Advance Design of RC Structure

1. Advance Design of RC Structure Lecture 9 Design of Raft Foundation Advance Design of RC Structure

2. Raft Foundations • Raft foundations • A raft is essentially a very large spread footing that usually encompasses the entire footprint of the structure. They are also known as mat foundation. • Foundation engineers often consider mats when • The structure loads are so high or the soil conditions so poor that the spread footings would be exceptionally large. If spread footings would cover more than about one third of the building area, a mat will be more economical. • The soil is very erratic & prone to excessive differential settlements. The continuity & rigidity of the mat foundation helps in reducing differential settlement of individual columns relative to each other. • Lateral loads are not uniformly distributed through the structure & thus may cause differential horizontal movements in spread footing. The continuity & rigidity of the mat will resist such movements. • The uplift loads are larger than spread footing can accommodate. Advance Design of RC Structure

3. Raft Foundations • The bottom of the structure is located below the groundwater table, so waterproofing is an important concern. The weight of the mat also helps resist hydrostatic uplift forces from the groundwater. • Types of raft foundation • Cellular raft foundation • Used on site where, poor ground must resist high bending moments • Crust raft foundation or blanket mat • Slab with thickening under the columns & walls • Plane raft foundation • Piled rafts Advance Design of RC Structure

4. Raft Foundations • Methods of designing raft foundation • The conventional rigid method • This method is easy to apply & the computations can be carried out using hand calculations. • The application of this is limited to rafts with relative regular arrangement of columns. Advance Design of RC Structure

5. Raft Foundations • The finite element method • This method can be used for the analysis of raft regardless of the column arrangements, loading conditions, & existence of cores & shear walls. • Commercially available computer programs like SAP2000 & SAFE can be used. Advance Design of RC Structure

6. Geotechnical Design • Bearing capacity • The allowable bearing capacity of a raft footing is given by For B < 1.2m Where qall is allowable bearing capacity in kilopascals N is standard penetration test (SPT) blow count B is the width of the footing s is the settlement in millimeters Df is the depth of the footing in meters Advance Design of RC Structure

7. Geotechnical Design • Settlement of raft foundation • The settlement of raft footing can be estimated in a manner similar to that of spread footings. • Immediate Settlement; based on the theory of elasticity can be used to estimate the corner settlement of a rectangular footing with dimensions of L' and B', Where q is the contact stress B’ is the least dimension of the footing vs is the Poisson ratio of the soil Es is the elastic modulus of the soil I1 , I2 & IF are obtained from, the table & the figure attached, in terms of the rations N = H/B’ (H = layer thickness), M = L’/B’ & D/B Advance Design of RC Structure

8. Structural Design of Raft Foundation • Conventional rigid method Step 1: Check soil pressure The resultant of columns working loads equals: The soil pressure at any point can be obtained; Where; A = area of the raft (BL) Ix = moment of inertia of the raft about x-axis = BL3/12 Iy = moment of inertia of the raft about y-axis = LB3/12 Advance Design of RC Structure

9. Structural Design of Raft Foundation Mx = moment of applied loads about the x-axis = Ptotal  ex + Mx My = moment of applied loads about the x-axis = Ptotal  ey + My The ex & ey are the eccentricities of the resultant from the C.G. of the raft. The coordinate of the eccentricities are given by: Compare the maximum soil pressures with net allowable soil pressure Advance Design of RC Structure

10. Structural Design of Raft Foundation Step 2: Draw the shear force & bending moment diagrams Divide the raft into several strips in the X-direction (B1, B2, B3) & in the Y-direction (B4, B5, B6, B7) The soil pressure at the center-line of the strip is assumed constant along the width of the strip. All the loads has to be factored The average pressure equals: This value shall be used in the analysis of the strip The total soil reaction (RB-E) for the strip B-E is equal to: The total soil reaction (RB-E) for the strip B-E is equal to: Advance Design of RC Structure

11. Structural Design of Raft Foundation The achieve equilibrium, columns loads & soil reaction must be modified such that the sum of the forces is equal to zero The modified soil pressure equals: The modified columns loads are obtained by multiplying each of the applied loads by the factor  given by; The shear & bending moment can be computed using regular structure analysis The same process should be carried out for all the strips in the raft foundation Advance Design of RC Structure

12. Structural Design of Raft Foundation Step 3: Design for flexure The maximum positive & negative moments can be obtained. The negative moments need top reinforcement & positive moment needs bottom reinforcement. Advance Design of RC Structure

13. Discussions Any Question? • Notes See you Wednesday All the best Advance Design of RC Structure

14. I1 and I2 for Settlement Equation Advance Design of RC Structure

15. I1 and I2 for Settlement Equation Advance Design of RC Structure

16. IF For Settlement Equation Advance Design of RC Structure