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

146 Chapter Five
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FIGURE 5-17
Bracing system for concrete formwork.
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geometry of a bracing system. The brace is positioned a distance x
from the base of the form and the height of the form is a distance y.
The diagonal length z of the brace can be calculated from the equation,
2
z= x + y 2
.
As shown in Figure 5-17, the horizontal force H at the top of the
form is applied in a direction to the right, which causes an axial compression
in the brace, a horizontal reaction to the left, and an upward
vertical reaction at the bottom of the brace. However, the horizontal
force may be applied to the top of the form in a direction to the left,
which would reverse the direction of the forces, causing axial tension
in the brace, a horizontal reaction to the right, and a downward reaction
at the bottom of the brace.
The horizontal load H is cause by wind pressure acting against
the form. Wind pressures and resulting forces on a structure should
be calculated according to local codes. The publication by the American
Society of Civil Engineers, Minimum Design Loads for Buildings and
Other Structures, SEI/ASCE-7, provides a comprehensive coverage of
wind loads based upon substantial research.
Example 5-19
Diagonal braces will be placed horizontally at 9 ft on centers along
the top of a 10-ft-high wall form. Each brace will be attached at the
top of the wall form and the bottom of each brace will be attached at
a distance of 6 ft horizontally from the bottom of the form. The local
code prescribes a 25 lb per sq ft wind pressure. For this wind pressure,
Table 5-7 shows a minimum lateral load of 125 lb per lin ft acting
on the top of the wall form. Calculate the axial force in each brace and
the reactions at the bottom of each brace.
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