Building Design and Construction Handbook - Merritt - Ventech!

10.24 SECTION TEN
Deflection of wood beams is calculated by conventional elastic theory. For example,
for a uniformly loaded, simple-span beam, the maximum deflection is computed
from
where w � the uniform load
L � span
E � modulus of elasticity
I � moment of inertia
4 � � 5wL /384EI (10.19)
Deflection should not exceed limitations specified in the local building code nor
industry-recommended limitations. (See, for example, K. F. Faherty and T. G. Williamson,
‘‘Wood Engineering and Construction Handbook,’’ McGraw-Hill Publishing
Company, New York.) Deflections also should be evaluated with respect to
other considerations, such as possibility of binding of doors or cracking of partitions
or glass.
Table 10.15 gives recommended deflection limits, as a fraction of the beam span,
for timber beams. The limitation applies to live load or total load, whichever governs.
Glulam beams may be cambered to offset the effects of deflections due to design
loads. These members are cambered during fabrication by creation of curvature
opposite in direction to that of deflections under load. Camber, however, does not
increase stiffness. Table 10.16 lists recommended minimum cambers for glulam
beams.
Minimum Roof Slopes. Flat roofs have collapsed during rainstorms even though
they were adequately designed for allowable stresses and definite deflection limitations.
The failures were caused by ponding of water as increasing deflections
permitted more and more water to collect.
Roof beams should have a continuous upward slope equivalent to 1 ⁄4 in/ft between
a drain and the high point of a roof, in addition to minimum recommended
TABLE 10.15 Recommended Beam-Deflection Limitations, in
(In Terms of Span, l, in)
Use classification
Roof beams:
Live load only
Dead load plus
live load
Industrial
Commercial and industrial:
l/180 l/120
Without plaster ceiling l/240 l/180
With plaster ceiling l/360 l/240
Floor beams:
Ordinary usage* l/360 l/240
Highway bridge stringers l/200 to l/300
Railway bridge stringers l/300 to l/400
* Ordinary usage classification is intended for construction in which walking
comfort, minimized plaster cracking, and elimination of objectionable springiness
are of prime importance. For special uses, such as beams supporting vibrating
machinery or carrying moving loads, more severe limitations may be
required.