structure is supporting at a particular instant. The live
loads in a structure are transmitted through the various
load-bearing structural members to the ultimate support
of the earth.
Horizontal members provide immediate or direct
support for the loads. These in turn are supported by
vertical members, which in turn are supported by
foundations and/or footings, which are finally supported
by the earth.
The ability of the earth to support a load is called
the soil-bearing capacity. It is determined by test and
measured in pounds per square foot. Soil-bearing
capacity varies considerably with different types of soil,
and a soil with a given bearing capacity will bear a
heavier load on a wide foundation or footing than it will
a narrow one.
Columns are high-strength vertical structural
members; in buildings they are sometimes called pillars.
Outside-wall columns and bottom-floor inside columns
usually rest directly on footings. Outside-wall columns
usually extend from the footing or foundation to the roof
line. Bottom-floor inside columns extend upward from
footings or foundations to horizontal members that
support the first floor. Upper floor columns usually are
located directly over lower-floor columns.
A pier in building construction might be called a
short column. It may rest directly on a footing, or it may
be simply set or driven in the ground. Building piers
usually support the lowermost horizontal structural
members. In bridge construction a pier is a vertical
member that provides intermediate support for the
The chief vertical structural members in light-frame
construction are called studs. They are supported on
horizontal members called sills or sole plates, and are
topped by horizontal members called top plates or stud
caps. Corner posts are enlarged studs located at the
In early full-frame construction, a
corner post was usually a solid piece of larger timber.
Built-up corner posts are used in most modern
construction. They consist of two or more ordinary
studs nailed together in various ways.
In technical terminology, a horizontal load-bearing
structural member that spans a space and is supported at
both ends is called a beam. A member that is fixed at
one end is called a cantilever. Steel members that
consist of solid pieces of regular structural steel shapes
are called beams. However, one type of steel member
is actually a light truss (discussed later) and is called an
open-web steel joist or a bar-steel joist.
Horizontal structural members that support the ends
of floor beams or joists in wood-frame construction are
called sills, girts, or girders. The choice of terms
depends on the type of framing being done and the
location of the member in the structure. Horizontal
members that support studs are called sills or sole plates.
Horizontal members that support the wall ends of rafters
are called rafter plates or top plates, depending on the
type of framing. Horizontal members that support the
weight of concrete or masonry walls above door and
window openings are called lintels.
A beam of given strength, without intermediate
supports below, can support a given load over only a
certain maximum span. If the span is wider than this
maximum, the beam must have intermediate supports,
such as columns. Sometimes it is not feasible to install
intermediate supports. In these cases, a truss may be
used instead of a beam.
A truss is a framework consisting of two horizontal
(or nearly horizontal) members joined together by a
number of vertical and/or inclined members to form a
series of triangles. The loads are applied at the joints.
The horizontal members are called the upper or top
chords and lower or bottom chords. The vertical and/or
inclined members that connect the top and bottom
chords are called web members.
WELDED AND RIVETED STEEL
The following paragraphs will discuss welded and
riveted steel structures and will give examples of both
methods used to make trusses.
WELDED STEEL STRUCTURES
Generally, welded connections are framed or seated
just as they are in riveted connections, which we will
discuss later. However, welded connections are more
flexible. The holes used to bolt or pin pieces together
during welding are usually drilled in the fabrication
shop. Beams are not usually welded directly to
columns. The procedure produces a rigid connection