Civil Engineering Reference
In-Depth Information
2000 psf maximum
wall pressure
3000 psf maximum
column pressure
3200
50º
40º
60º
2800
30º
70º
80º
90º
100º
2400
2000
1600
1200
800
400
0
2
4
6
8
10
12
14
16
18
20
22
24
pour rate, ft per h
all walls, columns with pour rate less than 7 ft per h
columns with pour rate greater than 7 ft per h
Figure 20.3
Lateral pressure of fresh concrete in columns and walls. (Courtesy of American
Plywood Association.)
In addition to the lateral pressures on the forms caused by the fresh concrete, it is
necessary for the braces and shoring to be designed to resist all other possible lateral
forces, such as wind, dumping of concrete, movement of equipment, bumping of equip-
ment, guy cable tension, uneven placing of concrete, and so on.
20.9
ANALYSIS OF FORMWORK FOR FLOOR AND ROOF SLABS
This section is devoted to the calculations needed for analyzing formwork for concrete
floor and roof slabs; its design is presented in the following section. Because wood form-
work is the most common type in practice, the discussion and examples of this chapter are
concerned primarily with wood.
Wood members have a very useful property with which you should be familiar, and
that is their ability to support excessive loads for short periods of time. As a result of this
characteristic, it is a common practice to allow them a 25% increase over their normal al-
lowable stresses if the applied loads are of short duration.
Formwork is usually considered to be a temporary type of structure because it re-
mains in place only a short time. Furthermore, the loads supported by the forms reach a
peak during pouring activity and then rapidly fall off as the concrete hardens around the
reinforcing and begins to support the load. As a result, the formwork designer will want to
use increased allowable stresses where possible. The ability of wood to take overloads is
