Two more contributors have joined this discussion. Richard Harris writes from Bournemouth:
I have read with interest the recent correspondence on mezzanine floor structures and
the effective lengths of their columns. For some years, I have been refusing to accept a factor of 1.5 for unbraced frames. The designers are always unable to justify the figure. Occasionally, when an impasse is reached, I visit the site. Surprisingly, even
when there appears to be no adjacent supporting structure according to the plans, there is often actually something robust enough to provide lateral restraint, bringing the effective length down below 1.0. I suspect that this, and vigilance by building control bodies, is an important factor in preventing collapse.