Page 351 - Forensic Structural Engineering Handbook

10.38 CAUSES OF FAILURES

putlog failed in a ductile manner (consistent with the pause between initial failure and total
collapse while the top chord yielded).
Analysis of the platform using the scaffolding supplier’s load tables showed that the
dead weight alone resulted in the putlogs being loaded to the published capacity.
Therefore, based on the supplier’s tables, the allowable superimposed loading on the
platform supported by the putlogs was essentially zero, and the platform was grossly
underdesigned for its intended use. Even though the platform was improperly designed,
that condition did not explain the failure because the published capacities appropriately
include factors of safety. A structural analysis showed that although loaded beyond the
allowable loadings, these putlogs should still take the additional load of the gypsum wall-
board and a worker without failing. The putlogs under the stacks of gypsum wallboard
were nearly at yield stress under the applied load. Using the reduced capacity for the
damaged putlog based on the conclusions of the metallurgical analysis and the computa-
tional results for the applied loads, the repaired putlog was loaded to the ultimate strength
of the repaired location under the weight of the gypsum wallboard. The weight of the
worker approaching the stack of gypsum wallboard was enough additional load to break
the repaired putlog.
In conclusion, the combination of the seriously under-designed scaffold platform
and the weight of the stack of gypsum wallboard plus the worker on an area supported
by a poorly repaired putlog caused the failure. The collapse spread to a larger area
beyond the broken putlog because the platform was not properly braced and tied into
the building.
How did this happen? The platform design was inappropriate; the scaffolding supplier
and the contractor never discussed the intended purpose of the platform, so the scaffold-
ing supplier did not know what load the platform was to carry. Also, the supplier’s
designer said the design was based on “experience,” and he never looked at load tables or
performed calculations of any kind. This resulted in a scaffold platform with no capacity
for any superimposed loads. Another contributing factor for the collapse was that some of
the scaffold components were not properly handled and maintained. The supplier’s policy
stated that damaged components were to be discarded and not repaired; consequently, the
putlog with the broken chord should have been discarded. Finally, the scaffold supplier
did not follow the company bracing and tie requirements, which led to a general collapse
of the temporary structure (see Fig. 10.5).

Case History 6: Masons’ Scaffold
A seven-frame-high masons’ scaffold collapsed during construction of a multistory
brick veneer facing on a building in British Columbia, Canada. Three workers who
were working on the fifth scaffold level rode down with the frames and other materi-
als. At the time of the failure in addition to the three workers and their materials on the
fifth level, bricks were stockpiled on the seventh level. The weight of the stockpiled
materials was approximately double the safe working load of the scaffold.
Investigators of the Worker’s Compensation Board of British Columbia (similar to a
state OSHA in the United States) found that the horizontal bracing members of the
scaffold were incomplete and the scaffold’s ties to the permanent structure were inad-
equate. In addition, they indicated that the contractor was not aware of the maximum
allowable load that could be placed on the scaffold, nor did he have manufacturer’s
documentation available to perform reasonable assessment of the maximum allowable
superimposed load (see Fig. 10.6).