Cross Frame Fatigue Check in MDX
Cross Frame Fatigue Check in MDX
(OP)
I am designing a skewed plate girder bridge with continuous cross frames (not staggered) in MDX using a the Plate & Eccentric Beam System Analysis. I am working on the design of the intermediate K shape cross frames and the sizes of the angle members are getting pretty big, L5x5x3/4, due to the check for fatigue stress. I turned off the fatigue check and changed the member sizes to L4x4x1/2 and they are good for strength.
I think the problem is is the fatigue category for the gusset plate to angle member weld. I selected Category E because the thickness of the connected members is less than 1 inch. I know there are a lot of bridges out there with cross frames composed of members much smaller than 5x5 angles that don't have a bunch of cracked welds. Has anyone else ran into a similar problem? I am contemplating ignoring fatigue for the intermediate cross frames.
I think the problem is is the fatigue category for the gusset plate to angle member weld. I selected Category E because the thickness of the connected members is less than 1 inch. I know there are a lot of bridges out there with cross frames composed of members much smaller than 5x5 angles that don't have a bunch of cracked welds. Has anyone else ran into a similar problem? I am contemplating ignoring fatigue for the intermediate cross frames.
RE: Cross Frame Fatigue Check in MDX
RE: Cross Frame Fatigue Check in MDX
RE: Cross Frame Fatigue Check in MDX
Regarding OPs problem, it does sound almost exactly like that TxDOT bridge they mentioned. It's worth noting that the reason they found over-estimates of stiffness was due to the eccentricity in single-sided connections giving them the 50-80% over-estimate when comparing analytical models to line elements in canned software. Shell element FEA analysis included the eccentricity which allowed for almost perfect correlation between the FEA model and the analytical models. However, when you looked at connections without eccentricity (knife plate connections to tubes, double angle braces, etc.) they found that there was no over-estimation.
It's also worth noting that while over estimating the stiffness is overly conservative for fatigue, it's under conservative for actual bracing.
Try running your braces as double angles and see what you get for member size and fatigue. See if you can watch a copy of that webinar or find the research papers cited in it. I found the webinar very interesting and informative and will definitely watch out for more presentations from Mr. Helwig.
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
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