Building Design and Construction Handbook - Merritt - Ventech!

7.36 SECTION SEVEN
ities, size, span, and also on sound judgment. Usually every third or fourth bent
should become a braced bay. Participation of bents adjoining the braced bay can
be assured by insertion of bracing designated ‘‘intermediate’’ in Fig. 7.20b. This
bracing is of greater importance when knee braces between trusses and columns
cannot be used. When maximum lateral stiffness of intermediate bents is desired,
it can be obtained by extending the X bracing across the span; this is shown with
broken lines in Fig. 7.20b.
Buildings with flat or low-pitched roofs, shown in Fig. 7.12d and e, require little
bracing because the trusses are framed into the columns. These columns are designed
for the heavy moments induced by wind pressure against the building side.
The bracing that would be provided, at most, would consist of X bracing in the
plane of the bottom chords for purpose of alignment during erection and a line or
two of sway frames for longitudinal rigidity. Alignment bracing is left in the structure
since it affords a secondary system for distributing wind loads.
7.11.4 Bracing Craneway Structures
All building framing affected by overhead cranes should be braced for the thrusts
induced by sidesway and longitudinal motions of the cranes. Bracing used for wind
or erection may be assumed to sustain the lateral crane loadings. These forces are
usually concentrated on one bent. Therefore, normal good practice dictates that
adjoining bents share in the distribution. Most effective is a system of X bracing
located in the plane of the bottom chords of the roof trusses.
In addition, the bottom chords should be investigated for possible compression,
although the chords normally are tension members. A heavily loaded crane is apt
to draw the columns together, conceivably exerting a greater compression stress
than the tension stress obtainable under dead load alone. This may indicate the need
for intermediate bracing of the bottom chord.
7.11.5 Bracing Rigid Frames
Rigid frames of the type shown in Fig. 7.14 have enjoyed popular usage for gymnasiums,
auditoriums, mess halls, and with increasing frequency, industrial buildings.
The stiff knees at the junction of the column with the rafter imparts excellent
transverse rigidity. Each bent is capable of delivering its share of wind load directly
to the footings. Nevertheless, some bracing is advisable, particularly for resisting
wind loads against the end of the building. Most designers emphasize the importance
of an adequate eave strut; it usually is arranged so as to brace the inside
flange (compression) of the frame knee, the connection being located at the midpoint
of the transition between column and rafter segments of the frame. Intermediate
X bracing in the plane of the rafters usually is omitted.
7.12 BRACING FOR INDIVIDUAL MEMBERS
For an ideally straight, exactly concentrically loaded beam or column, only a small
force may be needed from an intermediate brace to reduce the unbraced length of