Hello,
I am running my steel design and seismic provisions in the Frame Module and I am getting an odd output for the beam design. I have BRB frames in a K-Brace configuration. When I click view/update I get the following output:
Beam ParametersStory: Level 2 Frame No: 2 Member No: 36Fy (ksi): 50.00 Size: W21X93Frame Type: Buckling Restrained Braced Frame
F4.3 Non-V and Non-Inverted V-Braced Frame Unbalanced ForcesBeam must be designed for unbalanced brace axial horizontal force component on beam ends.
F4.5a Basic Requirements (D1.1 Highly Ductile) --- OKFlange b/tf = 4.53 Limit = 7.22 OKWeb h/tw = 32.31 Limit = 35.88 OKNote: Beam axial load from F4.3 Analysis not determined; Web h/tw limit assumed to be 1.49 sqrt(E/Fy).”
Verify brace properties with the brace manufacturer prior to finalizing structural design.
Is there a reason that RAM does not calculate the unbalanced brace axial horizontal force component on the beam? If so, is there a way to turn it on to design the beam for the axial load?
It's common in Ram Frame to use a rigid diaphragm and in the X brace configuration the beam will not strain or experience any axial loads since both ends are connected to that diaphragm. You can play with the nodal connections or manually verify that the beam can take the axial force, but it's not yet automated. See also: https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/18616/ram-frame---criteria---diaphragms
Thank you for confirming that Seth.
Seth,
I have a similar question to Patrick's. The only differences being that my brace configuration is non-chevron and diaphragm is semi-rigid. However I am seeing the same message. Beam ParametersStory: L4 Frame No: 4 Member No: 53Fy (ksi): 50.00 Size: W18X65Frame Type: Buckling Restrained Braced Frame
F4.5a Basic Requirements (D1.1 Moderately Ductile) --- OKFlange b/tf = 5.06 Limit = 9.18 OKWeb h/tw = 35.78 Limit = 36.05 OKNote: Beam axial load from F4.3 Analysis not determined; Web h/tw limit assumed to be 1.57 sqrt(E/RyFy).
Can you tell me why I may be seeing this message even though I do not have a chevron brace configuration? See image of my frames below.
Thank you.
The program is reminding you in the F4.3 note to check the beam for axial loads. In the presence of a rigid (or semi-rigid) diaphragm, the axial force in the beam from analysis will be zero (or small) so a hand check is needed (I like to use the frame story shear report - change in shear data for this).
Thank you, Seth! Are there plans to incorporate this check into future versions?