Building Design and Construction Handbook - Merritt - Ventech!

STRUCTURAL STEEL CONSTRUCTION 7.37
a column or the unsupported length of the compression flange of a beam. But there
is no generally accepted method of calculating that force.
The principal function of a brace is to provide a node in the buckled configuration.
Hence, rigidity is the main requirement for the brace. But actual members
do contain nonuniform residual stresses and slight initial crookedness and may be
slightly misaligned, and these eccentricities create deformations that must be resisted
by the brace.
A rule used by some designers that has proved satisfactory is to design the brace
for 2% of the axial load of columns, or 2% of the total compressive stress in beam
flanges. Studies and experimental evidence indicate that this rule is conservative.
7.12.1 Column Bracing
Interior columns of a multistory building are seldom braced between floor connections.
Bracing of any kind generally interferes with occupancy requirements and
architectural considerations. Since the slenderness ratio l/r in the weak direction
usually controls column size, greatest economy is achieved by using only wideflange
column sections or similar built-up sections.
It is frequently possible to reduce the size of wall columns by introducing knee
braces or struts in the plane of the wall, or by taking advantage of deep spandrels
or girts that may be otherwise required. Thus the slenderness ratio of the weak and
strong axis can be brought into approximate balance. The saving in column weight
may not always be justified; one must take into account the weight of additional
bracing and cost of extra details.
Column bracing is prevalent in industrial buildings because greater vertical clearances
necessitate longer columns. Tall slender columns may be braced about both
axes to obtain an efficient design.
Undoubtedly, heavy masonry walls afford substantial lateral support to steel
columns embedded wholly or partly in the wall. The general practice, however, is
to disregard this assistance.
An important factor in determining column bracing is the allowable stress or
load for the column section (Art. 7.19). Column formulas for obtaining this stress
are based on the ratio of two variables, effective length Kl and the physical property
called radius of gyration r.
The question of when to brace (to reduce the unsupported length and thus slenderness
ratio) is largely a matter of economics and architectural arrangements; thus
no general answer can be given.
7.12.2 Beam Bracing
Economy in size of member dictates whether laterally unsupported beams should
have additional lateral support between end supports. Lateral support at intermediate
points should be considered whenever the allowable stress obtained from the reduction
formulas for large l/r t falls below some margin, say 25%, of the stress
allowed for the fully braced condition. There are cases, however, where stresses as
low as 4.0 ksi have been justified, because intermediate lateral support was impractical.
The question often arises: When is a steel beam laterally supported? There is
no fixed rule in specifications (nor any intended in this discussion) because the