2. Learning out-comes
At the end of this lecture, the student should be able to:
a) List out the relevant codes of practice used in
concrete design
b) Explain the concept of SLS and ULS
c) Explain a basic concept of concrete design.
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B.Sc.Eng (Hons), C&G (Gdip)
3. 1.0 Design in Reinforced Concrete
1.0 Introduction
Importance of RC, it being one of the principal materials
used in structural design.
Composite material consisting of steel reinforcing bars
embedded in concrete.
Complimentary properties and cost consequences.
Overall economy with the advantages of corrosion and
fire resistance.
Type of application: Structural Frames Retaining Walls
Water Retaining structures. Highways Bridges
Prepared by: M.N.M Azeem Iqrah
B.Sc.Eng (Hons), C&G (Gdip)
4. These structures are normally designed in accordance
with a variety of CODES:
BS 5400:
Code of Practice for the Design of Steel, Concrete and
Composite Bridges.
BS 8007:
Code of Practice for the Design of Concrete structures
for Retaining Aqueous Liquids.
BS 8110: (Which is to be the basis of the work for this
semester)
Code of Practice for the Structural use of Concrete
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B.Sc.Eng (Hons), C&G (Gdip)
5. BS8110 is divided into 3 parts;
Part 1: Code of Practice for Design and Construction.
Covers material commonly required for everyday
design.
Part 2: Code of Practice for Special Circumstances.
Torsion, Deflection and Elastic Deformation
Part 3: Design Charts for Singly Reinforced Beams,
Doubly Reinforced and Rectangular Columns.
Charts for the design of singly reinforced beams,
doubly reinforced beams and rectangular columns
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B.Sc.Eng (Hons), C&G (Gdip)
6. Irrespective of the element being designed a designer
will need an understanding of :
The symbols used
The Basis of Design
Material Properties
Loading
Stress Strain Relationships
Durability and
Fire Resistance Symbols
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B.Sc.Eng (Hons), C&G (Gdip)
7. Design Approach used must ensure that the
probability of a Limit State being reached in the
Design/Service Life of a structure is within
acceptable limits;
However, complete elimination of probability of a
Limit State being achieved in the service life of a
structure is impractical as it would result in
uneconomical designs.
Limit States
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B.Sc.Eng (Hons), C&G (Gdip)
Skills College of Technology
8. There are three broad classification of limit states:
1. Strength limit states
2. Serviceability limit states
3. Special limit states
8
Limit States
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B.Sc.Eng (Hons), C&G (Gdip)
11. 11
Limit States
Special Limit States:
Damage or collapse in extreme earthquakes.
Structural effects of fire, explosions, or vehicular
collisions.
Prepared by: M.N.M Azeem Iqrah
B.Sc.Eng (Hons), C&G (Gdip)
12. Design Considerations
• Structure and Structural Members should have
adequate strength, stiffness and toughness to
ensure proper functioning during service life
• Reserve Strength should be available to cater for:
– Occasional overloads and underestimation of loads
– Variability of strength of materials from those
specified
– Variation in strength arising from quality of
workmanship and construction practices
12
Prepared by: M.N.M Azeem Iqrah
B.Sc.Eng (Hons), C&G (Gdip)
13. 13
Structural Design must provide adequate margin of
safety irrespective of Design Method
Design Approach should take into account the
probability of occurrence of failure in the design
process
Design Considerations
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B.Sc.Eng (Hons), C&G (Gdip)
14. 14
An important goal in design is to prevent limit state
from being reached.
It is not economical to design a structure so that none
of its members or components could ever fail. Thus, it
is necessary to establish an acceptable level of risk or
probability of failure.
Design Considerations
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B.Sc.Eng (Hons), C&G (Gdip)
15. Brittle behavior is to be avoided as it will imply a
sudden loss of load carrying capacity when elastic
limit is exceeded.
Reinforced concrete can be made ductile by limiting
the steel reinforcement.
