Building Design and Construction Handbook - Merritt - Ventech!

7.30 SECTION SEVEN
Composite construction actually occurs whenever concrete is made to assist steel
framing in carrying loads. The term composite, however, often is used for the
specific cases in which concrete slabs act together with flexural members.
Reinforced-concrete columns of conventional materials when employed in tall
buildings and for large spans become excessively large. One method of avoiding
this objectionable condition is to use high-strength concrete and high-strength reinforcing
bars. Another is to use a structural-steel column core. In principle, the
column load is carried by both the steel column and the concrete that surrounds
the steel shape. Building codes usually contain an appropriate formula for this
condition.
A number of systems employ a combination of concrete and steel in various
ways. One method features steel columns supporting a concrete floor system by
means of a steel shearhead connected to the columns at each floor level. The shallow
grillage is embedded in the floor slab, thus obtaining a smooth ceiling without
drops or capitals.
Another combination system is the lift-slab method. In this system, the floor
slabs are cast one on top of another at ground level. Jacks, placed on the permanent
steel columns, raise the slabs, one by one, to their final elevation, where they are
made secure to the columns. When fireproofing is required, the columns may be
boxed in with any one of many noncombustible materials available for that purpose.
The merit of this system is the elimination of formwork and shoring that are essential
in conventional reinforced-concrete construction.
For high-rise buildings, structural-steel framing often is used around a central,
load-bearing, concrete core, which contains elevators, stairways, and services. The
thick walls of the core, whose tubular configuration may be circular, square, or
rectangular, are designed as shear walls to resist all the wind forces as well as
gravity loads. Sometimes, the surrounding steel framing is cantilivered from the
core, or the perimeter members are hung from trusses or girders atop the core and
possibly also, in very tall buildings, at midheight of the core.
FRAME AND MEMBER BRACING SYSTEMS
7.10 BRACING DESIGN CONSIDERATIONS
Bracing as it applies to steel structures includes secondary members incorporated
into the system of main members to serve these principal functions:
1. Slender compression members, such as columns, beams, and truss elements
are braced, or laterally supported, so as to restrain the tendency to buckle in a
direction normal to the stress path. The rigidity, or resistance to buckling, of an
individual member is determined from its length and certain physical properties of
its cross section. Economy and size usually determine whether bracing is to be
employed.
2. Since most structures are assemblies of vertical and horizontal members forming
rectangular (or square) panels, they possess little inherent rigidity. Consequently,
additional rigidity must be supplied by a secondary system of members or by rigid
or semi-rigid joints between members. This is particularly necessary when the
framework is subject to lateral loads, such as wind, earthquakes, and moving loads.
Exempt from this second functional need for bracing are trusses, which are basically