Week 13 Foundation Design

It doesn’t matter what the subject is; once you’ve learnt how to
study, you can do anything you want.
• Foundation is the part of a structure which transmits the
loads from the structure to the underlying soil or rock.
• It is usually placed below the surface of the ground.
• All soils compress noticeably when loaded and caused the
supported structure to settle.
• The two essential requirements in the design of foundation
o The total settlement of the structures shall be limited to a tolerably
small amount
o The differential settlement of various parts the structure shall be
eliminated as nearly as possible.
• To limit settlement, it is necessary to transmit the load of
the strucutre to a soil stratum of sufficient strenght and to
spread the load over a sufficiently large are of the stratum
to minimize bearing pressure
• Figures below show pressure distribution under footing.
• Types of foundation depends on a number of factor such as:
o Soil properties and condition,
o Type of structure and loading,
o The permissible amount of deferential settlement.
• The choice is usually made from experience but comparative
designs are often necessary to determine the most economical
type to be used.
• The principal types of reinforced concrete foundation for building
o Isolated or pad footing
o Combined footing
o Raft foundation
o Strip footing
o Strap footing
o Pile foundation
• Pad Footing
o Used to support and transmit
the loads from piers and
o Tend to be the simplest and
cheapest foundation type
o Used when the soil is relatively
strong or when the column loads
are relatively light.
o Usually square or rectangular on
plan, and uniform thickness
• Combined Footing
o Where 2 columns are closed
together or convenient to
combined their footing to form a
continuous base.
o The shape maybe rectangular or
• Strap Footing
o Used where the base for exterior
column must not project beyond
the property line
o To restrain the overturning force
due to eccentric load on exterior
• Strip Footing
o Commonly used for the
foundations to load-bearing
o Also used when pad footings for
a number of column in line are
so closely spaced.
o Used on weak ground to
increase foundation bearing
• Raft foundations
o A combined footing which covers
the entire and supports all the walls
and columns.
o May consist of a slab of uniform
thickness or slab stiffened by
beams either above or below the
strap or an inverted flat slab floor.
o Useful for column with heavy loads
or low bearing capacity.
• Pile foundations
o When a solid bearing stratum (e.g:
rock) is deeper than about 3 m
below the base level.
o Loads are transmitted either to a
stiff bearing layer or to soil by
friction along the pile.
• Design of Pad footing
o Thickness and size of footing
o Effective depth of pad
o Design of flexure
• Bending of pad footing
o Check for shear
• Vertical shear:
• at a distance d from the face of column
• Ved < VRd,c  no shear reinforcement is required
• Punching shear
• Critical section at the perimeter 2d from face of column
o Cracking and detailing requirements
• Use rules for slabs in clause 9.3 EC2
• A rectangular pad foundation is required to support a single
transferring an axial service load which consists of 600 kN
permanent load and 450 kN variable load. Using the data
provided, determine the suitable size of pad footing and
design the reinforcement required.
• Design a rectangular pad footing of uniform thickness for
reinforced concrete column of size 300 x 300 mm and
carrying an ultimate axial load of 1500 kN and ultimate
bending moment 50 kNm