Seismic Provisions Handouts_4per_Landscape

AISC Live Webinar: Introduction to the 2005 AISC Seismic Provisions

American Institute of Steel Construction 1

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Today’s AISC Live Webinar

Introduction to the 2005 AISC Seismic Provisions

written and presented by

Thomas A. Sabol, Ph. D., S.E.Principal, Englekirk & Sabol

Consulting Engineers, Inc, Los Angeles, CA.



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Introduction to 2005 AISC Seismic Provisions

Part I

WebinarVersion

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Seminar Highlights

Seminar addresses selected, key content from:Seismic Provisions for Structural Steel Buildings(ANSI/AISC 341-05)Prequalified Connections for Special and Intermediate Steel Moment Frames for Seismic Applications (ANSI/AISC 358-05)Seismic Design Manual (First Edition, 2006)

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Seminar Highlights

2005 Seismic Provisions (ANSI/AISC 341-05)Presents seismic design and detailing requirements for different structural steel systemsNational, consensus standard referenced in 2006 model building codes

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Seminar Highlights

2005 Seismic Provisions (ANSI/AISC 341-05)NEW: Combines Allowable Strength Design (ASD) and Load and Resistance Factor Design (LRFD) into a unified formatNEW: Introduces design provisions for Buckling Restrained Braced Frames (BRBF) and Special Plate Shear Walls (SPSW)NEW: Introduces quality assurance and special welding requirements for steel seismic systems



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Seminar Highlights

Moment Frame Connection Prequalification Standard (ANSI/AISC 358-05)

First national consensus standard to replace FEMA 350 for design of moment frame connections(FEMA 350 is a moment frame connection design guideline developed after 1994 Northridge Earthquake based on multi-year research program)

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Seminar HighlightsMoment Frame Connection Prequalification

Standard (ANSI/AISC 358-05)Provides design requirements, design limitations, and design procedures for:

• Reduced Beam Section (RBS) • Bolted End Plate (BEP) connections

Supplement 1 (2009) contains liberalized requirements for BEP and new provisions for Bolted Flange Plate (BEP), Welded Unreinforced Flange-Welded Web (WUF-W), and Kaiser

Bolted Bracket (KBB)

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Seminar Highlights

Seismic Design Manual (Second Printing, 2006)Resource to help designers apply 2005 Seismic Provisions and Prequalified Connection StandardContains a copy of 2005 Seismic Provisions and Prequalified Connection Standard

But, without Supplement 1

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Seminar Highlights

Seismic Design Manual (Second Printing, 2006)Provides practical examples to illustrate

• basic seismic concepts in structural steel • design examples for braced frames, moment

frames, and other system components



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Questions

Please ask (type-in) questions when they occur to you – don’t wait until

the end of the seminar!

We may not be able to answer every question, but all of them

help us understand what content might not be

sufficiently clear.

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Seismic Design Manual


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Conventional Building Code Philosophy Objective: Prevent collapse in the extreme earthquake likely to occur at a building site. Objectives are not to necessarily:

• limit damage• maintain function• provide for easy repair

Seismic Design ManualI-16


Conventional Building Code Philosophy To prevent building collapse, design for ductile behavior


VEa

rthq

uake

Loa

d, V Ductility = Inelastic Deformation

Deformation, Δ

Δ



I-17

Completely elastic response


As required elastic strength goes down (i.e. larger “R”-factor) required inelastic deformation increases


V

Eart

hqua

ke L

oad,

V

Deformation, Δ

Δ

Δyield Δmax

Velastic

0.75Velastic

0. 5Velastic

0.25Velastic

As required elastic strength goes down (i.e. larger “R”-factor) required inelastic deformation increases

I-18

Completely elastic response



V

Eart

hqua

ke L

oad,

V

Deformation, Δ

Δ

Δyield Δmax

Velastic

0.75Velastic

0. 5Velastic

0.25Velastic

As elastic design load decreases, required inelastic deformation increases

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Seismic Provisions attempt to develop ductile behavior in steel seismic systems

Ductility is provided by yieldingFracture or instability reflect non-ductile behavior


V

Eart

hqua

ke L

oad,

V

Ductility = Inelastic Deformation

Deformation, Δ

Δ

Failure (fracture or instability)

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Seismic Provisions attempt to develop ductile behavior in steel seismic systemsChoose frame elements ("fuses") that will yield in an earthquake:

• Beams in moment resisting frames• Braces in concentrically braced frames• Links in eccentrically braced frames, etc.




