A composite beam, pillar or truss comprises polygonal cross-section modular units 11 connected end to end with adjacent units 11 reversed, the units 11 being held together by stringers 12 and end caps 13, and the whole beam being pre-tensioned by tie rods 21 passing through the stringers 12 or by threaded inserts. A connector 110 of cube form comprises face plates 111 having screw threaded connectors for the beams, the face plates being held together by a plurality of internal corners 113 and external corners 114. The beam 10 and connector 110 are apertured to receive services from any direction and can be assembled to form a pre-fabricated building or other structure.