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 sidered since a wood which is relatively strong with respect to one strength property may be weaker
in a different property when compared to another species.
should not sag. The Modulus of Elasticity (MOE or E) is a measure of stiffness or rigidity. Figure
2-11 presents the MOE values for a number of species, as well as the allowable design values. Al-
lowable design values are keyed to an average MOE value without reduction for property variability
because stiffness is most important from an appearance standpoint and not from the point of life
safety. That is, the member, if not properly designed, could sag or otherwise deflect without physi-
cally hurting someone.
2.5.2 Modulus Of Rupture. The Modulus of Rupture (MOR) is the measure of the
ability of a beam to support an applied load. Since the modulus of rupture represents the point at
which a beam will fail, it is considered a life endangering property and the allowable design values
(Figure 2-12) are substantially reduced from those given for small clear wood specimens.
wood to being crushed. A common example of where compression strength along the g-rain is util-
ized occurs in studs in house walls. The studs, usually 2x4's, hold up the roof with any snow or
wind loads that may arise and are loaded in compression parallel to the grain. Tie plates on railroad
ties exert a compression force perpendicular to the grain. If extremely light or soft wood species
were used for ties and with heavy railroad traffic, the weight of the trains, borne on the plates,
would literally crush the wood.
2.5.4 Tensile Strength. Tensile strength is basically the opposite of compression strength.
In this case, the load is applied in such a way that the wood member is being pulled from end to end
(tension parallel to the grain) or across the grain (tension perpendicular to the grain).
2.5.5 Shear. The shear strength of a member parallel to the grain is a measure of the
ability of wood to resist the slipping of one part along another. Shear failure is important in beam
design. Shear strength, like modulus of rupture, is a life endangering property. Consequently, sub-
stantial differences exist between the allowable design values, and those for small clear wood
specimens (Figure 2-13)
Side hardness relates to the ability of wood to resist denting. One
2.5.6 Side Hardness.
example includes flooring.
2.5.7 Toughness. Toughness is a measure of the amount of work expended in breaking a
small specimen by impact bending. This property is important when impact loads are applied.
Toughness is reduced by the early stages of decay and even stain fungi.
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