Vertical bracing

The stability of the building is dependent on the form and location of the vertical bracing, or other shear-resisting elements which are linked by floors or horizontal bracing.

For simplicity, vertical bracing is located in the façade or internal separating walls. Ideally, the bracing line would be on the centre-line of the main columns, but this may conflict with the location of the inner skin of external walls. Discussion between the architect and the structural engineer at an early stage can resolve this difficulty. Often, flat steel bracing elements are located in the cavity of the masonry wall to minimise these dimensional problems.

The most common arrangements of bracing in multi-storey construction is 'X', V or 'K' bracing using steel angle or circular hollow sections (see Figure 3.18). Inverted V bracing is preferred where substantial openings, e.g. doors, are required in the braced bay. To reduce its visual impact, bracing is often positioned around

3.15 Different overall forms of the frame by varying type and location of pinned connections

3.16 Portal-frame effect created using inclined pinned members

3.17 Examples of rigid and braced frames Partially braced frame vertical cores, which usually house the lifts, stairs, vertical service ducts and/or toilets, or on the external face of the building within the cavity wall.

Figure 3.18 also illustrates the forces in the individual members. In the X-braced form, the members may be designed to resist both tension and compression, or tension only, which leads to more slender members. Tension rods or flat plates are largely ineffective in compression, and, therefore, forces are resisted only in tension when using these elements. In the K- and V-braced forms, the members must be designed to resist tension and compression, depending on the direction of the forces on the building. Tension ties are not possible in this case.

Tension tie members are generally used in exposed steelwork because of their apparent 'lightness'. In X-braced frames, special

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X-bracing V-bracing K-bracing

X-bracing V-bracing K-bracing

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3.18 Different forms of bracing and their forces brackets may be included to allow connection of the four tie members at the cross-over points. An example of an X-braced structure using CHS sections with a connecting plate is illustrated in Figure 3.19.

A 'hybrid' between a rigid frame and a braced frame can be achieved by the use of 'knee' bracing. In this case, the corner junction between a beam and column is stiffened by a short bracing member, which is designed to resist either tension or compression (see Figure 3.17). The bracing member transmits a force to the beam or

3.19 X-bracing using CHS sections used at a sports centre in Hampshire (architect: Hampshire County Council)

columns, which is resisted by bending in these members. If necessary, knee bracing can be expressed as an architectural feature by curving the members or by using cast inset pieces.

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