PAD FOUNDATIONSThe axially loaded pad base in Design Example 2
is to be redesigned as a reinforced base, founded in the weathered
sandstone. Assuming settlements have been judged to be
satisfactory, the base will have an allowable bearing pressure, na
= 550 kN/m2.
Loadings
Since dead and imposed loads are approximately equal, acombined
partial load factor of P= 1.5 will be used.
Area of base
Adopt a 3.0 m 3.0 m square base, i.e. L = B = 3.0 m (see Fig.
11.25). Reactive design pressure on base for concrete design
Fig. 11.25Reinforced pad base design example.
Depth of baseThe base and its reinforcement must be capable of
resisting bending, beam shear and punching shear. At rst glance it
is not always possible to judge which is critical. The process of
selecting a suitable depth for the base is simplied by use of the
charts for estimating effective depth. The effective depth will be
checked for each case, assuming a typical reinforcement
percentageof between 0.25% and 0.50%. The results are shown inTable
11.1.
Table 11.1Estimating effective depth for reinforced pad base
design example
This indicates that bending is critical, i.e. it requires the
greatest effective depth, for low percentages of reinforcement.
For this particular example an average effective depth in both
directions of d = 600 mm will be selected.
Overall depth of base is, h = 600 + 25 (bar diameter) + 50
(cover) = 675 mmBendingFromFig. 11.25, the cantilever moment at
face of base plate is
ShearThe base should be checked for both beam shear and punching
shear, since either may be critical. Grade C40 concrete has been
specied.
Local shear at column faceThe shear at the face of the column
should be checked.
Allowable concrete shear stress, vc = 0.57 N/mm2
From BS 8110: Part 1: 3.4.5.8, the critical location for beam
shear is at a distance 2d = 2 600 = 1200 mm from the face of the
load (i.e. from the edge of the base plate in this example). The
shear force acting across this failure plane is
Vbeam= (design pressure) (area of base beyondcritical
location)
Punching shearThe critical location for punch- ing shear for a
square load is a square perimeter a distance 1.5d = 1.5 600 = 900
mm from the face of the load.
The length of one side of this perimeter is
Area of base outside of perimeter
Comparison with vbeam= 0.12 N/mm2 indicates that, in this
instance, punching shear is more critical than beam shear.
This is normally the case with square pad foundations. If
however a foundation size of say 2 m 4 m had been chosen in this
example, beam shear may well become critical.
Local bondAlthough not covered by BS 8110, local bond can be a
problem in foundation design, and should therefore be checked at
sections with high shear stress. Local bond is given by
where us is the sum of the bar perimeters at the section being
considered.Punching shear, Vu = 2979 kN
The length of the punching shear is u = 8800 mm.T25 bars @ 175
centres each way are proposed. The total number of bars crossing
the shear perimeter is u/175 = 50.The local bond stress is
where lais the lever arm which CP 110 approximates to the
effective depth d.
