Column Design

Column : 

1. A member in compression are called column and struct. or 
2. A reinforced concrete column is a structural members designed to carry compressive loads, composed of concrete with an embedded steel frame to provide reinforcement.
3. A column is that which transfer load to the ground and whose height/length not morethan 3 times its lateral least dimension.
4. A term strut is used to the compression member in any direction as those on the truss.
5. column are the important part of the structure a beam and or slab may fail without any serious demage but failure of the column engengers the whole structure so column must be carefully designed.

1. IS 456 Classified rectangular column as a short column when Slenderness ratio  [ the effective length to it least dimension (Le/d <12) ] called short column.
2. Minimum 4 bars are used in rectangular column.
3. Minimum 6 bars are used in circular column.
4. Minimum dia of bars in column is provided 12 mm
5. short column is designed on the bases of material strength and applied loads.
6. A long column has Slenderness ratio  [ the effective length to it least dimension (Le/d >12) ]
7. A long column is designed to resist the [ applied load + additional bending moment induced due to its tendancy to buckle ]
8. however Slenderness ratio should not exceed more than 60.
9. The crossectional are of  longitudnal reinforcement should not less than 0.8% of gross crossectional area of the column and not morethan 6% of gross crossectional area of the column.
10. For the rcc column have helical reinforcement must have at least 6 bars of longitudinal reinforcement witin the helical reinsforcement
11. in a helical rcc column longitudinal bars shall be in contact with the helical reinforcement and equidistance around its inner circumferemces.
12. the spacing of longitudnal measured along the periphery of the column shall not exceed 300 mm.
13. in case of pedestals in which longitudnal is not taken in account in strength calculation, nominal longitudinal reinforcement not  lessthan 0.15% of the crossectional area shall be provided.

Use of stirrups

1. We design reinforced concrete members and often we are asked to observe the steel reinforcement in field before the concrete is placed.  It is our job to make sure the concrete foundations, beams, columns, etc. are built the way they were designed.  During our observations we often find that steel beam stirrups, used in reinforced concrete design, are not installed correctly and it isn’t always clear to the installer why they are important.
2. Historically, beam stirrups had been used sparingly in residential construction.  However, in recent years concrete beam sizes have gotten shallower and spans have increased.  In our experience, this has been the result of architectural design and building occupant requirements. The increased cost of foundation elements, such as drilled piers, has also been a factor.  Increasing concrete beam spans, to reduce the need for additional piers, has resulted in the need for the use of steel stirrups.
3. Concrete beams vary in depth.  The deeper the beam, the more shear capacity.  When the depth is not adequate, steel stirrups must be added to increase the shear capacity of the beam.  These stirrups are usually one piece of steel that is bent into a rectangular shape.  Often small diameter steel is used, such as #3 and #4 rebar.  The stirrup typically wraps around the bottom and top bars of the beams.
4. A designer should specify the size, spacing and location along the length of the beam where the stirrups are required.  We like to specify the stirrup dimensions in our sections, so that the stirrup can be manufactured prior to installation.  Stirrups will be required at areas of high shear, such as bearing points and below large point loads.

1. The installer should be careful to fabricate the stirrup from one piece of steel and adequately overlap each end (contact the Structural Engineer or refer to the ACI code for variations).  Too often the stirrup is not pre-fabricated and the installer tries make the stirrup in the field, after the horizontal bars are already in place.  This is usually obvious, because the stirrup is constructed from two pieces with inadequate lap splice.  It is much easier and efficient to install a stirrup at the same time the horizontal reinforcement is being installed.  Always contact the Structural Engineer with any questions about size, shape, spacing and installation of stirrups prior to inspection.  This will help prevent last-minute changes, while the concrete truck is waiting.

Difference Between Singly Reinforced and Doubly Reinforced Beam

1. For a Beam, It is necessary to provide Reinforcement(Steel Bars) in Compression and Tension zone.
2. In a beam, If the reinforcement is only in tension zone then it is called Singly Reinforcement Beam and if the reinforcement is both at Tension and Compression zone then it is called Doubly reinforced Beam. 
3. In both case, there will be rod in tension and compression zone. This is because, it is not possible to form a Beam structure without stirrups.
4. To hold the stirrups in standing position, it is necessary to place two reinforcement in compression zone of singly reinforced Beam.
5. However, those two will never carry or pass loads in its body and it is just dummy. 
6. In a Beam, the top section is called Compression zone and the bottom section is called Tension zone. 

1. Singly reinforced section; 
   Steel at bottom of section to take TENSION force.
   
   
   Doubly reinforced section;
   Steel is provided at top & bottom zone i.e (Tension & 
   Compression zone resp.)
   
   
   
   Reason:
   
   Steel provided in top zone reduces cross sectional 
   dimension, section is safe against REVERSAL moment. 

Singly reinforced beam

Assumption Are made for calculation of Ultimate moment of resistance :

1. plane section should be remain plane in bending up to the point of failure.(i.e in zbove figure strains are proportional to the distance from the neutral axis)

Working stress Method OR Modular ratio method

1. This was the first theoritical method developed in 1900's for the design of rcc structure.
2. In this assumed that both concrete and steel act togather and prefectly elastic at all stages so that modular ratio (Modili of elastic of steel and concrete) can be used to determine the stresses in steel and concrete.
3. This method adopts permissible stresses which are obtained by applying factor of safety on material for design.
4. factor safety used for cubic strength of concrete = 3  and 1.8 for yeild stregth steel
5. this method has three defects :

• This method neighter show real strenght nor gives true factor of safety againt the structure failure.
• it gives uneconomic section.section are consrtucted big than limited state design
• concrete does not have definite modular of elasticity as in steel.

1. Design moment and shear in structure are calculated by elastic analysis with respect to service load applied on structure.
2. stresses in the concrete and steel are calculated on the basis elastic behaviour of the section.
3. Modular ration is taken may be either a conatant value for all strenght of concrete or may be vary according to strenght of concrete and steel.
4. IS 456 recommands modular ratio of elasticity of concrete which varies with rescepct to the strenght of concrete.