Diaphragm Basics - The Analysis of Irregular Shaped Structures - page 58

which causes an eccentrically applied force into the blocking and produces vertical forces
at the ends of the block, which are usually negligible. In current practice, the length of
the strap embedment into the main diaphragm section is usually an arbitrary distance
that is based on the shear capacity of the basic diaphragm nailing at that location. As an
example, assume that the existing diaphragm shear capacity is based on nailing that will
allow a unit shear of 320 plf. The embedment length would then be calculated by divid-
ing the collector force by 320 to get the required length. That calculation would be incor-
rect because the basic diaphragm shear is already applied at that location and the shear
from the transferred force is in addition to the basic diaphragm shear. Both shears must
be added. It is commonly assumed that simply lapping the strap collector with the offset
diaphragm chord will automatically and safely transfer the chord force across the offset.
This assumption is also incorrect and is discussed in detail in Chap. 3.
Occasionally, a tie strap and blocking is called out on the roof framing plan with a
total strap length and a minimum lap of the strap to the wall. However, calculations are
usually not provided to verify that the transfer has effectively been made. They should
be part of the construction documents provided for plan review and fully detailed on
the drawings. Figure 2.26 shows a drag strut or collector connection to a shear wall. The
collector, in this case, consists of flat blocking and a tie strap. The connection must resist
tension and compression forces as previously discussed. The details showing the
F i g u r e 2.26 Strut and collector with blocking.