Braced frame ductility

Each of the three main types of braced frames has a different ductile mechanism as summarized in Table 5.4.

▼ 5.4 Braced frame types and comments on their ductility

Frame type

Material

Comments

Tension-only

Steel

Wood

Capacity Designed diagonal tension members should yield before any connections fail or the compression chord buckles. Compression diagonals buckle due to their slenderness while tension yielding members are permanently elongated. During an earthquake as both braces lengthen incrementally over successive seismic pulses, the structure becomes floppy and possibly unstable (Fig. 5.30). Tension-only bracing is best suited to low-rise construction.

The strength of tension members is usually limited by connection strengths. Wood braces with conventional steel connections lack ductility and need to be designed to full elastic code forces.

Compression and tension

Steel In a Capacity Designed system connection details are stronger than the tensile strength of a brace. Compression braces absorb very little earthquake energy. Ductility improves if approximately the same number of braces in tension and compression are in each line of framing.6 Ductile fuses can also be inserted (Fig. 5.31). A fuse must be long enough to absorb inelastic axial deformations without tensile fracture yet short enough to compress plastically without buckling. Bucking-restrained braces that possess the characteristics of ductile fuses are discussed in Chapter 14.

Wood

No significant ductility can be expected.

Eccentrically braced

Steel

In this highly ductile system the structural fuse is the short length of beam between inclined struts, or struts and adjacent columns. All connections and members are designed stronger than the fuse region to ensure it alone absorbs the earthquake energy. Closely-spaced vertical stiffeners along the fuse prevent the beam web from buckling.

Before quake

Brace yields Brace buckles

Deflection - first pulse

Brace yields Brace buckles

Before quake

Deflection - first pulse

After first pulse

Brace buckles Brace yields

Deflection - second pulse

After first pulse

Brace buckles Brace yields

Deflection - second pulse

After second pulse

Both braces are slack

Both braces are slack

Deflection before tension braces are taut

Unrestrained deflections

Deflection before tension braces are taut

After second pulse

Unrestrained deflections

▲ 5.30 Under cyclic loading, yielding of tension-only braces result in a floppy structure.

▲ 5.31 Structural fuses in tension and compression bracing. Educational institution, Wellington.

Gravity force

Deflected shapes

Shear force diagrams

\

1

L ^

\

\

Í

k

k

(e) Bending moment diagrams

Gravity force

; '

' 1

▲ 5.32 Comparison between a horizontally loaded moment frame and a frame supporting gravity forces.

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