10 bracing connections

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MIT 1.582 DESIGN OF STEEL STRUCTURES

Topic 9: Bracing Connections

MIT 1.582Spring 2011

Erik Anders Nelson PE SE

Bracing Connections (Fillet Welded HSS)


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Braced Frames and Rigid Frames


Braced Frames


Diagonal bracing creates stable triangular configurations within the steel buildingframe (AISC 2002) “Braced frames are often the most economical method ofresisting wind loads in multi-story buildings (AISC 1991).”

Some structures, like the one pictured above, are designed with a combinationbraced and rigid frame to take advantage of the benefits of both

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Concentric Braced Frames

X brace (above left) Chevron (above right)


X brace (above left)

Two story X’s

X bracing is possibly the most common type of bracing

Bracing can allow a building to have access through the brace linedepending on configuration

( g )

Single diagonals

X Bracing

C ti f X b i l t d t b t l j i t


Connections for X bracing are located at beam to column joints

Bracing connections may require relatively large gusset plates at the beam tocolumn joint

The restriction of space in these areas may have an impact on the mechanicaland plumbing systems as well as some architectural features

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Chevron Bracing

Chevron bracing members use two types of connections


The floor level connection may use a gusset plate much like the connection on Xbraced frames. The bracing members are connected to the beam/girder at thetop and converge to a common point

If gusset plates are used, it is important to consider their size when laying-outmechanical and plumbing systems that pass through braced bays (AISC 2002)

Eccentrically Braced Frames

Link

Gusset

Beam or Girder

Stiffeners

Eccentric bracing is commonly used in seismic regions and allows for doorways andcorridors in the braced bays. The difference between Chevron bracing and

Eccentric Brace

Gusset

Eccentric brace with typical brace to beam connection


eccentric bracing is the space between the bracing members at the top gussetconnection

In an eccentrically braced frame bracing members connect to separate points on thebeam/girder. The beam/girder segment or “link” between the bracing membersabsorbs energy from seismic activity through plastic deformation (AISC2002)

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Eccentrically Braced Frames


Eccentrically braced frames look similar to frames with Chevron bracing

A similar V shaped bracing configuration is used

(AISC 2002)

Bolts and Weld not sharing


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Concentrically Braced Frames (CBFs)

Beams, columns and braces arranged to form a vertical truss. Resist lateral earthquake forces by truss action.

Develop ductility through inelastic action in braces.

- braces yield in tension- braces buckle in compression

Advantages

- high elastic stiffness

Disadvantages

less ductile than other systems (SMFs EBFs BRBFs)


- less ductile than other systems (SMFs, EBFs, BRBFs) - reduced architectural versatility

Single Diagonal Inverted V- Bracing V- Bracing


X- Bracing Two Story X- Bracing

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Pu sin Consider load path through connection region

P cos


Pu cos

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The KISS Method


Parallel Force Method


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The Uniform Force Method


The Uniform Force Method


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Inelastic Response of CBFs under Earthquake Loading


Tension Brace: Yields(ductile)

Compression Brace: Buckles(nonductile)

Columns and beams: remain essentially elastic

Inelastic Response of CBFs under Earthquake Loading


Compression Brace (previously in tension): Buckles(nonductile)

Tension Brace (previously in compression): Yields(ductile)

Columns and beams: remain essentially elastic

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SDM

Pu= Ry Fy

Ag

Limit state: fracture of net section

Pn = (0.75) Ae (Rt Fu)


Per Section 6.2: use expected tensile strength Rt FU when checking net section fracture of bracing member, since Ry Fy of the same member is used to computed the required strength

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SDM

Pu= Ry Fy Ag

Limit state: fracture of net section

For A36 Angles:


For A36 Angles:

03.1ksi582.175.0

ksi365.1

A

A

g

e

SDM


10 bracing connections

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