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Basic design considerations

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Basic structural design considerations according to British standards

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Basic design considerations

  1. 1. BASIC STRUCTURAL DESIGN Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip) Skills College of Technology
  2. 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. Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
  3. 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. 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 Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
  5. 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 Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
  6. 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 Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
  7. 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 Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip) Skills College of Technology
  8. 8.  There are three broad classification of limit states: 1. Strength limit states 2. Serviceability limit states 3. Special limit states 8 Limit States Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
  9. 9. 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)
  10. 10. 10 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. 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. 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. 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 Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
  14. 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 Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
  15. 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 Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
  16. 16. 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)
  17. 17. 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)
  18. 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. 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 Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
  20. 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. 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. 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)
  23. 23. Stress-Strain Curves Stress-Strain Curve for Concrete A typical curve for concrete is shown below: Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
  24. 24. 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)
  25. 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 Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
  26. 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 Prepared by: M.N.M Azeem Iqrah B.Sc.Eng (Hons), C&G (Gdip)
  27. 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. 28. 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)
  29. 29. 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)
  30. 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. 31. 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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