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Reinforcement at top, in the mid span location of a beam.
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Abishek_Siingh
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PostPosted: Wed Jan 19, 2011 6:00 am    Post subject: Reinforcement at top, in the mid span location of a beam. Reply with quote

Dear all,

I was checking a beam detailing sheet by a reputed consultant and I have a query on which if you could throw light, then the confusion would be sorted.

In a continuous beam, the software STAAD shows the same reinforcement at top, in the mid span ,equal to (or sometimes slightly less by 20%) the reinforcement at mid span at bottom. I fail to understand why?

If it is a doubly reinforced section, then still the top reinforcement cannot be equal to the bottom reinforcement( I am talking about mid span section only and not the supports). The top reinforcement is equal to (from basics), reinforcement due to incoming moment minus the balanced section capacity. How can the top reinforcement be equal to the bottom?

Secondly, the consultant alleges that it is because of the reversal of moments. My knowledge (limited although), says that even in the case of reversal of moments, the DL and LL will still be acting downwards, only EQ Bending moment diagram gets reversed. So, in no case can the top reinforcement be equal to the bottom reinforcement.

Kindly advise. I might be wrong in my basics...

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Abishek Siingh

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sakumar79
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PostPosted: Wed Jan 19, 2011 6:56 am    Post subject: Reply with quote

Dear Sir,
   Has the consultant given the STAAD input file? If you open the file and run the analysis and design, you should be able to find out the moments in various load combinations and cross-check why the reinforcement comes in same for top and bottom - this usually comes only when they are both nominal steel qty...

   There is a design parameter called track which can be used to get a more detailed design report... But even this is not needed if you can run the staad file and review the analysis results (BMD) graphically...

   Without the staad file it is impossible to check and comment where the error might be...

Hope that helps
Arun
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gpsarathyy
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PostPosted: Wed Jan 19, 2011 11:18 am    Post subject: Reply with quote

Dear Sefian,

As already specified by the Er. Kumar it is difficult to come to conclusion about the rebar requirement/Provided.
The following may be the some reasons:

  1. The minimum rebar may be governing.
  2. Please check wheather he has used seismic analysis (using SRSS) method. It will also give all as absolute values. Please check this case also.

We have to do thorugh study on the analysis, then we can easily get the solution.
1. Check with Track 2 report
2. Check with the bending moment selecting the beam for a particular load case.
By which we can sort out it.

Regards,
G.Parthasarathy
Chennai
Emai: GPSARATHYY@GMAIL.COM
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prof.arc
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PostPosted: Wed Jan 19, 2011 11:46 pm    Post subject: Reinforcement at top, in the mid span location of a beam. Reply with quote

You are Correct. There is no way that the BM at centre of span change sign from that of DL+LL to that of DL+LL+EQ

unless the vertical component EQ is so great [not possible with the low values of IS-Code values]
I congratulate you for being alert
ARC

On Wed, Jan 19, 2011 at 11:30 AM, Kumar_Abhishek_Singh <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           Dear all,

I was checking a beam detailing sheet by a reputed consultant and I have a query on which if you could throw light, then the confusion would be sorted.

In a continuous beam, the software STAAD shows the same reinforcement at top, in the mid span ,equal to (or sometimes slightly less by 20%) the reinforcement at mid span at bottom. I fail to understand why?

If it is a doubly reinforced section, then still the top reinforcement cannot be equal to the bottom reinforcement( I am talking about mid span section only and not the supports). The top reinforcement is equal to (from basics), reinforcement due to incoming moment minus the balanced section capacity. How can the top reinforcement be equal to the bottom?

Secondly, the consultant alleges that it is because of the reversal of moments. My knowledge (limited although), says that even in the case of reversal of moments, the DL and LL will still be acting downwards, only EQ Bending moment diagram gets reversed. So, in no case can the top reinforcement be equal to the bottom reinforcement.

Kindly advise. I might be wrong in my basics...
     


