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Course Title: Design of Concrete Structures II
Course Code: CE 3217
Reference Books
1. Design of Concrete Structures” by – Winter & Nilson (7th Edition)
2. Design of Concrete Structures” by – Nilson, David & Dolan (14th
Edition)
3. Design of Prestressed Concrete by T.Y. Lin
4. Pre-stressed Concrete by Krishna RajuCourse Content
• Design of columns under uniaxial and
biaxial loading, introduction to slender
column
• Structural design of footings
• Structural forms
• Introduction to prestressed concrete
• Analysis and preliminary design of prestressed beam sections
Design of Concrete Structures II (CE 3217)
Chapter: Footings
Reference Book: Design of Concrete Structures” by – Nilson, David & Dolan (14th Edition)
Lecture prepared by
Md Nuruzzaman
Lecturer, Department of Civil Engineering
Bangladesh Army University of Engineering and Technology (BAUET)
E-mail: [email protected], [email protected]
Phone: +8801719456829
Foundation
A foundation is the element of a structure which connects it to theground, and transfers loads from the structure to the ground.
Footings
1
TYPES OF FOUNDATIONS
Foundation Systems
Shallow Foundation Deep Foundation
Pile Foundation Pier (Caisson)
Foundation
Isolated spread
footings Wall footings Combined
footings
Cantilever or
strap footings
Raft or Mat
foundation
2
Shallow Foundations – are usually located no more than 6 ft below the lowest
finished floor. A shallow foundation system generally used when (1) the soil close the
ground surface has sufficient bearing capacity, and (2) underlying weaker strata do not
result in undue settlement. The shallow foundations are commonly used most
economical foundation systems.
Footings are structural elements, which transfer loads to the soil from columns, walls
or lateral loads from earth retaining structures. In order to transfer these loads properly to
the soil, footings must be design to
• Prevent excessive settlement
• Minimize differential settlement, and
• Provide adequate safety against overturning and sliding.
Types of Footings
Isolated spread footings under individual columns.
These can be square, rectangular, or circular.
Footing
Column
3
Wall footing is a continuous slab strip along the
length of wall.
Footing
Wall
4
Property line
Combined footings support two or more columns. These can
be rectangular or trapezoidal plan.
Combined Footing
Footing
Columns
5
Property line
Cantilever or strap footings: These are similar to combined
footings, except that the footings under columns are built
independently, and are joined by strap beam.
6
Raft or Mat foundation: This is a large continuous footing
supporting all the columns of the structure. This is used when
soil conditions are poor but piles are not used.
Mat or Raft
Footing
Columns
7
Deep Foundations – The shallow foundations may not be economical or even
possible when the soil bearing capacity near the surface is too low. In those cases deep
foundations are used to transfer loads to a stronger layer, which may be located at a
significant depth below the ground surface. The load is transferred through skin friction
and end bearing (Figure below).
Fig. 1(a) Axial Compressive Load transfer in deep foundations
Skin friction
Hard soil/
Bedrock
End bearing
Wf
P
Fig. 1(b) Axial Compressive Load transfer in deep foundations
Skin friction
End bearing (negligible)
Wf
P
8
Fig. 2 Axial Tension Load transfer in deep foundations
Skin friction
Wf
P
Fig. 3 Lateral Load transfer in deep foundations
M V
9
Piles
Piles cap
Column
Piles
Piles cap
Column
Fig. Pile Foundation- (a) Vertical Piles; (b) Battered Piles
(a) (b)
10
Pile Foundation
Column
Pile Cap
Piles
11
Bell
Column
Pier shaft
Pier Foundation (Caisson)
15
Soil Bearing Pressure at base of Footings
A. Concentrically Loaded Footing
P
L
W
Bearing pressure, p = P/A = P/(W x L)
Design of Concrete Structures II (CE 3217)
Chapter: Design of Columns
Reference Book: Design of Concrete Structures” by – Nilson, David & Dolan (14th Edition)
Lecture prepared by
Md Nuruzzaman
Lecturer, Department of Civil Engineering
Bangladesh Army University of Engineering and Technology (BAUET)
E-mail: [email protected], [email protected]
Phone: +8801719456829
Columns
Tied Column Spiral Column
Columns
Tied Column Spiral Column
Columns
Columns
Why strength reduction factors are lower in column than beam?
The ACI strength reduction factors are lower for columns than forbeams, reflecting their greater importance in a structure. A beamfailure would normally affect only a local region, whereas a columnfailure could result in the collapse of entire structure.
A basic 𝟇 factor of 0.7 is used for spirally reinforced columns and 0.65for tied columns vs 𝟇=0.90 for most beams.
Columns
Specification for Lateral tie bars
For longitudinal bars upto #10 (32 mm bars), use #3 (10 mm) lateral tie bar.
For longitudinal bars greater than #10 (32 mm bars), use #4 (12 mm) lateral tie bar.
Spacing
S = 16 db
S = 48 dt
S = Least dimension of column
The smallest value is to be selected.
Columns