Beams - Multi-Storey Buildings.

Structural steel floor systems consist of prefabricated standard components, and columns should be laid out on a repetitive grid which establishes a standard structural bay. For most multi-storey buildings, functional requirements will determine the column grid which will dictate spans where the limiting criterion will be stiffness rather than strength (Fig. 2.15).

Fig. 2.15 Typical floor layout

Steel components are uni-directional and consequently orthogonal structural column and beam grids have been found to be the most efficient.The most efficient floor plan is rectangular, not square, in which main, or ‘primary’, beams span the shorter distance between columns and closely-spaced ‘secondary’ floor beams span the longer distance between main beams. The spacing of the floor beams is con- trolled by the spanning capability of the concrete floor construction (Fig. 2.16).

Beam and shallow deck layout: (a) inefficient; (b) efficient
Fig. 2.16 Beam and shallow deck layout: (a) inefficient; (b) efficient

Having decided on the structural grid, the designer must choose  an economic structural system to satisfy all the design constraints. The choice of system and its depth depends on the span of the floor (Fig. 2.17). The minimum depth is fixed by practical considerations such as fitting practical connections.As the span increases, the depth will be determined by the bending strength of the member and, for longer spans, by the stiffness necessary to prevent excessive deflection under imposed load or excessive sensitivity to induced vibrations (Fig. 2.18). For spans up to 9m, shallow beams with precast floors or deep composite metal deck floors can be used to 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 12m. 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 10–12m. They are particularly appropriate where heavier loading is required and overall depth is limited.

Span ranges for different composite floor systems
Fig. 2.17 Span ranges for different composite floor systems

 Structural criteria governing choice
Fig. 2.18 Structural criteria governing choice

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 15m, composite steel trusses may be economic. As the span increases, the depth and weight of the structural floor increase, and above 15m 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.

Beams with web openings for service penetrations
Fig. 2.19 Beams with web openings for service penetrations

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.

Fabricated plate girders
Conventional universal beams span a maximum of about 15m. Recent advances in automatic and semi-automatic fabrication techniques have allowed the economic production of plate girders for longer span floors. Particularly  if a non-symmetric plate girder is used, it is possible to achieve economic construction well in excess of 15m (Fig. 2.20). Such plate girders can readily accommodate large openings for major services. If these are in regions of high stress, single-sided web stiffening may be used.Away from regions of high stress, stiffening is usually not required.

The use of intumescent paints, applied offsite, is becoming increasingly popular.

One major fabricator is now offering an integrated design and fabrication service for customized plate girders which can achieve a fire resistance of 2 hours when applied as a single layer in an off-site, factory-controlled process.

Taper beams
Taper beams (Fig. 2.20) are similar to fabricated plate girders except that their depth varies from a maximum in mid-span to a minimum at supports, thus achieving a highly efficient structural configuration. For simply-supported composite taper beams in buildings the integration of the services can be accommodated by locating the main ducts close to the columns.Alternative taper beam configurations can be used to optimize the integration of the building services.

Fabricated plate girders and taper beams
Fig. 2.20 Fabricated plate girders and taper beams

Composite steel floor trusses
Use of composite steel floor trusses as primary beams in the structural floor system permits much longer spans than would be possible with conventional universal beams.The use of steel trusses for flooring systems is common for multi-storey buildings in North America but seldom is used in Britain. Although they are considerably lighter than the equivalent universal beam section the cost of fabrication is very much greater, as is the cost of fireproofing the truss members. For maximum economy, trusses should be fabricated from T-sections and angles using simple welded lap joints.The openings between the diagonal members should be designed to accept service ducts, and if a larger opening is required a Vierendeel panel can be incorporated at the centre of the span. Because a greater depth is required for floor trusses, the integration of the services is always within the structural zone (Fig. 2.21).

Composite truss
Fig. 2.21 Composite truss

Stub girder construction
Stub girders were developed in North America in the 1970s as an alternative form of construction for intermediate range spans of between 10 and 14m.They have not been used significantly in the UK. Figure 2.22 shows a typical stub girder with a bottom chord consisting of a compact universal column section which supports the secondary beams at approximately 3-metre centres. Between the secondary beams a steel stub is welded on to the bottom chord to provide additional continuity and to support the floor slab. The whole system acts as a composite Vierendeel truss.A disadvantage of stub girders is that the construction needs to be propped while the concrete is poured and develops strength. Arguably, a deep universal beam with large openings provides a more cost-effective alternative to the stub girder because of the latter’s high fabrication content.

 Stub girder
Fig. 2.22 Stub girder

0 comentarios:

Post a Comment