disposed at right angles to each other the arms 168a through 168f are made to engage the slot 4 of six support elements 2 thereby connecting all six internally through connector 166 fig1 b shows another internal connector 170 having two arms 172a and 172b for connecting two support elements at an angle arm 172a would be disposed in slot 4 of a support element 2 arm 172b would then be fitted into another support element 2 and fastened thereto by means of bolts not shown and nuts such as nut 134 fig6 through holes 174a and 174b fig1 a shows a cross section through an arm 168a of internal connector 166 the arm has flanges 176a and 176b and recesses 178a and 178b which interlockingly engage the flanges 6a and 6b of support element 2 fig1 and 3 fig1 b shows an alternative cross section for arm 168a which has flanges 180a and 180b which interlock with flanges 6a and 6b of support element 2 another similar set of flanges 182a and 182b is located on the top of the connector arm 168a allowing the union of two support elements arranged with the slots 4 facing each other fig1 shows an external connector 184 for connecting or splicing two support elements a support element 200 having longitudinal flanges 189a and 198b is disposed within external connector 184 the support element 200 is fastened to external connector 184 by a bolt 186 through channel fitting 188.