2.5 AXIALLY LOADED PAD FOOTING - Note

2.0: ANALYSIS AND DESIGN

2.5 Axially Loaded Pad Footing

2.0: ANALYSIS AND DESIGN

2.5 AXIALLY LOADED PAD FOOTING
a) Describe types of foundation.

b) State factors in selection types of foundation.

c) Determine plan dimension of base.
d) Sketch sectional, plan and isometric views showing reinforcement

arrangement for pad footing.

a) Describe types of foundation.

Introduction

▪ Foundations are the supporting part of the structure which transfer the
applied loads to the earth.

▪ A foundation is the base on which a building rests and its purpose is to
safely transfer the load of a building to a suitable subsoil.

Safely sustain and transmit to the ground the combined
dead load and imposed loads so as not to cause any
settlement or other movement in any part of the building
or of any adjoining building or works.

Types of Foundation

◼ Foundations can be considered under two groups:

 Shallow Foundations
 Deep Foundations

 Shallow Foundations
1. Spread Foundations / Footings
- Pad @ Isolated Foundations, Strip Foundations
and Combine Both
2. Raft Foundation

 Deep Foundations
- Pile, Pier and Caissons

Types of Foundation

i. Shallow Foundations

Shallow foundations are those constructed near to the finished ground
surface; generally where the founding depth is less than the width of
the footing and less than 3m

ii. Deep Foundations

Deep foundations are those constructed too deeply below the
finished ground surface for their base bearing capacity to be affected
by surface conditions, this is usually at depths >3 m below finished
ground level

Foundations

◼ The principal types of RC Foundation
for buildings are :

I. Strip Foundation
II. Pad @ Isolated Foundation
III. Raft Foundation
IV. Piled Foundation

i. Strip Foundations

1. Used to support load bearing walls or under a line of closely spaced
columns

2. - constructed from concrete with typical mix ratio of 1:2:4.
3. Can be divided into three types:

i. - Ordinary strip foundation
ii. - Wide strip foundation
iii. - deep strip foundation

ii. Pad Foundations

▪ Normally used as foundation to building or residential building
not exceeding four-storey on stiff soil such as gravel.

▪ normally constructed from reinforced concrete with mix ratio
1:2:4.

▪ Steel reinforcement is added to strengthen and reduce the
thickness of the foundation.

iii. Raft Foundations

▪ required for low bearing capacity soil that tends to settle
unevenly when subjected to load.

▪ Can be divided into three types:
i. - solid slab raft foundation
ii. - beam and slab raft foundation
iii.- cellular raft foundation

iv. Pile Foundation

1. required when building load is too high or bearing capacity of
upper soil strata is low.

2. building load will then transmit to a stiff soil strata deep in the
ground.

3. Can be divided into two types:
(i) friction pile (depends entirely on the friction between pile and
soil surrounding it)
(ii) end bearing pile ( driven down to the stiff soil where the end
bearing of the pile transmit the building load directly to it)

Pile Foundation

Pile Foundation (Pile Cap)

b) State factors in selection types of foundation.

Foundation

The type of foundation to be used depends on a number of
factors such as:
(i) The soil properties and conditions
(ii) The type of structure and loading
(iii) The permissible amount of differential settlement

Causes of settlement are;
1. Deformation of the soil due to imposed loads.
2. Volume changes of the soil as a result of seasonal conditions.
3. Mass movement of ground in an unstable areas.

Foundation Terminology

1.Bearing capacity – safe load per unit area which the
ground can carry.

2.Subsoil – soils below the top soil; the topsoil being
about 300 mm deep.

3.Settlement – ground movement
4.Backfill – materials excavated from site and if suitable

used to fill in around the walls and foundations.

Foundation Failure

Bearing Strength of Soils



Typical allowable bearing values

TYPES OF SOIL TYPICAL BEARING VALUES
kN/m2
Massive igneous bedrock 10,000
Sandstone
Shales and mudstone 2000 to 4000
Gravel, sand and gravel 600 to 2000
Medium dense sand
Loose fine sand 600
Hard clay 100 to 300
Medium clay Less than 100
Soft clay 300 to 600
100 to 300
Less than 75

c) Determine plan dimension of base.

*stump size and soil properties such as bearing pressure is given.

Assumptions in design :

i. When the base is axially loaded the load may be assumed to be
uniformly distributed. The actual pressure distribution depends on
the soil type.

ii. When the base is eccentrically loaded, the reactions may be
assumed to vary linearly across the base.

