column and it's types

Muhammad Asif
Institute of Southren Punjab Multan Pakistan

1. Group members 1-M. Ismail Joiya 2- M.Adnan doger 3-M.Asif 4-Tariq Rasool 5-Sajjad Ahmad 6-Irfan Hussan

2. WHAT IS COLUMN? Column is a vertical structural member. It transmits the load from ceiling/roof slab and beam, including its self-weight to the foundation. Columns may be subjected to a pure compressive load. R.C.C. columns are the most widely used now-a-days.

3. Columns Columns carry primary Axial Loads and therefore are designed for compression. Additional loads from snow, wind or other horizontal forces can cause bending in the columns. Columns then need to be designed for Axial Load and Bending.

4. Columns Longitudinal rebar runs vertically and is held in place by ties Longitudinal bars are typically about 4% of the gross column area; ties are usually #3 or #4 bars Typically designed for compression, but must be able to resist bending Photo courtesy of John Gambatese

5. Column Forces F ( External ) W COL (External ) R 1 (Internal) R 2 (Internal) R Soil (External) W FTG (External) Horizontal loads caused by wind, snow, seismic or internal building load

6. COLUMN LOAD TRANSFER FROM BEAMS

7. Types Of Columns Long Columns Short (Strut) Columns Intermediate Columns

8. Long Column When the ratio of effective length to the least radius of gyration is greater than 45, then it is called a long column. A long column is subjected to bending moment in addition to direct compressive stress. The load carrying capacity of a long column is less than a short column The load carrying capacity of a long column depends upon slenderness ratio (slenderness ratio increases then the capacity of the column decreases)

9. LONG COLUMN : When length of column is more as compared to its c/s dimension, it is called long column. Long Column L e / r min > 50 Where, L e = effective length of column r min = Minimum radius of gyration

10. Real world example: Here in picture we can see long columns on front of building in “The White house” Washington D.C(USA).

11. Short Column: When the ratio of effective length to the least lateral dimensions of the column is less than 12, then it is called a short column. (or) When the ratio of effective length to the least radius of gyration is less than 45, then it is called a short column

12. SHORT COLUMN : When length of column is less as compared to its c/s dimension, it is called Short column. Short Column L e / r min <50 Or, L e /d < 15 Crushing Load : The load at which short column fails by crushing is called crushing load.

13. INTERMEDIATE COLUMN: Column is intermediate when 4d < L < 30d and 30 < L e /r min < 100 or Critical slenderness ratio.

14. What is the definition of the slenderness ratio of a column? Slenderness ratio is the ratio of the length of a column and the least radius of gyration of its cross section  Often denoted by lambda λ = l e / r min

15. Uses of slenderness ratio It is used extensively for finding out the design load as well as in classifying various columns in short/intermediate/long Example-  Short Steel column - lambda is less than 50. Intermediate - 50 -250 Long - 250 above.

16. Why this is important? Long columns under compression can fail via both buckling (bending side ways) as well as crushing. Various formulas to calculate such failure characteristics extensively use the use of this ratio.

17. Radius of gyration   Radius of gyration is used to describe the distribution of cross sectional area in a column around its censorial axis. The radius of gyration is given by the following formula. R.g= i/A Where I is the second moment of inertia. and  A  is the total cross-sectional area.

18. Calculating the radius of gyration To calculate the radius of gyration for the cross-section of the beam in the diagram, start with the values of  I  that were calculated earlier. I xx  = 33.3 x 10 6  mm 4 I yy  = 2.08 x 10 6  mm 4 Refer to the diagram for the values of b and d that are used in the calculation of A. A = Area of cross-section = 50 mm x 200 mm = 10,000 mm 2 Substitute  I  and A into the formula for  r  to give: This is the value of the radius of gyration about the x-x axis.

19. R.C.C. Columns Columns of square, rectangular and circular sections

20. Steel Columns or Stanchions Sta ndard Structural Steel Sections

21. Build-up Column Sections

22. Footing Column Girder Beam Partial View of 2 nd floor Framing For Clarity the Ground Floor Slab, 2 nd Floor Slab and Roof Framing and Roof Deck are not shown

23. 3D View of Retail Building Steel Framing and 1st Floor Slab Shown

24. Précautions of Columns construction Process of construction Vertical Height of concrete Columns for more then one storey Load on the column of the top storey

25. Failure Modes of COLUMN Column may fail in one of three condition Compression failure of concrete or steel reinforcement Buckling Combination of buckling and compression failure Compression failure is likely to occur with columns which are short and stocky. Buckling is probable with column which are long and slender

26. Failure modes of columns Comp r ession failure Buckling

27. CRIPPLING LOAD OR BUCKLING LOAD The load at which, long column starts buckling(bending) is called buckling load or crippling load. Buckling of column depends upon the following factors. 1. Amount of load. 2. Length of column 3. End condition of column 4. C/s dimensions of column 5. Material of column.

