In this article, you will learn about bracing systems and the types of bracing systems used in steel structures.
The selection of the right lateral force-resisting system has a significant effect on the performance of a steel structure. The longitudinal structural rigidity of steel frames is relatively weak in the length direction, so it is necessary to set up bracings along the longitudinal direction to ensure longitudinal rigidity and stability.
What is bracing?
Bracing systems are an essential element, generally subjected to lateral loads caused mainly by earthquake or wind.
Beams and columns are designed to carry the vertical loads acting on a structure, whereas bracings are used to handle the lateral loads. Bracings help to reduce the lateral deflection of the structure. The most common reason for providing bracing is to control the buckling of the main beams. A small tonnage of steel bracing adds significant resistance against the bending of the beam.
What does brace mean?
Bracing is one of the most essential and crucial parts of a structure — it helps to improve the stability of the building and resist the effects of earthquakes.
A braced frame is a structural system designed to withstand high wind and the forces of earthquakes. The members in a braced frame are not allowed to sway laterally. The purpose of providing bracings is to ensure additional safety against external loads. Braced buildings are reinforced by steel members that increase the tensile and compressive strength of the building.
Bracings are primarily used in steel structures to resist the lateral force acting on the structure due to wind, earthquake, and similar causes. Bracing is a construction technique used to improve the structural performance of a building. Bracing systems include wood or steel components that help evenly distribute loads and increase the safety of the structure.
Types of bracing
There are two main types of bracing systems used in the construction of buildings:
- Horizontal bracings
- Vertical bracings
Horizontal bracing system
The horizontal bracing system consists of bracings provided at each floor in the horizontal planes. It provides the load paths so that horizontal forces can be transferred to the planes of the vertical bracings.
Horizontal bracing systems are designed to transfer horizontal loads from columns at the perimeter of the structure to the planes of vertical bracing. The horizontal forces on perimeter columns are developed due to wind pressure on the structure. There are two major types of horizontal bracing systems generally used in the multistorey braced steel structure: diaphragms and discrete triangulated bracing.
With diaphragms, there are different types of floor systems that provide a sound horizontal diaphragm, such as composite floors, whereas others — such as precast concrete slabs — need specific measures to serve their purpose. For example, the steelwork and the precast concrete slab should be joined together properly to avoid relative movement.
Vertical bracings
In the vertical planes, the bracings lie between the column lines, providing the load paths used to transfer the horizontal forces to ground level. This system transfers the horizontal loads to the foundations and withstands the overall sway of the building.
These are the bracings placed between two lines of columns. There are two subtypes of vertical bracings:
Cross bracings
Cross bracings are slender members that withstand tension forces only, not compression forces, providing the necessary lateral stability mainly depending on the direction of the loading.
Single diagonal bracings
Vertical bracings are designed to resist the wind forces and the equivalent horizontal forces acting on the structure. They provide the load paths to transfer the horizontal forces to ground level, which provides lateral stability.
Single diagonal bracings are designed to resist both tension forces and compression forces. Here, a diagonal member is inserted into the rectangular areas of the structural frame, which is good for stabilisation of the frame. The bracing systems are placed at close to 45 degrees for an efficient system, and this arrangement is strong and compact.
A minimum of two vertical planes in each of the orthogonal directions is provided to prevent disproportionate collapse. The installed vertical planes of bracing help to improve structural stability.
Advantages of bracing systems
There are various benefits for which bracing systems are used in structures:
- Bracing systems are used to resist the buckling of the main beam in the structure.
- They help to equally distribute the lateral loads as well as the vertical loads between the main beams.
- They are cost-effective and flexible to design while delivering the required strength and stiffness.
- Braced frames have good resistance to high-speed winds and seismic forces compared to non-braced buildings.
- Reduction in lateral displacement is one of the major advantages of using a bracing system.
Disadvantages of bracing systems
There are also some disadvantages of bracing systems:
- The span length of the structure is restricted to around 40 feet when reinforced.
- The construction of bracing systems requires skilled workmanship.
Bracings in truss
Bracings are installed to hold trusses in a stable position and prevent the structure from buckling under the pressure of high winds and heavy rainfall.
Structures less than 12 feet in width do not require truss bracings. Truss braces should be installed inside the truss sections.
Designing bracing connections to code
Once a bracing layout is set, every brace-to-gusset and gusset-to-frame connection still has to be designed and code-checked for the tension and compression forces it carries.
The tool for this
Code-check any steel connection to AISC & Eurocode.
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FAQs
What is the main purpose of bracing systems?
Bracing systems are used in steel structures to resist lateral loads caused by wind and earthquakes, helping to control the buckling of main beams.
What are the two main types of bracing systems?
The two main types of bracing systems used in construction are horizontal bracings and vertical bracings.
How does horizontal bracing work?
Horizontal bracing provides load paths at each floor to transfer horizontal forces from perimeter columns to the planes of vertical bracing.
What is the function of vertical bracing?
Vertical bracing transfers horizontal forces from the structure to ground level and withstands the overall sway of the building.
What is the difference between cross bracing and single diagonal bracing?
Cross bracings primarily withstand tension forces, while single diagonal bracings are designed to resist both tension and compression forces.
Why is bracing important for steel structures?
Bracing improves stability, distributes loads evenly, and offers resistance to high-speed winds and seismic forces compared to non-braced buildings.
Are there disadvantages to using bracing systems?
Yes, bracing systems can restrict the span length of structures to around 40 feet and require skilled workmanship for construction.