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Seismic Provisions attempt to develop ductile behavior in steel seismic systemsDetail "fuses" to sustain large inelastic deformationsprior to the onset of fracture or instability

• Detail fuses for ductility

Seismic Design ManualI-22


Seismic Provisions attempt to develop ductile behavior in steel seismic systemsDesign all other frame elements to be stronger than the fuses

• All other frame elements develop the plastic capacity of the fuses

• Generally, this means other elements remain elastic or nearly elastic


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Alternatively, in some areas of the country, you may design to a higher force (i.e. use R = 3) and you do not have to detail the seismic elements as required by the Seismic Provisions.

Thus, you must either• Use R > 3 and seismic detailing from Seismic

Provisions• Use R = 3 and you need not use seismic detailing


You can’t use R > 3 and skip the

seismic detailing!

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Seismic Provisions for Structural Steel Buildings

Seismic Provisions



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Organization of the Seismic Provisions DocumentPart I: LRFD and ASD ProvisionsPart II: Composite Structural Steel and Reinforced Concrete BuildingsCommentary for Part I and Part II

Seismic Provisions

An unappreciated resource in the AISC

Seismic Provisions

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Major emphases of this SeminarPart I of AISC Seismic ProvisionsMoment frames and braced framesR > 3 seismic system requirements

Seismic Provisions

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Part I ContentsSymbolsGlossary1. Scope2. Referenced Specifications, Codes and Standards3. General Seismic Design Requirements

Seismic ProvisionsI-28


Part I Contents (continued)

4. Loads, Load Combinations, and Nominal Strengths5. Structural Design Drawings and Specifications, Shop Drawings and Erection Drawings6. Materials7. Connections, Joints and Fasteners8. Members

Seismic Provisions



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Part I Contents (continued)Provisions Specific to Steel Seismic Systems

9. Special Moment Frames (SMF)10. Intermediate Moment Frames (IMF)11. Ordinary Moment Frames (OMF)12. Special Truss Moment Frames (STMF)

Seismic Provisions

Moment frame

systems

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Part I Contents (continued)Provisions Specific to Steel Seismic Systems

13. Special Concentrically Braced Frames (SCBF)14. Ordinary Concentrically Braced Frames (OCBF)15. Eccentrically Braced Frames (EBF)16. Buckling-Restrained Braced Frames (BRBF)17. Special Plate Shear Walls (SPSW)

Seismic Provisions

Braced systems

New shear wall system

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Part I Contents (continued)Other Sections/Appendices

18. Quality Assurance PlanAppendix P: Prequalification of Beam-to-Column and Link-to-Column ConnectionsAppendix Q: Quality Assurance PlanAppendix R: Seismic Design Coefficients ad Approximate Period Parameters



Part I Contents (continued)Other Sections/Appendices

Appendix S: Qualifying Cyclic Tests of Beams-to-Column and Link-to-Column ConnectionsAppendix T: Qualifying Cyclic Tests of Buckling-Restrained BracesAppendix W: Welding ProvisionsAppendix X: Weld Metal/Welding Procedure Specification Notch Toughness Verification Test

Seismic Provisions



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Glossary

Terms listed in glossary are generally italicized where they first appear in a subsection


1. Scope

Seismic Provisions intended for use in buildings and “other structures”

“Other structures” have vertical and lateral systems similar to buildings and are designed, fabricated and erected in a manner similar to buildingsSeismic Provisions apply when R > 3 or when otherwise required by the building code

Seismic Provisions

e.g. for cantilevered column systems where

R = 2.2

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1. Scope

Seismic Provisions

Shows where Seismic Provisions are required based

on Soil Class

Seismic Provisions not required in

“gray”areas

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1. Scope

Seismic Provisions are used in conjunction with AISC Specification for Structural Steel Buildings (ANSI/AISC 360-05, March 9, 2005)

Seismic Provisions focus on seismic issuesDefers to the Specification for available and nominal strength, etc. for most elements

Seismic Provisions



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3. General Seismic Design Requirements

Seismic Provisions defer to applicable building code for

Required seismic strength (see slides on Section 4 for exception)Determination of Seismic Design Categories and OccupancyDesign story drift limits


4. Loads, Load Combinations, and Nominal Strengths

Seismic Provisions

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4.1. Loads and Load Combinations

