Workers at Bluesfest, where a July 17 storm collapsed the main stage, faced a similar situation a week earlier, when crews cut ties holding the stage's side screens due to fierce winds, witnesses say.
Adam Dietrich, a photographer with Carleton University's campus paper, was at Sunday evening's Cheap Trick show, when high winds collapsed the structure moments after the band left the stage, injuring three people.
Dietrich said the incident was similar to weather before the July 8th main-stage performance of the Black Keys.
"The scenario was the exact same, the fears coming from the crew on the ground were the exact same, it honestly felt the exact same," said Dietrich.
The Akron-Ohio duo's performance was delayed for over an hour when a violent storm brought wild wind gusts and drenching rain.
Dietrich — along with three other backstage workers who asked not to be identified — told CBC News that people backstage were ordered away from the structure because the stage crew feared it could collapse.
Ties that hold down the stage's side screens but designed to release under 70 km/h winds also did not, according to Dietrich and other witnesses. Stage workers had to cut the ties loose manually to alleviate pressure on the so-called "wind wall" of the stage, they said.
The Black Keys took the stage after the storm had passed and winds had returned to normal.
Stéphane Berger, the director of the Montreal company Groupe Berger that owns, assembles and manages the stage, said the winds during the storm on July 8 reached 90 km/h.
"It's possible that they had to cut some of the ties. That's possible," said Berger.
But Berger would not comment on the July 17 collapse, pending the results of an investigation from the Ontario Ministry of Labour.
The specifications of the Mark III stage with dual bays used in both shows say it can withstand gusts of wind as high as 120 km/h with the wind screens in place and steady winds of 80 km/h.
Groupe Berger began to dismantle the stage Tuesday after ministry investigators completed their examination of the collapsed structure.
Ministry officials have ordered festival organizers to provide copies of any videos taken by their contractors of the collapse of the stage and also engineering documentation for the stage, said ministry spokesman Matt Blajer.
The band Cheap Trick took the stage at about 7 p.m. Sunday and played for 20 minutes when the storm clouds rolled in. The band hastily left the stage and, moments later, high winds caused the structure to collapse.
Some 10,000 people in attendance took shelter, many at the nearby Canadian War Museum. Three people were injured but released from hospital on Monday.
Berger said it was "freakish weather" and not a structural defect that caused the collapse.
"Our explanation is that the wind was too strong," he said.
Winds at the Ottawa Airport were recorded at 96 km/h, but Environment Canada said a downburst — a strong downdraft of air from a thunderstorm that spreads out in all directions when it hits the ground at much faster speeds — was likely the trigger for the collapse of the main stage.
Downbursts can potentially cause as much damage as a tornado.