Hi all,
I'm designing a gusseted connection for a braced frame and cannot eliminate a warning: "Connector length less than allowable Table D3.1". The warning relates specifically to the interface of the gusset to the brace. From what I can discern from AISC design examples, the connector length should be defined by the center-to-center length of the outer bolts. I have increased this parameter to no avail and continue to get the warning. I tried reviewing the report with formulas and cannot find which parameter is not satisfying the requirements of Table D3.1 in the AISC steel manual. I have attached the report, with and without formulas. Unfortunately, Bentley isn't allowing me to upload the design file itself. Please see the google drive link below for the design file. I would really appreciate any kind of help determining what's wrong with my design.
drive.google.com/.../view
PDFPDF
I don't think U was ever intended to be 1.0 for cases with the weld length L less than the brace nominal size, but rather that condition was to be considered outside the range of applicability because it's a weak connection (U=0 at L=1 7/8"). However, the AISC 2022 Specification Table D3.1 is significantly revised from the 2016 edition and uses a new formula for slotted HSS members. Notice the new formula doesn't have the restriction of "L > H" and will actually not reduce the capacity as much when putting in lengths below H as the previous formula. The new formula with a 5" HSS doesn't reduce to U = zero until L = x_bar = 1.55". L=1" would still be below the new formula limit, but now the formula addresses the full range of potential values from zero capacity to the unreduced capacity.
Dear Engineer;
The model contains a gusset connection with a HSS brace in tension. In the tension rupture check for the brace, the shear lag parameter U is calculated as 1-x/l, where l = 1". In Table D3.1 of AISC 360-16, l is limited to the H (HSS width), which would be at least 5” in this model. U= 1 should be used when l < H.
Table D3.1 does not address this condition so you will have to rely on your own engineering judgment to modify in the program somehow the length l. The following steel interchange addresses a possible approach that you could use for Case 6. The arrangement might also fall outside the scope of the Specification, and you decide how best to proceed, Read the next: http://msc.aisc.org/globalassets/modern-steel/steel-interchange/2014/032014_si.pdf Carlo Lini, P.E., LEED AP BD+C Staff Engineer American Institute of Steel Construction.
The best approach, I found to solve the issue was considering the next:
Attached and image for better approach, Hopefully, this helps you on the solution.
Edgar García-Hernández
Structural Engineer, PE/M.S.C.E.