There are different types of bracing systems commonly used in multi-storey steel structures between orthogonally arranged beams and columns to transfer horizontal forces imposed on the structure.
Different bracing systems used in the construction of multistory steel structures are discussed in the following sections.
Fig.1: Types of Bracing Systems in Multistory Steel Structures
Types of Bracing Systems Used in Multi-Storey Steel Structures
There are two major bracing systems:
- Vertical bracing system
- Horizontal bracing system
Vertical Bracing System for Multi-Storey Steel Structures
Vertical bracing as shown in Figure-2 are diagonal bracings installed between two lines of columns. Not only does it transfer horizontal loads to the foundations (create load path for horizontal forces) but also it withstands overall sway of the structure.
Fig.2: Vertical Diagonal Bracing Provided Between Two Lines of Columns
Configurations of vertical bracings include cross diagonals (cross bracing) and single diagonal. In the former case, bracings are slender and withstand tension forces only, so they will not resist compression forces.
Therefore, tensile diagonals provide necessary lateral stability in addition to the floor beams that act as a part of bracing system. Figure-2 shows the placement of cross bracings between two lines of columns.
Fig.3: Single Bracing Provides Resistance Against Compression and Tension
As far as the single diagonal bracing is concerned, it is designed to resist both tension forces and compression forces. The arrangement of diagonal bracing is illustrated in Figure-3.
Bracing elements are commonly placed at nearly 45o because it not only offers an efficient system compare with other systems but also strong and compact connections between bracing member and beam-column juncture will be achieved.
It is worth mentioning that, if the bracing member inclination is smaller than 45o (angle from vertical), then the sway sensitivity of the structure would be increased whereas wider bracing member arrangement provide greater structural stability.
What Are the Forces that Vertical Bracing Should Be Designed to Resist?
Vertical bracing systems are required to be designed to resist wind forces, equivalent horizontal forces that represent the influence of initial imperfections and second order effects caused by frame sway in the case of the flexible frame.
How Many Vertical Bracing Planes Should be Installed?
It is recommended to provide at least three vertical bracing planes to provide adequate resistance in both directions in plan and against torsion forces around vertical axis of the structure.
If higher number of vertical planes of bracing is provided, it would enhance structure stability. Practically, the number of vertical plans of bracing is installed for multistory steel structures.
It is recommended to employ minimum two vertical planes of bracing in each orthogonal direction to avoid disproportionate collapse. Important part of the structure should be braced using more than one plane of vertical planes to prevent progressive collapse.
Locations of Vertical Bracing System in Multi-Story Structure
The location of vertical planes of bracing should be determined carefully. It is advised to place the vertical bracing planes at furthest point of the structure to withstand torsion forces that may occur due to horizontal forces.
Horizontal Bracing System for Multi-Storey Steel Structures
Horizontal bracing systems purpose is the transfer of horizontal loads from columns at the perimeter of the structure to the planes of vertical bracing.
The horizontal forces on perimeter columns are generated because of wind force pressure on the cladding of the structure.
There are two major types of horizontal bracing systems which are used in the multistory braced steel structure namely: diaphragms and discrete triangulated bracing.
Regarding diaphragms, there are various types of floor systems that some of them provide perfect horizontal diaphragm such as composite floors whereas others such as precast concrete slabs need specific measures to satisfactory serve their purpose.
For example, steel work and precast concrete slab should be joint together properly to avoid relative movements.
As far as discrete triangulated bracing is concerned, this type of bracing is considered when floor system cannot be used as a horizontal bracing system.
It is a horizontal system of triangulated steel bracing placed in each orthogonal direction. The horizontal bracing are placed between supports which commonly are locations of vertical bracings
Regarding bracing at roof level, wind girder is used to resist horizontal forces at the top of the columns.
Fig.4: Horizontal Bracing Placement