Building Design and Construction Handbook - Merritt - Ventech!

WOOD CONSTRUCTION 10.29
There is an exception, however, applicable when holes or other reductions in area
are present in the critical part of the column length most susceptible to buckling;
for instance, in the portion between supports that is not laterally braced. In that
case, ƒ c should be based on the net section and should not exceed F c, the design
value for compression parallel to grain, multiplied by applicable adjustment factors,
except C P; that is,
ƒ � P/A � F (10.24)
c n c
where A n � net cross-sectional area.
The stability factor represents the tendency of a column to buckle and is a
function of the slenderness ratio. For a rectangular wood column, a modified slenderness
ratio, L e/d, is used, where L e is the effective unbraced length of column,
and d is the smallest dimension of the column cross section. The effective column
length for a solid column should be determined in accordance with good engineering
practice. The effective length L e may be taken as the actual column length
multiplied by the appropriate buckling-length coefficient K e. For the solid column
in Fig. 10.2a, the slenderness ratio should be taken as the larger of the ratios L e1/
d 1 or L e2/ds 2, where each unbraced length is multiplied by the appropriate value of
K e. For solid columns, L e/d should not exceed 50, except that during construction,
L e/d may be as large as 75.
The column stability factor C P is given by
where F* c �
� �
2
cE c cE c cE c
P
2c � 2c c
1 � (F / F*) 1 � (F / F*) F /F*
C � � � (10.25)
design value for compression parallel to the grain multiplied by all
applicable adjustment factors except C P
F cE � K cEE�/(L e/d) 2
E� � modulus of elasticity multiplied by adjustment factors
K cE � 0.3 for visually graded lumber
� 0.384 for machine evaluated lumber (MEL)
� 0.418 for products with a coefficient of variation less than 0.11
c � 0.80 for solid sawn lumber
� 0.85 for round timber piles
� 0.90 for glulam timber
For a compression member braced in all directions throughout its length to prevent
lateral displacement, C P � 1.0.
10.8.2 Built-up Columns
These often are fabricated by joining together individuals pieces of lumber with
mechanical fasteners, such as nails, spikes, or bolts, to act as a single member (Fig.
10.2d). Strength and stiffness properties of a built-up column are less than those of
a solid column with the same dimensions, end conditions, and material (equivalent
solid column). Strength and stiffness properties of a built-up column, however, are
much greater than those of an unconnected assembly in which individual pieces act
as independent columns. Built-up columns obtain their efficiency from the increase
in the buckling resistance of the individual laminations provided by the fasteners.
The more nearly the laminations of a built-up column deform together—that is, the