28. Failure Modes Short Columns – fail by crushing (“compression blocks or piers” Engel) f c = Actual compressive stress A = Cross-sectional area of column (in 2 ) P = Load on the column F c = Allowable compressive stress per codes Intermediate Columns – crush and buckle (“columns” Engel) Long Columns – fail by buckling (“long columns” Engel) E = Modulus of elasticity of the column material K = Stiffness (curvature mode) factor L = Column length between pinned ends (in.) r = radius of gyration = ( I /A) 1/2

29. Find the Effective length 1.      Both ends pinned                                         Effective length = actual length x 1.0 2.      Both ends fixed                                             Effective length = actual length x 0.5 3.      One end pinned other end fixed                                            Effective length = actual length x 0.7 4.   One end fixed other end completely free                       Effective length = actual length x 2.0

30. (L e ) for different support conditions.

31. Selecting Columns In order to select the correct column for a particular application Determine the effective length of the column required Select a trial section Using the radius of gyration value for this trial section calculate the slenderness ratio. If the slenderness ratio is greater than 180, try a larger cross section trial section. Using the slenderness ratio obtain the compressive strength from tables. (from the Y-Y axis)

32. Selecting Columns - Example A column, pin ended, length 5m has an axial load of 1500kN. The steel has a yield stress of about 265MPa Effective L = 1 x 5 = 5m Choose a trial section – 203 x 203 / 86 From table 1 - Rad . Of Gyration = 5.32 SR = 5 / 0.0532(Needs to be in metre) = 94

33. Extract from Table 2 Axis of Buckling – Y – Y Yield ( MPa ) SR = 94 Comp Strength = 200MPa (Approx) SRatio 265 275 340 25 258 267 328 50 221 228 275 75 187 192 221 100 138 141 153 150 74 74 77

34. From Table 2 – Compressive strength is approx 150MPa The actual stress ( s ) = Load / CSA = 1500 x 10 3 / 110.1 x 10 -4 = 132 MPa This value is 18MPa below the required stress value for this section We should now repeat the process until we have a stress value JUST below the 150 MPA value TRY IT YOURSELF

35. WHAT IS UNIAXIALLY LOADED COLUMN? WHEN A COLUMN IS SUBJECTED TO EITHER COMBINED AXIAL COMPRESSION (P) AND MOMENT (M) AS IN FIG- 1 OR ONLY AXIAL LOAD (P) APPLIED AT AN ECENTRICITY e=(M/P) AS IN FIG- 2 SO THAT THE COLUMN IS TRYING TO BEND ABOUT ONLY ONE AXES OF THE COLUMN CROSS SECTION IS KNOWN AS UNIAXIALLY LOADED COLUMN.

36. CROSS SECTION OF UNIAXIALLY LOADED COLUMN

37. IN THIS CASE,COLUMNS ARE SUBJECT TO TENSION OVER A PART OF THE SECTION AND IF OVERLOADED MAY FAIL DUE TO TENSILE YIELDING OF THE STEEL ON THE SIDE FARTHEST FROM THE LOAD.

38. 43 Steel Reinforcement in Columns The limiting steel ratio ranges between 1 % to 8 %. The concrete strength is between 25 MPa to 45 Mpa. Reinforcing steel strength is between 400 MPa to 500 Mpa.

39. 44 Design procedure 1. Calculate factored axial load Pu 2. Select reinforcement ratio 3. Concrete strength = 30 MPa , steel yield strength = 420 MPa 4. Calculate gross area 5. Calculate area of column reinforcement, As, and select rebar number and size.

40. 45 Guidelines for Column Reinforcement Long Reinforcement Min. bar diameter Ø12 Min. concrete covers 40 mm Min. 4 bars in case of tied rectangular or circular Maximum distance between bars = 250 mm Short Reinforcement ( Stirrups) Least of: (16)×diameter of long bars least dimension of column (48)×diameter of ties d c S A sp

41. 46 Reinforcement of Columns