Basic design considerations

BASIC STRUCTURAL DESIGN
Prepared by: M.N.M Azeem Iqrah
B.Sc.Eng (Hons), C&G (Gdip)
Skills College of Technology

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.

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

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

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

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

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
Skills College of Technology

 There are three broad classification of limit states:
1. Strength limit states
2. Serviceability limit states
3. Special limit states
8
Limit States

9
Strength Limit
States:
• Flexure
• Torsion
• Shear
Limit States
• Fatigue
• Settlement
• Bearing

10
Serviceability Limit
States:
• Cracking
• Excessive Deflection
• Buckling
• Stability
Limit States

11
Limit States
Special Limit States:
Damage or collapse in extreme earthquakes.
Structural effects of fire, explosions, or vehicular
collisions.

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

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

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.

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

Material Properties:
Characteristic Strength of Concrete fcu
Concrete Grade Characteristic Strength
(N/mm2) fcu
C25 25
C30 30
C35 35
C40 40
C45 45
C50 50

Characteristic Strength of reinforcement fy
Reinforcement Type Characteristic Strength
(N/mm2) fy
Hot Rolled Mild steel 250
High Yield Steel 460

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

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

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.

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

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

Stress-Strain Curves
Stress-Strain Curve for Concrete
A typical curve for concrete is shown below:

BS 8110 makes use of a modified stress-strain curve as
shown

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

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

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

Fire Protection
Fire protection of reinforced concrete members is
largely achieved by specifying limits for:
Cover to reinforcement
minimum dimensions for section

This table shows the nominal cover to ALL
reinforcement to meet the specified period of fire
resistance

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):

Minimum dimensions of reinforced concrete members
for fire resistance

Basic design considerations

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Viewers also liked

Viewers also liked (20)

Similar to Basic design considerations

Similar to Basic design considerations (20)

Recently uploaded

Recently uploaded (20)

Basic design considerations