Civil Engineering Reference
In-Depth Information
Shear:
0.9
w
2
h
Hbh
180
3.50
0.9
300
1.50
2
3.50
12
(12)
(12)
49.00
12
Deflection:
3
1,700,000
5.36
300
1.62
50.55
For practical design work, many tables are available to simplify and expedite the de-
sign calculations. Table 20.4 which shows the maximum permissible spacings, center to
center, of supports for joists, stringers, and other beams continuous over four or more sup-
ports for one particular grade of lumber, is one such example.
20.11
DESIGN OF SHORING
Wood shores are usually designed as simply supported columns using a modified form of
the Euler equation. If the Euler equation is divided by a factor of safety of 3, and if
r
is re-
placed with 0.3 times
d
(the least lateral dimension of square or rectangular shores), as
follows,
P
A
2
E
/0.3
d
)
2
3(
the so-called National Forest Products Association formula results:
P
A
0.3
E
(
/
d
)
2
/d value normally specified is 50. If a round shore is being used, it
may be replaced for calculation purposes with a square shore having the same cross-
sectional area. Should a shore be braced at different points laterally so that it has dif-
ferent unsupported lengths along its different faces, it will be necessary to calculate
the
The maximum
/d ratios in each direction and use the largest one to determine the allowable
stress.
The allowable stress used may not be greater than the value obtained from this equa-
tion, nor greater than the allowable unit stress in compression parallel to the grain for the
grade and type of lumber in question. If the formwork is to be used only once, the allow-
able stress in the column can reasonably be increased by 25%. Example 20.3 shows the
calculation of the permissible column load for a particular shore. Tables are readily avail-
able for making these calculations for shores, as they are for sheathing and beam mem-
bers. For example, Table 20.5 gives the allowable column loads as determined here for a
set of simple shores.
