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
    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.
                     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
      to obtain total 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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