Text (16 of 20) in CHEMCLAD: Product Description
Deflection Limits and Factor of Safety
Design specifications for roofing and siding panels must include appropriate deflection limits and
Fiberglass reinforced plastic building panels can handle larger deflections than metals without
experiencing permanent deformation. However, large deflections for any roofing or siding material,
including FRP, can result in leaks, or panels blowing off buildings.
When the ratio between the Length of span and maximum Deflection (L/D) is too low, excessive
panel deflection can cause enlarging of the fastener holes. In addition, excessive deflection
caused by panel buffeting during windstorms can break the seals at panel end and side-laps.
Both of these conditions will lead to building leaks.
In cases of extreme deflections and fastener hole enlargement or washer damage, panels can
experience premature failure due to fastener pullover. In this situation, the chance for panels being
sucked off the building in a severe windstorm is increased.
The L/D limits and Factors of Safety applied in the following Load/Span tables are appropriate for
FRP roofing and siding panels according to ASTM D-3841, Standard specification for glass
reinforced plastic panels.
To ensure quality performance, panel specifications should include these deflection limits (L/D)
and Factors of Safety as minimum standards for acceptable performance.
Some manufacturers report spans based on more liberal Length/Deflection limits. To convert for
comparison with industry practice, use the following factors: where L equals Span length L20 =
1.15 x L30 and L45 = 1.1 x L60. In other words, for siding applications span limits defined by L/20
have to be reduced by 15% to equal span limits calculated at L/30. Roofing applications typically
require less deflection than siding, hence the higher L/D ratios. Spans limited by L/45 have to be
reduced by 10% to equate to L/60.
Span calculation used by our engineers follows the accepted form of
S = ((F x El)/144 x U x L/D)E1/3
Where S equals Span in inches
F equals the constant derived to account for span multiple and fasteners
U equals Uniform load in pounds
E equals Flexural Modulus
I equals Area Moment of Inertia
Thus to transform one span based on L/D of 30 to L/D of 20, the ratio of these two has to be
raised to the power of .33. For example, (30/20) to the power of .33 equals 1.10 so the span at
L/D=20 would be 110% of the Span at L/D=30.
FRP Roofing Panels
Deflection Allowable: L/60
Factor of Safety: 2.5
FRP Siding Panels
Deflection Allowable: L/30
Factor of Safety:1.88
Deflection Allowable: L/180
Factor of Safety: 1.88
The load/span tables in this section have been developed based on the results of large scale,
vacuum box testing, in accordance with modified test method ASTM E-72. This testing using full
size panels simulates actual service conditions, and produces reliable values. The load/span data
tables are to be used as a guide only, as specific temperature and chemical exposures are not
considered. Graham's technical representatives can assist with specific applications. Panel
materials, testing, and technical data are produced under strict guidelines as required under
Graham's ISO 9001.