25 January 2005
A flexural member can fail by reaching its full plastic moment capacity, or it
can fail by lateral-torsional buckling (LTB), flange local buckling (FLB), or web local
buckling (WLB). Calculate nominal moment strength, Mn, in accordance with AISC
LRFD 2003. If a flexural member's capacity is governed by a buckling mode of failure,
remove the element when the internal moment reaches the nominal moment strength.
Distribute the loads associated with the element in accordance with 3-2.4.3. If the
member strength is not governed by buckling, the strength will be governed by
plastification of the cross section and it may be possible for a plastic hinge to form.
Verify that deformation of primary members will not cause premature failure in
secondary members, due to geometric interference; for instance, torsional rotation of a
girder should not cause excessive deformation and stresses in any beam that frames
into the girder with a simple shear tab connection.
5-126.96.36.199 Formation of Plastic Hinge.
If hinge formation, i.e. material non-linearity, is included in the AP analysis,
the requirements of Section A5.1 of the AISC LRFD 2003 for plastic design must be
met. AISC LRFD 2003 permits plastic analysis only when the structure can remain
stable, both locally and globally, up to the point of plastic collapse or stabilization.
Where the analysis indicates the formation of multiple plastic hinges, ensure each cross
section or connection assumed to form a plastic hinge is capable of not only forming the
hinge, but also capable of the deformation demands created by rotation of the hinge as
additional hinges are formed in the element or structure. Since the element could be
required to undergo large deformations as plastic hinges are being formed, special
lateral bracing is required. The magnitude of the plastic moment, Mp, used for analysis
must consider the influence of axial or shear force when appropriate. Further
information on plastic design is provided in The Plastic Methods of Structural Analysis
(Neal 1963) and Plastic Design of Steel Frames (Beedle 1958).
5-188.8.131.52 Modeling of a Plastic Hinge.
For Linear Static analyses, if the calculated moment exceeds the nominal
moment strength and it is determined the element is capable of forming a plastic hinge,
insert an "equivalent" plastic hinge into the model by inserting a discrete hinge in the
member at an offset from the member end and add two constant moments, one at each
side of the new hinge, in the appropriate direction for the acting moment; see Figure 3-
9. The magnitude of the constant moments is equal to the determined plastic moment
capacity of the element. Determine the location of the plastic hinge through engineering
analysis and judgment or with the guidance provided for seismic connections in FEMA
350, Recommended Seismic Design Criteria for New Steel Moment-Frame Buildings
and AISC 341-02, Seismic Provisions for Structural Steel Buildings. For Nonlinear
Static and Dynamic Analysis, use software capable of representing post-peak flexural
behavior and considering interaction effects of axial loads and moment. Ensure that
shear failure will not occur prior to developing the full flexural design strength.