Applicable Building CodeDetermines loads and load combinations for required strength of steel seismic systems using provisions in ASCE 7 except Seismic Provisions may impose additional requirements…



Applicable Building Code… except Seismic Provisions may impose additional requirements:

When demand from one member can impose higher loads on another member

0.9D + 1.0E (note that E is assumed to have both a positive and negative sign in this combination)

Seismic Provisions

Investigates presence of “net tension”



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Applicable Building CodeDetermines overstrength factor, Ωo, to multiply horizontal earthquake load, E, when amplified seismic loads are required by the Seismic ProvisionsΩo is estimate of maximum load that can be imposed on a member by another member

Seismic Provisions

This is not the same as Ω (ASD

Factor of Safety)

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Applicable Building CodeOverstrength factor, Ωo, is estimate of maximum loadthat can be imposed on a member by another member

Pseudo “mechanism load”Tries to account for “unaccounted strength”in seismic system

Seismic Provisions

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5. Structural Design Drawings and Specifications, Shop Drawings, and Erection Drawings

Seismic Provisions

Significant change in 2005 Seismic

Provisions

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5.1 Structural Design Drawings and Specifications

The engineer, not the contractor or inspector, is in the best position to know which components are part of the seismic system and which require special consideration

The engineer must communicate the design intentto the contractor and inspector via the structural design drawings

Seismic Provisions



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Structural design drawings need to indicateType of Seismic Load Resisting System (SLRS) (e.g. SMF, EBF, etc.)Members and connections that are part of SLRSConfiguration of the connections



Structural design drawings need to indicateMember/connection material specifications and sizesLocation of “demand critical welds”

Sections 5.2 and 5.3 contain similar requirements for shop and erection drawings

Seismic Provisions

Welds likely to experience inelastic

demand – See Section 7.3b

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Locations in seismic system with special

limitations related to fabrication and

attachments – See Section 7.4


Structural design drawings need to indicateLocation and dimensions of protected zonesWelding requirements as specified in Appendix W, Section W2.1


6. Materials

Seismic Provisions



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6.1. Material Specifications

Specified minimum yield strength (Fy) for members with anticipated inelastic behavior shall not exceed 50 ksi (unless suitability is proven by testing)

Limitation does not apply to columns where inelastic behavior is assumed to be limited to column base.


e.g. Ru = RyFyAg

6.2 Material Properties for Determination of Required Strength of Members and Connections

When specified in Seismic Provisions, required strength shall be based on “Expected Yield Strength,” RyFy, of an adjoining member

Underlying assumption is that actual yield strength is greater than minimum specified strengthIn seismic design, it is not appropriate (i.e. not “conservative”) to underestimate demand on one member created by another

Seismic Provisions

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6.2 Material Properties for Determination of Required Strength of Members and Connections

Material Specification Ry RtASTM A36 (shapes) 1.5 1.2ASTM A572 Gr. 42 1.3 1.1ASTM A500 HSS 1.4 1.3ASTM A53 ( Pipe) 1.6 1.2ASTM A36 (plate) 1.3 1.2ASTM A992 (shapes) 1.1 1.1

Table I-6-1(Abridged)Ry and Rt Values for Different Member Types

Seismic Provisions

New in 2005: Used for “tensile

strength”

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7. Connections, Joints and Fasteners

Seismic Provisions



Seismic Provisions

7.2. Bolted Joints

All bolts in SLRS shall be pretensioned high-strength bolts (i.e. no A307 bolts)

Faying surfaces shall be prepared as slip-critical with a Class A surfaceEven though you prepare joint as if it were “slip-critical,” you may use the higher bolt “bearing” values (with some exceptions)

Faying surface is where steel plies come into

contact

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7.2. Bolted Joints

Bolts and welds shall not be designed to share forcein a joint or same force component in a connection

Seismic Provisions

Bolts

WeldsVertical force (and possibly the horizontal force) is resisted by bolts and welds, but designed so that eitherwelds or bolts take total load

Line of action of vertical force

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7.3. Welded Joints

Welding shall be performed in accordance withAppendix WWelding Procedure Specification (WPS) per AWS D1.1 and approved by the Engineer of Record


7.3a. General Requirements

All welds in members and connections within SLRS shall use filler metal with minimum CVN value of 20 ft-lbs at 0oF*

Seismic Provisions

*See Section 7.3b for additional CVN requirements for demand critical

welds



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7.3b. Demand Critical Welds

Where frame is normally at 50oF or higher (i.e. most conditioned structures), all welds designated as “demand critical” shall use filler metal with minimum CVN value of

20 ft-lbs at -20oF40 ft-lbs at 70oF per Appendix X


7.3b. Demand Critical Welds

Although demand critical welds are identified in the Seismic Provisions, there may be other welds that warrant this designation by the designer.

