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Q: Are there deflection limits for the structural backup for portland
cement plaster?
A: A structural frame must be strong
enough to resist bending so that the finish attached to it is not
subjected to more flexural stress than it can handle. This helps
maintain structural integrity and control performance of the finish.
For plaster, deflection limits and proper sizing of panels are helpful
in restraining unwanted cracking.
An in-plane load that acts on buildings and wall panels is the force
of gravity (acting on the building’s weight). Out-of-plane
loads include wind and seismic loads. All of these forces can make
panels bend, or deflect.
Concrete and masonry substrates are very stiff, so when plaster
is directly applied to either of these surfaces, deflection criteria
of the backup is assumed to be within allowable limits. Frame systems,
such as wood or steel framing, are less rigid. They will deflect
due to loads, so limitations are needed to maintain function and
appearance of the plaster panels.
In ASTM C926, in Annex A2 on design considerations, it states: “Maximum
allowable deflection for vertical or horizontal framing for plaster,
not including cladding, shall be L/360.” In ASTM C1063, Standard
Specification for Installation of Lathing and Furring to Receive
Interior and Exterior Portland Cement-Based Plaster, there
is a footnote to Table 2 - Allowable Support or Hanger Wire Spacing
ft-in. (mm) and Cold-Rolled Channel Main Runner Spans, that limits
deflection of the backup to L/360. Both documents are available
from ASTM.
An example of the deflection criteria is as follows: for a 10-ft
panel (3.05 m), the maximum deflection is 10 ft divided by 360 equals
0.0277 ft, or 0.33 in. (8.5 mm). Designers should choose framing
members of adequate stiffness when using plaster finishes.
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