Civil Engineering Reference
In-Depth Information
Chapter 4
Strength of Timbers
The various mechanical properties of woods have been investigated
by exhaustive testing in many laboratories, most notably the Forest
Products Laboratory of the U.S. Department of Agriculture in Madi-
son, Wisconsin. In addition, much research has been done in civil
and agricultural engineering laboratories at many state universities.
Tension
A tension test is made as indicated in Figure 4-1. Although mod-
ern testing machines are by no means as simple as the apparatus
shown, it serves well to show how such tests are made. The speci-
men is placed in the machine, gripped at each end, and the load is
progressively increased until the material reaches its failure point.
Usually, the elongation is not of any significance unless it is desired
to determine the modulus of elasticity. The allowable fiber stresses
indicated by modern stress grading (such as 1200f or 1450f) apply
equally to extreme fiber stress in bending and to tensile stresses.
G
W
S
M
a
c
L
R
B
F
d
b
A
MOVABLE HEAD
FIXED END
Figure 4-1 The tension test. The specimen R is placed in the
wedge grips a, b, c, and d, thus applying tension between the
fixed end and the movable head of the machine. The movable
head is connected to the scale lever G on which the weight
W slides. This arrangement is similar to an ordinary weighing
scale. Two center marks (L and F) are punched on the specimen
at a standard distance (A) apart. When testing, the pull on the
specimen is gradually increased by moving W to the left. Di-
mensions A and B are then measured after each load increase.
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