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BASIC STRUCTURAL DESIGN
Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)Skills College of Technology
Learning out-comesAt 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 ULSc) Explain a basic concept of concrete design.
Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
1.0 Design in Reinforced Concrete 1.0 IntroductionImportance 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)
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
Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
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
Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
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
Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
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
Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)Skills College of Technology
There are three broad classification of limit states:
1. Strength limit states2. Serviceability limit states3. Special limit states
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Limit States
Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
9
Strength Limit States:• Flexure• Torsion• Shear
Limit States• Fatigue• Settlement• Bearing
Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
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Serviceability Limit States:• Cracking• Excessive Deflection• Buckling• Stability
Limit States
Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
11
Limit StatesSpecial 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)
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 practices12
Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
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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
Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
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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
Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
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.
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Design Considerations
Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
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
Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
Characteristic Strength of reinforcement fy
Reinforcement Type Characteristic Strength (N/mm2) fy
Hot Rolled Mild steel 250
High Yield Steel 460
Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
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)
Partial safety factors for Strength of Material
Material and Stress type
Partial Safety Factor, γm
Reinforcement 1.15Concrete - Flexure or Axial Load
1.5
Concrete - Shear, unreinforced
1.25
Concrete - bond 1.4Concrete - other e.g. bearing
>1.5
Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
Loading:Characteristic LoadsGk - 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)
Design LoadsIn 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)
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)
Stress-Strain CurvesStress-Strain Curve for ConcreteA typical curve for concrete is shown below:
Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
BS 8110 makes use of a modified stress-strain curve as
shown
Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
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
Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
DurabilitySigns 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
Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
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)
Fire Protection
Fire protection of reinforced concrete members is
largely achieved by specifying limits for:
Cover to reinforcement minimum dimensions for section
Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
This table shows the nominal cover to ALL reinforcement to meet the specified period of
fire resistance
Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
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)
Minimum dimensions of reinforced concrete members for fire resistance
Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
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