Axially loaded pad bases

Symbols:
Gk : characteristic permanent load from the column (kN)
Qk : characteristic variable load from the column (kN)
W : weight of the base (kN)
q : safe bearing pressure (kN/m2)

Design parameter

To design size of pad footing base use serviceability limit:

P = 1.0Gk + 1.0 Qk

P = service load
Gk= dead load
Qk= imposed load

BEARING PRESSURE , q

• Load ( P ) depends on location or load position
on the foundation base

• Bearing pressure ( q ) acts on the
base of the foundation

Load ( P )
Area of foundation base ( A )

q = P/A

Section of pad foundation

750mm 6R8-225 G.L

300mm Column stump

4Y20

Reinforced
concrete pad

25mm Thick
lean concrete

PLAN AND CROSS SECTION

Exercise 1:

A 400 mm square column carries a dead load of 1050 kN
and imposed load of 300 kN. The safe bearing capacity of
the soil is 170 kN/m2. Determine the size for square pad
footing in mm. Assume weight of footing is 130 kN.

Solution:

Given: weight of footing = 130 kN
q = 170 kN/m2

Total dead load = 130 + 1050 Gk = 1050 kN z
= 1180 kN Qk = 300 kN

Ultimate axial load, P ; ◦. ◦ ◦ ◦
= 1.0 Gk + 1.0 Qk
= 1.0(1180) + 1.0(300) l xl
= 1180 + 300
= 1480 kN.

q = P/A = 1480/A

170 = 1480/A
A = 2132/170 = 8.71 m2
l = √ 8.71 = 2.951 m = nearest 25 mm

Hence, use a 2.975 m x 2.975 m square footing

d) Sketch sectional, plan and isometric views showing reinforcement
arrangement for pad footing.

Exercise 2:

A 350 mm square column carries a dead load characteristic of 1000 kN and
a live load characteristic of 330 kN. The safe bearing pressure on the soil is
195 kN/m2. Assume a pad footing weighing 130 kN

a) Calculate the required pad footing area in mm2.

b) Propose a suitable plan dimensions for a square pad footing to the
nearest 25 mm.

c) Sketch an isometric view of the isolated pad footing assuming a
thickness of 500 mm with dimensions.

Solution: Gk = 1000 kN z
Qk = 330 kN
Given: weight of footing = 130 kN
q = 195 kN/m2 ◦. ◦ ◦ ◦

Total dead load = 130 + 1000 l xl
= 1130 kN

Ultimate axial load, P ;
= 1.0 Gk + 1.0 Qk
= 1.0(1130) + 1.0(330)
= 1460 kN.

a) q = P/A = 1460/A
195 = 1460/A
A = 1460/195 = 7.49 m2

b) l = √ 7.49 = 2.736 m = nearest 25 mm
c ) Hence, use a 2.750 m x 2.750 m square footing

c. ISOMETRIC LAYOUT

500 mm

Exercise 3 :

A column 400mm x400mm carries a dead
load of 800kN and an imposed load of
300kN.

The safe bearing pressure is 200kN/m2.
Design a square base to resist the loads.

Solution:
Given:

q = 200 kN/m2

Total dead load = 800 kN Gk = 800 kN
Total live load = 300 kN Qk = 300 kN

Ultimate axial load, P ; z
= 1.0 Gk + 1.0 Qk
= 1.0(800) + 1.0(300) ◦. ◦ ◦ ◦
= 1100 kN.
l xl
a) q = P/A = 1110/A

200 = 1110/A
A = 1110/200 = 5.50 m2

b) l = √ 5.50 = 2.345 m = nearest 25 mm
c) Hence, use a 2.350 m x 2.350 m square footing

c. ISOMETRIC LAYOUT

2350 mm 500 mm
2350 mm

Mohd Suhaimi bin Ahmad is a Senior Lecturer of Civil Engineering at Kolej Matrikulasi Kejuruteraan
Kedah, Pendang, Kedah. He graduates B. Civil Eng. from UTHM and MSc. Civil Engineering from
UNIMAP. He also obtained Diploma & Cert. in Civil Eng. (Construction-Polytech) and Edu. Diploma
(Domestic Water Supply-MPBP). His Registered as Graduate Engineer with BEM and Graduate
Technologist with MBOT. Have an experience in civil engineering field and teaching since 2004.

Nor Azah binti Aziz is a Senior Lecturer of Civil Engineering at Kolej Matrikulasi Kejuruteraan Kedah,
Pendang, Kedah. She obtained B. Tech. & Edu. (Civil Eng.) & Diploma Civil Eng. from UTM and M.
Edu. Tech. from USM. Have an experience in teaching Civil Engineering since 2001. She is actively
involved in the Civil Engineering syllabus drafting program at the matriculation division level.