Arup director David Lewis said late architect Enric Miralles insisted on a complicated decorative roof for the building's chamber, instead of a simpler alternative.
The comments over the incident - which saw a loose beam swing down from the building's roof earlier this month - come in the wake of an outburst from RMJM design director Paul Stallan, who claimed that it was an 'engineering issue'.
Lewis said at a speech of the Institution of Structural Engineers: 'We offered [Miralles] a more efficient alternative structure. But you couldn't say no to [Miralles].'
On Thursday, Stallan responded to reports that there was a 'fundamental flaw' with the building's roof by claiming it was an 'engineering issue'.
He said: 'Arup have got to look at it.'
Meanwhile, the Scottish Parliament has released the details of Arup's investigation into the cause of the incident.
The engineer claimed that one of two bolts to secure the lower end of the beam was missing, while the other was broken and had damaged threads.
It said the damage was caused by the bolt jamming before someone tried to remove it.