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Structural Steel Frames 289
A disadvantage of this layout is that the increased span of main beams requires an
increase in their depths so that they will project below the underside of the secondary
beams. Heating, ventilating and electrical services, which are suspended and run below the
main beams, are usually hidden above a suspended ceiling. The consequence is that the
requirement for comparatively unobstructed floor space causes an increase in the overall
height of a building because of the increase in depth of floor from floor finish to suspended
ceiling at each floor level. Where there is a requirement for a large floor area unobstructed
by columns, either a deep long-span solid web beam, a deep lattice girder or Vierendeel
girders are used.
The advantage of using deep lattice girders or Vierendeel girders is that they may be
designed so that their depth occupies the height of a floor and does not, therefore, increase
the overall height of construction. The Vierendeel girder illustrated in Figure 5.12 is fabricated
from mild steel plates, angles, channels and beam sections, which are cut and welded
together to form an open web beam. The advantage of the open web form is that it can
accommodate both windows externally and door openings internally, unlike the diagonals
of a lattice girder of the same depth. The solid parts of the web of this girder are located
under the columns they are designed to support. The specialist fabrication of this girder
together with the cost of transporting and hoisting into position involves considerable cost.
The conventional structural steel frame comprises continuous columns which support
short lengths of beam that are supported on shelf angles bolted to the columns. Where
there is a requirement, for example, for the structural frame to overhang a pavement, the
frame has to be cantilevered out, as illustrated in Figure 5.13. To support the columns on
the external face of the cantilever, it is necessary to use shorter lengths of the main external
Structural steel frame
Figure 5.12 Vierendeel girder.
Vierendeel girder in
depth of first floor
supports frame above