A filler is a plate inserted in a splice between a gusset or splice plate and stress-carrying members to fill a gap between them. Requirements for fillers included in the AISC specifications for structural steel for buildings are as follows.
In welded construction, a filler 1⁄4 in or more thick should extend beyond the edge of the splice plate and be welded to the part on which it is fitted (Fig. 5.10). The welds should be

able to transmit the splice-plate stress, applied at the surface of the filler, as an eccentric load. The welds that join the splice plate to the filler should be able to transmit the splice plate stress and should have sufficient length to prevent overstress of the filler along the toe of the welds. A filler less than 1⁄4 in thick should have edges flush with the splice-plate edges. The size of the welds should equal the sum of the filler thickness and the weld size necessary to resist the splice plate stress.
In bearing connections with bolts carrying computed stress passing through fillers thicker than 1⁄4 in, the fillers should extend beyond the splice plate (Fig. 5.11). The filler extension should be secured by sufficient bolts to distribute the load on the member uniformly over the combined cross section of member and filler. Alternatively, an equivalent number of bolts should be included in the connection. Fillers 1⁄4 to 3⁄4 in thick need not be extended if the allowable shear stress in the bolts is reduced by the factor 0.4(t  0.25), where t is the total thickness of the fillers but not more than 3??„4 in.
The AASHTO specifications for highway bridges require the following: Fillers thicker than 1⁄4 in, except in slip critical connections, through which stress-carrying fasteners pass, should preferably be extended beyond the gusset or splice material. The extension should be secured by enough additional fasteners to carry the stress in the filler. This stress should be calculated as the total load on the member divided by the combined cross-sectional area of the member and filler. Alternatively, additional fasteners may be passed through the gusset or splice material without extending the filler. If a filler is less than 1⁄4 in thick, it should not be extended beyond the splice material. Additional fasteners are not required. Fillers 1⁄4 in or more thick should not consist of more than two plates, unless the engineer gives permission.
The AREMA does not require additional bolts for development of fillers in high-strength bolted connections.

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