Bridge Manual (NZTA, A linear elastic critical buckling analysis was performed using Microstran. Download ColdSteel v2.1 Demo Setup (exe), Download user manual (pdf). Single-user licence - $1,500 + GST Multi-user licence (5 copies).-Microstran -Space Gass -Concept steel structural design -Concrete structural -Footpath Rating. Title, Microstran V8: User's Manual. Contributor, Engineering Systems Pty Limited. Publisher, Engineering Systems, 2002. Length, 338 pages. Export Citation.

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Hi, I'm hoping someone can clarify how Microstran uses the Elastic Critical Load analysis to determine compression effective length in its steel design module. I have a 125*5 square hollow section truss chord (Ixx = Iyy = 5.44E6 mm4) that has a maximum axial compression of 7.73 kN in a particular load case. The elastic critical load analysis gives a buckling load factor of 55.38 for this load case. Looking at the buckling shape from the elastic critical load analysis, the buckling mode is about the Y axis (out of the truss plane) between rafters at centres that vary but a bit under 5m maximum.

I thought the design module would use an axial force of 7.73 kN * 55.38 = 428 kN to calculate an effective length of 5.01m, for both X and Y axis buckling (since Ixx = Iyy). However it's actually using Lx = 38.0m and Ly = 6.8m. The chord is about 14m long. Lx corresponds to buckling in the plane of the truss in which diagonals intersect the chord at 1.5m centres. Because of the long effective length, the steel design module give a fail result for this chord due to low member capacity in compression. I'm struggling to understand how this could be the case since the buckling load factor was 55.38.

Appreciate your thoughts. Edit: The load case is earthquake and the chord axial force is a little odd, going from compression to tension to compression. Could the program be averaging the axial load along the member length for calculation of effective length, then using the maximum compression for the design check? Still not sure that would explain different lengths for the X & Y axes.

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RE: Microstran - Automatic calculation of steel member effective length (Structural) 23 Apr 17 19:19. The Lx and Ly values were calculated by Microstran using the elastic critical load analysis results and the restraint data. I played a bit last night and removed the restraint data. (I had previously entered them for manual effective length calculation and not deleted them when switching to ECL method, assuming they would be ignored).

The result was that both Lx and Ly are now 5.01m and the member passes the design check, both as expected. So, it turns out that nominating restraints may reduce the calculated member capacity when using the ECL method. I haven't got to the bottom of it yet, but I suspect that Microstran used a small axial force to calculate the effective length, then used the largest axial force for the design check.

Inconsistent on the conservative side if so. RE: Microstran - Automatic calculation of steel member effective length (Civil/Environmental) 27 Apr 17 20:27. I asked one of my colleagues in Australia for any additional insight and he said: I think this relates to this excerpt from the Microstran Manual and also Bentley Communities article: Results from ECL Analysis If results are available from an elastic critical load (ECL) analysis, you may enter “ECL” in the kx and ky fields to have effective lengths computed from the elastic buckling load factors (c). The results from an ECL analysis will only be applicable if the model includes all member restraints relevant to column action. If you change column restraints during member design ECL analysis results may not be correct.