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Flenn Hale
Construction Manager
Asked a question last year

Can we place a Column over Beam mid-span?

Sir, Can we place column in the clear span of a beam without mid column support as shown in the figure. I was designing this mini football stadium and even though i feel that it is not totally OK to do so but nonetheless is there a way we can do so? Reason for doing so is to create a open basement area for car parking. are there any fundamental knowledge required while executing such work like -depth of beam required -special detail of reinforcement or any other requirements. Thank you

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Wilma Caldwell
Architectural technologist

Well, when we talk about an existing contract being then it’s not possible. 

The answer can be best understood with the help of the bending moment diagram. 

The moment of an integrated column is placed in the mid-span of an existing beam for its support then it starts to behave in as a continuous beam and negative moments develop on it both sides of the column for which there would not be any reinforcement available at the top level.

Which means midspan of the beam contains only positive reinforcement at the bottom of the beam unless and until it has been designed for reversal of stresses which is not a common design practice. 

My honest opinion is, in such type of critical situation The Beam may develop a crack on top of both sides of the column and may also fail in such type of case.

Thank You.

Beams:

Top reinforcement in beam is lapped at mid span as the beam does not experience any negative moment at midspan and so lapping is great in that region. In case of bottom reinforcement it is lapped near the ends and again the same concept applies as there will be no positive moment at the ends of the beam.

But if we look at moment frame beams, these beams because of cyclic loading experience both negative and positive moments during earthquake as moment frame is the LFRS of the building resisting earthquakes. All the diaphragm load is dumped on these moment frames which carries the lateral shear.

So in case of moment frames both top and bottom bars are spliced at the center of the beam and sometimes at third the span of the beam so that reinforcement bond slip does not occur at the beam ends which acts as the plastic hinge zone to absorb and dissipate all the earthquake energy.

In case of lap zone in moment frame beams and columns, a higher confinement of stirrups or tie is required so that bar does not buckle and concrete also does not spall as the bond between steel and concrete is weak.

Columns:

Rebars in column should be lapped at around 5 feet from the story. The most important reason behind this is workability.

We know that during lateral forces and because of interstory drifts columns experience maximum moments at the top and bottom of the story. Now we cannot lap the bars into the joint as those are the locations of maximum stresses. So the best possible location is around 5 ft from story height. This way, when the floor below is constructed the workers can easily stand on the floor and work on the column without worrying about the formwork and heights. Thus the construction process is accelerated.

There is one more reason of providing closely spaced ties in the end zones of columns. It is because of these splices the strength of the section gets slightly reduced and the ductility is also reduced. So to increase ductility and column confinement we have to provide smaller spacing of ties.

In case of Moment frames reinforcement should be spliced at the middle height of the column as moment frame is LFRS system and it will experience very high moments. So the splicing is particularly shifted to middle height in moment frames. No matter how high is the story height it should be in the center of the column as it is a region of zero moment.

Lillian Holmes
Director of Public Works

Hi Flenn,

To analyze the beam structure and weight of the top floor please provide details of dimensions and length of column and beams. so that I will be analyzed according to IS 456:2000 and give you a final result of what is an ultimate load you must carry on beam and column if provided at midspan.

Yes, you can do that. These are called hung columns. These are columns, having no foundation/ footing, rest over beams, or slabs. These are also termed as hanging columns/ floating column. They can not transfer load directly to the foundation or footing. Load from these columns is the point load on the beam over which it rests.These beams are generally deep beam need high percentage of reinforcement, also termed as transfer beam. Load is transferred through the transfer beam to the supporting columns, ultimately to the foundation. These columns are designed to take vertical load , considering it a compression member. These are abundantly provided to get more space in the ground floor to keep provisions for parking, conference hall, architectural requirement , banquet hall in hotels, commercial building etc.These are also provided in intermediate floor for the similar purpose. These columns are designed for vertical load only. In earthquake prone areas these may prove unsafe. If at all provided, they will not take lateral forces, all seismic forces shall be taken by the main columns supporting the transfer beam. Reinforcement detailing is an important issue in the design of column- beam junction.