The basic roof truss consists of two rafters or struts and a restraining tie. Both rafters are pivoted at their apex.
The other end of one of the rafters is pivoted to a free standing base, whilst the remaining rafter end runs on ball bearings along a track.
When a load is hung from the apex, the free end of the rafter moves sideways, restrained by a spring balance tie.
Both rafters also include spring balances so that all internal loads can be directly measured.
Re-adjustment of the geometry back to its original unloaded configuration is easily made before taking measurements. The length of the tie can be varied to change the angles of the truss.