LOAD REVERSAL was identified as the most likely reason why a glulam strut swung loose in the roof above the Scottish Parliament debating chamber last Thursday.
Sources close the project suggested that the strut had been subject to unforeseen tensile forces which had pulled it free from its stainless steel socket.
The strut formed part of a complex three dimensional roof structure and should have been in compression at all times.
The nearly 4m long strut formed part of a complex structure comprising laminated oak compression members and steel rods in tension spanning up to 24m.
Stainless steel nodes welded up from plate and tube hold the structure together. (NCE 5 June 2003).
Last Thursday one end of the glulam member came free from the lower yoke-shaped connection, allowing it to rotate on its pin-jointed upper connection. It was left dangling above the heads of MSPs who later evacuated the chamber.
The strut was removed on Friday and Scottish Parliament structural engineer Arup has been involved since then in a re-analysis of the three dimensional roof structure.
The failure occurred towards one the end of the roof, where spans are much shorter, but where member sections are the same as on the long span areas.
The glulam strut was located in the stainless steel yoke during erection by two bolts passing through the rear of the yoke into sockets glued into the end grain.
These were not designed to take tension, NCE has learned.
The strut would have had to move a significant distance out of the yoke before it could swing free, a factor which suggests tensile forces were involved.
Arup was given until Wednesday evening to produce a report on the most likely cause of the incident.
Its analysis will be checked by Atkins, which was appointed independent structural consultant by the Scottish Parliament on Tuesday evening.
Debates in the Parliament have been transferred to another nearby building until further notice.