Design of Raft Foundation

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)

D_f = 𝑞_𝑢/𝛾_𝑠× (1−𝑠𝑖𝑛Ø)²/(1+𝑠𝑖𝑛Ø)²

Where,

D_f = depth of foundation

q_u = safe bearing capacity of soil

 𝛾_𝑠 = 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^’) = ∑P_i * x_i / P_total , ∑P_i * y_i / P_total

Eccentricity about both axis are,

e_x = L/2 – X’ , and

e_y = B/2 – Y’

Calculate soil pressure at corner of each strip on both side

For the calculation of moment

F= (P_tot / A) ± (M_y/I_y) x ± (M_x/I_x) y < q_na

Where, f = soil pressure at the point of x,y

x and y are distance of a point from y and x-axis respectively

M_x = moment about x-axis = P_tot * e_y

M_y = moment about x-axis = P_tot * e_x

I_x = Moment of inertia about x-axis = L B³/12

I_y = 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):

M_u = 0.133 f_ck b d² [for fe500]

where,

M_u = maximum strip moment

f_ck = 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 K_s×τ_c. i.e. τ_v ≤ K_s×τ_c. (IS 456 : 2000, Cl. 31.6.3.1)

Where,

K_s = (0.5 + β_c) but not greater than 1, βc being the ration of short side to long side of the column/capital; and

τ_c = 0.25 √𝑓_𝑐𝑘 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)

M_u = 0.87×f_y×A_st×(d – 𝑓_𝑦×𝐴_𝑠𝑡/𝑓𝑐𝑘×𝑏)

Calculate development length

The development length (L_d) is given by (IS 456 : 2000, Cl. 26.2.1)

𝐿_𝑑=∅×𝜎_𝑠/4×𝜏_𝑏𝑑

where,

 ∅ = diameter of reinforcement bar

𝜎_𝑠= 0.87 f_y = stress on the steel bar

𝜏_𝑏𝑑= bond strength which can be obtained from IS 456: 2000, cl. 26.2.1.1

𝐿_𝑑≤ 1.3×𝑀1/𝑉+𝑙_𝑜 (IS 456 : 2000, Cl. 26.2.3.3)

where,

l_o = 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 (σ_br) = P_u/A_c

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)

ALSO READ

RAFT FOUNDATION

EXAMPLE OF RAFT FOUNDATION DESIGN