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 supporting towers. Intermediate support to the truss is provided in the plane of the roof through
a diaphragm, dedicated horizontal truss bracing or a combination of the two.
The header truss is an efficient system to span intermediate lengths and provides a relatively
simple erection system.
The structural efficiency and stiffness decrease exponentially as the span of the truss increases.
This can be offset by increasing the depth of the truss but the practical limits of transporting the
fabricated hardware, erecting the assembled truss and lateral bracing of the system and its
individual components limit the truss depth to something on the order of 7.5 meters. A header
truss virtually precludes the use of expansion joints, therefore the hangar door span may be
limited by the thermal response of non-structural components. However, a three hinged arch
system allows an expansion joint along the centerline of the hangar bay and extends the practical
thermal expansion limit. The header truss system also requires that flightline frontage be
dedicated to structural supports. Finally, the potential expansion of the hangar is constrained by
the presence of towers and lateral load resisting systems.
Construction documents for a header truss system must clearly indicate the camber requirements
as well as supply the necessary information for the fabricator and erector to predict the truss's
response at various states of construction, handling and loading.
The header truss may be designed as fixed, pinned or partially restrained at its supports in order
to balance the strength and deflection characteristics of the header truss with the complexity of
detailing and erection. The designer of a statically indeterminate truss must carefully consider the
influence that temperature and erection rigging will have on the difficulty of completing
connections as well as final camber as this complexity must be communicated on the design
documents.
Conventional Framing Systems
Conventional framing systems are represented by relatively short spans where a single hot rolled
shape, pre-engineered frame element or built-up plate girder can span the entire flightline
opening. Such a system requires the relatively frequent placement of columns along the
flightline face of the hangar.
A conventional framing system is the most efficient structural design approach, is easily
expanded by repetition of the selected elements and simplifies the structural erection but
significantly reduces the operational flexibility.
Repetitive Truss System
Repetitive truss system is composed of a series of trusses, spanning the width of the hangar at
regular intervals. Secondary members span from truss to truss to enclose the building envelope.
A repetitive truss system is essentially a series of header truss and the issues faced by the
designer are similar.
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