Page 337 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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FLOOR AND ROOF SYSTEMS

FLOOR AND ROOF SYSTEMS 7.21

deformations under gravity loads (Fig. 7.24). These bending stresses can be significantly reduced by
cambering the trusses, thereby preloading the columns. An alternative is to provide slotted bottom-
chord connections that are tensioned or welded after dead load is applied.

7.14 CASTELLATED BEAMS

A special fabrication technique is applied to wide-flange shapes to produce castellated beams. This
technique consists of cutting the web of a wide-flange shape along a corrugated pattern, separating
and shifting the upper and lower pieces, and rewelding the two pieces along the middepth of the
newly created beam (Fig. 7.25). The result is a beam with hexagonal openings having a depth,
strength, and stiffness greater than the original wide-flange shape, but that maintains the same weight
per foot as the original wide-flange shape. A similar technique, with some additional trimming of the
web, can be used to create a beam with round openings. The numerous openings, or castellations,
that are formed in the beam web can accommodate mechanical ductwork, thereby reducing the overall
floor depth.
Castellated beams can be designed to act compositely with the floor deck. Economical spans typ-
ically range from about 35 to 70 ft. For composite design, it is structurally more efficient to fabri-
cate the beam from a heavier wide-flange shape for the lower portion than for the upper portion. As
a rule of thumb, the deflection of a castellated beam is about 25% greater than the deflection of an
equivalent beam with the same depth but without web openings, primarily due to increased shear
deformations.
The load capacity of a castellated beam is frequently dictated by the local strength of the web
posts and the tee portions above and below the openings. Therefore, these beams are more efficient
for supporting uniform loadings than for concentrated loadings. The latter produce web-shear distri-
butions that tend to be less favorable because the perforated web has less capacity than the solid web.

7.15 LRFD EXAMPLES FOR COMPOSITE FLOORS

Examples of composite beam and girder designs for the floors of typical interior bays of office build-
ing floor systems are shown in Fig. 7.26 (30-ft by 30-ft bay) and 7.27 (30-ft by 45-ft bay). The
designs are based on the following:
• Beams and girders are ASTM A992 steel (50-ksi yield stress).
• Floor is 3-in, 20-ga composite steel deck with 3.25 in of lightweight concrete fill with a total
2
weight of 47 lb/ft .
2
• Total dead load (floor slab, partitions, ceiling, and mechanical) is 77 lb/ft .
2
• Live load is 80 lb/ft , with live-load reductions in accordance with size of loaded areas supported
(Art 4.4.3).
• Cambering compensates for approximately 75% of dead-load deflections.
• Live-load deflections are limited to 1/360 of the span.
• Percentage of full composite action is limited to 90.

For the 30-ft by 30-ft bay shown in Fig. 7.26, beams are W14 × 22 and girders are W18 × 40.
However, the design shown would not meet acceptable vibration criteria for an open-office layout
having a damping ratio of 2.5% (see Art. 7.18). This would require that the typical beam be increased
from W14 × 22 to W16 × 26, and the typical girder be increased from W18 × 40 to W18 × 46. The
2
steel weight would increase from 3.5 to 4.1 lb/ft .
For the 30-ft by 45-ft bay shown in Fig. 7.27, beams are W18 × 35 and girders are W21 × 50. The
2
steel weight is 4.6 lb/ft . In this case, the design shown would meet acceptable vibration criteria for
an open-office layout having a damping ratio of 2.5%.

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