Risa- (Boundary cond.)

[palk7 EIT]

Hi,
I am new to risa-2D and playing around with it, attached the image when the floor system is connected to the beam (connected via hangers) should I modify the boundary conditions at A,B,C as restrained in horizontal axis (roller) otherwise my deflection is very large for that horizontal load. The releases at the members start & end kept it as PIN-PIN (very large deflection), fix-fix(considerable defl.)

So my question is apart from fixing the bases as fixed, should there be horizontal restrain at A,B,C due to floor system?

Thank you!

 

[jayrod12]

What is your lateral force resisting system? If it is intended on being this moment frame, then no, you cannot put a horizontal reaction there.

If the lateral system is shear walls or bracing or moment frames elsewhere intended on taking the horizontal load to the foundation, then yes you could consider having a reaction there. However your diaphragm would need to be designed accordingly.

 

[palk7 EIT]

Hi Jayrod12,

The moment frame (fixed connection) is not in this bay but is on the bay back of this. And this particular bay has a plwd. sheathed wood stud wall to the left of this frame.

 

[jayrod12]

Well again, if there is a competent lateral system including diaphragm, then you should be able to dump your lateral load into the diaphragm and then into the intended lateral system.

Since we (strangers on the web) aren't fully privy to all of the details of the structure, we can't speak in definitives.

If you have somewhere else for the lateral load to go, then I don't see the need to design for it.

What is the source of the lateral load you were applying to the frame?

 

[palk7 EIT]

Yes, thats o.k, its a wind load. That's fine just learning the software so wanted to know whether or not a simple floor system when connected to beam should be to modify the boundary conditions to restrain it in horiz. direction.

But now I understand that if that beam-column is a fixed connection, I just leave it as it is. And that fixed connection is going to take the reaction. And only when I put a braced frame or there is going to be wood shear wall then I need to put a restraint in the horizontal direction.

 

[jayrod12]

If you're talking about the beam to column connection for this frame, I apologize as then I misunderstood the entire question.

Part of your question was whether you should have a reaction at the floor level. The answers above should suffice for that portion.

The second part of your question, whether the beam to column joints should be modelled as fixed or pinned, depends a lot on the type of construction and the intended connection. If this is a wood frame, then I'd be inclined to only use pinned connections as moment connections in wood members are difficult. If concrete, I'd typically go with fixed due to the fact that generally speaking they provide some level of fixity regardless of whether you wanted them to or not. If steel, it could be either as the connection in real life could be constructed as a simple shear connection (pinned in the model) or as a moment connection (fixed in the model).

Which direction to choose is up to you as the designer.

 

[palk7 EIT]

Yes so I assume that the beam to column connection is automatically assigned as fixed by the program and if we need to change the release to pin-pin we could change it, but my concern was on the overall boundary condition at that intersection of beam & column due to the floor system.

 

[jayrod12]

Sounds like you've got it.

 

[johnie134]

correct.