Seismic-Resistant Steel Design - EBFs

8/12/2019 Seismic-Resistant Steel Design - EBFs

1/21

1

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)


2/21

2

e

e

e

e

Some possible bracing arrangement for EBFS

e e e e

e

e


3/21

3


4/21

4


5/21

5


6/21

6

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.


7/21

7

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


8/21

8

e

V V

Mp Mp


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


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


9/21

9

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


10/21

10

-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.


11/21

11

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


12/21

12

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


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


13/21

13



(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


14/21

14



(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


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



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


15/21

15



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.


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


16/21

16

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)


17/21

17

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


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)


18/21

18


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.


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.


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.


19/21

19


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


20/21

20


21/21

21

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) .

Seismic-Resistant Steel Design - EBFs

Documents

Transcript of Seismic-Resistant Steel Design - EBFs