is hung are called spandrels.  The web of a spandrel girder is


often wider than that of an interior girder to help it resist bending

despite the fact that its tributary area may be less.  It has to bear

the weight of the curtain wall cladding which is often comprised

of windows and thin panels that cannot tolerate bending induced

stresses.  Also spandrels are subjected to lateral forces such as

wind.  Perimeter columns often have larger cross-sections than

interior columns for the same reasons.  Since the skeletal frame

click image to enlarge

is carrying the entire load of the building, the columns must be

Fig. 220 Exterior cladding

spaced relatively close together.

(scale visualization model)


Long span construction


Structures like aircraft hangers, coliseums, or atriums, that require greater open areas

between columnar supports than standard wood or steel structural members can span, are

built with long span construction technology.  Many designs have been used successfully

depending on the area to be spanned, the anticipated roof loads, occupancy use, etc.

This lesson will cover only bar joists, trusses, space frames, and folded shell structures.


Bar joists



Bar joists are simply long floor or ceiling truss joists.  They

usually span from girder to girder, which themselves are

supported by widely spaced columns or bearing walls. 

Prefabricated bar joists are available that are designed for

specific load requirements.

Fig. 221 - Bar joist peaked roof






Standard truss designs, like those shown previously for

bridges, can also be used to provide a clear span inside

buildings since they require no intermediate supports.  A

triangulated truss has a higher H/S ratio than a truss joist.

Fig.  222 - Hanger with

Also, it is fabricated from structural steel shapes like T-

long span truss roof

shapes and angles rather than small diameter rods.  The
  design of the truss depends on the anticipated loads, the

amount of open floor space required, and the overall shape of the building.  If the truss

must bear a downward acting roof load, a design that is efficient in supporting a load

applied to the top chord, like the Howe, Pratt, and Warren deck truss designs, is standard.


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Page 131 - Building stability - Long-span construction

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