The grouping of loads in Table 4.2 is not meant to imply that secondary
loads are less important or of lesser magnitude as in many cases they are not.
However, dead and wind loads are always present and must be considered in
every design. Individual loads in the secondary category may or may not apply.
The incidence of structural failure of curtain wall is far lower than failures
where a system fails to provide some other performance characteristic such
as weathertightness or condensation resistance. Nonetheless, they do occur.
The most common type of failure results from negative wind loads pulling
panels or glass out of the wall. Other common failures include dislodging
of snap caps and panels due to temperature induced movement, failure of
anchors under stacking dead load, disengaging of split mullions under high
load and failure of connections and framing under extreme but predictable
loading.