3348 Design of Welds Between Stub and Bottom Chord

The welds that are needed to fasten the stubs to the top flange of the bottom chord are primarily governed by the shear forces that are transferred between these components of the stub girder. The shear

«es = Ves/(0.9 X Qn) = Ves/(jconnQn) = 265/(0.9 X 22.7) = 13.0

«is = Vis/(0.9 X Qn) = Vis/(jconnQn) = 77/(0.9 X 22.7) = 3.8

force distribution gives these stress resultants, which are equal to those that must be transferred between the slab and the stubs. Thus, the factored forces Ves and Vis that were developed in Section 33.4.7 are used to size the welds.

Axial loads also act between the stubs and the chord; these may be compressive or tensile. In Figure 33.7, it is seen that the only axial force of note occurs in the exterior vertical of the exterior stub (load = 18 kip); the other loads are very small compressive or tensile forces. Unless a significant tensile force is found in the analysis, it will be a safe simplification to ignore the presence of the axial forces insofar as the weld design is concerned.

The primary shear forces that have to be taken by the welds are developed in the outer regions of the stubs, although it is noted that in the case of Figure 33.5, the central vertical element in both stubs carry forces of some magnitude (63 and 19 kip, respectively). However, this distribution is a result of the modeling of the stubs; analyses of girders where many more verticals were used have confirmed that the major part of the shear is transferred at the ends [5,6,11]. The reason is that the stub is a full shear panel, where the internal moment is developed through stress resultants that act at points toward the ends, in a form of bending action. Tests have also verified this characteristic of the girder behavior [5,7]. Finally, concentrating the welds at the stub ends will have significant economic impact [4-6].

In view of these observations, the most effective placement of the welds between the stubs and the bottom chord is to concentrate them across the ends of the stubs and along a short distance of both sides of the stub flanges. For ease of fabrication and structural symmetry, the same amount of welding should be placed at both ends, although the forces are always smaller at the interior ends of the stubs. Such U-shaped welds were used for a number of the full-size girders that were tested [4,5,7], with only highly localized yielding occurring in the welds. A typical detail is shown in Figure 33.9; this reflects what is recommended for use in practice.

Prior to the research that led to the change of the welded joint design, the stubs were welded with all-around fillet welds for the exterior as well as the interior elements. The improved, U-shaped detail provided for weld metal savings of approximately 75% for interior stubs and around 50% for exterior stubs.

For the sample stub girder, W16 x 26 shapes are used for the stubs. The total forces to be taken by the welds are

Exterior stub: Ves = 265 kip

Interior stub: Vis = 77 kip

Using E70XX electrodes and 16-in. fillet welds (the fillet weld size must be smaller than the thickness of the stub flange, which is | in. for the W16 x 26), the total weld length for each stub is Lw, given by (refer to Figure 33.9)

since U-shaped welds of length (bfs + 21) are placed at each stub end. The total weld lengths required for the stub girder in question are therefore

Exterior stub:

Interior stub:

Stub flange

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