15
Design Considerations
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B.Sc.Eng (Hons), C&G (Gdip)
17. Characteristic Strength of reinforcement fy
Reinforcement Type Characteristic Strength
(N/mm2) fy
Hot Rolled Mild steel 250
High Yield Steel 460
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B.Sc.Eng (Hons), C&G (Gdip)
18. Design Strength
In order to take account of the difference between
actual and laboratory values, local weaknesses and
inaccuracies in the assessment of the resistance of
sections, the Characteristic Strengths, fk are divided by
an appropriate partial safety factor for strength,γm .
Design Strength = fk / γm
Prepared by: M.N.M Azeem Iqrah
B.Sc.Eng (Hons), C&G (Gdip)
19. Partial safety factors for Strength of Material
Material and Stress type Partial Safety Factor, γm
Reinforcement 1.15
Concrete - Flexure or Axial
Load
1.5
Concrete - Shear,
unreinforced
1.25
Concrete - bond 1.4
Concrete - other e.g. bearing >1.5
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B.Sc.Eng (Hons), C&G (Gdip)
20. Loading:
Characteristic Loads
Gk - Dead load
Qk - Imposed Load
Wk - Wind Load
BS 648:Schedule of Weights for Building Materials
BS 6399: Design Loadings for Buildings, Part 1: Code
of Practice for Dead and Imposed loads
are useful documents in which to find values to be used
in calculations.
Prepared by: M.N.M Azeem Iqrah
B.Sc.Eng (Hons), C&G (Gdip)
21. Design Loads
In order to account for Variation in Loads due to:
Errors in the analysis and Design
Constructional inaccuracies
Possible load increases
The Characteristic Loads Fk are multiplied by the
appropriate partial safety factor for loads, γf to give the
Design Loads acting on the structure
Design Load = Fk.γf
Prepared by: M.N.M Azeem Iqrah
B.Sc.Eng (Hons), C&G (Gdip)
22. Load Combination Load Type
Dead Load, Gk Imposed Load, Qk
Wind
Load,
Wk
Adverse Beneficial Adverse Beneficial
Dead + Imposed 1.4 1 1.6 0 -
Dead + Wind 1.4 1 - - 1.4
Dead + Wind + Imposed 1.2 1.2 1.2 1.2 1.2
Generally the adverse factors are used to determine the design loads acting on
a structure. Elastic analyses then allow the determination of maximum BM’s
and Shears for which sections must be capable of sustaining
Prepared by: M.N.M Azeem Iqrah
B.Sc.Eng (Hons), C&G (Gdip)
24. BS 8110 makes use of a modified stress-strain curve as
shown
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B.Sc.Eng (Hons), C&G (Gdip)
25. DURABILITY & FIRE RESISTANCE
As well as the need to design structures to withstand the
applied loads due consideration must be given to both
durability and fire resistance. In fact it can be seen that
the
design of an element can not begin without considering
these factors in some way
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B.Sc.Eng (Hons), C&G (Gdip)
26. Durability
Signs of concrete deterioration are nowadays far too
common. Ex’s..... Repair can be very costly and difficult.
Improved durability is therefore paramount.
How can this be achieved:
cover to reinforcement
minimum cement content
maximum water/cement ratio
maximum crack widths
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B.Sc.Eng (Hons), C&G (Gdip)
27. The table gives nominal (min+5) depths of cover
to be used for a variety of exposure conditions. Note
linkage with Max. water/cement ratio, Min. cement
content and concrete grade
Prepared by: M.N.M Azeem Iqrah
B.Sc.Eng (Hons), C&G (Gdip)
28. Fire Protection
Fire protection of reinforced concrete members is
largely achieved by specifying limits for:
Cover to reinforcement
minimum dimensions for section
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B.Sc.Eng (Hons), C&G (Gdip)
29. This table shows the nominal cover to ALL
reinforcement to meet the specified period of fire
resistance
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B.Sc.Eng (Hons), C&G (Gdip)
30. In addition to cover
we must also
consider minimum
section dimensions
which vary
depending upon the
element considered
and it’s location as
indicated(BS8110):
Prepared by: M.N.M Azeem Iqrah
B.Sc.Eng (Hons), C&G (Gdip)
31. Minimum dimensions of reinforced concrete members
for fire resistance
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B.Sc.Eng (Hons), C&G (Gdip)