LRFD-Steel Design

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LRFD-Steel Design

• Dr.
• Ali I. Tayeh
• First Semester

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Steel DesignDr. Ali I. Tayeh

• Chapter 6-B

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Beam-Columns
Example 6.5
Solution
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Beam-Columns
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Beam-Columns
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Beam-Columns
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Beam-Columns
Example 6.6
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Beam-Columns
Solution
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Beam-Columns
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Beam-Columns
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Beam-Columns
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Beam-Columns
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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

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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

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Beam-Columns
Design of beam column The procedure can be
plained as following
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Beam-Columns
Evaluation of Cm
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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 Ch1.0.
• See Example 6.8

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Beam-Columns

• See Example 6.8
• Solution

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Beam-Columns
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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

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Beam-Columns
See Example 6.11
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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.

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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

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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 distributed gravity loads
  on its top chord. To account for loadings of 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.

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Beam-Columns

• TRUSSES WITH TOP-CHORD LOADS BETWEEN JOINTS
• The following procedure is recommended.
• 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.
• Add the reactions from this fixed-end beam to the
  actual joint loads to obtain total joint loads.
• 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.

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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.
• See Example 6.13

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Beam-Columns -Steel Design

• End