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(3) Dynamic Load Factor. The dynamic load factors listed in Table
23 may also be used as a rational starting point for a preliminary design of
a braced frame. In general, the sidesway stiffness of braced frames is
greater than unbraced frames and the corresponding panel or sidesway dynamic
load factor may also be greater. However, since Table 23 is necessarily
approximate and serves only as a starting point for a preliminary design,
refinements to this table for frames with supplementary diagonal braces are
not warranted.
(4) Loads in Frame Members. Estimates of the peak axial loads in
the girders and the peak shears in the columns of a braced rigid frame are
obtained from Figure 46. It should be noted that the shear in the blastward
column and the axial load in the exterior girder are the same as the rigid
frame shown in Figure 45. The shears in the interior columns V2 are not
affected by the braces while the axial loads in the interior girders P are
reduced by the horizontal components of the force in the brace FH. If a
bay is not braced, then the value of FH must be set equal to zero when
calculating the axial load in the girder of the next braced bay. To avoid
an error, horizontal equilibrium should be checked using the formula:
EQUATION:
Ru = V1 + nV2 + mFH
(118)
where,
Ru, V1, V2 and FH are defined in Figure 46
n = number of bays
m = number of braced bays
In addition, the value of Mp used in Figure 46 is simply the design
plastic moment obtained from the controlling panel or combined mechanism.
(a) An estimate of the peak loads for braced frames with
nonrigid girder to column connections may be obtained using Figure 46.
However, the value of Mp must be set equal to zero. For such cases, the
entire horizontal load is taken by the exterior column and the bracing.
There is no shear force in the interior columns.
(b) Preliminary values of the peak axial loads in the columns
and the peak shears in the girders are obtained in the same manner as rigid
frames. However, in computing the axial loads in the columns, the vertical
components of the forces in the tension braces must be added to the vertical
shear in the roof girders. The vertical component of the force in the brace
is given by:
EQUATION:
Fv = AbFdysin [gamma]
(119)
(c) The reactions from the braces will also affect the load on
the foundation of the frame. The design of the footings must include these
loads.
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