Thank you,
Kumar Abhishek Singh(M.Tech.,IIT Delhi)
Deputy Manager-Structures, DLF Ltd.

     



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shiven2004
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PostPosted: Thu Jan 20, 2011 8:51 am    Post subject: hii Reply with quote

Positive bending moment at mid span will be never equal to negative moment unless we consider earthquake parameter higher or equal than g ....but in case of minimum steel engineer can use same steel at top as well as bottom .......but in other case it is not  reliable ...sometimes habit of designing structure for maximum moment without noting the direction can create a panic and we designer should not simply design but we should analyse with fundamental intact ....


thanks
regards
shivendra kumar
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Abishek_Siingh
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PostPosted: Thu Jan 20, 2011 1:56 pm    Post subject: Reply with quote

Thank you Prof. ARC and Mr. Shiven for supporting my view. But it is really hard to digest that many consultants do not abide by this, but rather prefer to provide for, whatever reinforcement STAAD feels like displaying in the results.

In my view good engineering is the ability to interpret the results of the software and compare them with standard experience based and academic facts, and then provide for the same.

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kapildingare
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PostPosted: Fri Jan 21, 2011 5:09 am    Post subject: positive moment at mid span Reply with quote

hallo,
    

             I am not sure about your case but I have seen cases when positive moment in beam at column joint is larger than nigative moment in beam at that column joint while designing building frames for a load case with earthquake load acting and hence steel at bottom in a beam at column jt more than steel at top at that location. So be careful before making any decision.
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kapildingare
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PostPosted: Fri Jan 21, 2011 5:55 am    Post subject: Reply with quote

Resp Prof Arc,
     
                     Sir,your openion regarding nigative moment and hence steel at midspan of beam at top with earthquake load is correct when there are DL+LL+EQL but I doubt about your opinion when we consider load case of .9DL+1.5 EQL.
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Abishek_Siingh
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PostPosted: Fri Jan 21, 2011 4:35 pm    Post subject: Reply with quote

Dear Mr. Kapil,

My replies to your two posts are seriatim below:

1. The point of concern is not the support but the mid span of the beam.

2. EQ bending moment diagram is linear with zero or negligible value of bending moment at mid span(positive or negative). 0.9 dead load bending moment diagram is parabolic generally and  will still be having greater value at mid span, that is positive moment at mid span, in magnitude than the negative moment at mid span due to 1.5EQX. SO the net will still lead to reinforcement at bottom face of the beam.

3. Many of the structural engineers are of the opinion (all those with whom I have interacted on load combinations) that 0.9DL+1.5EQX is not a realistic load case as, EQ is an instantaneous load and should not be multiplied by 1.5 while reducing DL by 10%. But, as our code mentions it, we have to use it.

Kindly share you views on this.

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Abishek Siingh

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anshugoel
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PostPosted: Fri Jan 21, 2011 5:59 pm    Post subject: Reply with quote

Kumar_Abhishek_Singh wrote:
Dear Mr. Kapil,

My replies to your two posts are seriatim below:

1. The point of concern is not the support but the mid span of the beam.

2. EQ bending moment diagram is linear with zero or negligible value of bending moment at mid span(positive or negative). 0.9 dead load bending moment diagram is parabolic generally and  will still be having greater value at mid span, that is positive moment at mid span, in magnitude than the negative moment at mid span due to 1.5EQX. SO the net will still lead to reinforcement at bottom face of the beam.

3. Many of the structural engineers are of the opinion (all those with whom I have interacted on load combinations) that 0.9DL+1.5EQX is not a realistic load case as, EQ is an instantaneous load and should not be multiplied by 1.5 while reducing DL by 10%. But, as our code mentions it, we have to use it.

Kindly share you views on this.


As I remember, one of the explanation for 0.9Dl+1.5 * Lateral used to be to check for overturning and uplift forces - not so much as beam design.
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