According to Eurocode 1993-1-8:2005 clause 2.4 (3), if a connection is composed of welds and bolts, the fastener with the highest stiffness should carry the whole load. In that case bolts, and in particular normal bolts with hole clearance, will not contribute to the joint resistance at all. However, there as some exceptions:
Clause 3.9.3 (1) of the same code says that: "As an exception to 2.4(3), preloaded class 8.8 and 10.9 bolts in connections designed as slip-resistant at the ultimate limit state (Category C in 3.4) may be assumed to share load with welds, provided that the final tightening of the bolts is carried out after the welding is complete".
It should also be noted that National Annex to BS EN 1993-2:2006 (Design of steel bridges) states that the ultimate capacity of slip resistant bolts acting in friction and welded connections should be taken as no greater than 90% of their combined capacities.