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Beam - Design Fundamentals

Beams are structural members loaded at right angles (perpendicular) to Deflection
their length Most beams are horizontal and subjected to gravity or vertical
loads, e.g. a shelf support. However a vertical member can act as a beam
under certain conditions, such as, a curtain wall mullion subjected to wind
loading. The bending moment developed in a beam is dependent on:
(a) The amount of load applied
All beams deflect under load. The amount of deflection is dependent on:
(b) The type of loading applied
(a) The amount of load,
(c) The support conditions
(b) The support conditions,
(c) The stiffness of the beam’s cross-sectional shape,
(d) The stiffness of the beam material
The stiffness of the beam’s cross-sectional shape is measured by its
Beam Loading - Point Load “Moment Of Inertia” or "I". The larger a beam’s "I", the stiffer it is and the
less it will deflect. A beam’s "I" can change for each major axis. The "I" of
A load concentrated onto a very small length of the beam is a point load. both major axes (I 1-1 and I 2-2) are provided.
The stiffness of a beam’s material is measured by its “Modulus of
Beam Loading - Uniform Load Elasticity” or "E". The larger a material’s "E", the stiffer it is and the less
it deflects. For example, steel is about three times stiffer than aluminum
and as a result, deflects only one-third as much Do not confuse stiffness
with strength. Two materials may have identical strengths yet still have
A load spread evenly over a relatively long length of the beam is a different "E’s". A high-strength aluminum may be as strong as steel and
uniform load. still deflect three times as much.
Point and uniform loads can be placed on a beam in any combination. A The load charts and tables give calculated deflections for the loads
series of point loads can approximate a uniform loading. The load charts shown. In many cases, a final design will be determined by the maximum
and tables are based on a uniform load unless identified otherwise. deflection, not the maximum load.
Support Conditions - Simple Beam Bending Moment
A simple beam has supports that prevent movement left and right, or A beam must not only hold up the anticipated loads, but must also have
up and down, but do not restrain the beam from rotating at the supports sufficient additional capacity to safely hold unforeseen variations in
into a natural deflected curve. Most connections produce simple beams. applied loads and material strengths. This additional capacity is called
The load charts and tables are based on simple beams unless identified a safety factor and is usually regulated by the various design codes
otherwise. and standards. A beam’s strength is usually measured by an allowable
bending moment or an allowable stress. The traditional approach is
Support Conditions - Continuous Beam the allowable stress method, where a beam is determined to have a
maximum allowable stress (in pounds per square inch) which is not
to be exceeded.
The approach of the current AISI “Specification For The Design Of
Cold-Formed Steel Structural Members” is to use a maximum allowable
Any simple beam that is supported at one or more intermediate points bending moment (in inch-pounds) which is not to be exceeded. Bending
is a continuous beam. A mezzanine joist that passes over three or more
columns is an example of a continuous beam. moment divided by a beam’s section modulus or "S" equals stress.

Support Conditions - Fixed-End Beam

Supports that prevent the beam from rotating into a natural deflected
curve produce a fixed-end beam. A welded end connection to very rigid
support produces a fixed-end beam.

Support Conditions - Cantilever Beam

A cantilever beam is a fixed-end beam that is supported at one end only,
while the other end is unsupported. Brackets are examples of cantilever
beams.

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