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Design Guide: Steel Bracing Systems - CalcTree
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Design Guide: Steel Bracing
Systems
Steel Bracing Systems
Selecting and designing a lateral force restraining system for a steel structure
has significant implications on its overall stability. As well as resisting vertical
loadings, all structures must resist lateral loading as a result of wind or
earthquakes which are simplified into a static lateral load for design.
Whether it be a tall tower resisting seismic loading, a bay of a stadium resisting
torsion from cantilever seating, or even a humble shed resisting the wind, all
structures need a lateral stability system.
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Examples of lateral load resisting systems
For most large buildings, the primary lateral resistance system is a "core",
typically made up of a series of RC shear walls enclosing elevators and stairs. In
some cases, the core alone is unable to resist the immense loads caused by
wind and earthquakes. One method to increase the lateral stiffness, which is the
ability to resist lateral deflection, of a building is to incorporate steel bracing
systems.
Deflected shape of braced frames
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Design Guide: Steel Bracing Systems - CalcTree
Deflected shape of unbraced frames
Tension members are preferred for bracing elements since compression
members can be subject to buckling (and therefore the steel section may need
to be bigger). Bracing elements are often placed at 45⁰ to achieve a beamcolumn juncture. Inclinations less than 45⁰ will increase the sway sensitivity of
the structure.
Braced (diagonal) members in tension and compression
In this article, we go through the common types of steel-braced frame systems
and provide a step-by-step guide on how to design them. Braced frames are
mainly suitable for low-rise to mid-rise buildings since they are not incredibly
effective in resisting torsional loads compared to shear walls.
The primary objectives when designing steel-braced frames are to:
Direct lateral load in horizontal beams and diaphragms to angled struts.
Thereby leveraging the axial strength of brace members and providing load
paths transferring the lateral load into vertical elements and down to
foundations.
Reduce deflection and sway caused by lateral loading by stiffening parts of
the structure. This could be done with a roof diaphragm, a belt and braces
outrigger or a vertical wall. Stiffening the structure also defines a clear load
path along where you as the designer want the load to go, e.g. stiffening a
roof plane with steel bracing and adding vertical brace bays, provides a clear
load path of lateral roof loads into the brace bays and down to the
foundations.
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of of
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four sides
a building. For local buckling and torsion of the
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Design Guide: Steel Bracing Systems - CalcTree
steel bracing, implement a direct lateral restraint for the compression flanges
or torsional restraint to the whole beam.
💡What are Diaphragms?
They are structural elements that can transfer lateral loads to the vertical
resisting elements of a structure (i.e. shear walls and columns) primarily
through in-plane shear stress. They are typically horizontal and are formed in
roofs and floors.
Applications of Steel Bracing
In all structures, lateral resistance is required in the X (horizontal) and Y (vertical)
primary planes. This is because lateral load must be taken by horizontal
structural elements (a diaphragm) to then be transferred to vertical structural
elements, so that the load can make it's way down to the foundations. Here, we
will discuss vertical and horizontal bracing systems.
Vertical Bracing Systems
Example of a cross-braced frame structure
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Design Guide: Steel Bracing Systems - CalcTree
Vertical bracing bays consist of angled steel strut and/or tie members that span
diagonally between orthogonal beams and columns. They are typically positioned
in vertical planes, between lines of columns or within partition walls. In
buildings, they provide load paths for lateral forces generated in floor and roof
diaphragms undergoing seismic and wind actions down to the foundations. They
work to transform horizontal loads to vertical ‘push-pull’ loads in primary
column groups through axial loads in the diagonal bracing elements.
Depending on the structure, brace bays will be used in conjunction with other
lateral stability elements, like cores, moment frames or shear walls.
Structurally, vertical bays are ideally placed at the extremities of the structure in
order to resist global torsion. Architecturally, vertical bays are often placed within
partition walls or otherwise so they can be hidden.
Load path for lateral loads in vertical braced bays, showing the vertical push (T = tension)
and pull (C = compression) forces induced by the bracing frame action.
Horizontal Bracing Systems
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Horizontal bracing system (SteelConstruction.info)
Horizontal steel bracings can be used to form flat or angled diaphragms, typically
in floor and roof systems. Here the bracing works to stiffen the floor or roof and
hence creates a diaphragm. This stiffness increase provides an effective path for
lateral loads to travel to vertically stiff elements.
In buildings, the structure that forms floors and roofs has an inherent lateral
resistance which is often stiff enough to establish a diaphragm. Nonetheless, in
situations where the floor or roof doesn't have required lateral stiffness, steel
bracing can be added. They are often referred to as an "exo-cross-brace" and are
diagonal tension/compression members positioned beneath the floor and roof.
For roofs, horizontal bracing is typically cross-ties (tension-only rod elements
arranged in an X pattern bay to bay). This is because lateral loads generated at
the roof plane are notably less pronounced compared to those imposed on the
structural floor plane. Restricting loading exclusively to tension is achieved by
using ties and flat plates, which are components that poss either negligible or no
compressive capacity.
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Load paths for horizontal bracing
Types of Braces
Concentric Braced Frames
Concentric bracing involves diagonal braces positioned within the frame's plane.
Both ends of the brace connect at the end point of other framing members,
culminating in the formation of a truss-like structure to build a stiff frame. While
concentric bracing is efficient in resisting lateral forces, it can be susceptible to
buckling under large loads.
Concentric Bracing
Eccentric Braced Frames
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Design Guide: Steel Bracing Systems - CalcTree
Like concentric bracing, eccentric bracing is arranged in a way that diagonal
bracing is positioned in the plane of the frame. The primary difference you may
have noticed is that they do not connect at the endpoints of another framing
member. The eccentricity of the brace connection helps prevent buckling under
large loads since their effective length is reduced. Eccentric bracing can undergo
large deformations without collapsing compared to concentric bracing.
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