How to design if beam axes does not coincide with column center line in reinforced concrete building.

[Aye Min Khaing]

How to design if beam axes does not coincide with column center line in reinforced concrete building. It can be do in etabs by using insertion point tool. But It is not easy in complex structure. Please suggest me how to do and reference book and experence that you have done. Thank you all.

[Simple Structures]

Most software nowadays inputs physical member sizes, i.e. column size and beam size, including centre lines. The analysis programme is therefore able to calculate any additional moment due to eccentricity. 

Also, in concrete frame buildings there is the slab, which is built into both beams and columns, and helps mitigate eccentricity. 

You can also cross check with a hand calculation knowing the eccentricity of loading - this hand calc check should assure you.

Remember the beam width does not always match the column plan size - especially around the perimeter, where the edge beams may be set to align with the outer edge of column.

Keep things simple. Remember In the old days there were no computers and all calculations were by hand! 

[Aye Min Khaing]

If so, Should I ignore this effect in all conditions? I would like to know practices of structural engineer in cause of this effect and have this any limitation and engineering judgement? In some paper, this effect make torsion in column and should be avoided in high seismic zone.  

Thanks for your answer.

Edited March 17, 2020 by Aye Min Khaing

[Simple Structures]

1 hour ago, Aye Min Khaing said:

If so, Should I ignore this effect in all conditions? I would like to know practices of structural engineer in cause of this effect and have this any limitation and engineering judgement? In some paper, this effect make torsion in column and should be avoided in high seismic zone.  

You cannot generalise, as eccentricities can vary and heavily loaded beams at greater eccentricity, can generate significant bending moment in the column, and the increase the reinforcement (bar size or number) in the column; The column would need to be deigned for both axial and bending. There is no hard fast rule for this, each beam/column location needs to be looked at and decision made.

A concrete frame can come in many configurations:

1. The floor plate say with 2-way spanning floor slab supported on beams in both direction; Here slab load (dead and imposed) is shared by all four beams; One beam being slightly eccentric would be less onerous;

2. The floor plate say with one-way spanning slab with beams in one direction only, would mean greater loading on the beams, greater reaction onto the column meaning, then eccentricity consideration is important;

3. Say you have 300mm thick 'flat-plate' or 'flat slab' then the slab load comes into the column, and eccentricities become less critical . In this situation assuming slab punching shear etc has been checked, eccentricity of slab loading becomes less critical.

Where torsion occurs, say a beam going pas and connected to one face of the column, then the torsional moment - and torsion reinforcement - would need to be designed for.

With the question being asked, I am assuming you are designing a simple structure, hence not too much to worry – but do talk to senior engineer in your office and take his/her view.

 

In highly seismic areas the reinforcement continuity is more critical, and detailing for load/stress reversal is critical. My advise would be:

(1)   to close up the spacing of the shear links/stirrups closer to the column joint, half the spacing to what you have in middle of beam. Say if you have shear links at 300mm centre at the middle of the beam, then make them 150mm centres closer to the beam/column joint - for a length of say 1/8 span length from face (in both beam and column either side of joint)

(2)  Provide sufficient tension lap lengths (say 40 times bar diameter, as a rule of thumb)

Talk to a senior engineer in your office for job specific consideration.

[Aye Min Khaing]

Thanks 😊