3343Design Checks for the Bottom Chord

The size of the bottom chord is almost always governed by the stress resultants at midspan or Point 3 in Figure 33.5 and Figure 33.7. This is also why the preliminary design procedure focused almost entirely on determining the required chord cross-section at this location. As the stress resultant distributions in Figure 33.7 show, the bottom chord is subjected to combined positive bending moment and tensile force at Point 3, and the design check must consider the beam-tension member behavior in this area.

Combined bending and tension must also be evaluated at Point 2, the exterior end of the interior stub. The local bending moment in the chord is generally larger here than at midspan, but the axial force is smaller. Only a computation can confirm whether Point 2 will govern in lieu of Point 3. Further, although the location at the interior end of the exterior stub (Point 2a) is rarely critical, the combination of negative moment and tensile force should be evaluated.

At Point 1 of the bottom chord, which is located at the exterior end of the exterior stub, the axial force is equal to zero. At this location the bottom chord must therefore be checked for pure bending as well as shear.

The preceding applies only to a girder with simple end supports. When it is part of the lateral load-resisting system, axial forces will exist in all parts of the chord. These must be resisted by the adjacent structural members.

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