Strength of Timbers - Carpenter's and Builder's Math, Plans, and Specifications - page 106

less than 20 pounds per square foot of live load, it will not safely
support workers.
In the past, there was considerable confusion regarding wind
loads against sloping roofs, and some rather weird formulas were
evolved to calculate their imaginary intensities (although no one had
ever seen a roof blow in , and many persons have seen roofs blow
off ). In other words, wind loads are negative (that is, they create
suction ). This is now well recognized, and insurance companies are
well informed on the subject.
In regions where hurricanes are not common, the insurance com-
panies recommend that an uplift allowance of 30 pounds per square
foot be provided for, acting at right angles to the slope of the roof.
This will include the slight positive pressure commonly found inside
buildings during windstorms. If eaves are wide and overhanging, it
is recommended that a gross lifting force of 45 pounds per square
foot be provided for. If the weight of the roof itself is appreciable, it
may be deducted from the gross uplift allowance.
It should be recognized that a maximum snow load plus a maxi-
mum wind load could hardly occur at the same time. Thus, it cannot
be considered compensating.
Definitions
In the shop and in the field, the fitness of any species of wood for
a given purpose depends on various properties. When treating the
strength, stiffness, hardness, and other properties of wood, many
technical terms are used. For an understanding of these terms, the
following definitions are worth noting:
Bending forces —Forces that act on some members of a struc-
tural frame. They tend to deform them by flexure.
Brittleness —Breaking easily and suddenly, usually with a
comparatively smooth fracture. The opposite of toughness
(sometimes incorrectly called brashness ) that refers more to
brittleness. Old and extremely dry wood is inclined to brash-
ness. Green or wet wood is tougher, though not as strong in
most cases.
Compression —The effect of forces that tend to reduce or
shorten the dimensions of a member.
Deformation —A change of shape or dimension. Disfigure-
ment, such as the elongation of a structural member under
tension.