# Steel Design

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```					LRFD-Steel Design

Dr.
Ali I. Tayeh

First
Semester
Steel Design
Dr. Ali I. Tayeh
Chapter 6-B
Beam-Columns
Example 6.5

Solution
Beam-Columns
Beam-Columns
Beam-Columns
Beam-Columns
Beam-Columns
Example 6.6
Beam-Columns
Solution
Beam-Columns
Beam-Columns
Beam-Columns
Beam-Columns
Beam-Columns
MEMBERS IN UNBRACED FRAMES

• In a beam column whose ends are free to translate, the maximum
primary moment resulting from the side sway is almost always at one
end. As was illustrated in the next Figure the maximum secondary
moment from the sides way is always at the end. As a consequence of
this condition, the maximum primary and secondary moments are
usually additive and there is no need for the factor Cm; in effect, Cm =
1.0.
• The amplification factor for the sides way moments, B2, is given by two
equations.
• Either may be used; the choice is usually one of convenience:

OR
Beam-Columns
Evaluation of Cm
• The summations for Pu and Pe2apply to all columns that are in the same
story as the column under consideration. The rationale for using the
summations is that B2
Beam-Columns
Design of beam column
The procedure can be plained as following :
Beam-Columns
Evaluation of Cm
Beam-Columns
The detailed procedure for design is:
• Select an average b value from Table 6-1 (if bending appears more
dominant than axial load, select a value of m instead). If weak axis
bending is present, also choose a value of n.
• From Equation 6.5 or 6.6, solve for m or h.
• Select a shape from Table 6-2 that has values of b, nt, and n close to
those needed. These values are based on the assumption that weak axis
buckling control the axial compressive strength and that Ch==1.0.

• See Example 6.8
Beam-Columns
• See Example 6.8

Solution:
Beam-Columns
Beam-Columns
Beam-Columns
Beam-Columns
Design of Bracing
• A frame can be braced to resist directly applied lateral forces or to
provide stability. The latter type, stability bracing, The stiffness and
strength requirements for stability can be added directly to the
requirements for directly applied loads .
• Frame bracing can be classified as nodal or relative. With nodal bracing,
lateral support is provided at discrete locations and does not depend on
the support from other part of the frame.
• The AISC requirements for stability bracing are given in Section C3 of
the Specification. For frames, the required strength is
Beam-Columns

See Example 6.11
Beam-Columns
Design of Unbraced Beam-Columns
• The preliminary design of beam-columns in braced frames has been
illustrated. The amplification factor BI was assumed to be equal to 1.0
for purposes of selecting a trial shape; B I could then be evaluated for
this trial shape. In practice, BI with almost always be equal to 1.0. For
beam-columns subject to sides way, the amplification factor B2 is based
on several quantities that may not be known until all column in the
frame have been selected. If AISC Equation C 1-4 is used for B2, the
sides way deflection oh may not be available for a preliminary design.
If AISC Equation Cl-5 is used,  Pe2may not be known. The following
methods are suggested for evaluating H2.
Beam-Columns
Design of Un braced Beam-Columns

• in the United States contains a limit on the drift index, values of 1/500
to 1/200arc commonly used (Ad Hoc Committee on Serviceability,
1986). Remember that oh is the drift caused by IH, so if the drift index
is based on service loads, then the lateral loads H must also be service
loads. Use of a prescribed drift index enables the designer to determine
the final value of B2 at the outset.
• See Example 6.12
Beam-Columns
TRUSSES WITH TOP-CHORD LOADS BETWEEN JOINTS
•   If a compression member in a truss must support transverse loads
between its ends, it will be subjected to bending as well as axial
compression and is therefore a beam-column. This condition can occur
in the top chord of a roof truss with purlins located between the joints.
The top chord of an open-web steel joist must also be designed as a
beam-column because an open-web steel joist must support uniformly
this nature, a truss can be modeled as an assembly of continuous chord
members and pin-connected web members. The axial loads and
bending moments can then be found by using a method of structural
analysis such as the stiffness method. The magnitude of the moments
involved, however, does not usually warrant this degree of
sophistication, and in most cases an approximate analysis will suffice.
Beam-Columns
TRUSSES WITH TOP-CHORD LOADS BETWEEN JOINTS
•   The following procedure is recommended.
1. Consider each member of the top chord to be a fixed-end beam. Use
the fixed end moment as the maximum bending moment in the
member. The top chord is actually one continuous member rather
than a series of individual pin-connected members, so this
approximation is more accurate than treating each member as a
simple beam.
2. Add the reactions from this fixed-end beam to the actual joint loads
3. Analyze the truss with these total joint loads acting. The resulting
axial load in the top-chord member is the axial compressive load to
be used in the design.
Beam-Columns
TRUSSES WITH TOP-CHORD LOADS BETWEEN JOINTS
• This method is represented schematically in the next figure .
Alternatively, the bending moments and beam reactions can be found by
treating the top chord as a continuous beam with supports at the panel
points.
Beam-Columns -Steel Design

End

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