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8/12/2019 Seismic-Resistant Steel Design - EBFs
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Design of SeismicDesign of Seismic--Resistant SteelResistant Steel
Building StructuresBuilding Structures
Design of SeismicDesign of Seismic--ResistantResistant
Steel Build ing StructuresSteel Building Structures
1 - Introduction and Basic Principles
2 - Moment Resisting Frames
3 - Concentrically Braced Frames
4 - Eccentrically Braced Frames
44 -- Eccentrically Braced FramesEccentrically Braced Frames EBFsEBFs))
Description of Eccentrically Braced Frames
Basic Behavior o f Eccentrically Braced Frames
AISC Seismic Provisions for Eccent rical ly Braced
Frames
Eccentrically Braced Frames (Eccentrically Braced Frames (EBFsEBFs))
Framing system with beam, columns and braces. Al least
one end of every brace is connected to isolate a segment
of the beam called a link.
Resist lateral load through a combination of frame action
and truss action. EBFs can be viewed as a hybrid syst embetween moment frames and concentrically braced
frames.
Develop ductility through inelastic action in the links.
EBFs can supply high levels of ductility (similar to
MRFs), but can also p rovide high levels of elastic
stiffness (similar to CBFs)
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e
e
e
e
Some possible bracing arrangement for EBFS
e e e e
e
e
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Energy Dissipation Mechanisms
MRF CBF
EBF
M
V
P
Forces in Beams and Links
e e
Inelastic behavior of link controlled by:
Flexural yielding
Shear yielding
A combinat ion of f lexural and shearyielding
Will shear or flexure control the behavior of the
link?
The length of the link (e) serves as a key
design parameter.
Shorter links expected to yield in shear andlonger links are expected to yield in flexure.
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How long is long and how short is short? e
V V
M M
Shear vs. Flexural Yielding L inks:
Static equilibr ium of link: Ve = 2M or e2M
V=
Shear yielding occurs when: V = V = 0.6F (d 2 t ) tp y f w
Flexural yielding occurs when:
= fully plastic shear
M = M = Z Fp y
= fully plastic moment
e
Vp Vp
Mp Mp
Shear Vs. Flexural Yielding L inks:
Shear and flexural yielding occur simultaneously
when V=Vp and M=Mp
or, when: e2M
V
p
p
=
e
Vp Vp
M M
Shear Vs. Flexural Yielding Links:
Shear yielding only will occur when V=Vp and M < Mp
or, when: e2M
V
p
p
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e
V V
Mp Mp
Shear Vs. Flexural Yielding L inks:
Flexural yielding only will occur when V < Vp and M = Mp
or, when: e2M
V
p
p
Shear Vs. Flexural Yielding Li nks:
e2M
V
p
p
Simple Plastic Theory (assume no strain hardening and
no shear - flexure interaction):
SHEAR YIELDING LINK:
FLEXURAL YIELDING LINK: e2M
V
p
p
Shear Vs. Flexural Yielding L inks:
e1.6M
V
p
p
Real Behavior - accounting for strain hardening:
PREDOMINANTLY SHEAR YIELDING LINK:
PREDOMINANTLY FLEXURAL YIELDING LINK: e2.6 M
V
p
p
COMBINED SHEAR AND FLEXURAL YIELDING:1.6M
Ve
2 .6 M
V
p
p
p
p
Example: W18x40 A992
kipsinksiin 3920504.78ZFM 3yp
kips159
531055229175060
260
....
. wfyp ttdFV
52159
3920kips
kipsin p
p
V
M
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Example: W18x40 A992 (cont )
52V
M
p
p 0461 p
p
V
M. 5662
p
p
V
M.
PREDOMINANTLY SHEAR YIELDING LINK: e 40"PREDOMINANTLY FLEXURAL YIELDING LINK: e > 65"
COMBINED SHEAR AND FLEXURAL YIELDING LINK: 40" < e 65"
Shear Yielding Links
e1.6M
V
p
p
Provide best overall structural performance for:
strength
stiffness
ductility
V
e
=e
Link Deformation: (radian)
Experimental Response of a Shear Link:
W10x33 (A992) e = 23" = 1.1Mp/Vp
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-800
-600
-400
-200
0
200
400
600
800
-0.15 -0.10 -0.05 0.00 0.05 0.10 0.15
Link Rotation, (rad)
Link
ShearForce
(kN)
2002 AISC Seismic Provisions
Section 15
Eccentrically Braced Frames
15.1 Eccentrically Braced Frames (EBF) Scope
EBF are expected to withs tand signifi cant
inelastic deformation in the links when
subjected to the design earthquake.
Diagonal braces, columns, and beamsegments outside of link shall be designed
to remain essentially elastic under maximum
forces that can be generated by the fully
yielded and strain-hardened link.
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15.2 Links
Links shall comply with width-thickness ratios
in Table I-8-1.
Specified minimum yield stress of steel used
in link shall not exceed 50 ksi.
Web of a link shall be single thickness withou t
doubler-plate reinforcement and without webpenetrations.
15.2 Links
Except as limited below, required shearstrength of Link Vushall not exceed thedesign shear strength of the link Vn.
Where:
= 0.9Vn= nominal shear strength of link (lesserof Vpor 2Mp/e) (kips)
Vp= 0.6FyAw(kips)
Aw= (db 2tf)tw
15.2 Links
Link rotation angle is inelastic angle betweenlink and beam outside link when total storydrift equals design story d rift,.
