Reinforced concrete column

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1. Reinforced concretecolumn
Prepared by: M.N.M Azeem Iqrah
B.Sc.Eng(Hons), C&G (Gdip)
Skills College of Technology

2. Introduction tocolumn
•Columns act as vertical supports to beams and
slabs, and to transmit the loads to the
foundations.
•Columnsareprimarilycompressionmembers,
althoughtheymayalsohavetoresistbending
momenttransmittedbybeams.
•Columns may be classified as short or slender,
braced or unbraced depending on various
dimensional and structuralfactors.

3. Columnsections
•Common column cross sections are: (a)
square, (b) circular and (c) rectangularsection.
•The greatest dimension should notexceed
four times its smaller dimension.(h≤4b).
•For h>4b, the member should be regarded as
a wall for designpurpose.

4. Failure modes ofcolumns
•Columns may fail in one of threemechanisms:
1.Compression failure of the concrete or steel
reinforcement;
2.Buckling
3.Combination of buckling andcompression
failure.
•Compression failure is likely to occurwith
columns which are short andstocky.
•Buckling is probable with columns which are long
andslender.

5. Failure modes ofcolumns
Compression
failure
Buckling

6. Short and slender columns
(Clause
3.8.1.3, BS8110)
•A braced column is classified as being short if:

7. Braced and unbraced columns
(clause
3.8.1.5, BS8110)
•A column may be considered braced in a given
plane if lateral stability to the structure as a
whole is provided by wall or bracing or
buttressing designed to resist all lateral forces
in that plane. It should otherwise be
considered asunbraced.

8. Braced and unbraced columns (clause 3.8.1.5,
BS8110)

9. Braced and unbraced columns (clause 3.8.1.5,
BS8110)

10. Effective height of column(clause
3.8.1.6, BS8110)
•The effective height, le of a column in a given
plane may be obtained from the following
equation:
Whereisacoefficientdependingonthefixity
atthecolumnendsandloistheheightofthe
columns.
•Effective height for a column in two plane
directions may bedifferent.

11. Effective height of column
(clause
3.8.1.6, BS8110)
•for braced column can be obtained from
Table3.19.
•End condition 1 signifies that the column end is fully
restrained.
•End condition 2 signifies that the column end is partially
restrained.
•End condition 3 signifies that the column isnominally
restrained.

12. End conditions (clause 3.8.1.6.2, BS8110)
•End condition 1 –the end of the column is
connected monolithically to beams on either side
which are at least as deep as the overall dimension
of the column in the plane considered. Where the
column is connected to foundation, it should be
designed to carrymoment.

13. •End condition 2 –the end of column is connected
monolithically to beams or slabs on either side which
are shallower than the overall dimension of the
column in the planeconsidered.
End conditions (clause 3.8.1.6.2, BS8110)

14. •End condition 3 –the end of the column is
connected to members which, while not specifically
designed to provide restraint to rotation of the
column will nevertheless, provide some nominal
restraint.
End conditions (clause 3.8.1.6.2, BS8110)

15. Example 3.17 classification of column (Arya,
2009)
•Determine if the column shown below isshort.

16. Example 3.17 classification of column (Arya,
2009)

17. Short columndesign
•The short column are divided into three
categories:
1.Columns resisting axial loadonly,
2.Columns supporting anapproximately
symmetrical arrangement ofbeams,
3.Columns resisting axial loads and uniaxial or
biaxialbending

18. •B2 will resist an axial load only, as it supports beams
equal in length and symmetricallyarranged.

19. •C2 supports a symmetrical arrangement of beams
but which are unequal in length. If (a) the loadings
on the beam are uniformly distributed,(2)thebeam
spans do not differ by more than 15 percent, the
column C2 belongs to category2.
•If the column does not meet criteria (a) and (b), then
the column belongs to category3.

20. Theoretical strength of reinforcedconcrete
column
The equation is derived on the assumption that the axial load is
applied perfectly at the centre of thecolumn.

21. Clause 3.8.4.3 Nominal eccentricity of shortcolumns
resisting moments and axialforce
•To allow for nominal eccentricity, BS 8110
reduce the theoretical axial load capacity by
about10%.
•Design maximum axial load capacity of short
columnis:

22. Clause 3.8.4.4 Short braced columns supportingan
approximately symmetrical arrangement ofbeam
•The column is subjected to axial and small
moment when it supports approximately
symmetrical arrangement ofbeams:
•
•The design axial loadcapacity:

23. Column resisting an axial loadand
uniaxialbending
•For column resisting axial load and bending moment
at one direction, the area of longitudinal
reinforcement is calculated using design charts in
Part 3 BS8110.
•The design charts are available for columns having a
rectangular cross section and symmetrical
arrangement ofreinforcement.

