Civil Engineering Reference
In-Depth Information
based on the total time of application of the loads. In other words, if forms are to be used
many times, the amount of overload they can resist is not as large as if they are to be used
only one time.
The paragraphs that follow discuss flexure, shear, and deflections in formwork.
Flexure
Normally, sheathing, joists, and stringers are continuous over several spans. For such
cases it seems reasonable to assume that the maximum moment is equal to the maximum
moment that would occur in a uniformly loaded span continuous over three or more
spans. This value is approximately equal to
2
10
w
M
Shear
The horizontal and vertical shear stresses at any one point in a beam are equal. For materi-
als for which strengths are the same in every direction, no distinction is made between
shear values acting in different directions. Materials such as wood, however, have entirely
different shear strengths in the different directions. Wood tends to split or shear between
its fibers (usually parallel to the beam axis). Since horizontal shear is rather critical for
wood members, it is common in talking about wood formwork to use the term
horizontal
shear
.
The horizontal shearing stress in a rectangular wooden member can be calculated by
the usual formulas as follows:
VQ
(
V
)[
b
(
h
/2)
(
h
/4)]
3
V
3
V
2
A
f
v
Ib
2
bh
(
12
bh
3
)(
b
)
In this expression,
V
is equal to
w
/2 for uniformly loaded simple spans. As in earlier
chapters relating to reinforced concrete, it is permissible to calculate the shearing stress
at a distance
h
from the face of the support. If
h
is given in inches and if
w
is the uni-
form load per foot, the external shear at a distance
h
from the support can be calculated
as follows:
h
V
0.5
w
12
w
2
12
V
0.5
w
The sheathing, joists, stringers, wales, and so on for formwork are normally continu-
ous. For uniformly loaded beams continuous over three or more spans,
V
is approximately
equal to 0.6
w
. At a distance
h
from the support, it is
assumed
to equal the following
value
2
12
V
0.6
w
