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Thread: Assorted queries - non linear analysis for part braced frame and minimum required reinforcement query

  1. #1
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    Default Assorted queries - non linear analysis for part braced frame and minimum required reinforcement query

    Hi all,
    A couple of questions as follows:

    1.Non-linear analysis.
    If understand this right, this refers to the incremental loading procedure that occurs for the situation when one considers large overall deformation/second order effects and / or truss members. Could I get some clarification on the following:
    a) If no truss members are present in a structure then selection of non-linear static analysis is a way of accounting for the second order effects (I've checked a few simple examples by hand and they seem correct).
    b) I have a frame which is part tension braced (cross bracing in elevation), but also acts as a part sway frame at higher levels. Obviously I am getting the default switch to non linear analysis (due to the tension members), but does this also automatically account for the geometric non-linearity due to the sway deflection?
    c) I also have convergence problems with my model, but only when the bracing members are modelled as tension only - if truss or compression members then convergence is fine. Is this most likely due to a modelling error. We are modelling the columns as continuous members through floor levels, which I now understand to be incorrect? (structure is a simple 3d frame only with no plates).

    2. Calculation of required reinforcement for slabs.
    a) I have noticed a discrepancy between the required reinforcement results for a simple plate (reinforced for bending only) when comparing the RSA output to a hand calculation to both BS8110 and EC2 using the W&A moments obtained from the software. Is it possible to provide a worked example of the RSA calculation method to a particular code, or let me know how I can display this in full. Typically I end up with a bigger area of reinforcement when doing the hand calc.


    Aside from that, good software, thanks for your help!

  2. #2
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    Default Re: Assorted queries - non linear analysis for part braced frame and minimum required reinforcement query

    Quote Originally Posted by scottknight321957 View Post
    Hi all,
    A couple of questions as follows:

    1.Non-linear analysis.
    If understand this right, this refers to the incremental loading procedure that occurs for the situation when one considers large overall deformation/second order effects and / or truss members. Could I get some clarification on the following:
    a) If no truss members are present in a structure then selection of non-linear static analysis is a way of accounting for the second order effects (I've checked a few simple examples by hand and they seem correct).
    b) I have a frame which is part tension braced (cross bracing in elevation), but also acts as a part sway frame at higher levels. Obviously I am getting the default switch to non linear analysis (due to the tension members), but does this also automatically account for the geometric non-linearity due to the sway deflection?
    c) I also have convergence problems with my model, but only when the bracing members are modelled as tension only - if truss or compression members then convergence is fine. Is this most likely due to a modelling error. We are modelling the columns as continuous members through floor levels, which I now understand to be incorrect? (structure is a simple 3d frame only with no plates).

    2. Calculation of required reinforcement for slabs.
    a) I have noticed a discrepancy between the required reinforcement results for a simple plate (reinforced for bending only) when comparing the RSA output to a hand calculation to both BS8110 and EC2 using the W&A moments obtained from the software. Is it possible to provide a worked example of the RSA calculation method to a particular code, or let me know how I can display this in full. Typically I end up with a bigger area of reinforcement when doing the hand calc.

    Aside from that, good software, thanks for your help!
    ad 1.a)
    Yes, it is true.

    ad 1.b)
    No, it is considering only the non-linear behavior of tension-only bracings. Higher order geometrical non-linear effects are not taken into account is such case (if "Non-linear" and "P-delta" check-boxes are not active in any load case)

    ad 1.c)
    For the load case where convergence problems are observed please try to change from default BFGS method to full Newton-Raphson one - activating "Matrix update after each iteration" in options.
    Convergence problems may be also observed for symmetrical structures with tension-only X bracings defined in such way that they are unstable when no bracing is active. In such case for vertical downward loads adding very small lateral load may improve convergence - because it will activate some bracings.

    ad 2.a)
    I would prefer your model and calculations to search for the source of differences

    Regards,
    Pawel Pulak
    Support Specialist
    Autodesk, Inc.

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    Default Re: Assorted queries - non linear analysis for part braced frame and minimum required reinforcement query

    Hi Pawel,
    Sorry its taken me a while to get back to you.
    So, just to confirm, if we want to account for geometrical non-linearity in a structure with tension-only elements, then we simply have to ensure that the 'non-linear' box is ticked? Or both 'non-linear' AND 'P delta'?

    I will send over the slab comparison results as soon as I get five minutes

    Thanks again for your help

  4. #4
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    Default Re: Assorted queries - non linear analysis for part braced frame and minimum required reinforcement query

    Yes, to account for geometrical non-linearity in a structure with tension-only elements it is necessary to tick 'non-linear' box (2nd order effects) or 'non-linear' and 'P delta' boxes (2nd and 3rd order effects).

    2nd order effects are responsible for instance for increased lateral displacements and additional moments when vertical downwards load coexists in frame with lateral load
    3rd order effects are responsible for instance for occurrence of tension in slender simply supported horizontal beam (with at least one internal node) under vertical load.

    As you see the names 'non-linear' and 'P delta' are somehow misleading here - resulting from backward compatibility with previous versions of the software

    Regards,
    Pawel Pulak
    Support Specialist
    Autodesk, Inc.

  5. #5
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    Default Re: Assorted queries - non linear analysis for part braced frame and minimum required reinforcement query

    Pawel,
    You are a legend....you have no idea how much confusion this has caused

    Thanks again!

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