Seismic checks - Beams (Seismic: AISC 341)
Classification
In all cases if the given “width to thickness ratio” is less than or equal to the given limit, then the seismic classification is satisfied.
AISC 341-16 and AISC 341-10
Beams in OMF and OCBF – No additional requirements.
Beams in IMF and SCBF – Beams must satisfy the requirements of clause D1.1b for “moderately ductile” members.
Beams in SMF – Beams must satisfy the requirements of clause D1.1b for “highly ductile” members.
The loading conditions affect the seismic classification in the following way,
- Axial tension only – no classification required.
- Any other loading condition – the appropriate rules in the section classification table are applied.
See: AISC 341-16 and AISC 341-10 seismic classification - all members
AISC 341-05
Beams in OMF, SCBF and OCBF – No additional requirements.
Beams in IMF – Beams must satisfy clause 8.2a i.e. the requirements for Compact sections to AISC 360-05 in Table B4.1.
Beams in SMF – Beams must satisfy clause 8.2b for “seismically compact” sections.
The loading conditions affect the seismic classification in the following way,
- Axial tension – no classification required.
- Major axis bending only – applies to beams in SMF only.
- Any other loading condition – apply appropriate rules in the section classification table.
Stability bracing
AISC 341-16 and AISC 341-10
Beams in certain SFRS frame types must be provided with “stability bracing” to restrain lateral torsional buckling. There are two “levels” of requirement; one for ‘Moderately ductile members and one for “Highly ductile members”. The use of these depends upon the SFRS frame type as defined below.
Beams in OMF and OCBF – No additional requirements.
Beams in IMF and SCBF – Beams must satisfy clause D1.2a for “moderately ductile” members. For SCBF this only applies in the presence of V or A braces.
Beams in SMF – Beams must satisfy clause D1.2b for “highly ductile” members
Moderately Ductile
Beams shall be braced per D1.2a for moderately ductile members, i.e. maximum spacing per D1.2a(3)
Lpd = 0.17 x (E/Fy) x ry
The design condition is,
Lb ≤ Lpd
Lb = the laterally unbraced length of the compression flange taken as the beam length between locations where both the top flange and bottom flange are restrained for LTB.
Highly Ductile
Beams shall be braced per D1.2b for highly ductile members, i.e. maximum spacing,
Lpd = 0.086 x (E/Fy) x ry
The design condition is,
Lb ≤ Lpd
Note: In both cases:
- The position of lateral braces is not checked for the location of points of concentrated force or positions of plastic hinge
- Lateral braces are not designed to meet the additional criteria for strength and stiffness.
AISC 341-05
OCBF and SCBF
Reference AISC 341-05 14.3 (2) and 13.4a (2) respectively.
For doubly symmetric I sections,
Lpd = (0.12 + 0.076 x (M1/M2)) x (E/Fy) x ry
Where
M1 = the smaller moment at end of unbraced length – min(abs(Ma), abs(Mb))
M2 = the larger moment at end of unbraced length – max(abs(Ma), abs(Mb))
Note: Lateral braces are assumed to
meet the strength and stiffness requirements of Equations A-6-7 and A-6-8 of
AISC 360-05, Appendix 6.
Note: Lateral braces are assumed to
be provided at the intersection of the V/A brace and the beam or the beam has
sufficient stiffness to satisfy the criteria in the “User Note” to AISC 341-05,
13.4a. In both cases the program assumes this intersection to be a braced point
(with both flanges braced).
OMF
Reference AISC 341-05 11.8 – no additional requirements.
IMF
Reference AISC 341-05 10.8.
Max spacing of coincident restraints along top and bottom flange, continuous bracing is included while determining unbraced length for each flange,
Lpd = 0.17 x (E/Fy) x ry
The design condition is,
Lb ≤ Lpd
Lb = the laterally unbraced length of the compression flange taken as the beam length between locations where both the top flange and bottom flange are restrained for LTB.
Note: The position of lateral braces
are not checked for the location of points of concentrated force or positions of
plastic hinge.
Note: Lateral braces are not designed
to meet the additional criteria for strength and stiffness.
SMF
Reference AISC 341-05 9.8.
Max spacing of coincident restraints along the top and bottom flange, continuous bracing is included while determining the unbraced length for each flange,
Lpd = 0.086 x (E/Fy) x ry
The design condition is,
Lb ≤ Lpd
Lb = the laterally unbraced length of the compression flange taken as the beam length between locations where both the top flange and bottom flange are restrained for LTB.
Note: The position of lateral braces
are not checked for the location of points of concentrated force or positions of
plastic hinge.
Note: Lateral braces are not designed
to meet the additional criteria for strength and stiffness.
Design for brace forces in SCBF and OCBF
In both variants of the code the beams in SCBF and OCBF that are configured with “V” or “A” braces are required to resist a “push-pull” force generated by the brace pair. In general terms the compression brace is assumed to retain a percentage of its resistance post-buckling and the tension brace is assumed to have a level of defined “overstrength”.
Note: In meeting the requirements of F1.4 and
F2.3, the design of beams is separated into two distinct approaches – those
beams with V&A braces at mid-span and those with diagonal braces at their
ends. Clearly, frames that are braced using V&A braces have to meet both
requirements e.g. in chevron systems but each is checked individually and this
is believed to be conservative.
AISC 341-16 and AISC 341-10, V and A braces
OCBF
Reference AISC 341-10 F1.4a.
Note: The tension yield strength/amplified seismic + post buckling strength in AISC
341-10 is replaced by overstrength seismic load + post buckling strength in AISC
341-16.
Note: The lower bound on the force in the tension
brace, “The maximum force that can be developed by the system”, according to
F1.4a (1) (i) (c) is not applied.
SCBF
Reference AISC 341-10 F2.3.
AISC 341-16 and AISC 341-10, all other braces
OCBF
No requirements.
SCBF
Reference AISC 341-10 F2.3.
The expected tension strength, the expected compression strength and the expected post buckling strength are determined in the same way as for V and A braces.
AISC 341-05, V and A braces
OCBF and SCBF
Reference AISC 341-05 14.3 and 13.4a respectively.