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Castellated and Cellular Beams

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CONTENTS


COMPILED BY:
C. S. Harper, British Steel Sections:"Plates & Commercial Steels":
Structural Advisory Service.

Introduction

The commercial property market demands a maximum return on money invested. This is achieved by providing the type and quality of building required by prospective tenants, on time and on budget, whilst allowing maximum flexibility for changes to the building to accommodate future advances in technology.

The steel frame is ideally suited to satisfy the most stringent commercial, architectural and engineering demands for quality, speed, economy and flexibility.

This guide is intended to give the steel designer general guidance on the selection of a long span floor system based on the use of cellular or castellated beams without the need for detailed calculations at the scheme design stage. It does not however alleviate the need for a full structural analysis to be carried out by suitably qualified personnel. Further guidance on the solutions suggested in this guide is available from The Steel Construction Institute and from Westok Structural Services on cellular beams in particular. These are indicated in the references.

Structural steelwork is particularly suited to medium and long span flooring systems because of its high stiffness and excellent strength to weight ratio.

A review of the common systems used to achieve medium to long spans in steel is given before looking in detail at cellular and castellated beams.

Slimflor systems are described in British Steel publications "Design in Steel 2" and "3".


Beams and Girders

Universal Beams

These can be used non-compositely or compositely with the concrete floor slab and have a useful span range between 6 to 12m depending on loading requirements. The possibility of cutting holes in the webs of beams; to allow for the passage of services, is an option.

Dual Plane Grillage

This has been detailed in an earlier British Steel publication "Design in Steel 1 The Parallel Beam Approach". The system comprises two levels of beams in two orthogonal directions, allowing simple service integration and simplified connections.

Haunched Beams

A reduced beam size is achieved by taking into account the moment capacity of the connection. It is particularly suited to rigid frame designs.

Lattice Girders

Several types are available giving different forms. A typical arrangement would be top and bottom chords from universal beam or column sections separated by light vertical or diagonal bracing. Services may be located between the bracing or beneath the structure.

Stub Girders

These offer a potential economic solution for spans ranging from 12-20m where the column grids are regular. Services can be located within the structural zone offering savings in building height for wide open areas.

Tapered Girders

These are fabricated from plate and have a practical span range of 10-20m. They allow distinct service zones to be located in bands at the edges and at column locations within the building.

Castellated and Cellular Beams

The aim of this guide is to provide information on the use and design of both castellated and cellular beams. However, it is advantageous to look at the difference between the two. The obvious difference is the opening shape and this is where the major advancement in technology has been made. In the former pseudo-hexagonal holes are formed by the fabrication of universal beam or universal column sections. The openings are limited in size and cannot accommodate large services without some modification. The cellular beam approach seeks to resolve some of the problems associated with castellated beams by forming circular penetrations at regular centres.

When a castellated beam is fabricated (Fig. 1), the cut is made on a half hexagonal line down the centre of the beam, the halves are moved across by one spacing, then welded together to form the completed beam.


Figure 1

Cellular beams are fabricated by cutting the beam using a double pass process (patented by Westok) and again separating, moving and welding the two halves together (Fig. 2). Fig. 3 shows the completed beams.


Figure 3

The cellular process has several advantages over the castellated process, namely:

  • For the same beam section several different diameters of cell are possible without change in the fabrication process and therefore at no extra cost.

  • The spacing of the cells can be changed between limits (1.08d 1.5d), giving great flexibility in the positioning of the apertures. The beam can therefore be designed so that the secondary beam connections coincide with the full web section of the cellular beams (Fig. 4). The result of this is that fewer holes will require infilling to accommodate secondary beams.

  • Because of the variable diameter and spacing of cells the cellular beam can have a range of depths. In effect a customised beam can be designed at no extra cost.

  • Previously it was necessary to infill openings at supports and other areas of high shear with blanks to achieve the required capacity. The cellular process however allows the use of ring stiffeners in those areas, therefore keeping more zones free for services. In addition fewer infill plates will be required (See Fig. 5).

Possible Layouts

Castellated and cellular beams may be used in a variety of layouts. Two of the more common ones are shown in Figs 6 & 7. Typically for Layout 'A' (Fig. 6) a composite main floor beam has a practical span range of 10- 20m with secondaries at 3m c/c. For a 16m x 8m grid with an imposed loading of 5KN/m', one would expect castellated main beams in the 914 x 305 range (OA depth 1371mm) and secondaries in the 457 x 192 range.

The cellular beams would be of a similar size with an overall depth of around 1200mm. For the alternative span arrangement (Layout 'B': Fig. 7) beams in the 686 x 254 range would be expected for both main and secondary beams with overall depths of around 900mm. When considering roof layouts, spans of up to 35m are possible with the spacing of beams depending on the type of roof membrane or cladding but typically in the range of 5m 6m (Fig. 8).

Advantages of Castellated and Cellular Beams

  • The major advantage is the accommodation of services within the structural zone (Fig. 9 & 10). The effect is that the overall building height can be reduced compared to the simple beam solution where the services are supported beneath the beams. Typical savings in these terms could be up to 500mm per floor.

  • With recent changes in technology, long span floor beams can now be used compositely with the structural concrete floor, giving considerable savings in beam depth. Cellular beams in particular may be designed asymmetrically with the top tee of the beam being of smaller section size than the bottom tee. (Fig. 11). The bottom tee in this case acts in tension with the concrete slab providing the compressive resistance through normal shear connector arrangements. The depth of the beams also produces savings in the number of shear connectors required to provide composite action, due to the increased lever arm.

  • The implied need to cut the beam into two brings inherent advantages in that it allows precambering to be carried out at no extra cost. This can be allowed for in the design, so that the effects of dead load deflection can be negated. Circular sections may be produced with dramatic effect (Fig.12). This guide gives a brief outline of where castellated and cellular beams can be used to advantage in commercial office type buildings of today. They have also enhanced the visual appearance of various types of buildings by using the beams in non conventional locations, for example as columns (Fig.13).

    Design Process

    The detailed design of castellated beams has in the past been complicated and time consuming, particularly for heavy loads or floor beams. However since the advent of computer systems the process has been simplified.

    A Steel Construction Institute publication (SCI), Ref. 8, gives detailed guidance on the design of cellular beams. A computer programme "CELLBEAM" also by the SCI is available to analyse cellular beams. This is available from Westok Structural Services, free of charge.

    To aid the process of design, preliminary design tables have been produced and are inserted in the back cover of this publication. They are for noncomposite simply supported beams and should be used as a general guide to the required section size. The designer would then use the beam as an initial size to put into the computer programme, or carry out manual detailed design.


    Fire Protection

    The requirements of BS 5950 Pt 8 should be followed in determining fire protection requirements of castellated and cellular beams. This recommends that the thickness of fire protection material should be 1.2 times the thickness required for the original section based on the Hp/A value of this section. Several materials are possible; such as boards sprays and intumescents, sprays being the most common because of the ease of application and relatively low cost. (Fig. 14).





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