A presentation on “Braced Connections”

A presentation on “Braced Connections”

Benefits of Structural Steel

  • Some benefits associated with use of structural steel for owners are:
  • Steel allows for reduced frame construction time and the ability to construct in all seasons
  • Steel makes large spans and bay sizes possible, providing more flexibility for owners
  • Steel is easier to modify and reinforce if architectural changes are made to a facility over its life
  • Steel is lightweight and can reduce foundation costs
  • Steel is durable, long-lasting and recyclable

Unique Aspects of Steel Construction:

Procurement and management of structural steel is similar to other materials, but there are some unique aspects to steel construction:

  • Steel is fabricated off-site
  • On-site erection is a rapid process
  • This gives use of structural steel some scheduling advantages
  • Coordination of all parties is essential for achieving potential advantages

Forces on Structures (Fig. 1):

  • Forces from gravity, wind, and seismic events are imposed on all structures
  • Forces that act vertically are gravity loads
  • Forces that act horizontally, such as stability, wind and seismic events (the focus of this discussion) require lateral load resisting systems to be built into structures
  • As lateral loads are applied to a structure, horizontal diaphragms (floors and roofs) transfer the load to the lateral load resisting system
Forces acting on structures
Fig.1: Forces acting on structures
  • The type of lateral load resisting system to be used in a structure should be considered early in the planning stage
  • Lateral stability as well as architectural needs must be met

The three common lateral load resisting systems are:

  • Braced Frames (Fig. 2)
  • Rigid Frames (Fig. 2)
  • Shear Walls
Braced and Rigid Frames
Fig. 2: Braced and Rigid Frames

Steel Frame Connection Types:

  • Simple Connections
  • Moment Connections: Fully-Restrained and Partially-Restrained
  • All connections have a certain amount of rigidity
  • Simple connections (A above) have some rigidity, but are assumed to be free to rotate
  • Partially-Restrained moment connections (B and C above) are designed to be semi-rigid
  • Fully-Restrained moment connections (D and E above) are designed to be fully rigid
Steel frame connection types
Fig. 3: Steel frame connection types

Simple Connections:

  • Designed as flexible connections
  • Connections are assumed to be free to rotate
  • Vertical shear forces are the primary forces transferred by the connection
  • Require a separate bracing system for lateral stability
  • The following few slides show some common simple framing connections

Moment Connections:

  • Designed as rigid connections which allow little or no rotation, Used in rigid frames.
  • Moment and vertical shear forces are transferred through the connection.
  • Two types of moment connections are permitted: Fully-Restrained and Partially-Restrained (Fig. 4).

 

Moment Connections
Fig. 4: Moment Connections

 

Rigid Frames:

  • Rigid frames, utilizing moment connections, are well suited for specific types of buildings where diagonal bracing is not feasible or does not fit the architectural design
  • Rigid frames generally cost more than braced frames

Braced Frames:

  • Diagonal bracing creates stable triangular configurations within the steel building frame
  • “Braced frames are often the most economical method of resisting wind loads in multi-story buildings.”
  • Some structures, like the one pictured above, are designed with a combination braced and rigid frame to take advantage of the benefits of both

Temporary Bracing:

  • Structural steel frames require temporary bracing during construction
  • Temporary bracing is placed before plumbing up the structural frame
  • This gives the structure temporary lateral stability
  • Temporary bracing is removed by the erector
  • In a braced frame, temporary bracing is removed after final bolt-up is complete and the permanent bracing system is in place
  • In a rigid frame, temporary bracing is removed after final bolt-up is complete

Concentric Braced Frames:

Bracing is concentric when the centre lines of the bracing members intersect Common concentric braced frames used in buildings today include:

  • X brace
  • Two story X’s
  • Chevron
  • Single diagonals

X bracing is possibly the most common type of bracing. Bracing can allow a building to have access through the brace line depending on configuration

X Bracing:

  • The diagonal members of X bracing go into tension and compression similar to a truss.
  • The multi-floor building frame elevation shown above has just one braced bay, but it may be necessary to brace many bays along a column line
  • With this in mind it is important to determine the locations of the braced bays in a structure early in a project
  • Connections for X bracing are located at beam to column joints
  • Bracing connections may require relatively large gusset plates at the beam to column joint
  • The restriction of space in these areas may have an impact on the mechanical and plumbing systems as well as some architectural features
Different types of Bracing
Fig. 5: Different types of Bracing

 

Chevron Bracing:

  • The members used in Chevron bracing are designed for both tension and compression forces
  • Chevron bracing allows for doorways or corridors through the bracing lines in a structure
  • Chevron bracing members use two types of connections
  • The floor level connection may use a gusset plate much like the connection on X braced frames
  • The bracing members are connected to the beam/girder at the top and converge to a common point
  • If gusset plates are used, it is important to consider their size when laying-out mechanical and plumbing systems that pass through braced bays

Eccentrically Braced Frames:

  • Eccentric bracing is commonly used in seismic regions and allows for doorways and corridors in the braced bays
  • The difference between Chevron bracing and eccentric bracing is the space between the bracing members at the top gusset connection
  • In an eccentrically braced frame bracing members connect to separate points on the beam/girder
  • The beam/girder segment or “link” between the bracing members absorbs energy from seismic activity through plastic deformation
  • Eccentrically braced frames look similar to frames with Chevron bracing
  • A similar V shaped bracing configuration is used

Combination Frames:

 

Figure showing Combination Frames
Fig. 6: Figure showing Combination Frames

 

  • As shown Fig. 6 (left) a braced frame deflects like a cantilever beam while a moment resisting frame deflects more or less consistently from top to bottom
  • By combining the two systems, reduced deflections can be realized
  • The combination frame is shown above right

 

 

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