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-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


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

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


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Strength Limit States:• Flexure• Torsion• Shear

Limit States• Fatigue• Settlement• Bearing


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

Limit States


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Limit StatesSpecial 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 practices12


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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



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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.



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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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 1.15Concrete - Flexure or Axial Load

1.5

Concrete - Shear, unreinforced

1.25

Concrete - bond 1.4Concrete - other e.g. bearing

>1.5


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:


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


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


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


Engineering

Basic design considerations