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Steel Designers' Manual - 6th Edition (2003)
62 Multi-storey buildings
E
0
=
C-) =
1000
500
Minimum
Strength
Deflection
Vibration
0 5 10 15 20
Fig. 2.18 Structural criteria governing choice
Beam span (m)
minimize the floor zone. Simple universal beams with precast floors or composite
metal deck floors are likely to be most economic for spans up to 12 m. A range of
section capacities for each depth enables a constant depth of construction to be
maintained for a range of spans and loading. As with column components, plated
beams and fabricated girders may be used for spans above 10–12 m. They are particularly
appropriate where heavier loading is required and overall depth is limited.
For medium to lightly loaded floors and long spans, beams may also take the form
of castellated beams fabricated from standard sections, cellular sections or plates.
Above 15 m, composite steel trusses may be economic. As the span increases, the
depth and weight of the structural floor increase, and above 15 m spans depth predominates
because of the need to achieve adequate stiffness.
Castellated and cellular beam sections
Castellated beams (Fig. 2.19(a)) have been used for many years to increase the
bending capacity of the beam section and to provide limited openings for services.
These openings are rarely of sufficient size for ducts to penetrate without significant
modification, which increases fabrication cost. The cellular concept is a
development of castellated beams that provides circular openings and greater shear
capacity. Since their introduction in 1990, they have proved to be increasingly
popular for longer span solutions where services and structure have to be integrated.
Bespoke openings for services can be cut in the webs of universal beams and
fabricated plate girders.