UFC 4-211-01N
25 October 2004
and the truss 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.
4-2.2.1
Advantages. The header truss is an efficient system to span
intermediate lengths and provides a relatively simple erection system.
4-2.2.2
Disadvantages. The structural efficiency and stiffness decrease
exponentially as the span of the truss increases. Offset this by increasing the
depth of the truss. However, 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 m
(25 ft.) 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.
4-2.2.3
Considerations. 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.
4-3
MATERIALS.
4-3.1
Weathering Steel. Weathering steel, if considered, should not be
used where exposed to recurrent wetting by salt water or airborne abrasives.
Weathering steel should not be used at or below grade. Careful detailing should
be maintained throughout the design to ensure that weathering steel does not
trap and hold water. Pockets that hold water will not form the required oxide
coating that gives the steel its enhanced corrosion protection. In areas where
weathering steel is acceptable, proper detailing and use of materials should be a
requirement to prevent staining of adjacent building components.
4-3.2
Hollow Structural Sections. Hollow Structural Sections (HSS) are
an attractive design choice for their weak axis stability or bi-axial properties of the
truss members. There may also be additional benefits derived from efficiency in
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