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

Downloaded From : www.EasyEngineering.net
Design of Wood Members for Formwork 129
Shores are column members with axial loads that induce compression
stresses that act parallel to the grain. Values of compression
stresses are presented in Tables 4-2 and 4-3 for stresses parallel to
grain. However, these compression stresses do not consider the length
of a member, which may affect its stability and strength. A shore must
be properly braced to prevent lateral buckling because its strength is
highly dependent on its unbraced length.
The slenderness ratio for a shore is the ratio of its unbraced length
divided by the least cross-sectional dimension of the shore, l u
/d. It is
used to determine the allowable load that can be placed on a shore.
The allowable load on a shore decreases rapidly as the slenderness
ratio increases. For this reason, long shores should be cross-braced in
two directions with one or more rows of braces.
Figure 5-12 illustrates the calculations of l u
/d for a 4 × 6 rectangular
column member. For this 4 × 6-in. column, it is necessary to calculate
two values of l u
/d to determine the value that governs for buckling.
For strong-axis buckling, the unbraced length is 9 ft, or 108 in.,
and the dimension d = 5½ in. Therefore, l u
/d = 108/5.5 = 19.6.
For weak-axis buckling, the unbraced length is 6 ft, or 72 in., and
the dimension d = 3½ in. Therefore, l u
/d = 72/3.5 = 20.6.
Because 20.6 is greater than 19.6, the value of 20.6 must be used in
calculating the allowable compression stress in the member. Thus,
there can be different bracing patterns for a given size compression
member. For each particular bracing pattern, the governing l u
/d ratio
must be determined.
In Figure 5-12 the bracing is shown as a small member to illustrate
the concept of unbraced length. Bracing must be of a sufficient
size and must be securely attached to a rigid support in order to prevent
lateral movement of the compression member.
ww.EasyEngineering.n
FIGURE 5-12
Illustration of unbraced lengths for compression members.
Downloaded From : www.EasyEngineering.net