Fin and Diaphragm Walls in Tall Singlestorey Buildings

The authors' experience on sports halls, gymnasia, stadia, assembly halls and structures of similar form has shown that fin and diaphragm walls are well suited to tall single-storey buildings enclosing large open areas. Such buildings account for a large number of the projects constructed in Britain, and throughout the rest of the world, and their importance is particularly relevant with the present trend in this country towards providing facilities for public recreation and leisure. The vast majority of these structures have a steel or reinforced concrete framework supporting the roof loads. The framework columns are then enveloped by a cladding material, backed up by an insulating barrier and protected on the inner face by a hard lining. Frequently, the cladding, insulation and lining require a subsidiary steel framework to provide support, and both the main frame columns, and sometimes the subsidiary frame also, require fire protection. The specification for painting the structural framework depends upon its degree of exposure and accessibility, and in unfavourable conditions the costs against this item can be unexpectedly high. The resulting 'wall' thus requires up to six different materials and several sub-contractors, suppliers and trades. The framework and cladding require frequent maintenance and do not provide the durability afforded by the use of masonry for the same purpose, neither do they possess the same aesthetic qualities which are natural in masonry construction and which can be greatly enhanced by imaginative detailing.

The fin or diaphragm wall forms the structure, cladding, insulation, lining, and fire barrier in one material, using one trade carried out by the main contractor. Maintenance is minimal, applied protective coatings are eliminated and durability is virtually ensured. They also have obvious applications to industrial structures where robustness to resist the hard wear of the associated operations is of prime importance. Vandal resistance is an added bonus to all projects employing fin and diaphragm wall construction.

Masonry, like all other structural materials, requires a full understanding of its strengths and weaknesses in order to employ it economically. Masonry's previously stated main weakness, low tensile strength, can be compensated for in design by providing a high Z/A ratio when bending stresses are involved. It is equally important to take full advantage of the gravitational forces involved, and the combination of these two aspects of masonry design led to the development of the diaphragm wall. An alternative solution to overcome masonry's poor tensile resistance is to provide precompression in the wall through post-tensioning rods spaced at designed centres and torqued to provide the axial loading which is usually missing from tall single-storey structures. This alternative is discussed later in this chapter and in Chapter 15.

In order to exploit both the highest Z/ A ratio and gravitational resistance, the geometric distribution of the materials should be similar - that is, to place the material at its largest practical lever arm position. In arriving at the most suitable geometric profile, due consideration must be given to the shear forces involved and to the buckling tendency of the material in the compression zone of the profile.

For practical considerations, the geometric arrangement of the wall must also relate to multiples of standard brick or block dimensions.

A diaphragm wall comprises two parallel leaves of brickwork or blockwork spaced apart and joined by perpendicular cross-ribs placed at regular intervals to form box or I sections (see Figure 13.1 and 13.2).

The two parallel leaves of the wall act as flanges in resisting the bending stresses and are stiffened by the ribs acting as webs mainly resisting shear forces. The length of the parallel leaves, which may be considered to act with the cross-ribs, is often limited by their tendency to buckle and, therefore, the section is best appraised as an I section. The length of the flange of the I section is established in a similar way to that of the T beam in reinforced concrete design, which should be familiar to many designers. The depth between the flanges is designed to meet the individual structural and other requirements of each project. Costs and space are usually minimised by designing the shallowest depths practicable.

The fin wall was developed from the diaphragm wall and its general form is shown in Figure 13.3. The masonry T section formed by the projecting fin and the bonded leaf of the cavity wall provides the main supporting member of the structure, while the other leaf of the cavity wall provides either the lining or the cladding depending on whether the fins are externally or internally exposed. The whole fin plus the cavity wall is used in determining the slenderness ratio of the section, and a calculated length of the wall is considered to act with the fin as the flange of the T profile in resisting the lateral loading. It is more common to expose the projecting fins externally, as this is usually the preference of the architectural designer and greater structural economy can be achieved. However, they can be exposed internally, and the design principles involved are similar, although careful consideration must always be given to the direction of the loading, and the section available at a particular level to resist it. Disproportionate collapse regulations capping beam outer leaf capping beam outer leaf

Diagram Diaphragm Wall

decking roof beam strip footing lintel door opening inner leaf

Figure 13.1 General arrangement of diaphragm wall profiles decking roof beam strip footing lintel door opening inner leaf

Figure 13.1 General arrangement of diaphragm wall profiles

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Figure 13.2 Diaphragm wall box and I section

normal wall ties in accordance with BS 5628

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(3) Large voids are available for distribution of services, etc.

(4) No cavity ties in bonded walls.

(5) Symmetrical section for simplicity of analysis.

(6) Fewer vertical plumbing lines reduces labour costs.

(7) Smaller site area is required - beneficial on restricted sites.

(8) Slight cost saving. Fin Wall

(1) Less roof area is required (see Figure 13.4).

(2) Less foundation area is required (see Figure 13.5).

Figure 13.3 Fin wall arrangement need to be considered in choosing the particular form to use (see Chapter 8).

13.1 Comparison of Fin and Diaphragm Walls

Having concluded that, for a particular tall single-storey project, masonry is the most suitable structural material, the next decision to be made is what form: fin, diaphragm or any other, to use for the structure. Regarding fin and diaphragm walls, each has some advantages over the other and a summary of the basic considerations is given below, from which the form most suited to the function or aesthetics of the particular project can be assessed.

Diaphragm Wall

(1) Smooth, finished face both internally and externally.

