the link beam by permitting doublers or allowing

(5) Link beam rotation. The link beam rotation, at a

frame drift 0.4*R *times the drift calculated from

holes within it.

prescribed seismic forces, is limited to the values

(e) Brace sizing. Once the link beam size

given in Paragraph 15.2g of the AISC Seismic

has been selected, the brace size is determined by the

Provisions.

The procedure for calculating the

requirement given in the AISC Seismic Provisions

rotations is as follows (refer to Figure 7-24):

that its compressive strength be at least 1.5 times the

axial force corresponding to the controlling strength

(a) Perform an elastic analysis of the frame

of the link beam. The controlling strength is either

for the prescribed seismic forces, being certain that

the shear strength *V*s or the reduced flexural strength

the analysis includes the contribution of the elastic

shear deformation of the link beam.

force in the brace. Note that once the link beam is

selected, the brace forces are determined from its

(b)

Calculate 0.4R times the drift angle

strength, and the brace forces calculated in the elastic

obtained from the analysis in (1).

This angle is

analysis will not govern, and will not be used in the

denoted as *" *in Figure 7-24.

brace design.

(c) Calculate the rotation angle *2 *, as shown

(f) Brace-to-beam connection. The AISC

in Figure 7-24, for the appropriate configuration.

Seismic Provisions require that the brace-to-beam

This simplified procedure is slightly conservative,

connection develop the compressive strength of the

since the elastic curvature of the beam segments

brace, and that no part of the brace-to-beam

between hinges and of the brace deformations have

connection extend into the web area of the link. The

been ignored, and would contribute a minor amount

required development may be at the strength level of

of the required deformation. It should be noted that

the connection. The prohibition of the extension of

calculation of the rotation by multiplying the elastic

the brace-to-beam connection into the link beam is

deflections of the link beam by 0.4*R *would be

intended to prevent physical attachments that might

unconservative, since these deflections include elastic

alter the strength and deflection characteristics of the

effects, such as the axial deformation of the braces,

link beam. It is not intended to prevent the centerline

that would not increase proportionally after the link

intersection of brace and link beam from intersecting

begins to yield.

within the link.

(d)

Link-beam web.

Link-beam web

(g) Column sizing. FEMA 302 requires

doubler plates are prohibited in AISC Seismic

that the columns remain elastic at 1.25 times the

Provisions because tests have shown that they are not

forces causing yield of the link beam.

"Remain

fully effective. The performance of eccentric braced

elastic at" means the same as "have the strength to

frames relies on the predictability of the strength and

resist." The strength, including bending moments,

strain characteristics of the link beam.

It is not

considered advisable to complicate the behavior of

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