Wood Compression Members

The design of wood columns or other types of compression members requires consideration of compressive strength parallel to the grain, end bearing, and stability, or resistance to buckling. Compressive strength considerations are the same regardless of the type of column, since the maximum compressive stress Æ’c induced by loads must not exceed the design value for compression parallel to the grain, Fc, multiplied by applicable adjustment factors for service conditions (Art. 10.5).
(For design for end bearing, see Art. 10.11.1, and for stability, see Art. 10.8.1).
Wood compression members may be a solid piece of lumber or timber (Fig. 10.2a), or spaced columns, connector joined (Fig. 10.2b and c), or built-up (Fig. 10.2d).

Solid Columns

These consist of a single piece of lumber or timber or of pieces glued together to
act as a single member. In general,

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, Æ’c should be based on the net section and should not exceed Fc, the design value for compression parallel to grain, multiplied by applicable adjustment factors,
except CP; that is, 

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  smaller the slip between laminations, under compressive load the greater is the relative capacity of the column compared with an equivalent solid column.

When built-up columns are nailed or bolted in accordance with provisions in the National Design Specification for Wood Construction, American Forest & Paper Association, the capacity of nailed columns exceeds 60% and of bolted builtup columns, 75% of an equivalent solid column for all L/d ratios. The NDS contains criteria for design of built-up columns based on tests performed on built-up columns with various fastener schedules.

Spaced Columns

A wood spaced column consists of the following elements: (1) two or more individual, rectangular wood compression members with their wide faces parallel; (2) wood blocks that separate the members at their ends and one or more points between;
and (3) steel bolts through the blocks to fasten the components, with splitring or shear-plate connectors at the end blocks (Fig. 10.2b). The connectors should be capable of developing required shear resistance.
The advantage of a spaced column over an equivalent solid column is the increase permitted in the design value for buckling for the spaced-column members because of the partial end fixity of those members. The increased capacity may range from 21⁄2 to 3 times the capacity of a solid column. This advantage applies only to the direction perpendicular to the wide faces. Design of the individual members in the direction parallel to the wide faces is the same for each as for a solid column. The NDS gives design criteria, including end fixity coefficients, for spaced columns.

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