Page 5 - ZSi-Foster Channel Loads

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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
“Moment Of Inertia” or "I" The larger a beam’s "I", the stiffer it is and the
Beam Loading - Point Load 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 different
A load spread evenly over a relatively long length of the beam is a "E’s" A high-strength aluminum may be as strong as steel and still deflect
uniform load 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 a
The load charts and tables are based on simple beams unless identified safety factor and is usually regulated by the various design codes and
otherwise standards A beam’s strength is usually measured by an allowable bending
moment or an allowable stress The traditional approach is the allowable
Support Conditions - Continuous Beam 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
bending moment (in inch-pounds) which is not to be exceeded Bending
Any simple beam that is supported at one or more intermediate points moment divided by a beam’s section modulus or "S" equals stress
is a continuous beam A mezzanine joist that passes over three or more
columns is an example of a continuous beam
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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