This subject is unclear to me...AISC 89 min weld size based on THICKER part JOINED...AISC 2005...min weld based in THINNER part....ok so far..
when I read the commentary in 2005 for the reasing behind this is where I get totally confused which is a common occurance in my experience in using the 2005 code..
As I read and reread their explanation, it is still clear as mud.
Question:..their logic is based on the quenching effect of the thicker part causing rapid cooling in SMALL welds that can lead to lack of ductility, cracking,etc
but, in my mind this new requirement leads to overall smaller fillet welds which goes against their logic for making this change in the first place...
ofcourse "low-hydrogen" weld rods are slipped in their to make it all more clear...they never quantify or identify what a low-hydrogen rod is..and ofcourse the statement that the engineer and welder must coordinate what type of rod etc....
maybe I am going senile..does anyone have a clear logical interpration of this....thanks

First off, AWS (the welding code) changed from basing this on the thicker part to thinner part a long time ago, AISC is just now happening to catch up and most structural engineers don't make a habit of reading AWS so it came as a shock to us.

The basis for the change has to do with the circumstances of the original requirement. The minimum weld size was originally based on the fact that at that time there was a higher probability of weld cracking due to use of electrodes with a higher hydrogen content as well as the lack of defined preheat requirements and poor regulation of heat input. Therefore having a bigger weld (basing it on the thicker part joined), was desirable to help offset these issues (cools more slowly).

Nowadays, almost all SMAW welds are considered low-hydrogen (for an extended explanation of what this really means see http://www.lincolnelectric.com/en-us/support/weldi...), we have defined pre-heating requirements, and modern welding practices allow more regulation of heat input. All of these together mean it is much less likely for a weld to crack, thus it was felt that basing the minimum weld thickness on the thinner part joined, and thus providing slightly smaller (more economical) weld sizes, was warranted. If you still want to base it on the thicker part joined, knock your socks off, it won't hurt.

Thanks for the info WillisV...I get it now....clear,
concise with all the pertinent info...refreshing in this age of "Moon-talk"