The formwork trades refer to the practice of using single-post shores in two tiers as double decking; occasionally triple decking (three tiers) has been used. Over the past 30 years this characteristically unstable system has been the cause of innumerable formwork-shoring collapses.
EXTRA PRECAUTIONS NEEDED WITH DOUBLE-DECKED SHORES
A double-deck system, by definition twice the height of ordinary shoring, is particularly vulnerable to lateral forces such as those induced by wind, moving equipment, or unbalanced loads of fresh concrete. Lack of rigid connections and adequate bracing--a common occurrence in formwork construction--makes it possible for the failure of a single shore to trigger collapse of the whole system. If for some reason single-post shores must be double decked, design and inspection of the system should be done by a professional structural engineer and qualified formwork designer.