What is "compression member"

Member \Mem"ber\, n. [OE. membre, F. membre, fr. L. membrum; cf. Goth. mimz flesh, Skr. mamsa.]

1. (Anat.) A part of an animal capable of performing a distinct office; an organ; a limb.

   We have many members in one body, and all members have not the same office.
   --Rom. xii. 4.

2. Hence: A part of a whole; an independent constituent of a body; as:

   1. A part of a discourse or of a period or sentence; a clause; a part of a verse.

   2. (Math.) Either of the two parts of an algebraic equation, connected by the sign of equality.

   3. (Engin.) Any essential part, as a post, tie rod, strut, etc., of a framed structure, as a bridge truss.

   4. (Arch.) Any part of a building, whether constructional, as a pier, column, lintel, or the like, or decorative, as a molding, or group of moldings.

   5. One of the persons composing a society, community, or the like; an individual forming part of an association; as, a member of the society of Friends.

   6. (Math.) one of the elements which, taken together, comprise a set.

   7. (Math.) one of the individual objects which comprise a group or class.

      Compression member, Tension member (Engin.), a member, as a rod, brace, etc., which is subjected to compression or tension, respectively.

Compression members are structural elements that are pushed together or carry a load, more technically they are subjected only to axial compressive forces. That is, the loads are applied on the longitudinal axis through the centroid of the member cross section, and the load over the cross sectional area gives the stress on the compressed member. In buildings posts and columns are almost always compression members as are the top chord of trusses.