Difference in Staad Code Checking and Steel Design (under new tab)

Hi!!

I have a simple 2-D portal frame analysis of steel ISMB (Indian Steel Section).

Now code checking has given that the beam section has passed as matched with my manual calculation. But in the separate steel design tab, it shows beams as failed.

The difference what I have seen, in design tab while calculating the permissible bending stress, its takes the minimum ry values and not the rz value even if it was checking the moment about Z axis, This results in failure while in CODE CHECKING, the permissible bending stress about z axis calculated by taking the rz, even though the rz is greater that ry.

Why is it, will it not make more conservative. Please reply. Thanks

Note, For code checking in order to ignore the intermediate node points a suitable factor kZ is applied so that Lz X KZ will always be the length of the beam for each beam segments.

Note 2, It is assumed that main beam is connected to the secondary beams at intermediate points, so, the LZ is actually the length of each beam segments.Floor System Steel_Mt Connection.std

  • Hey!! someone please help me to find out the reason for the difference between two design checks, not a big thing for the experts, I believe :)
    Thanks
    Rajarshi
  • Hi Rajarshi,

    I believe that the discrepancy you're seeing is a result of how you are trying to define the KL/r beam slenderness. Rather than using the unbraced length of the members about each local axis (parameters LY and LZ), you are trying to modify the K values. While in theory this would produce the same results, I think by not inputting the unbraced length you are forcing the program to assume a much shorter beam length with an atypically high effective length factor (which is intended to modify the unbraced length).

    If you use an LY and LZ parameter of 6m for both beams in the "batch" mode design for you model input, you'll find that the beams are shown to fail due to high slenderness ratio in bending about the weak axis. The actual LY and LZ lengths used should reflect how the actual structure will be braced in those directions (e.g., if you have a lateral brace at midspan of the beams, LY = 3m). The Steel Designer mode is simply using the full member length as the unbraced length and a effective length factor (K) of 1 in each direction, which then gives the same KL/r values as you'll find in the batch design code check.

    -Jason Coleman

    Jason Coleman, PE

    Bentley Systems
    Senior Manager Technical Content