3346 Design of Stubs for Shear and Axial Load

The shear and axial force distributions indicate the governing stress resultants for the stub members. It is important to note that since the Vierendeel members are idealized from the real (i.e., continuous) stubs, bending is not a governing condition. Given the sizes and locations of the individual vertical members that make up the stubs, the design checks are made for axial load and shear. For example, referring to Figure 33.7, it is seen that the shear and axial forces in the exterior and interior stubs, and the axial forces in the verticals that represent the floor beams, are the following:

Exterior stub verticals:

Shear forces: Axial forces:

103 kip -18 kip

63 kip 0.4 kip

99 kip 3 kip

Interior stub verticals:

Shear forces: Axial forces:

19 kip 0.8 kip

20 kip 4 kip

Floor beam verticals:

Exterior: Axial force _ -39 kip

Shear forces are zero in these members.

The areas and moments of inertia of the verticals are known from the modeling of the stub girder. Figure 33.7 also shows the shear and axial forces in the bottom and top chords, but the design for these elements has been addressed earlier in this chapter.

The design checks that are made for the stub verticals will also indicate whether there is a need for stiffeners for the stubs, since the evaluations for axial load capacity should always first be made on the assumption that there are no stiffeners. However, experience has shown that the exterior stubs always must be stiffened; the interior stubs, on the other hand, will almost always be satisfactory without stiffeners, although exceptions can occur.

The axial forces that are shown for the stub verticals in the preceding are small but typical, and in all probability only the exterior end of the exterior stub really requires a stiffener. This was examined in one of the stub girder research studies, where it was found that a single stiffener would suffice, although the resulting lack of structural symmetry gave rise to a tensile failure in the unstiffened area of the stub [5].

Although this occurred at a very late stage in the test, the type of failure represents an undesirable mode of behavior, and the use of single stiffeners therefore was discarded. Further, by reason of ease of fabrication and erection, stiffeners should always be provided at both stub ends.

It is essential to bear in mind that if stiffeners are required, the purpose of such elements is to add to the area and moment of inertia of the web, to resist the axial load that is applied. There is no need to provide bearing stiffeners, since the load is not transmitted in this fashion. The most economical solution is to make use of end-plate stiffeners of the kind that is shown in Figure 33.1; extensive research evaluations showed that this was the most efficient and economical choice [4,5,7].

The vertical stub members are designed as columns, using the criteria of the AISC Specification [3]. For a conservative solution, an effective length factor of 1.0 may be used. However, it is more realistic to utilize a K-value of 0.8 for the verticals of the stubs, recognizing the end restraint that is provided by the connections between the chords and the stubs. The K-factor for the floor beam verticals must be 1.0, due to the pinned ends that are assumed in the modeling of these components, as well as the flexibility of the floor beam itself for buckling of the vertical member.

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