Moment Capacity variation between STAAD result and Manual Calculation

Hello All,

I was manually calculating the bending capacity of the built up I sections as per IS-800 and found that the results for compact and semi-compact webs were not matching with the STAAD results.

I have attached the snap of my manual calculation, as well as the STAAD result. The sectional properties are matching exactly.

Kindly provide your expertise in this regard. It will be very much helpful.

Also, if we see the section classification for the attached member [450x5(web) + 175x8(flanges)], the web is classified as "Compact" in STAAD, whereas actual classification has to be "Semi-Compact". Clarify this as well.

Regards,

Parishith J

PDFPDF8875.Frame.std

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  • Can you please upload the STAAD file here?

    Thanks & Regards,
    Abhisek Mandal
    Global Technical Support

  • Hello Mr. Abhisek,

    Have Uploaded the model with the initial post, kindly check the design bending strength for member no. 4.

    Regards,

    Parishith J

  • Member number 4 is a tapered type of section for which the cross section dimension is varying throughout the length. STAAAD computes design results at 13 sections along the length and report the critical one. Here the critical section is marked as Zero which means start of the member.

    The depth of the member at start end is 750 mm(web only) and yu should take that in calculation. But in your calculation you have taken 450mm and hence the difference. Please use 750x5mm web in the calculation and the value should match.

    Regarding the other question related to section classification, can you show me your calculation? In the attached file I did not see any detailed calculation. Here as the member is subjected to tension, r1 will be negative and hence the limit will be high. Please try at your end and let me know. Below equations will be used to calculate.

    Hope this helps.

    Thanks & Regards,
    Abhisek Mandal
    Global Technical Support

  • Appreciating your effort Mr. Abhishek, but still i do have some questions regarding the capacity calculation, because the start of the member is 750 as you were saying, then probably i should have an higher moment capacity around 287 kN.m.

    And moreover see the attached snap, which depicts the start and end of the member. Start of the member will be shown in green and end will be shown in blue if i am not wrong.

    If you see the start for member 4, it is 450 mm only. 

    Secondly regarding the section classification, if it is for 750mm depth, then the d/tw ratio is definitely a slender web.

    I was calculating the section classification for Neutral Axis at mid depth criteria. If i was calculating as per general criteria and 450mm may fall under "Compact".

    Kindly calculate the capacity for 450 mm depth section as it is the start of the member and help me to match the staad result.

    Thanks in Advance,

    Parishith Jayan

Reply
  • Appreciating your effort Mr. Abhishek, but still i do have some questions regarding the capacity calculation, because the start of the member is 750 as you were saying, then probably i should have an higher moment capacity around 287 kN.m.

    And moreover see the attached snap, which depicts the start and end of the member. Start of the member will be shown in green and end will be shown in blue if i am not wrong.

    If you see the start for member 4, it is 450 mm only. 

    Secondly regarding the section classification, if it is for 750mm depth, then the d/tw ratio is definitely a slender web.

    I was calculating the section classification for Neutral Axis at mid depth criteria. If i was calculating as per general criteria and 450mm may fall under "Compact".

    Kindly calculate the capacity for 450 mm depth section as it is the start of the member and help me to match the staad result.

    Thanks in Advance,

    Parishith Jayan

Children
  • Yes you are correct, my bad. The design section is having 450x5mm dimensions. Let me check the capacity and get back to you.

    Regarding the section classification can you provide your calculation. As you said, it will be slender in the case of compression being the axial force. But as the member is subjected to tension (see report -88.25 (T)), r1 will be negative and that will increase the limit.

    Thanks & Regards,
    Abhisek Mandal
    Global Technical Support

  • If you provide the design capacity check for the section it would be very much helpful.

    For the section classification of the web, I used the category under "Neutral Axis at Mid depth", which would yield 450/5 = 90.

    Which is greater than 89.25 [105 X 0.85 (105E)]. So, the web is falling between 105E and 126E, hence "Semi-Compact".

    This was my consideration.

    Based on the STAAD result, the web classification is reported as "Compact". So, my query is whether we should not use "Neutral Axis at mid depth" criteria to find the classification for the web?

    And in my case, I am getting a "Semi-Compact" web, whereas the result produced by STAAD is "Compact". - So following the code, we are getting two different classification for a single case. The safe option is to go with the conservative one, right. In that case, why STAAD is not adopting "Semi-Compact"..?