Adding node to existing (main) beam to create secondary beam

[vickings99]

Hi!

My name is Vicky and I am a civil engineer.
Staad is very new for me although I used to work on other Structural programs and I have a few problems that I had not came across in the past. One of them is the following:

I have a main beam and I want to create five secondary ones being supported on the former one. When I just added new nodes on the main one and I created the secondary ones, my results were full of instabilities since - as I found out later - the created intersections (between main and secondary beams) are considered to be UNJOINTED.
So, what I did was to divide/break my main beams into smaller ones at the intersection points that I wanted. My model was then working just fine. However, I find this phenomenon very unpractical for things like adding point loads to the main beam, getting bending moments, shear forces and deflection values for it and so on.
I then thought that another way would be to leave the main beam as it is and then assign/apply supports to the intersection points so that Staad "understands" that the nodes are really joints between the members.

I would be very grateful if someone could advice me on the issue and tell me what they think as at the end I am not sure at all which method is the correct one, which one is better and if there is any other method in front of my nose that I dont see...

Thank you very much in advance.

 

[omel18181]

can you post a sketch of what you are modeling?

 

[vickings99]

Of course, thank you for the help in advance

So, using the sketch, I am talking about beams with numbers:
13, 14, 15 and 16 that should be connected to the longitudinal beams. The latter ones are in this model split/broken as in any other case the intersections between transverse and longitudinal beams are considered UNJOINTED.

 

[Gumpmaster]

In STAAD, you can't just add a node that coincides with a member and expect the member to be jointed with the node. You have to split the member and tell it to connect to the node. You can then later define the two new members as a physical member.

The problems with defining a physical member are that you're limited on the load types you can apply to the member, and you can only design the steel phyiscal member in the steel designer module (which has a host of limitations).

I don't think that assigning supports at the intersection points will have the effect you're looking for.

The method that you used is the correct method. When designing the beams you need to be careful to input the correct unbraced lengths for compression and bending by defining the proper parameters and applying those to your split up member.

As for looking at an overall shear or bending moment diagram, I don't think there's a real elegant way of displaying that. You can show only the members you're interested in and then show the overall structure bending and shear diagrams, but that's cumbersome.

 

[vickings99]

Gumpmaster,

Thank you very much for your reply. It has been really helpful.
I really appreciate it.