Building Design and Construction Handbook - Merritt - Ventech!

WOOD CONSTRUCTION 10.25
TABLE 10.16 Recommended Minimum Camber for
Glued-Laminated Timber Beams*
Roof beams† 11 ⁄2 times dead-load deflection
Floor beams‡ 11 Bridge beams:§
⁄2 times dead-load deflection
Long span 2 times dead-load deflection
Short span 2 times dead-load plus 1 ⁄2 applied-load deflection
* Camber and Deflection, AITC 102, Appendix B, American Institute of Timber
Construction.
‡ The minimum camber of 1 1 ⁄2 times dead-load deflection will produce a
nearly level member under dead load alone after plastic deformation has occurred.
Additional camber is usually provided to improve appearance or provide necessary
roof drainage.
‡ The minimum camber of 1 1 ⁄2 times dead-load deflection will produce a
nearly level member under dead load alone after plastic deformation has occurred.
On long spans, a level ceiling may not be desirable because of the optical illusion
that the ceiling sags. For warehouse or similar floors where live load may remain
for long periods, additional camber should be provided to give a level floor under
the permanently applied load.
§ Bridge members are normally cambered for dead load only on multiple spans
to obtain acceptable riding qualities.
camber (Table 10.16), to avoid ponding. As a general guideline, when flat roofs
have insufficient slope for drainage (less than 1 ⁄4 in/ft), the stiffness of supporting
members should be such that a 5-lb/ft 2 load will cause no more than 1 ⁄2-in deflection.
Because of ponding, snow loads or water trapped by gravel stops, parapet walls,
or ice dams magnify stresses and deflections from existing roof loads by
1
Cp � (10.20)
3 4
1 � W�L /� EI
where C p � factor for multiplying stresses and deflections under existing loads to
determine stresses and deflections under existing loads plus ponding
W� � weight of 1 in of water on roof area supported by beam, lb
L � span of beam, in
E � modulus of elasticity of beam material, psi
I � moment of inertia of beam, in 4
(Kuenzi and Bohannan, ‘‘Increases in Deflection and Stresses Caused by Ponding
of Water on Roofs,’’ Forest Products Laboratory, Madison, Wis.)
10.7.4 Bearing Stresses in Beams
Bearing stresses, or compression stresses perpendicular to the grain, in a beam occur
at the supports or at places where other framing members are supported on the
beam. The compressive stress in the beam ƒ c� is given by
ƒ � P/A (10.21)
c�
where P � load transmitted to or from the beam and A � bearing area. This stress
should be less than the design value for compression perpendicular to the grain F c