CASE CC4 - Acceptance of Off-the-Shelf Structures, J. V. Tyrrell
Acceptance of structures not designed for the site.
Structural distress; change orders; increases in contract cost.
Collection of Facts: Sometimes off-the-shelf items will provide the most
economical solution to a design problem. Some of these items are actually
pre-engineered but not stockpiled in great quantities and instead fabricated
when ordered. For this type of structure, manufacturer's data is usually
available showing what loadings were considered in design. If the design
loading is not carefully checked against the intended use and the prevailing
site conditions, the structure may prove unsatisfactory.
The need for verifying the design may seem self-evident, but we have many
cases where inadequate structures were acquired or contracted for before the
inadequacies were discovered. Apparently there is some inclination to accept
whatever is commercially available without carefully considering the
Solution: When using off-the-shelf or pre-engineered structures, check them
for the loadings intended including environmental loadings at the site (wind,
snow, earthquake, soil conditions, etc.). If the structure is not adequate,
do not use it unless the manufacturer modifies it or you expect to reinforce
it separately. The cost of modification should be included in the project.
CASE CC5 - Erection of Cantilever Hangar, J. V. Tyrrell
Collapse during erection.
Collection of Facts: A typical cantilever type hangar with tension strut
suspender was being erected, at NAS Miramar, using scaffolding and jacks to
position the cantilever ends at the correct elevation before connecting the
suspender. This process had apparently been successfully used previously. In
this case, one of the trusses slipped off of the jack creating a domino
collapse and resulting in injury to personnel. Several causes were possible.
This method inherently involves lateral forces when two adjacent trusses are
jacked to different elevations. The scaffolding is designed to minimize
weight and has little reserve strength. It was not capable of sustaining the
truss when it toppled and, consequently, massive collapse followed.
Solution: This method of erection is not recommended. Instead, it is
suggested that the tension suspenders be connected immediately with a
provision for adjustment in the splice. The trusses can then be jacked to
proper elevation without the possibility of dropping the structure. Erection
methods are usually the contractor's domain; however, at a minimum, any
contractor should be made aware of the potential difficulties and at least be
required to submit a detailed plan and sequence for approval. If jacking is
attempted before connection of the suspenders, the scaffolding should be
carefully checked, provision should be made to brace the scaffolding and the
jacks, and the maximum difference in elevation permitted should be established
by structural calculations.