Civil Engineering Reference
In-Depth Information
perpendicular to the grain. The allowable stresses for compression perpendicular to the
grain are much less than they are for compression parallel to the grain and, in fact, will
often be less than the allowable compression stresses in shores as determined by the Na-
tional Forest Products Association formula.
As a first illustration, the bearing stresses in stringers resting on shores are considered.
One particular point that should be noted is that the bearing area may well be less than the
full cross-sectional area of the shore. For instance, in Examples 20.1 and 20.3, the 2-
8-in.
stringers are supported by 4-
6-in. shores, as shown in Figure 20.6. Obviously, the bear-
ing area does not equal the full cross-sectional area of the 4
19.25 in.
2
),
6 (3.50
5.50
8.25 in.
2
).
The bearing stress can be calculated by dividing the total load applied to the shore
(3600 lb from Example 20.3) by the hatched area shown in Figure 20.6.
but rather equals the smaller crosshatched area (1.50
5.50
3600
Bearing stress
8.25
436 psi
From Table 20.3, the allowable bearing stress perpendicular to the grain in the
stringer is 490 psi for the Douglas fir, coastal construction lumber used in the examples in
this chapter. The values given in this table for compression perpendicular to the grain are
actually applicable to bearing spread over any length at the end of a member and for bear-
ing at 6 in. or more in length at interior locations.
Should the bearing be located more than 3 in. from the end of a beam and be less than
6 in. long, the allowable compressive stresses perpendicular to the grain may be safely in-
creased by multiplying them by the following factor, in which
is the length of bearing
measured along the grain. Should the bearing area be circular, as for a washer, the length
of bearing is considered to equal the diameter of the circle.
8
Multiplier
If the shore is bearing at an interior point of the stringer in Figure 20.6, the allowable
compression stress perpendicular to the grain equals
8
5.50
0.375
5.50
(490)
(490)
523 psi
436 psi
OK
Figure 20.6
