Diaphragm Basics - The Analysis of Irregular Shaped Structures - page 53

The magnitude of the shear would suggest that an increase in the wall length is
warranted.
Determination of net shears at the collector:
v net =- 433 - - 542 975 plf

Note: Since both diaphragms are trying to slide off of their support at grid line 2 in
the direction of the applied loads, the diaphragm shears transferred into the collector
are acting in the negative direction or toward the shear wall. They are additive and
therefore are both negative in value. The shear wall shears act in the opposite direction
and are therefore positive. The net shears at the shear wall are

v =--=+ v vv

WL

1462 - 975 =+ 487 plf

net S 2 2 R

Determination of the collector force:
Summing from grid line A, collector force at grid line B:

F B =- 975 10 () =- 9750 lb compression

The direction of the shears transferred into the collector is pushing on the wall as
expected.
Force at grid line C:

F C =- 9750 + 487 20 () = 10 lb

This is close enough (round-off error), therefore the force diagram closes to zero.
Note: All force diagrams must close to zero, or an error exists. The determination of
the direction of the shears that are applied from a simple diaphragm and distributed
into the shear walls is based on a good understanding of how simple diaphragms work.
Making a determination of the direction of the shears distributed in a complex dia-
phragm would not be as easy. For complex diaphragm layouts, using the visual shear
transfer method eliminates the need to assume the direction of the shears, because they
are shown.
Part B—Using the Visual Shear Transfer Method: The sheathing elements for the left and
right diaphragms are placed adjacent to the collector and shear wall on grid line 2. The
shears that are transferred into the boundary elements along the grid line are both headed
in the same direction and therefore should be added. Since they are pushing against the
wall, the collector will be in compression. The shear wall shears and diaphragm transfer
shears oppose each other and should be subtracted from each other to get a net shear. The
net shear is acting in the opposite direction to the collector shears. The force diagrams
are generated by multiplying the net shears by the length of the boundary elements and
summing the forces along the line. ▲

This quick comparison of the two methods shows that the major difference between
the two methods is basically a visual one. The main steps are the same. Irregular-shaped
diaphragms can get very complex because of the number of discontinuities and compli-
cated load paths required to transfer the discontinuous forces. The visual shear transfer
method will prove to be very advantageous in solving these problems.