“Industry recommendations for wall framing deflection tested at the
design pressures are L/175 (where L=span).” (Makepeace, Shooner,
Kyle, Wiseman, & deMiguel, 2004) The deflection of the transom
simultaneously moves inward under exterior positive pressure
(wind load) on one building face and outward under negative
pressure on the opposite face of the building, though the degree of
movement is different. “The L/175 deflection criterion is an industry
recommendation only and can be altered by the specifier if the
project demands. Deflection limits of L/200 are quite common and
deflection limits of L/240 are also not unusual. The less the allowable
deflection the deeper or heavier the framing and there is an
increased cost associated with this.” (Makepeace, Shooner, Kyle,
Wiseman, & de Miguel, 2004)
Vertical Movement
The vertical movement of the curtain wall is a result of live loads
and thermal differentials. “Live load movements result from all
occupants, materials, equipment, construction, or other elements
of weight supported in, on, or by structural elements that are likely
to move. Live load movements can cause upward or downward
motion. For example, a downward live load on a floor below can
result in disengagement of improperly designed curtainwall anchors
on a floor above that remains static, while resulting in a “crushing”
action at the floor below.” (WAUSAU, 2016) The following WAUSAU
drawing and caption below illustrates the vertical movement caused
by live loads. Depending on the type of mullion attachment and
mullion spacing the deflection of the floor assembly will create
vertical shear on the perimeter joint protection.
“Thermal movement is also an issue with mullions. A typical two-story
aluminum mullion placed in an exterior environment can oscillate up
to 13 mm (0.5 in.).” (Kazmierczak, 2008) Thermal movement of a
mullion can be expressed as follows:
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