Discontinuous Shear Walls - The Analysis of Irregular Shaped Structures - page 431

F i g u r e 11.21 Load reversal from discontinuous walls.
the beam is in compression and the bottom flange is in tension. When the load is applied
acting to the right, the bottom flange of the beam is in compression. The support beam
must be designed for bending in both directions due to load reversal. The compression
flange of the supporting beam acts as a column which tends to buckle about its weakest
axis when subjected to an axial compression force brought about by an applied axial
force and/or bending. If the compression flange is completely unbraced, it will tend to
buckle, as shown in Fig. 11.22. Steel beams are typically installed with a 2× or 4× wood
plate bolted to the top flange of the steel beam, as shown in Fig. 11.23. The floor sheath-
ing is nailed directly to the wood plate, which braces the beam for out-of-plane buck-
ling forces when the top flange is in compression. Ceiling material or other forms of
lateral bracing are rarely connected to the bottom flange. Therefore, if the bottom flange
goes into compression due to uplift loads, it is unbraced along its entire length. For glu-
lam beams, the issue lies in the layup of the beam, as shown in Fig. 11.23. The standard
layup commonly used for glu-lam beams is combination V4. Beams using this layup are
required to be braced at the top lam and are fabricated with the intent that the top
flange will always be in compression, which is the typical application. When subject to
bending action, the tension and compression stresses at the top and bottom faces of a
rectangular beam are equal. Since the allowable stress for compression parallel to the
grain is typically twice that of the allowable tension stress for a given species, a lower-
strength top lam can be used to balance the tension stress at the bottom of the beam.
A failure can occur if a V4 combination is used to support a discontinuous shear wall
because the top lam would have one-half of the required strength in tension when the