Hi everyone, I have been using RAM Structural Systems for just a week now. After laying out my gravity steel beam members in RAM Modeler, I have an option for Assigning beam fixity, but when I click to select my beams for different fixity options, which I do and then, when I click on (Options> Show Fixity > All) & select a beam, it doesn't show the fixity conditions (no window pops up with the beam's end fixity status).
I read in the manual that "When modeled, gravity members are by default considered as pinned (released) against bending and fixed against torsion at each end." Is there not a way to change this?
Could someone tell me why the show fixity does not show the fixity conditions after I prompt it to?
Thanks a lot!
Cheers,Ed
The manual is correct in this regard.
To elaborate, a steel gravity beam is always pinned at the ends, though it can be modeled with a cantilever at one or both ends.
Gravity beams that are “Other” or “Concrete” material can be assigned different fixity, but that stiffness is considered only in the Ram Concrete module analysis.
Lateral beams (of any material) should be used anywhere that a fixed connection is required. Keep in mind, the stiffness of those members is only going to be properly evaluated using Finite Element analysis in RAM Frame. Also note, any lateral beam must be supporting by other lateral members down to the base.
If you find yourself in a situation where you want a beam with end fixity, but you absolutely don't want it to participate in lateral load resistance, simply disconnect that member entirely from the diaphragm in RAM Frame (Assign menu – Nodes option).
Seth, so if I have a gravity beam that is fixed on one end because it will cantilever then I have to tell the program its a lateral beam and thus this lateral beam will contribute towards the lateral force resisting system correct? I assume that it wont impact a lot but is this what I need to do to get the analysis results I need?
My original reply was from a time before we had stub cantilever beams in the program. You can now create a gravity stub cantilever beam that is naturally fixed at one end and transfers moments to the support. I still prefer to model these as lateral beams in many cases however. One reason is to check the deflection at the tip of the cantilever including the effect of the support rotation.
Seth, If I make the beam a lateral beam I get presented with the error telling me that a lateral beam must be supported by lateral column, I dont want to artificially make some columns lateral that are not intended to be lateral, so how would I get around this issue of trying to fully fix a gravity beam because it is cantilevering off the building. The cantilever is not your typical cantilever however it spans 1 frame bay then cantilevers off the 2nd bay. Please let me know how you would model this accurately?
If you are going to run it in Ram Frame, then any lateral beam will need to be supported by another lateral member, and so on, all the way down to the ground. If you don't want those members to contribute to lateral stiffness, they can always be disconnected from the diaphragm (or hinged somewhere), but that increases the potential for instability. Gravity beams can also be used, as I noted above, I just have a preference for using lateral beams. Your beam sounds more like a standard beam with an extension cantilever to me. See also: RAM Modeling FAQ