Vertical bracing systems
A variety of structural forms can be used to provide lateral stability. The principal systems are shear walls, lattice frames and rigid frames, but more sophisticated systems may be needed for very tall buildings.
Shear walls resist wind forces in bending by cantilever action and where they already exist, for instance to provide a fire protected service core, are an efficient method of carrying lateral loads. Lattice frames act as vertical steel trusses. Rigid jointed frames are less effective in providing lateral rigidity because of shear distortion in the vertical members. The British Standard BS 5950 sets a limit on lateral deflection of columns as height/300 but height/600 is a more reasonable figure for buildings where the external envelope consists of sensitive or brittle materials such as stone facings.
Rigid frames resist lateral loads by bending in the beams, columns and connections.
Rigid frames resist lateral forces through the stiffness provided by rigid joints between the horizontal floor components and vertical columns. The need to resist bending moments from wind loads increases the size of the column members and the complexity of the framing connections. For these reasons, rigid frames are only used when there is a particular functional reason for their use, such as the need to provide unobstructed interior space with total adaptability.
One possible exception to this general rule, is the facade frame with a combination of closely spaced external columns and deep spandrel facade beams. Such a system is usually used for very tall buildings where the facade frame forms a rigid tube.
Rigid frames resist lateral forces through the stiffness provided by rigid joints
Rigid frames avoid any intrusion but are relatively expensive.
The advantages of the rigid frame are that:
- open bays between all columns are created,
- total internal adaptability is provided,
However, the disadvantages are that:
- They are almost always more expensive than other systems,
- Columns are larger than for simple connections.
- Generally, they are less stiff than other bracing systems with large complex connections.
For more information on shear walls please click here
Lattice frames act as vertical trusses and a number of different forms are commonly used.
Lattice frames act as vertical trusses which support 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 only tension, the bracing is made up of crossed diagonals. Depending on the wind direction, one diagonal will take all the tension while the other is assumed to remain inactive. Tensile bracing is smaller in cross-section than the equivalent strut and is usually made up of a back-to-back channel or angle sections. When designed to resist compression, the bracings become struts and the most common arrangement is the `V' brace.
Typical cross bracing and `K' bracing
Lattice frames are efficient and unobtrusive, if detailed carefully.
The advantages of lattice frames are that:
- lattice panels can be arranged to accommodate doors and openings for services,
- bracing members can be concealed in partition walls,
- they provide an efficient bracing system.
The disadvantages are that:
- diagonal members with fire proofing can take up considerable space,
- adaptability is limited.
However, with careful design these difficulties can be avoided
Reinforced concrete shear walls and cores act in a similar way to lattice frame bracing.
Shear walls are normally constructed in in-situ reinforced concrete, but may be either pre-cast concrete of brickwork. They are more rigid than other forms of bracing, and there is a need for fewer of them. Shear cores are shear walls in box form which provide torsional or twisting resistance as well as providing a highly effective bracing system.
Shear walls are effective but may create difficulties during construction.
The advantages of shear walling are that:
- concrete 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 that:
- they constitute a separate form of construction which may delay the contract programme,
- it is difficult to provide connections between steel and concrete to transfer the large forces generated.
For more information on advantages/disadvantages of shear walling please click here
The floor structure transfers lateral loads from the façade to the bracing system.
All stability systems use the floor plate as a diaphragm to transfer lateral loads from their point of application to the bracing elements. The designer should ensure that the floor is capable of performing this function.
Diaphragm action of floors
Bracing must ensure lateral stability in all directions and also torsional stability.
The bracing must be arranged on plan to ensure lateral stability in at least two directions, which should be approximately perpendicular - typically these correspond to the principal axes of the building. This will effectively ensure stability in all directions.
Torsional stability should also be ensured. This can be done by using an approximately symmetric plan arrangement, ideally with the bracing elements located close to the perimeter of the building.
Suitable location of bracing elements is therefore a fundamental requirement of the bracing system design.
Different locations of bracing elements