Lecture Chp-9&10 – Columns - PowerPoint PPT Presentation

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Lecture Chp-9&10 – Columns

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  1. Lecture Chp-9&10 – Columns

  2. Lecture Goals • Definitions for short columns • Columns

  3. Analysis and Design of “Short” Columns General Information Column: Vertical Structural members Transmits axial compressive loads with or without moment transmit loads from the floor & roof to the foundation

  4. Analysis and Design of “Short” Columns General Information • Column Types: • Tied • Spiral • Composite • Combination • Steel pipe

  5. Analysis and Design of “Short” Columns Tied Columns - 95% of all columns in buildings are tied Tie spacing h (except for seismic) tie support long bars (reduce buckling) ties provide negligible restraint to lateral expose of core

  6. Analysis and Design of “Short” Columns Spiral Columns Pitch = 1.375 in. to 3.375 in. spiral restrains lateral (Poisson’s effect) axial load delays failure (ductile)

  7. Analysis and Design of “Short” Columns Elastic Behavior An elastic analysis using the transformed section method would be: For concentrated load, P uniform stress over section n = Es / Ec Ac = concrete area As = steel area

  8. Analysis and Design of “Short” Columns Elastic Behavior The change in concrete strain with respect to time will effect the concrete and steel stresses as follows: Concrete stress Steel stress

  9. Analysis and Design of “Short” Columns Elastic Behavior An elastic analysis does notwork, because creep and shrinkage affect the acting concrete compression strain as follows:

  10. Analysis and Design of “Short” Columns Elastic Behavior Concrete creeps and shrinks, therefore we can not calculate the stresses in the steel and concrete due to “acting” loads using an elastic analysis.

  11. Analysis and Design of “Short” Columns Elastic Behavior Therefore, we are not able to calculate the real stresses in the reinforced concrete column under acting loads over time. As a result, an “allowable stress” design procedure using an elastic analysis was found to be unacceptable. Reinforced concrete columns have been designed by a “strength” method since the 1940’s. Creep and shrinkage do not affect the strength of the member. Note:

  12. Behavior, Nominal Capacity and Design under Concentric Axial loads 1. Initial Behavior up to Nominal Load - Tied and spiral columns.

  13. Behavior, Nominal Capacity and Design under Concentric Axial loads

  14. Behavior, Nominal Capacity and Design under Concentric Axial loads Let Ag = Gross Area = b*h Ast = area of long steel fc = concrete compressive strength fy = steel yield strength Factor due to less than ideal consolidation and curing conditions for column as compared to a cylinder. It is not related to Whitney’sstress block.

  15. Behavior, Nominal Capacity and Design under Concentric Axial loads 2. Maximum Nominal Capacity for Design Pn (max) r = Reduction factor to account for accidents/bending r = 0.80 ( tied ) r = 0.85 ( spiral ) ACI 10.3.6.3

  16. Behavior, Nominal Capacity and Design under Concentric Axial loads 3. Reinforcement Requirements (Longitudinal Steel Ast) Let - ACI Code 10.9.1 requires

  17. Behavior, Nominal Capacity and Design under Concentric Axial loads 3. Reinforcement Requirements (Longitudinal Steel Ast) - Minimum # of Bars ACI Code 10.9.2 min. of 6 bars in circular arrangement w/min. spiral reinforcement. min. of 4 bars in rectangular arrangement min. of 3 bars in triangular ties

  18. Behavior, Nominal Capacity and Design under Concentric Axial loads 3. Reinforcement Requirements (Lateral Ties) ACI Code 7.10.5.1 size # 3 bar if longitudinal bar # 10 bar # 4 bar if longitudinal bar # 11 bar # 4 bar if longitudinal bars are bundled

  19. s s s 16 db ( db for longitudinal bars ) 48 db ( db for tie bar ) least lateral dimension of column Behavior, Nominal Capacity and Design under Concentric Axial loads 3. Reinforcement Requirements (Lateral Ties) Vertical spacing: (ACI 7.10.5.2)

  20. 3. Reinforcement Requirements (Lateral Ties) 1.) At least every other longitudinal bar shall have lateral support from the corner of a tie with an included angle 135o. No longitudinal bar shall be more than 6 in. clear on either side from “support” bar. 2.) Behavior, Nominal Capacity and Design under Concentric Axial loads Arrangement Vertical spacing: (ACI 7.10.5.3)

  21. Behavior, Nominal Capacity and Design under Concentric Axial loads Examples of lateral ties.

  22. Behavior, Nominal Capacity and Design under Concentric Axial loads Reinforcement Requirements (Spirals ) ACI Code 7.10.4 size 3/8 “ dia.(3/8” f smooth bar, #3 bar dll or wll wire) clear spacing between spirals 1 in. 3 in. ACI 7.10.4.3

  23. Behavior, Nominal Capacity and Design under Concentric Axial loads Reinforcement Requirements (Spiral) Spiral Reinforcement Ratio, rs

  24. Behavior, Nominal Capacity and Design under Concentric Axial loads Reinforcement Requirements (Spiral) ACI Eqn. 10-5 where

  25. Behavior, Nominal Capacity and Design under Concentric Axial loads 4. Design for Concentric Axial Loads (a) Load Combination Gravity: Gravity + Wind: and Check for tension etc.

  26. Behavior, Nominal Capacity and Design under Concentric Axial loads 4. Design for Concentric Axial Loads (b) General Strength Requirement where, f = 0.65 for tied columns f = 0.7 for spiral columns

  27. Behavior, Nominal Capacity and Design under Concentric Axial loads 4. Design for Concentric Axial Loads (c) Expression for Design defined:

  28. Behavior, Nominal Capacity and Design under Concentric Axial loads or

  29. Behavior, Nominal Capacity and Design under Concentric Axial loads * when rg is known or assumed: * when Ag is known or assumed:

  30. Example: Design Tied Column for Concentric Axial Load

  31. Example: Design Tied Column for Concentric Axial Load Design tied column for concentric axial load Pdl = 150 k; Pll = 300 k; Pw = 50 k fc = 4500 psi fy = 60 ksi Design a square column aim for rg = 0.03. Select longitudinal transverse reinforcement.

  32. Example: Design Tied Column for Concentric Axial Load Determine the loading Check the compression or tension in the column

  33. Example: Design Tied Column for Concentric Axial Load For a square column r = 0.80 and f = 0.65 and r = 0.03

  34. Example: Design Tied Column for Concentric Axial Load For a square column, As=rAg= 0.03(15.2 in.)2 =6.93 in2 Use 8 #8 bars Ast = 8(0.79 in2) = 6.32 in2

  35. Example: Design Tied Column for Concentric Axial Load Check P0

  36. Example: Design Tied Column for Concentric Axial Load Use #3 ties compute the spacing < 6 in. No cross-ties needed

  37. Example: Design Tied Column for Concentric Axial Load Stirrup design Use #3 stirrups with 16 in. spacing in the column

  38. Behavior under Combined Bending and Axial Loads Usually moment is represented by axial load times eccentricity, i.e.

  39. Behavior under Combined Bending and Axial Loads Interaction Diagram Between Axial Load and Moment ( Failure Envelope ) Concrete crushes before steel yields Steel yields before concrete crushes Any combination of P and M outside the envelope will cause failure. Note: