UFC 4-023-03
25 January 2005
B-4.3
Damage Limits.
The Damage Limits given in Section 3-2.6 were taken from ITG 2001 and
similar values are used by the British.
B-4.4
Plastic Hinge Considerations for AP Modeling of Steel Structures.
Since an AP analysis that assumes plastic behavior is likely to experience
large deformations and potentially the formation of multiple plastic hinges, the analysis
is for all intents and purposes a plastic analysis or design even though a complete
mechanism may not be reached. Therefore, it is appropriate to employ the AISC
provisions for plastic analysis and design. The plastic analysis provisions are found in
AISC LRFD 2003 Section A5.1.
An element with a compact section is capable of reaching the full plastic
moment capacity (if adequately braced), however a compact section may not be
sufficient if the plastic hinge must sustain significant rotations following hinge formation.
See AISC LRFD 2003 Commentary B5 and AISC 341-02 Section C8.2 for a discussion
of preventing local buckling for various levels of rotation capacity and member ductility
factor (specific definitions of rotation capacity and ductility factor are provided in AISC
LRFD 2003 in the glossary). For higher levels of inelastic rotation, it may be necessary
to use a "seismically compact" section in accordance with AISC LRFD 2003 Table C-
B5.1 and AISC 341-02 Table I-8.1.
In addition, lateral bracing requirements for plastic analysis are more severe
and are required due to the large deformations possible in this type of behavior.
Although not specifically covered in the AISC provisions, the analyst must also consider
other load effects and their impact on the plastic moment capacity. For example, a
section with an applied compressive axial force will have a reduced plastic moment
since some of the cross section will be utilized to resist the axial force. This reduction is
small in many cases, but the effect should be verified before assuming a full plastic
moment is achievable. Beedle 1958 indicates the axial effect can be ignored for P/Py <
0.15 with small error in the value of Mp. Moy 1996, Neal 1963, and Beedle 1958 cover
the effect of such "secondary" effects in detail.
B-4.5
Beam-Column Considerations for AP Modeling of Steel Structures.
A determination must be made for a beam-column element on whether the
response will be deformation controlled and allow for inelastic action or if the response
will be force controlled and the element becomes ineffective when the limit state is
reached. Since any beam element may have some amount of axial force, most
elements will be treated as beam-columns. If the axial force is low (P/Py) and the
element is adequately braced, the response is not greatly affected by the presence of
the axial force and the element can be treated as a flexural element. However, as the
axial force increases, the response can be altered. It is worth noting that "strong
column, weak beam" design as specified for seismic design is appropriate when
considering progressive collapse as well. Early hinge formation in columns would tend
to reduce the resistance to progressive collapse and should be avoided. By ensuring
B-15
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