Bracings - Multi-Storey Buildings.

Three structural systems are used to resist lateral loads: continuous or wind-moment frames, reinforced concrete walls and braced-bay frames (Fig. 2.29). Combinations of these systems may also be used.

Bracing structures: (a) continuous frame; (b) reinforced concrete wall; (c) braced bay frames
Fig. 2.29 Bracing structures: (a) continuous frame; (b) reinforced concrete wall; (c) braced
bay frames

Continuous construction
Continuous frames are those with rigid moment-resisting connections between beam and columns. It is not necessary that all connections in a building are detailed in this way: only sufficient frames to satisfy the performance requirements of the building.

The advantage of a continuous frame is:

• Provides total internal adaptability with no bracings between columns or walls to obstruct circulation.

However, the disadvantages are:

• Increased fabrication for complex framing connections
• Increased site connection work, particularly if connections are welded
• Columns are larger to resist bending moments
• Generally, less stiff than other bracing systems.

Wind-moment frames are limited in application.

Shear walls
Reinforced concrete walls constructed to enclose lift, stair and service cores generally possess sufficient strength and stiffness to resist the lateral loading.

Cores should be located to avoid eccentricity between the line of action of the lateral load and the centre of stiffness of the core arrangement. However, the core locations are not always ideal because they may be irregularly  shaped, located at one end of the building or are too small. In these circumstances, additional braced bays or continuous frames should be provided at other locations (Fig. 2.30).

Core locations: (a) efficient; (b) inefficient
Fig. 2.30 Core locations: (a) efficient; (b) inefficient

Although shear walls have traditionally been constructed in in  situ reinforced concrete they may also be constructed of either precast concrete or brickwork.

The advantages of shear walls are:

• The beam-to-column connections throughout the frame are simple, easily fabricated and rapidly erected.
• Shear walls tend to be thinner than other bracing systems and hence save space in congested areas such as service and lift cores.
• They are very rigid and highly effective.
• They act as fire compartment walls.

The disadvantages are:

• The construction of walls, particularly in low- and medium-rise buildings, is slow and less accurate than steelwork.
• The walls are difficult to modify if alterations to the building are required in the future.
• They are a separate form of construction, which is likely to delay the contract programme.
• It is difficult to provide connections between steel and concrete to transfer the large forces generated.

Recent developments in steel–concrete–steel composite sandwich  construction  (Bi-steel®) largely eliminate these disadvantages and allow pre-fabricated and fully assembled lift shafts to be erected simultaneously with the main steel framing.

Steel–concrete–steel construction can also be used for blast-resistant walls and floors.

Braced-bay frames
Braced-bays are positioned in similar locations to reinforced concrete walls, so they have minimal impact upon the planning of the building.They act as vertical trusses which resist the wind loads by cantilever action.

The bracing members can be arranged in a variety of forms designed to carry solely tension or alternatively tension and compression. When designed to take tension only, the bracing is made up of crossed diagonals. Depending on the wind direction, one diagonal will take all the tension while the other remains inactive.

Tensile bracing is smaller than the equivalent strut and is usually made up of flatplate, channel or angle sections.When designed to resist compression, the bracings become struts and the most common arrangement is the ‘K’ brace.

The advantages of braced-bay frames are:

• All beam-to-column connections are simple
• The braced bays are concentrated in location on plan
• The bracing configurations may be adjusted to suit planning requirements (eccentric bracing)
• The system is adjustable if building modifications are required in the future
• Bracing can be arranged to accommodate doors and openings for services
• Bracing members can be concealed in partition walls
• They provide an efficient bracing system.

A disadvantage is:

• Diagonal members with fire proofing can take up considerable space.

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