Seismic Provisions

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7.3b. Demand Critical WeldsExamples of demand critical welds in SMF and IMFinclude following CJP groove welds:

Welds of beam flanges to columnsWelds of single plate shear connections to columnsWelds of beam webs to columnsColumns splice welds, including column bases and tapered transitions

Seismic Provisions

Example “demand critical”welds I-60

7.4. Protected Zone

Certain areas of a seismic system are designated as “protected zones”

Within the protected zone: Welded, bolted, screwed or shot-in attachments for perimeter edge angles, exterior facades, partitions, duct work, piping, or other construction are prohibited

Seismic Provisions



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7.4. Protected Zone

Location of protected zones in a moment frame

Seismic Provisions

Protected zones in a moment frame

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8. Members

Seismic Provisions

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8.2. Classification of Sections for Local Buckling

Seismic performance of members in the SLRS may require yielding and high levels of inelastic deformation

To facilitate this demand, Seismic Provisionsspecify for selected members that they be compact, λp (Specification Table B4.1) , or seismically compact , λps, (Seismic Provision Table I-8-1)

Seismic Provisions

More stringent than Specificationrequirements I-64

8.2b. Seismically Compact

Seismic Provisions

Seismically compact limits, λps, for “unstiffened elements”(e.g. flanges of wide flange sections)

bfb = bf /2



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8.2b. Seismically Compact

Seismic Provisions

Seismically compact limits, λps, for “stiffened elements”(e.g. webs of wide flange sections)

twh

I-66


Tables 1-2 through 1-6 of Seismic Design Manuallist structural sections that satisfy local buckling requirements (both “compact” and “seismically compact”) for SMF, SCBF, and EBF systems


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λps limits for Wide Flange Sections

This section satisfies local buckling requirements for all listed applications (shown by “•”)

This section does not satisfy local buckling requirements for indicated application (e.g. SMF beam and column) I-68

8.3. Column Strength

When axial loads on seismic columns are “large,” the Seismic Provisions require that these columns satisfy additional requirements.

Seismic Provisions



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Special requirements shall be met when Pu/φcPn > 0.4 (LRFD) or ΩcPa/Pn > 0.4 (ASD)

Seismic Provisions

φc = 0.9 (LRFD) Ωc = 1.67 (ASD)Pa = Required axial strength of a column using ASD load

combinationsPn = Nominal axial strength of a columnPu = Required axial strength of a column using LRFD load

combinations

Without using Ωo to calculate Pa or Pu for checking the load to

strength ratio

If you “fail” the test you then

have to use Ωoto calculate Pa

or PuI-70


Special requirements shall be met whenPu/φcPn > 0.4 (LRFD) or ΩcPa/Pn > 0.4 (ASD)…

If ratios are exceeded, axial compressive and tensile strength, considered in absence of applied moment, based on amplified seismic load(i.e. if Pu/φcPn > 0.4 use Ωo if required by the applicable building code load combinations)

Seismic Provisions

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8.4. Column Splices


8.4a. General

Centerline of splice made with fillet welds or PJP welds shall be located 4 ft. or more from beam-to-column connections or at column mid-height, whichever is less

Seismic Provisions

4 ft. or more from connection or at column midheight

Column splice



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8.4b. Columns Not Part of the Seismic Load Resisting System

Column splices in columns not in SLRS:Splice required shear strength with respect to both orthogonal axes shall be Mpc/H (LRFD) or Mpc/1.5H (ASD), where Mpc is based on the appropriate direction of applied load

Seismic Provisions

Mpc

H

Mpc

Vu

II-74

9. Special Moment Frames (SMF)

Seismic Provisions

II-75

9.1. Scope

SMF are expected to withstand significant inelastic deformations (R = 8) when subjected to design an earthquake

Seismic ProvisionsII-76

9.1. Scope

Seismic Provisions

Basic Design ProcedureCalculate demands based on building codeAnalyze frameSize “fuses” (i.e. frame girders)Size other members so fuses will governConfirm that frame satisfies drift criteria



