Design of raft foundation:
The Raft foundation is a sub-structure which supports an arrangement of columns or walls in a row or rows and transmitting the load to the soil by means of a continuous slab with or without depressions or openings. Here we discuss about the step wise procedure of design of raft foundation.
Safe bearing capacity of soil
As per IS 1893: 1 Cl 6.3.5.2allowable bearing pressure in soil can be increased depending upon type of foundation thus bearing capacity of soil is increased by 50% assuming it will be Raft foundation
Then, safe bearing capacity of soil is calculated by applying factor of safety of 1.2
Depth of foundation
Generally, the depth of raft foundation shall be not less than 1 m (IS 2950 Part 1, Cl. 4.3)
Df = 𝑞𝑢/𝛾𝑠× (1−𝑠𝑖𝑛Ø)2/(1+𝑠𝑖𝑛Ø)2
Where,
Df = depth of foundation
qu = safe bearing capacity of soil
𝛾𝑠 = unit weight of soil
Ø = angle of repose of soil
However, the lower face of the designed footing will be placed at a level of 1m below at which soil is free from seasonal volumetric change.
Calculation of service loads
Service loads includes all the loads from column, staircase, lift and other vertical or inclined structure that is connected to footing and transmit the load to the raft foundation. 10% is increased for self-weight of footing.
Area of foundation
Assumption: All above reaction from superstructure is taken as non-eccentric surcharge to the soil.
The required area of foundation can be calculated by using given formula,
Area of foundation = total service load / safe bearing capacity of soil
If the area of footing calculated is greater than 50% of plinth area, then only raft foundation is used. But normally raft is provided if the area calculated is greater than 70%. Raft becomes compulsory for those building which have underground basements.
Calculation of eccentricity
In almost all building, there is presence of eccentricity of load. For calculation of eccentricity, we have to find the center of gravity of foundation area and the load.
For rectangular footing area, CG of footing area = L/2 , B/2
CG of load (X’ , Y’) = ∑Pi * xi / Ptotal , ∑Pi * yi / Ptotal
Eccentricity about both axis are,
ex = L/2 – X’ , and
ey = B/2 – Y’
Calculate soil pressure at corner of each strip on both side
For the calculation of moment
F= (Ptot / A) ± (My/Iy) x ± (Mx/Ix) y < qna
Where, f = soil pressure at the point of x,y
x and y are distance of a point from y and x-axis respectively
Mx = moment about x-axis = Ptot * ey
My = moment about x-axis = Ptot * ex
Ix = Moment of inertia about x-axis = L B3/12
Iy = Moment of inertia about y-axis = L3 B/12
Calculation of thickness of raft
i. Calculation of Depth from Moment Criterion (IS 456 : 2000, ANNEX G 1.1):
Mu = 0.133 fck b d2 [for fe500]
where,
Mu = maximum strip moment
fck = characteristic strength of concrete at 28 days
b = width of that strip
d = effective thickness of raft foundation
ii. Calculation of Depth from Two Way Shear:
Depth of raft will govern by two-way shear at one of the exterior column. In case, location of critical shear is not obvious it may be necessary to check all locations. When shear reinforcement is not provided, the calculated shear stress at critical section shall not exceed Ks×τc. i.e. τv ≤ Ks×τc. (IS 456 : 2000, Cl. 31.6.3.1)
Where,
Ks = (0.5 + βc) but not greater than 1, βc being the ration of short side to long side of the column/capital; and
τc = 0.25 √𝑓𝑐𝑘 in limit state method of design.
Normally, thickness of raft foundation design is governed by punching shear. Thickness should be maximum value obtained from moment criteria or shear criteria.
Calculation of reinforcement on both axis
In both axis, reinforcement is calculated based on the maximum moment at that strip in both direction. For a axis, normally the strip with maximum moment is taken and same amount of reinforcement is placed in all footing area in that direction.
From (IS 456 : 2000, Annex G 1.1)
Mu = 0.87×fy×Ast×(d – 𝑓𝑦×𝐴𝑠𝑡/𝑓𝑐𝑘×𝑏)
Calculate development length
The development length (Ld) is given by (IS 456 : 2000, Cl. 26.2.1)
𝐿𝑑=∅×𝜎𝑠/4×𝜏𝑏𝑑
where,
∅ = diameter of reinforcement bar
𝜎𝑠= 0.87 fy = stress on the steel bar
𝜏𝑏𝑑= bond strength which can be obtained from IS 456: 2000, cl. 26.2.1.1
𝐿𝑑≤ 1.3×𝑀1/𝑉+𝑙𝑜 (IS 456 : 2000, Cl. 26.2.3.3)
where,
lo = Effective depth or 12∅, whichever is greater
M1 = moment of resistance of that section
V = shear force at the section due to design loads
Bearing of concrete
Load transfer from Column to footing:
Nominal bearing stress in column concrete (σbr) = Pu/Ac
Allowable bearing stress = 0.45×fck (IS 456: 2000, Cl. 34.4)
When permissible bearing stress on the concrete within the supporting or supported member exceeded, the reinforcement shall be provided for developed excess force by dowel bars. (IS 456: 2000, Cl. 34.4.1)
Dowel of at least 0.5% of the cross-sectional area of the supported column and a minimum of four bars shall be provided. Diameter of the Dowels shall not exceed the diameter of column bar by more than 3 mm. (IS 456 : 2000, Cl. 34.4.1)
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