DW KIM,
I have not come across a lateral tee for quite some time now, so have not used FEA to investgate SIF's for these tees. Even the new relief systems I look at these days have no laterals, with the only place I see laterals being on open drain systems, which is non critical not analysed piping. As MoverZ has mentioned above, FEPipe would be a good package to use for finding SIF's on piping components. Also as mentioned above the french code gives a factor close to 2 for a 45 degree tee which ties up nicely with the factor I have used in the past, and this would appear to be conservative. So in general I would use a factor of 2 for 45 degree laterals knowing that this will be conservative. However, in cases where the D/t ratio is getting close to code limits or above, large header to branch ratio (per MoverZ) and for more acute angled tees, I would look into using FEA/FEPipe to calculate SIF's.
Regards