II-77

9.2. Beam-to-Column Connections


9.2a. Requirements

All beam-to-column connections in SLRS shall satisfy:

An interstory drift angle at least 0.04 radian

Seismic Provisions

II-79

9.2a. Requirements

Seismic Provisions

Δ

Lbeam

θ

Interstory Drift Angle θ =Δ

Lbeam

Deformed shape of test specimen

II-80

9.2a. RequirementsAll beam-to-column connections in SLRS shall satisfy:

Measured flexural resistance of connection, at face of column, is at least 80% of Mp of connected frame beam at interstory drift angle of 0.04 radian

Seismic Provisions

-40000

-30000

-20000

-10000

0

10000

20000

30000

40000

-0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.08

Interstory Drift Angle (rad)

Bea

m M

omen

t at F

ace

of C

olum

n (in

-kip

s)

0.8 Mp

- 0.8 Mp

M0.04 ≥0.8 Mp

M0.04 ≥0.8 Mp



II-81

9.2b. Conformance Demonstration

Requirements of 9.2a shall be satisfied by one of the following:

SMF connection recognized by PrequalifiedConnection Standard (ANSI/AISC 358)Qualifying tests per Appendix S of Seismic Provisions

Relevant tests reported in the literatureRelevant project specific tests

Seismic Provisions

Project-specific

II-82

9.3. Panel Zone of Beam-to-Column Connections (beam web parallel to column web)

Seismic Provisions

II-83

9.3a. Shear Strength

Seismic Provisions

Panel zone must be strong enough to resist demand from connecting beam without excessive deformation

Yielding of panel zone recognized as an efficient method of providing ductility

Panel zone

II-84


Seismic Provisions

Panel Zone Required Shear Strength =

Required strength (shear) based on demands generated by beams framing into column

Beam 1 Beam 2

Mf1 Mf2

Vc

Vc

Mf2db -tf

Mf2db -tf

Mf1db -tf

Mf1db -tf

( ) cfb

fu V

tdM

R −−

= ∑



II-85


Seismic Provisions

When Pu ≤ 0.75 Py in column, shear strength of panel zone:

(AISC Spec EQ J10-11)

Where: dc = column depth

db = beam depth

bcf = column flange width

tcf = column flange thickness

Fy = minimum specified yield stress of column web

tp = thickness of column web including doubler plate

dc

d b

tp

tcf

bcf

⎥⎥⎦

⎤

⎢⎢⎣

⎡+=

pcb

cfcfpcyv tdd

tbtdFR2316.0

Use φ = 1.0

II-86

9.4a. Width-Thickness Limitations

Beam and column members shall meet requirements of Section 8.2b (i.e. seismically compact per Table I-8-1), unless otherwise qualified by tests

Seismic Provisions

II-87

9.6. Column-Beam Moment Ratio

Seismic Provisions

Strong Column – Weak Beam provision is intended to prevent global frame instability rather than prevent yielding of individual columns

Delaying column yielding helps force beam yielding at multiple levels and provides greater overall frame stability

II-88


Seismic Provisions

M*pc-1

M*pc-2

M*pb-2M*pb-1

Note:M*pc is based on minimum specified

yield stress of columnM*pb is based on expected yield stress

of beam and includes allowance for strain hardening

Use Fyfor

column

Use1.1RyFy

for beam

0.1MM

*pb

*pc >

∑∑



II-89


Exception: Eq. 9-3 need not apply if either (a) or (b) is true:

(a) Columns aren’t too heavily loaded and (i) they are located at the roof or (ii) there aren’t too manycolumns that don’t satisfy Eq. 9-3

(b) Columns are sufficiently strong compared to the columns on the floor above


9.7. Lateral Bracing at Beam-to-Column Connections

Seismic Provisions

II-91Seismic Provisions


These photographs show lateral torsional buckling in frame girders. This behavior can twist the column out-of-plane unless the column is adequately braced (see Section 9.7a.). Required frame girder bracing is discussed in Section 9.7b.

Lateral torsionalbuckling

II-92

9.8 Lateral Bracing of Beams

Both flanges of beams shall be laterally braced.Unbraced length between lateral braces shall not

exceed Lb = 0.086ryE/Fy

Braces need to possess sufficient strength and stiffness (Appendix 6 of Specification)

Seismic Provisions

Lb ≤ 0.086ryE/Fy

Lateral bracing



II-93


Both flanges of beams shall be laterally braced.

