Formwork for Concrete Structures by R.L.Peurifoy and G.D- By EasyEngineering.net

134 Chapter Five
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Size of
shore and
horizontal
member,
S4S
Net contact
bearing area
between
members,
sq in.
Allowable Compression Stress Perpendicular
to Grain, lb per sq in.
F c^
= 405 F c^
= 425 F c^
= 565 F c^
= 625
2 × 4 5.25 2,126 2,231 2,966 3,281
3 × 4 8.75 3,543 3,718 4,943 5,468
4 × 4 12.25 4,961 5,206 6,921 7,656
4 × 6 19.25 7,796 8,181 10,876 12,031
TABLE 5-6 Allowable Loads that May Be Transmitted from Horizontal Wood
Members to Vertical Wood Shores, lb
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on a stringer, compression stresses perpendicular to grain are created.
The contact area between two members is the bearing area. As shown
in Tables 4-2 and 4-3, the allowable compression stress perpendicular
to grain is less than the allowable compression stress parallel to grain.
Therefore, it is necessary to check the bearing stress of wood members.
The values given in Table 5-6 are the maxima that the shores will
support without danger of buckling. However, these values do not
consider the contact bearing stresses at the top or bottom of the shore.
Because a shore cannot support a load greater than the one that can
be transmitted to it at the top or bottom, it may be necessary to reduce
the loads on some shores to values less than those given in the table.
For example, if the allowable unit compressive stress of a wood member
that rests on a shore is 565 lb per sq in., then the maximum load on a
shore that supports the wood member will be limited to the product
of the net area of contact between the wood member and the shore,
multiplied by 565 lb per sq in.
The area of contact between a 4 × 4 S4S horizontal wood member
and the top of a 4 × 4 S4S shore will be 12.25 sq in. The maximum load
that can be transmitted to the shore will be 12.25 sq in. × 565 lb per
sq in. = 6,921 lb. If this load is exceeded, it is probable that the underside
surface of the horizontal wood member in contact with the shore will be
deformed permanently. This may endanger the capacity of transferring
loads between the horizontal wood member and the shore.
Table 5-6 gives the allowable loads that may be transmitted to
shores from horizontal wood members, based on the allowable unit
compressive stresses perpendicular to the grain of the wood member.
The same loads must be transmitted from the bottoms of the shores to
the bases on which the shores rest.
In Table 5-6, the area of contact between a horizontal wood member
and a shore is determined with the narrower face of the wood
member bearing on the major dimension of the shore if the two faces
of a shore are of unequal dimensions. For example, the area of contact
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