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 Designers are cautioned that the basic structural elements of a truss system should be "statically
determinate" during the adjustment phase (to avoid introduction of large and often unpredictable
stresses into a constrained system) unless a careful investigation has been made into the resulting
load condition.
Miscellaneous Considerations
Provide a means to "lock in" the final adjusted configuration once the system has been leveled.
If high strength bolts are used, ensure that they are fully tensioned. The use of load indicating
washers in final bolted assemblies of the principle load carrying members is encouraged. The
use of load indicating washers allows for more meaningful inspection of the primary connections
at a later time. In all cases, bolted assemblies that require fasteners to be loosened in order to
adjust the structure should have their bolts discarded and replaced before the construction is
complete.
l) Failure Mechanism: Single points of failure are undesirable in any facility but
are historically not uncommon in long span steel structures. The designer should pay particular
attention to the connections between major structural elements (truss supports, cantilever
anchors, etc). The designs should include secondary or backup load paths should the primary
system be damaged.
Construction and Erection
Hangar construction involves the creation of a long span, column free space. These requirements
complicate the erection of the building and make the steel erection contractor a much more
important partner in the process than is typical of most government construction. While the
structural engineer is typically advised to avoid interfering with the means and methods of the
construction professionals, he should have an understanding of the consequences which
accompany any chosen erection method. Two general approaches are applicable to the erection
of large hangar bays, ground assembly with heavy lift and aerial assembly with shoring towers.
Ground Assembly with Heavy Lift
While not unique to hangars, lifts of pre-positioned, pre-assembled hardware weighing 30 to 50
tons are uncommon in most construction and typical of hangar construction. Ground
construction is typical the most common means selected by contractors. Large cribbing is set
immediately adjacent to the lift location and leveled. Shims are set to adjust for elevation
differences and to establish the proper camber. The structural element is then constructed on top
of the cribbing. Once completed, the entire element is lifted into place and the final support
connections made. Fabricating on the ground allows for enhanced safety for the bulk of the
work as well greater control of quality and ease of access for inspectors. The drawbacks of the
approach include the cost associated with mobilizing a crane or cranes which can lift the
assembly. Tight quality control is essential to ensure that the final fit is made.
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