RCC (Reinforced cement concrete) Column/Strut
All details about the Column (a compression member in structure)
Column -Reinforced concrete columns are
used to transfer the load of the structure to its foundations. These are reinforced by means of main longitudinal bars to resist compression and/or bending; and transverse steel (ties) to resist bursting force.
The column or strut is a vertical compression member, the effective length of which exceeds three times its least lateral dimension(LLD).
(A) LongItudInaI Reinforcement-
In a reinforced column, the area of
longitudinal reinforcement shall not be less than 0.8 percent nor more than 6 percent of the gross cross-sectional area of the column.
The area of longitudinal reinforcement should normally not exceed 4 percent of the gross cross sectional area of the column. This percentage can be considered as the maximum from practical considerations.
However where bars from one column have to be lapped with those of another column above, the total maximum percentage of 6 percent may be allowed at the lapping. Proper placing and compacting of concrete should be ensured at the place of lapping.
A minimum number of 4 bars shall be
provided in a rectangular column and 6 bars in a circular column with helical reinforcement.
The bars shall be not less than 12 mm in diameter and spacing of the bars along the periphery of the column shall not exceed 300 mm.
In the case of pedestals in which the longitudinal reinforcement is not taken -into account in strength calculations, nominal longitudinal reinforcement of not less than 0.15 percent of the gross cross-sectional area shall be provided.
Note - Pedestal is a compression member. the effective length of which does not exceed 3 times the least lateral dimension.
#Dowels and Bar supports-
Dowel and bar supports, spacer bars, bar chairs etc.,should be specifically listed on the structural drawing and should be schedule in that portion of the structure in which they are first required so that they can,be delivered wtth reinforcement and are available for placement in time.
Footing dowles shall be scheduled with footings rather than in column.
A reinforcement concrete compression
member shall have transverse or helical
reinforcement so disposed that every longitudinal bar nearest to the compression face has effective lateral support against buckling. The effective lateral support is given b transverse reinforcement either in the form of circular rings capable
of taking up circumferential tension or by
polygonal links (lateral ties) with internal angIe not exceeding 135°.
# Pitch and Diameter of Lateral Ties
Pitch-The pitch of the transverse
reinforcement shall not be more than the least of the following distances(.see in fig. Below 7.6A)
a) the least lateral dimension of the compression member,
b) sixteen times the smallest diameter of the longitudinal reinforcing bar to be tied, and
c) forty eight times the diameter of the transverse reinforcement.
Diameter -The diameter of the polygonal links or lateral ties shall not be less
than one-fourth of diameter of the largest
longitudinal bar, and in no case less than 5 mm.
# Helical Reinforcement (Spirally Reinforced)(see in above fig. 7.6B)
Pitch - Helical reinforcement shall
be of regular formation with the turns of the helix spaced evenly and its ends shall be anchored properly by providing one and a half extra turns of the spiral preferably with a 135° hook.
Thepitch of the helical turns shall be not more than 75mm or one-sixth of core diameter of the column, nor less than 25 mm or 3 times thediameter of steel bar forming helix.
Tension lap length shall be provided at lap splices.
NOTE - It is important to note that when the ratio of the volume of helical reinforcement provided to the volume of the core greater than
the strength of the compression member may be increased by 1.05 times the strength of similar member with lateral ties.
As= gross area of the section,
Ac = area of the corn of the helically reinforced column measured to the outside diameter of the helix,
fck = characteristic compressive strength of the concrete
fy= characteristic strength of the helical reinforcement
but not exceeding 415 N/mm^2
Diameter -The diameter shall be not less than one-fourth of the diameter of the largest longitudinal bar, and in no case less than 5 mm.
At least two temporary stirruls should be provided to hold splices in position(see in fig. Below) or to stiffen the helically bound columns during fabrication.
The stirrups coming above the floor shall not be removed until the next column is erected.
Where reinforcement for very wide columns is to be fabricated in separate cages and erected in sections,they should be held together by at least 12 mm diameter bars spaced at double the stirrup spacing.(see in fig.below).
Mukesh kumar (Gyan of engineering)