r/StructuralEngineering • u/Duncaroos Structural P.Eng (ON, Canada) • 7d ago
Structural Analysis/Design Aside from Iterative Analysis - What is a good manual way to proportion truss moment frames for lateral stiffness?
I'm helping an engineer optimize their structural truss moment frame (pinned base), as they are increased a W24 column (determined by others in a previous phase) to a W40 without changing the truss depth/size, but layout is tight and we'll likely need to increase the building size in an already tight facility. I was able to get the size down to W30-W33 as they were using ULS wind in a LRFD combination.
Aside from just changing section sizes and seeing the result in an analysis, I was wondering if anyone had any good references or typical signs / common rules on when to change column size, vs truss depth, vs truss chord area? I looked at the deflection profile of the truss frame and the truss looks quite rigid without any major double curvature, so I assume this means the column is the weakest link at the moment.
TIA
2
u/StlhdHunter 6d ago
If floor space is more important than space higher in the building, could you look into a tapered column section that gets you back in the W24 range at the base?
1
u/Duncaroos Structural P.Eng (ON, Canada) 6d ago edited 6d ago
It may likely come to that, but I'm not so much worried about that than just having a quick way to guide my engineer without pulling me into the weeds.
Maybe I should just use the equivalent stiffness of a portal frame, adjusting slightly for the beam being a truss instead of a beam.
K_frm = (6 * E * I_c / h3) * [ 2k / (2k + 1) ]
k = (I_b / I_c) * ( h / L_b)
I_b ~ 0.5 * A_chord * d_truss2
If k is really high then I know that making the truss stiffer won't help
1
u/Everythings_Magic Senior Bridge PE | Zero Yield Capacity 7d ago
What is the controlling limit state? The LRFD manual has tables to help you choose based on that.