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Technical doubts in the analysis of RCC framed structure

 
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udayakumar
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PostPosted: Tue Nov 29, 2016 7:47 am    Post subject: Technical doubts in the analysis of RCC framed structure Reply with quote

Dear Sir,

I did 2 case studies using Staad pro. In the first case study I analyzed a (stilt+2) RCC framed structure and following are my doubts:


1. After analysis I calculated the stresses in each column and compared with the allowable bearing stress (0.446*Fck) as per IS 456. In some columns the actual stress exceeds the allowable stress and at site due to physical constraints there is no scope for further increasing the size of the column. Under this circumstances what is the solution to be adopted.

2. I also calculated the stiffness of the members to check whether the stiffness of column is more than stiffness of the beams. What is the effective length of the members to be considered for arriving at the stiffness for beams and columns for calculating stiffness (K=I/L)

In the second case study

1. I analyzed an existing building (G+1) which was constructed (without doing any design) by adopting thumb rules of adopting 9"x9" columns with the span of beams (whose size is also 9'x9") not exceeding 10'. When the structure is analyzed and designed, staadpro shows some of the column sizes are inadequate whereas the structure is stable and standing for the past 20 years. How it is so?

2. If second floor is proposed to be constructed in this building in near future by adopting same thumb rules whether it is advisable to do so.

Kindly help me in clarifying the above doubts. Thankyou sir.

Best Regards
R.Udayakumar.
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VPandya
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PostPosted: Wed Nov 30, 2016 5:46 pm    Post subject: Governing Buiding Codes loads may not have come on this Structure yet. Reply with quote

Dear Er.  Udaykumar,
I can answer your second case :

You said "
"In the second case study

1. I analyzed an existing building (G+1) which was constructed (without doing any design) by adopting thumb rules of adopting 9"x9" columns with the span of beams (whose size is also 9'x9") not exceeding 10'. When the structure is analyzed and designed, staadpro shows some of the column sizes are inadequate whereas the structure is stable and standing for the past 20 years. How it is so?

2. If second floor is proposed to be constructed in this building in near future by adopting same thumb rules whether it is advisable to do so. "


My answer:  
Just because a building is standing for 20 years does not mean it is done per governing building code. My LEAD Structural Engineer ( MY Boss) told me that there are many many  buildings  still standing because they have not been subjected to  the extreme governing loads as required by the governing Building codes. As an Structural Engineer take no comfort in this that it  is standing for 20 years. Return period of some Earthquakes loads  is 500 years. Probability of that load is once in 500 years. Same is the case with maximum Wind Loads of the Codes.  Do the design based on Governing Building Code and then only say it is safe or  fails.  


Regards.

Vasudeo Pandya  P.E.  
Structural Engineer
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udayakumar
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PostPosted: Thu Dec 01, 2016 10:20 am    Post subject: Reply with quote

Thankyou Er. Pandya Sir for clarifying my doubts of the second case study.

It would be helpful if some expert in this forum clarifies my doubts of first case study also.

Thankyou sir in advance.

Best Regards
R.Udayakumar.
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vegad
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Joined: 25 Dec 2013
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PostPosted: Fri Dec 02, 2016 4:43 am    Post subject: Reply with quote

Dear Engr. UdayaKumar,

Case I

1) Try, increasing the strength of the material, or use an entirely different and stronger material.
2) Clear span/ length.

Case II

1) Try accounting 'all active' load paths; and recalculate. The structure may fall into a category of heavily confined masonry structure. That's where majority of India is living into.
2) Depends on the eligibility criteria, established by point 1, for a particular thumb rule.

Idealize the structure, chose appropriate code and make it compliant.

Thank you

Not an 'expert'.
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udayakumar
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PostPosted: Wed Dec 07, 2016 8:47 am    Post subject: Reply with quote

Thankyou Er. Vegad. I also learnt that providing pedestal to the column also would help in overcoming the excessive stressed columns.

Best Regards
R.Udayakumar.
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prahladpareek
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PostPosted: Thu Dec 08, 2016 12:20 pm    Post subject: Reply with quote

udayakumar wrote:
Thankyou Er. Vegad. I also learnt that providing pedestal to the column also would help in overcoming the excessive stressed columns.

Best Regards
R.Udayakumar.
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vegad
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Joined: 25 Dec 2013
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PostPosted: Fri Dec 09, 2016 2:14 am    Post subject: Reply with quote

udayakumar wrote:
Thankyou Er. Vegad. I also learnt that providing pedestal to the column also would help in overcoming the excessive stressed columns.

Best Regards
R.Udayakumar.


So you eventually increased the size of column up to a small height (pedestal); glad that restraint imposed was relaxed and equally lucky that excessive stresses lied at lower level.

Cheers.
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