Welds - Instantaneous Center of Rotation

[skibuf23]

Hey all,

I was looking at Table 8-4 Coefficients C for Eccentrically Loaded Weld Groups for use in designing a stiffened seat connection that is a bit larger than usual. Followed the whole eccentric load @ 0.8W (width of seat plate) found a C-value plugged it in... pretty typical I thought.

Now to the question, this is the first time I am designing such a connection and I am doing a bit of research / looking up related information in the process. I stumbled upon an older 9th Edition ASD manual, and the Eccentric Load on Weld Groups Table XIX had significantly lower values for C for the same geometry. Were talking 2.29 versus .780. Was hoping to learn why this was. One methods shows a passing design and the other method fails miserably. Design loads are ASD by the way, that may be significant to mention.

Thanks in advance from a newbie.

 

[nutte]

The C values in the 13th and 14th edition manuals don't have the phi or omega value included. For ASD, omega=2, and your C/omega value would be 2.29/2=1.14. This is still higher than the 9th edition value of 0.780, because starting with the 2nd edition LRFD manual, the specification allows you to take advantage of the increased strength of a fillet weld loaded perpendicular to its axis. There were also some changes to the instantaneous center of rotation equations between the 9th edition and 2nd edition (and subsequent) manuals.

All that said, the stiffened seat procedure uses the vector analysis method to size the welds at the stiffener to the support, not the instantaneous center of rotation method. The vector analysis method will result in more weld. In keeping with the design methodology, I'd recommend the vector analysis method.