Page 362 - Forensic Structural Engineering Handbook

TEMPORARY STRUCTURES IN CONSTRUCTION 10.49

south side of the building. The wall was not fully sheathed, the sheathing that was in place
was only partially nailed, and it is likely that this wall was not adequately braced. No truss
drawings were available at the site. According to the workers, “the bracing didn’t make any
sense, the other guys on the other site were responsible for the bracing” (see Fig. 10.10).

Case History 11: Column Reinforcing Steel Bracing

A tall reinforcing bar cage for a concrete column collapsed in Vancouver, British
Columbia, in the morning of November 2, 1993, seriously injuring one worker. The col-
umn rebar cage consisted of 44 bars of 25-mm diameter and 38-ft length each from the
ground up, to which an additional 40 bars of 25-mm diameter and 24-ft length were spliced
starting at 9 ft aboveground, or 16 in above the top of the previous pour. The collapsed cage
was reported as weighing approximately 14,000 lb. The column was to be part of a ductile
moment-resisting frame. The reinforcing steel requirements and lap locations were detailed
on the structural engineering drawings. Five guy cables were stabilizing the cage. At the
time of the accident, the injured worker was on top of a previously poured concrete wall,
rigging a section of gang form to be moved by a tower crane. He was positioned such that
two of the five guylines were on either side of him, to his east and west.
Two possible scenarios were developed by investigators of the Worker’s Compensation
Board of British Columbia: “Either the moving gang form panel tripped the guyline to the
west of the worker, initiating the rebar cage’s collapse, or someone had undone one of the
guylines to the east, which sent the cage over in a northwesterly direction.”
They further opined, “Either scenario could have caused the other guylines to go down,
with the one behind the worker (to the east) catching and flinging him approximately 32 ft
across the site. Had the rebar cage been adequately braced, it would not have collapsed.”
Following this incident, and others in which reinforcing steel or its temporary supports had
collapsed, the Worker’s Compensation Board issued a Technical Commentary, stating that

(a)
FIGURE 10.11 Column reinforcing steel. (a) Tall reinforcing steel cage collapsed before erect-
ing formwork. (From Jozef Jakubowski, Workers’ Compensation Board of British Columbia,
Vancouver, BC, Canada.)