(2) Better structural use of materials.

diaphragm wall diaphragm wall

fin wall

Figure 13.4 Comparison of roofing areas fin wall

Figure 13.4 Comparison of roofing areas site boundary site boundary

Figure 13.5 Comparison of foundation areas

(3) Has greater visual impact - more scope for architectural effect.

(4) Marginally easier to post-tension when required.

(5) Less cutting of bricks for bonding can usually be achieved.

Both the fin and the diaphragm walls become more economical, in comparison with other structural forms such as steel or reinforced concrete frameworks, as the height of the wall increases, and they are of little advantage on lower heights where normal cavity brickwork can often satisfy all the structural requirements. For further discussion on the application of fin and diaphragm walls, see section 13.11 and Chapters 10, 11, 12 and 14.

13.2 Design and Construction Details

Thorough consideration of the structural behaviour of the roof of the building is imperative for the maximum economy to be achieved in the overall building costs. The wall may be designed as a cantilever and the structure covered with the simplest possible roof construction. However, it has generally been found that, to obtain the greatest economy, the roof should be detailed and constructed in such a way that it can act as a horizontal plate to prop and tie the tops of the walls and to transfer the resulting horizontal reactions to the transverse walls of the building, where these reactions can then be transferred to the building foundations through the racking resistance of these shear walls (see Figure 13.26). To satisfy this design analysis, the details must provide adequately for fixing the tops of the walls to the roof plate, the roof plate must be capable of spanning between the shear walls, and the forces must be transferred from the roof plate into the shear walls.

A capping beam can be used on top of the wall to transfer the prop and tie forces into the roof plate. This has the potential advantage of being able to resist uplift forces from a lightweight roof and also of transferring the roof plate horizontal bracing to girders horizontal bracing to girders

used as a girder beam

Figure 13.6 Roof girder to transmit wind forces to shear walls used as a girder beam

Figure 13.6 Roof girder to transmit wind forces to shear walls forces into the shear walls if the capping beam is continued all round the building. If, due to large roof openings or unsuitable decking, the plate action of the roof cannot be relied upon, a wind girder may be provided (see Figure 13.6), in which case the capping beams can often be used as booms for this girder.

The roof decking can be constructed from a variety of materials and supported in many ways. Generally, steel universal beams, castellated beams or lattice girders have been found to be the most economical means of support, spaced at centres to suit the selected decking. They do not necessarily need to relate to the centres of ribs or fins. However, in fin wall construction, the geometry of the building invariably leads to the roof supporting members lining up with the projecting fins. For long roof spans, a space deck can prove to be more economical, and the aesthetic value of this system combined with its economy, when applicable, makes it a popular proposition. Alternatively, timber laminated beams with solid timber decking may be used with considerable visual effect, although their economy would need to be balanced against the attractiveness of the finished product. The simplest solution in timber is, perhaps, provided by trusses with a suitably designed bracing system.

A capping beam is generally required at the top of diaphragm walls. However, for both fin and diaphragm walls where no capping beam is to be used, the main roof beam often requires strapping down to resist wind uplift forces. This can be quite easily done using rods cast into the padstone and taken down into the brickwork to a suitable level to ensure sufficient dead load, with an adequate factor of safety, to resist the uplift forces (see Figure 13.7).

When assessing the overall costs of the roof decking, it is necessary to take account of the value of its ability to act as a roof plate to resist the prop and tie forces discussed earlier. If an apparently less expensive roof decking is selected, any additional costs for strapping, bracing, etc., which would not necessarily have been required for apparently more expensive decking, must be included to arrive at the overall cost.

roof beam padstone anchor rod fixing through padstone and into roof beam

roof beam padstone anchor rod fixing through padstone and into roof beam anchor rod with endplate and nut section through fin Figure 13.7 Anchoring detail for main roof beams fins fins

opening

Figure 13.8 Typical simple building plan in fin wall construction opening

Figure 13.8 Typical simple building plan in fin wall construction

13.3 Architectural Design and Detailing

It is generally considered that the fin wall provides greater scope for architectural expression than the diaphragm wall. A typical simple plan layout for a fin wall building is shown in Figure 13.8 and an almost unlimited number of variations can be applied to this basic profile.

The sizes and spacing of the fins can vary, and the corner fins can be eliminated altogether as shown in Figure 13.9.

The fins themselves can be profiled on elevation, some examples of which are indicated in Figure 13.10.

The treatment at eaves level (see Figure 13.11) and the variety and mixture of the facing bricks and fin types can present unlimited and interesting visual effects.

A word of caution, however. When a mixture of bricks is to be introduced it is essential to ensure that the various bricks and/or blocks are compatible, particularly with regard to thermal and moisture movements. The structural design calculations must also take account of the differing design strengths of the masonry under these circumstances. The diaphragm wall also has possibilities for architectural expression and some examples of its treatment at roof level are shown in Figure 13.12.

It is not essential that diaphragm walls should be designed with flat faces on elevation and, particularly on tall buildings, a fluted arrangement as shown in Figure 13.13 can break up a large expanse of brickwork.

spacing can vary proportions can vary shape can vary (hollow fin) ——

■ rain water pipe corner fins omitted if required

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Responses

  • Orgulas
    What is a brick fin wall?
    5 months ago
  • Melvin Arteaga
    Why lining under the beam of maked inengoneering buildings?
    5 months ago
  • tewolde
    How are fin walls constructed?
    2 months ago

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