How to set deflection limit DFF?

[imsengr]

Hi all,

I am designing a steel space frame, and would like to set a deflection limit of L/240 for my members. I understand that the deflection command in STAAD is DFF, but how do I set my deflection limit for all the members, which are obviously of varying lengths.

Also, one of my nodes is experiencing a big deflection, in the order of a few inches. It is a point on a beam where several beams and a bracing meet. Does anyone have any suggestion how I can limit deflection at that node.

Thank you all. Please let me hear from all of you familiar with my problems.

TobyGA

 

[RockEngineer]

There are tutorials on this.

http://www.reiworld.com/Product/AllDemo.asp

 

[structuresguy]

See RockEngineer's repsonse to setting the DFF variable. However, my concern is about the node with big deflection. I have a couple of questions about it. Are you sure that all the members intersecting at the node are truly intersecting at the same node? Do all the members have the same deflection at this node? The reason I ask is I have had several instances where it looks like one node with several members framing into it. When in reality there might be two or even three nodes very close to each other. So you have a discontinuity of the members, which almost always leads to some excessive deflection at that node.

 

[JMorrison]

Easiest way I've found to check what's going on if there's excessive deformation at a node, turn on Animate mode if your version of Staad supports it. Once you see a graphic of how the building is actually deflecting, it's usually pretty obvious where the problem is, or where you need to add a little more beef.

I generally try to stay away from automatic deflection calcs (and design mode in general) in Staad - I typically use it as an analysis tool and use judgement to pick member sizes... but if you must do it this way, you'd just add a parameter of DFF 240, and don't forget to specify your DJ1 and DJ2 values if you have any nodes without a real support.