4



Corner A-4 sA-4



Corner C-1 sC-1



+



6300.00 10581.6 6.430



+



5.249



6.430



-



5.249



6.430



+



5.249



=



48.87



= =



48.87 60.547



= =



48.87 50.049



KN/m



= =



48.87 47.687



KN/m2



+ KN/m2 +



10.8



+



2



-



3000.00 3600.68



6.3



Corner A-1 sA-1



Grid B-4 sB-4



Grid B-1 sB-1



= =



48.87 37.189



= =



48.87 55.298



= =



48.87 42.438



6.430 KN/m2



+ KN/m



5.249



6.430



-



0.000



6.430



-



0.000



2



KN/m



-



2



60.547 KN/m Hence OK In the X-direction, the raft is divided in three strips :Maximum Soil Pressure =



(i)



Strip C-C Width Soil Pressure Span



= = =



Maximum moment



=



3.3 60.547 7.00 60.547



= 296.68 (ii)



Strip B-B Width Soil Pressure Span



= = =



Maximum moment



=



6 57.923 7.00 57.923



= 283.82 (iii) Strip A-A Width Soil Pressure Span Maximum moment



= = = =



3.3 52.674 7.00 52.674



= 258.10 Cantilever Moment along X-direction Soil Pressure = 60.547 Span = 0.30 Maximum moment



=



60.547



= 2.72 (iii) Strip 4-4 Maximum Soil Pressure = Span = Maximum moment



=



2