Double Angle Struts:
Compression members composed of two angles (channels, or tees), back to back in contact or separated through a gusset, shall be connected together (through rivets or welds) so that slenderness ratio (λ) of each member between connections is not greater than 40 or 0.6 times the most unfavourable slenderness ratio of the strut as a whole (whichever is less). At least there are two rivets/bolts (or their equivalent welding) should be used to connect them at their ends. Also there shall not be less than two additional connections equidistant in the length of the strut. Tacking rivets are provided in double struts, so in which they might act together. Their pitch (in line) should be not more than 32 t or 300 mm (whichever is less). Under exposed conditions these values are reduced up to 16 t or 200 mm, where t is the thickness of the outside plate.
Double angle struts may be connected to the gusset plate, either
(a) back to back to both sides of it by not less than two bolts or rivets (or equivalent welding). An effective length, leff, is then taken as between 0.7 to 0.85 times the distance between intersections, or
(b) back to back to a single side of the gusset by one or more rivets/bolts or equivalent welding in each angle and shall be designed as single angles.