Step 1: Understanding the concept of a beam.
A beam is a structural member designed primarily to resist loads that act perpendicular to its longitudinal axis. These loads cause bending and shear stresses within the beam. Beams are commonly used in structures such as bridges, buildings, and frames.
Step 2: Analysis of the given options.
(A) Axial tension or compression: This type of loading is mainly associated with columns or tie members, not beams. Hence, this option is incorrect.
(B) Transverse loads and couples: Correct — Beams are subjected to transverse loads (loads acting perpendicular to the axis) and bending moments or couples, which cause bending of the beam.
(C) Twisting moment: Twisting moments primarily act on shafts, not beams, so this option is incorrect.
(D) No load, but its axis should be horizontal and x-section rectangular or circular: This statement does not define the loading condition of a beam and is therefore incorrect.
Step 3: Conclusion.
A beam is mainly subjected to transverse loads and bending moments (couples). Therefore, the correct answer is (B) Transverse loads and couples.