High strength steels can soften as a result of welding and specialist advice on allowable stresses should be sought.
Fillet welds may fail in the weld throat due to shear. The allowable stress in fillet welds (based on the weld throat area) is therefore that for shear loading, which is usually about 70% of that for direct loading, depending on the design standard being used.
One important consequence of welding is the possible embrittlement of the steel. Its 'strength' can then be reduced, depending on a number of factors. Specialist advice can be obtained from the Structural Integrity Department.
For more information, see UK standard BS449 : Part 2 : 1969 Specification for the use of structural steel in building, or 'Welded Joint Design' by J G Hicks, 2nd Edition, Abington Publishing, 1997 (ISBN 1 85573 337 4)