The weld symbol with a dashed fillet weld symbol below (or above) is a supplementry contour symbol. The face of the fillet indicates weld should be welded flush without subsquent finishing.
The dashed fillet symbol that shows up in the AISC prequalified weld joints (Table 8-36 in the LRFD Manual) are their to indicate subsequent build-out welding beyond the profile of the full-pen weld. This is to reinforce the pen weld (see note J in the "Notes to Prequalified Welded Joints" for Table 8-36).
Pylko...this symbol is to show the configuration of the weld in the event you have a "Tee" configuration of the joint as compared to a "corner" configuration. Using JAE's example of Table 8-36 LRFD and the weld joint designation of TC-L1b, you will see that the joint is prequalified in either a "Tee" configuration or a "Corner" configuration.
If you are using the "Corner" configuration, you need only to weld the thickness of the horizontal component + any "cap" or reinforcement (note that weld capping or reinforcement is not to be used in computing stresses).
If you are using the "Tee" configuration, you will notice that the vertical component is extended with dotted lines to show the intersection as a "Tee". Correspondingly, you will see that a "fillet" configuration is used to transition the change from horizontal to vertical to reduce the potential for a stress raiser or notch effect at the joining of the horizontal to vertical. This fillet transition is accommodated in the symbol as an addition to the butt weld symbol. If you have a "Tee" configuration in actuality, the dotted fillet symbol would become solid in your weld symbol on the drawings.