Link rotation shall not exceed:
(a) 0.08 radian for links of length 1.6Mp/Vporless
(b) 0.02 radian for links of length 2.6Mp/Vporgreater
(c) Value obtained from linear interpolation forlink lengths between 1.6Mp/Vpand 2.6Mp/Vp
Link inelastic rotation angle (p):
eh
Lp
Story drift
e
p
h
L
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15.3 Link Stiffeners
Full depth web stiffeners shall be prov ided onboth sides of link web at diagonal braceends of link.
Stiffeners shall have a combined width notless than (bf2tw) and a thickness not lessthan 0.75twnor 3/8 in., whichever is larger
where bfandtware link flange width and link webthickness, respectively.
Link stiffeners atbrace ends of link
15.3 Link Stiffeners
Links shall be provided with intermediate webstiffeners as follows:
(a) Links lengths 1.6Mp/Vpor less shall be
provided with intermediate web stiffenersspaced:
(30tw-d/5) for = 0.08rad (52tw-d/5) for = 0.02rad or less
(linearly interpolate between 0.08 and 0.02)
Design Requirements for EBF Links
Link Stiffeners are required to prevent buckl ing
of the web in shear
Intermediatestiffeners
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15.3 Link Stiffeners
Links shall be provided with intermediate webstiffeners as follows:
(b) Links lengths greater than 2.6Mp/Vpand
less than 5Mp/Vpshall be provided withintermediate web stiffeners placed at adistance 1.5 bffrom end of link
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15.3 Link Stiffeners
Links shall be provided with intermediate webstiffeners as follows:
(c) Links lengths between 1.6Mp/Vpand
2.6Mp/Vpshall be provided with
intermediate web stiffeners meeting the
requirements o f (a) and (b), above
(d) Intermediate web stiffeners are not requiredin links lengths greater than 5Mp/Vp
15.4 Link to Column Connections
Link to column connections must be capableof sustaining maximum link rotation anglebased on link length (i.e. as specified inSection 15.2).
Strength of link connection, measured atcolumn face, must be equal to at least the
nominal shear strength of link, Vn
15.4 Link to Column Connections
Link to column connections shall demonstratesatisfaction of the above by:
(a) Using prequalified connection per
Appendix P(b) Providing project-specific tests or tests
from the literature per Appendix S
15.4 Link to Column Connections
Link to column connections shall demonstratesatisfaction of the above by:
Exception: Where reinforcement at beam-to-
column connection at link end precludesyielding of beam over the reinforced length,the link is permit ted to be the beamsegment from end of reinforcement to thebrace connectionand
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15.4 Link to Column Connections
Link to column connections shall demonstratesatisfaction of the above by:
Exception: Where such l inks are used andlink l ength does not exceed 1.6Mp/Vp, cyclictesting of connection is not required ifdesign strength of reinforced section and
connection equals required strength basedon s train-hardened link per Section 15.6
Full depth s tiffeners per Section 15.3 to beplaced at link-to-reinforcement interface.
15.4 Link to Column Connections
Comments: Currently no prequalified link-to-column
connections FEMA 350 prequalified moment connections
not necessarily suitable for link-to-columnconnections
Suggest avoiding EBF configurations withlinks attached to columns until furtherresearch available on link-to-columnconnections
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15.5 Lateral Bracing o f Link
Lateral bracing shall be prov ided at both topand bottom li nk flanges at each end of link .
Required strength of link end lateral bracing is0.06RyFybftf(i.e. link flange force)
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15.6 Diagonal Brace and Beam Outside Link
Required combined axial and flexural strength
of diagonal brace shall be forces generated
by 1.25RyVn(i.e. shear strength o f link ).
Design strength of b race shall be based onLRFD Specification Section H
Design Requirements for EBF Beam-Outside-Link
Beam outside link
15.6 Diagonal Brace and Beam Outside Link
Design of beam outside the link:
(1) Required strength of beam outside of linkshall be for ces generated by 1.1RyVn(i.e.
shear strength of link).For purposes of determining design
strength of beam outside of link, Rymaybe used to calculate available strength(i.e. you can use on bo th demand acapacity sides of equation)
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15.6 Diagonal Brace and Beam Outside Link
Design of beam outside the link:
(2) Beam shall be provided with lateral bracingwhere indicated as required by analysis.
Lateral bracing shall be provided at bothtop and bottom flanges and each braceshall have a required strength of at least
0.02Fybftf.
15.6 Diagonal Brace and Beam Outside Link
At connect ion between d iagonal brace andbeam at link end of brace, intersection ofbrace and beam centerlines shall be at theend of the link or in the link.
15.6 Diagonal Brace and Beam Outside Link
Required strength of diagonal brace-to-beam
connection at link end of brace shall be at
least expected nominal brace strength as
given i n Section 15.6.No part of this connection shall extend over
the link length.
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15.6 Diagonal Brace and Beam Outside Link
If brace resists por tion of link moment,
connection shall be designed as a FR
connection.
Brace width-thickness ratio shall satisfy pperLRFD SpecificationTable B5.1
Test on Full-Scale 6-Story EBF: Final failure by
buckling of gusset plate
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15.8 Required Column Strength
In addition to requirements in Section 8,
required strength of columns shall be
determined based on load combinations
from applicable building code
except that moments and axial loads
introduced into column at connection of a
link or b race shall not be less than those
generated by 1.1RyVn(i.e. expected nominal
strength of the link) .
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