24. Column resisting an axial loadand
uniaxialbending
•Design charts are derived based on yield stress of
460 N/mm
2
for reinforcement steel. They are
applicable for reinforcement with yield stress of
500 N/mm
2
, but the area of reinforcement
obtained will be approximately 10% greater than
required.
•Design charts are available for concrete grades–
25, 30, 35, 40, 45 and50.
•The d/h ratios are in the range of 0.75 to 0.95in
0.05increment.

25. Design chart for column resisting an axial loadand
uniaxial bending moment, (Part 3, BS8110)

26. Column resisting an axial loadand
biaxialbending
•The columns are subjected to an
axial and bending moment in bothx
and ydirections.
•The columns with biaxial moments
are simplified into the columns with
uniaxial momentbyincreasingthe
moment about one of the axesthen
design the reinforcement according
the increasedmoment.

27. Column resisting an axial load andbiaxial
bending (clause 3.8.4.5, BS8110)

28. Reinforcement details:longitudinal
reinforcement (clause 3.12.5, BS8110)
1.Size and minimum number of bars –bar size should not be
less than 12 mm in diameter. Rectangular column should
reinforced with minimum 4 bars; circular column should
reinforced with minimum 6bars.
2.The area of longitudinal reinforcement should lie inthe
limits:
3.Spacing of reinforcement –the minimumdistancebetween
adjacent bars should not be less than the diameter of the
bar or hagg + 5mm.

29. Reinforcement details –links (clause 3.12.7, BS8110)
•The axial loading on the column may cause buckling
of the longitudinal reinforcement and subsequent
cracking and spalling of concretecover.
•Links are passing round the bars to preventbuckling.

30. Reinforcement details –links (clause 3.12.7, BS8110)
1.Size and spacing of links –the diameter of
the link should be at least one quarter of the
largest longitudinal bar size or minimum 8
mm. The maximum spacing is 12 times of the
smallest longitudinalbar.
2.Arrangement oflinks

31. Example 3.20 axially loaded column (Arya,2009)
•Design the longitudinal and links for a 350mm square,short
braced column based on followinginformation.

32. Example 3.20 axially loaded column (Arya,2009)

33. Example 3.21 Column supporting anapproximately
symmetrical arrangement of beam ( Arya,2009)
•An internal column in a braced two-storey building supporting
an approximately symmetrical arrangement of beams
(350mm wide x 600 mm deep) results in characteristic dead
and imposed loads each of 1100 kN being applied to the
column. The column is 350 mm square and has a clear height
of 4.5 m. Design the longitudinal reinforcement andlinks.

34. Example 3.21 Column supporting anapproximately
symmetrical arrangement of beam ( Arya,2009)

35. Example 3.21 Column supporting anapproximately
symmetrical arrangement of beam ( Arya,2009)

36. Example 3.21 Column supporting anapproximately
symmetrical arrangement of beam ( Arya,2009)
•Links
•
link = diameter of largest longitudinalbar/4
•= 32/4 = 8 mm (equal to minimum bar size
of 8mm)
•The spacing of thelinks
•= the lesser of (12 smallest longitudinal bar or
the smallest cross sectional dimension of
column)
•= the lesser of (12x25 = 300 mm or 350mm)
•= 300mm

37. Example 3.22 Columns resistingan
axial load and bendingmoment
•Design the longitudinal and shear reinforcement for
a 275 mm square, short braced column which
supportseither
(a)An ultimate axial load of 1280 kN and a momentof
62.5 kNm about the x-xaxis
(b) An ultimate axial load of 1280 kN and bending
moment of 35 kNm about the x-x axis and 25 kNm
about the y-yaxis

38. Example 3.22 Columns resisting an axial load and bendingmoment

39. Example 3.22 Columns resisting an axial load and bendingmoment

40. Example 3.22 Columns resisting an axial load and bendingmoment

41. Example 3.22 Columns resisting an axial load and bendingmoment

42. Example 3.22 Columns resisting an axial load and bendingmoment