hello everyone
i am design the steel building i staad pro . now if the beam is continuous say 10 m and i am having the node at the ends. the say i'm getting the beam section as pass with iswb 450 and if i am inserting the node in between the beam say one at mid point the the staad gives beam as pass with value less than the above written value. say ismb 400 what is the reason. i have check all the parameters and they are ok . but design is not sufficient as per requirement.
Himmatlal,
Your question is not clear.
Can you attach both the models? You have attached only one.
STAAD uses the length of the member when calculating the allowable bending compression stress. If you use a node in the middle, you are reducing the effective length of the member. Hence, you are getting higher allowable stress which will result with a smaller pass ratio.
You have to give Ky and Kz values for the members to help STAAD identify what is the actual effective length to be taken for calculating allowable stresses. If there is a crossbeam at the middle node which provides restraint for the compression flange, then you can introduce the node and use the default Ky and Kz values. If not, then you have to enter Ky and Kz as 2 for the members with new node in center.
Calculation of Ky and Kz is Effective length factor x Actual Effective length of the beam of which the member is a part of / Length of the member...
For example, if you take a 10 m long member and cut into 4 and 6 m parts, and if you can take effective length as 0.85 x L, then
Ky and Kz for 4m part = 0.85 x 10 / 4
Ky and Kz for 6m part = 0.85 x 10 / 6
Hope that if you add these factors, your pass ratios come same in both cases...
Arun
Dear Arun ,
What ever you have said about Ky and Kz could not be perceived by me. I will have relook into it and try to understand
What I feel that in lieu of Ky and Kz facotos the members 1, 2 and 3 should be assigned Ly and Lz that is unsupported length, as 10 meter to all the three members. Please examine my views and give your valuable comments.
Dear Mr. Suresh Sharma,
Yes, Ly and Lz can be used in the place of Ky and Kz and in this case, it will be easier to use...
Dear Arun & Suresh,
Should we use UNB & UNT commands in lieu of LY & LZ? Because they correspond to the unsupported length of the Top & Bottom flanges respectively..
Will it make a huge difference in the design of the member?
Kindly elaborate..
Regards,
Arun.
Yours is not a case of unsupported i.e. UNB and UNT.
Dear Suresh,
Thanks for your reply. But it'll be much helpful to me if you can add some more clarity to the message.
I have a 10 m long beam(W16x40). It's split into 5 parts due to the secondary beams that are framing into the same. Now in the design parameters section, should I be using LZ = 10?
Or should I use UNB = 10? What difference will the two make?
As per the explanation of Design Parameters in STAAD, UNB, UNT parameters are used for calculating bending stresses for behavior like a beam. LY, LZ parameters are used for specifying effective lengths i.e., behavior similar to a column. Can these two be used interchangeably?
Kindly correct me if I'm wrong about the usage of these parameters.
In IS 800 the command is UNL in lieu of both UNT and UNB. In the present case that you have cited you will have to assign UNL as 10 m for all the 5 pieces of beam which are of 2 meter between node to node and forming a part of 10 meter long beam.