2. Introduction
▪ Spread foundations supporting two or more columns are classified as combined
footings or combined bases.
▪ Combined footings can become a necessity for two reasons
▪ Instances where two columns are so closely spaced such that designing isolated pad footings
under each column will result in a base overlap.
▪ Instances where the external columns are very close to a property line such that
constructing isolated pad bases is impossible without a projection beyond the line.
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3. Principles
▪ The design of combined bases can be done using two approach, the flexibility method
and the conventional rigid method. The latter is most common method.
▪ The conventional rigid approach makes two assumptions:
▪ The base is infinitely rigid and therefore the deflection of the footing does not influence the
pressure distribution.
▪ The footing is proportioned such that the resultant of all applied loads pass through the
centroid of the base.
▪ The design of combined bases in principle is the same procedure as for an isolated
pad foundation.
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4. Steps for Design
▪ Determine the required size of the combined base using the permissible bearing
stress and the critical loading at the serviceability limit state.
▪ Proportion the footing for uniform pressure distribution by finding the point of
application of the resultant of the column loads and adjusting the geometry of the
footing appropriately.
▪ Determine the bearing pressures associated with the critical loading arrangement at
the ultimate limit state.
▪ Assume a suitable thickness for the footing, h and effective depth, d and verify
punching shear at both column faces
▪ Analyze the footing in the longitudinal and transverse direction.
▪ In the longitudinal/transverse direction, the footing is idealized as an inverted overhanging
beam subjected to the earth pressure at the ultimate limit state.
▪ Design the bending reinforcement in the longitudinal/transverse direction
▪ Verify shear at the critical sections including punching shear at the basic control
perimeter 2d from the column face.
▪ Verify detailing requirements
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5. Worked Example
A combined footing is required in a proposed facility for two closely spaced
column. Design the footing completely using a presumed bearing capacity of
180kN/m2 using C20/25 concrete and reinforcing steel of 410Mpa bars. The design
data is presented as follows:
Table 1.0 Design Data
Figure 1.0: Column Layout
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Column Size Gk (kN) Qk (kN)
C1 275x275 365 185
C2 300x300 525 290
15. Solution Cont’d
▪ Punching Shear at Basic Control Perimeter 2.0d from column face
Punching at the basic control perimeter is not a critical check in combined bases, hence
the check is ignored in this example, however a check can be carried out using the same
procedures outlined in the design of isolated pad bases for each column.
▪ Transverse shear
The maximum shear force is used to carry out the check. By Inspection, this occurs in the
longitudinal direction.
▪ N:B The design shear force is taken at 1.0d from the supports.
𝑉𝐸𝑑 = 353.6 − 𝑝 1.0d = 353.6 − 226.24 × 0.442 = 253.6𝑘𝑁
𝑣 𝐸𝑑 =
𝑉 𝐸𝑑
𝑏𝑑
=
253.6×103
1000×442
= 0.57Mpa
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16. Solution Cont’d
▪ Transverse shear Cont’d
𝑣 𝑅𝑑,𝑐 = 0.12𝑘(100𝜌𝑓𝑐𝑘)1/3
≥ 0.035𝑘3/2
𝑓𝑐𝑘
𝜌 =
𝐴 𝑠
𝑏𝑑
=
1340
1000×442
= 0.0030 ; 𝑘 = 1 +
200
𝑑
= 1 +
200
442
= 1.67 < 2
𝑣 𝑅𝑑,𝑐 = 0.12 × 1.67(100 × 0.0030 × 20)1/3 ≥ 0.035 × 1.67
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2 20 = 0.36𝑀𝑝𝑎
𝑆𝑖𝑛𝑐𝑒 (𝑣 𝐸𝑑 = 0.57𝑀𝑝𝑎) > (𝑣 𝑅𝑑,𝑐= 0.42𝑀𝑝𝑎) 𝑁𝑜𝑡 𝑜𝑘
The section is not adequate in shear, hence the section depth can be increased to 600mm,
or reinforcement increased, or concrete grade increased and then redesigned.
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18. Quiz
A combined footing is required in a proposed facility for two closely spaced
column. Design the footing completely using a presumed bearing capacity of
150kN/m2 using C30/37 concrete and reinforcing steel of 460Mpa bars. The design
data is presented as follows:
Table 1.0 Design Data
Figure 1.0: Column Layout
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Column Size Gk (kN) Qk (kN)
C1 275x275 365 185
C2 300x300 525 290
