a function of the geometry of the frame (see Figure 7-

restrictive flange-width-to-thickness ratio of 52 / *F*y

24). Link beams can yield in shear, in bending, or in

are required for the beam portions of eccentric braced

both shear and bending at the same time. Which

frames in order to provide the beams with stable

yield mechanism governs is a function of the

inelastic deformation characteristics.

The same

relationship of link length to the ratio of its bending

requirement is used for the beams of special moment-

strength to shear strength. Where the length of the

resisting space frames.

link beam is less than 1.6 *M*s/*V*s, the yielding is

almost entirely in shear. Where the length is greater

(b)

Link beam strength.

The basic

than 2.6 *M*s/*V*s, the yielding is primarily in bending.

requirement for link beam strength is given in the

Where the length is between 1.6 *M*s/*V*s and 2.6 *M*s/*V*s,

AISC Seismic Provisions, which states that the shear

both shear and bending yield will occur. Since link

in the link beam web due to prescribed seismic forces

beams that yield in shear are considered to have the

be limited to 0.8 *V*s. Paragraph 15.2f of the AISC

most stable energy-dissipating characteristics, most

Seismic Provisions addresses the concern for the

of the EBF research has tested the cyclic inelastic

effect that substantial axial loads in the link beam

capacity of link beams with shear yielding at large

could have on its inelastic deflection performance. It

rotations.

Consequently, most of the design

presumes that in shear links, the web' capacity is

s

provisions are concerned with limiting the link beam

fully utilized in shear, and that flanges provide the

shear yield rotation to less than the maximum cyclic

needed axial and flexural capacity. Shear links with

test rotations, and then requiring details indicated by

a length less than 2.2 *M*s/*V*s are considered to be

the tests as necessary to ensure that this rotation can

controlled by shear. Substantial axial loads occur in

occur through a number of cycles without failure.

some EBF configurations when the link beam is

required to transmit horizontal forces to or from the

(4)

Design criteria.

The specific criteria

braces.

It is recommended that, insofar as it is

governing the design of eccentrically braced frames

possible, link beams be located so that they are not

are

given

in

the

AISC

Seismic

Provisions.

required to transmit the horizontal force component

Additional detail is provided in the following

of braces or drag struts. Where axial forces in the

paragraphs.

link cannot be avoided, the flexural strength shall be

reduced by the axial stress *f*a, giving *M*RS = *Z *(*F*y - *f*a).

(a) Link beam location and stability. Link

The *f*a should correspond to the lesser value of the

beams are the fuses of the EBF structural system, and

axial force corresponding to yield of the link beam in

are to be placed at locations that will preclude

shear, or that which, when combined with link

buckling of the braces. A link beam must be located

bending, causes the beam flanges to yield.

in the intersecting beam at least at one end of each

brace.

There are exceptions permitting concentric

bracing at the roof level and/or at the bottom level of

EBF over five stories in the AISC Seismic

Provisions.

Compact sections meeting the more

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