Shear Walls With Openings - The Analysis of Irregular Shaped Structures - page 377

above and below the opening. SDPWS Section 4.3.5.2 sets some limitations for force
transfer around an opening (FTO):
1. The length of each wall pier shall not be less than 2 ft.
2. A full-height wall segment shall be located at each end of the wall.
3. Where out-of-plane offsets occur, portions of the wall on each side of the offset
shall be considered as separate FTO shear walls.
4. Collectors for the shear transfer shall be provided through the full length of the
wall.
Limitation 1 is new. Previous codes did not specify a minimum pier width as long
as the required aspect ratio was maintained. As shown in Fig. 10.5, limitation 4 requires
tie straps and blocking to be installed across the full length of the wall at grid lines B
and C, effectively creating rigid member end connections and providing a continuous
load path across the wall. This limitation can require very long straps on apartment-
type structures. An alternate method will be presented in Example 10.2 to limit the
length of these straps.
As shown in Fig. 10.6, the direction of the shears acting on the ends of the vertical
piers and the sections above and below the opening produces contraflexure in the mem-
bers, as the lateral force is applied at the top of the wall. The point of inflection or contra-
flexure is assumed to occur at midheight and midlength of the members. The maximum
moment occurs at each end of each segment, grid lines 2 and 3, and the moment becomes
zero at the point of inflection. It is assumed that the force on the inside chord of each sec-
tion is zero where the moment is zero. These simplified assumptions reduce the number
of variables so that the wall can be analyzed as statically determinate.
Walls are often built to a height of 8 ft. In this case, the tops of window and door
openings are typically set at 6′-8″ above the floor, leaving only a 1′-4″ deep section
above the opening. To meet the maximum allowed aspect ratio of 3.5 : 1, in this case, the
opening can only be 4′-8″ wide. A 9 ft high wall would allow a maximum opening
width of 8′-2″. Since the aspect ratio cited exceeds 2 : 1, if seismic controls, the unadjusted
shear resistance must be adjusted by 2 b / h in accordance with SDPWS Section 4.3.4.
Wall heights containing opening widths that do not allow the header to comply with
the maximum allowable aspect ratio can be designed using the cantilever method out-
lined in Sec. 10.4 or with the frame method outlined in Chap. 14.
Most examples presented in available publications, as well as common standard
practice, ignore the application of gravity loads when analyzing these types of walls.
The determination of the tension force for the hold-down, in this case, would be conser-
vative. Technically, this approach is incorrect unless the gravity loads to the wall are
very small. The addition of gravity loads can significanly increase the shears applied to
the wall segments, increase the bearing perpendicular-to-grain stresses at several loca-
tions within the wall, and increase the axial forces in the chord members. The decision
to include gravity loads in the analysis should be based on the magnitude of the load
and good engineering judgment.
Example 10.1: FTO Method, Blocking and Strapping Full Width of Wall With Uniform Load
The wall shown in Fig. 10.7 has a 4500 lb lateral force applied at the top of the wall with a
uniform gravity load of 200 plf. The analysis will be based on the allowable stress design
(ASD), load combination W + D .