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

130 Chapter Five
Downloaded From : www.EasyEngineering.net
Square shores are often preferred to rectangular ones because the
minimum dimension of the cross section is the same about both the
x-x and y-y axes. This eliminates the requirement of calculating slenderness
ratios about two axes to determine the governing value.
Using square shores in lieu of rectangular ones also reduces the possibility
of orienting the shores improperly during erection of the
forms. Installing incorrect bracing patterns of shores can adversely
affect the safety of shoring.
Solid wood columns may be classified into three categories according
to their length: short columns, intermediate columns, and long columns.
Short columns are defined by small l u
/d ratios, the ratio of unbraced
length to the least cross-sectional dimension. Columns are generally considered
short when the l u
/d ratio is less than 10. The failure of short columns
is primarily by crushing of the wood. Therefore, the limiting stress
in a short column is the allowable compression stress parallel to grain.
For intermediate l u
/d ratios, the failure is generally a combination
of crushing and buckling. For long columns, large l u
/d ratios, the failure
is by lateral buckling of the member. The instability of long wood
columns is often referred to as long-column buckling. The equation
for determining the critical buckling load of long columns was
derived by Francis Euler and is discussed in many publications
related to column and compression members. For rectangular wood
members, the limiting value is l u
/d = 50. Columns with ratios greater
than this value are not permitted in formwork.
Equation (5-61) is the recommended equation to determine the
allowable compression stress in a wood column with loads applied
parallel to grain. This single equation takes into consideration modes
of failure, combinations of crushing, and buckling. Because a single
equation is used, many terms are included:
ww.EasyEngineering.n
F = ∗⎡
+ ∗
C
c ⎡
⎣ +
∗
F F F F F c⎤
c
[ 1 (
ce/ c
)]/2 [ 1 (
ce/ c
)]/2 2
∗ ⎤
⎦ − (F / )/
⎣⎢
ce
Fc c (5-61)
⎦⎥
where F c
= allowable compression stress, lb per sq in.
F * = compression stress parallel to grain, lb per sq in., obtained
c
by multiplying the reference design value for compression
stress parallel to grain by all applicable adjustment
factors except C P
; F * = F (C × C × C × C × C )
c C// D M F i t
F ce
= 0.822E′ min
/(l e
/d) 2 , lb per sq in., representing the impact of
Euler buckling
E′ min
= modulus of elasticity for column stability, lb per sq in.,
obtained by multiplying the reference design value of
E min
by all applicable adjustment factors; E′ min
= E min
(C M
• C T
• C i
• C t
)
l e
/d = slenderness ratio, ratio of effective length, in., to least
cross-sectional dimension, in. Note: For wood columns
l e
/d should never exceed 50
c = 0.8 for sawn lumber
Downloaded From : www.EasyEngineering.net