Discontinuous Shear Walls - The Analysis of Irregular Shaped Structures - page 420

The shear panels in the roof or floor cavity must act as mini-shear walls. When the
shear panels are spaced, 2× blocking and tie straps are required between the shear panels
to act as collectors or drag elements (boundary elements), which transfer the diaphragm
shears from panel to panel in accordance with ASCE 7-05 Sections 12.10.1 and 12.10.2.
The tie straps are required because code does not allow the roof and floor sheathing be
used to splice boundary elements. When shear panels are installed between each joist,
the vertical shear forces from adjacent shear panels are available to oppose the overturn-
ing forces of the panels. Because these opposing forces are not present in cases of partial
blocking, hold-down connections are required at the end of each shear panel if the over-
turning force exceeds the dead load of the resistive force. The dead load of the trusses or
joists can be used to resist overturning of the shear panels; however, a proper connection
of the panel to the joist must be made to accomplish this. The distribution of the forces
from the shear panels into a wall is shown in Fig. 11.12. The overturning forces at each
end of the panels must be tied to a vertical collector in the wall below to distribute the
force into the wall. The wall acts as a transfer diaphragm if hold-downs are not installed
in the foundation at each collector location. Special nailing of the sheathing to each 2×
stud in the collector is required. Included in the figure is the construction of the strut
force diagram that must be checked. Note that additional strut forces are applied at the
start and end of the shear wall. The dead load of the wall is included in the transfer dia-
phragm loading diagram. The net wall shears are determined by adding or subtracting
the transfer diaphragm shears to or from the basic wall shears.
The wall shown in Fig. 11.13 has a height of 8 ft and a length of 12 ft. The depth of
the roof joists is 24″ with a joist spacing of 2′-0″ o.c. Only two shear panels are installed
as shown in the figure. The uniform shear from the diaphragm is 175 plf, and the collec-
tor forces into and out of the wall are 2150 and 1250 lb, respectively. The dead load of
each joist is equal to 750 lb. The wall dead load is 80 plf. Analyze the wall for the loads
shown.
Sum of shear forces to each shear panel:

∑

V horiz = 2150 + 1250 + 175 12 () = 5500 lb

5500
2

2750
2

V = = 2750 lb perpanel v = = 1375 plf

panel panel

The unit shear in the blocking panels suggests that an additional blocking panel is
required.
Check the overturning of the wall, load case W + D:




80 12
2

() 

2




1
12

T = 5500 10 () - 750 246810 ( ++++ +- 12 ) = 1478 . b




2




80 12
2

()  1

12

C = 5500 10 () + 750 246810 ( ++++ ++ 12 ) = 7688 . b

Determine the collector forces at the roof diaphragm (see Fig. 11.14):

F strut =-2150 lb compression at startofwal l

F strut =+1250 lb tensionatend of wall