Beam or post for a construction system

A hollow beam or post for a construction system, the beam or post comprising:

• a core element formed of a straight profiled hollow central element, and
• straight surrounding elements;
• which are assembled around the core element in correspondence with the profile thereof, forming a square surrounding the core element,
• wherein
• the core element formed of a straight hollow central element having a cross-section provided with four diametrically opposed hollow outer projections in the shape of outwardly flaring dovetails, which are arranged crosswise, these four hollow outer projections having narrowed inner ends which are spaced apart from the narrowed inner ends of the adjacent hollow outer projections and are connected by segments of the hollow central element, and
• the straight surrounding elements are hollow and each has an inner wall whose internal shape is complementary to the four diametrically opposed outer projections of the core element and its segments when the elements are assembled around the core element.