Seismic Provisions

Lateral bracing provided by full-height perpendicular framing

Lateral bracing provided by shallow perpendicular framing and stiffener II-94


Required strength of lateral braces provided adjacent to plastic hinges:

Seismic Provisions

Bracing adjacent to plastic hinge

Strength of bracing > 0.06Mu/ho

Plastic hinge

II-95


Required strength of lateral braces provided adjacent to plastic hinges:

Seismic Provisions

Lateral bracing at RBS provided by structural slab

Lateral bracing provided angles – check stiffness of bracing) (Note: deck not in place)

II-96

9.9. Column Splices

When splices are not made with CJP weldsrequired flexural strength based on smaller column

RyFyZx (LRFD) RyFyZx/1.5 (ASD)

Seismic Provisions

Mu = RyFyZx

Splice not made with CJP (e.g. fillet welds

or bolts)



II-97

9.9. Column Splices

When splices are not made with CJP weldsrequired shear strength based onΣMpc/H (LRFD) ΣMpc/(1.5H) (ASD)

where ΣMpc is sum of nominal plastic flexural strengths of columns above and below the splice

Seismic Provisions

Mpc2

H

Mpc1

Vu

II-98

10. Intermediate Moment Frames (IMF)

Seismic Provisions


Seismic Design Manual Table 4-1Comparison of Requirements

for SMF and IMF Systems

Special Moment Frame (SMF)

Intermediate Moment Frame (IMF)

Interstory Drift 0.04 radian 0.02 radianConnection Flexural Strength 80% of nominal plastic moment of

the connection at interstory drift angle of 0.04 radian

80% of nominal plastic moment of the connection at interstory drift

angle of 0.02 radian

Connection Shear Strength

Vu for load combination 1.2D + 0.5L + 0.2S plus shear from

application of moment of 2[1.1RyFyZ/distance between

plastic hinge locations]

Vu for load combination 1.2D + 0.5L + 0.2S plus shear from

application of moment of 2[1.1RyFyZ/distance between

plastic hinge locations]― or ― ― or ―

Lesser Vu permitted if justified by analysis

Lesser Vu permitted if justified by analysis. See also the exception provided in Seismic Provisions

Section 10.2a.



Special Moment Frames (SMF)

Intermediate Moment Frames (IMF)

Panel Zone Shear Strength

For Pr < 0.75Pc

with φv = 1.00Rn = Per Specification Eqn. J10-

12, with φv = 1.00

No additional requirements beyond AISC Specification

Panel Zone Thickness t > (dz + wz)/90 No additional requirements beyond AISC Specification

Continuity Plates To match tested condition To match tested condition

Beam-Column Proportion No additional requirements beyond AISC Specification




II-101

11. Ordinary Moment Frames (OMF)


11.1. Scope

OMF are expected to withstand minimal inelastic deformations (R = 3.5) in their members and connections when subjected to design earthquake.

Model codes place significant limits on where OMF may be used

Seismic Provisions

II-103

11.1. Scope

Seismic Provisions

Seismic Design Category A or B C D E FMaximum Height No limit No Limit Not

permitted1,2Not

permitted1,2Not

permitted1,2

Maximum Building Height per Seismic Design Category per 2006 International Building Code

Notes1. OMF may be used in a single story building ≤ 60 ft. tall with bolted end

plates and roof dead load ≤ 15 psf and any dead load of any wall > 35 ft. is ≤ 15 psf

2. OMF may be used in a building ≤ 35 ft. tall with roof, floor and wall dead load ≤ 15 psf

II-104

11.2a. Requirements for FR Moment Connections

Seismic Provisions

Figure 11-1 Special Weld Access Hole Geometry

Special weld access hole



II-105

11.3. Panel Zone of Beam-to-Column Connections (beam web parallel to column web)

No additional requirements beyond those in the Specification


11.4. Beam and Column Limitations

No additional requirements beyond those in Section 8.1 of Seismic Provisions

Seismic Provisions

II-107

11.5. Continuity Plates

When FR connections use welds of column flanges to beam flanges or beam-flange connection plates, continuity plates shall be provided

Continuity plates also required when

or when

Seismic Provisionstcf

b bf

tbf

ycybbffcf FFtbt /54.0<

6/bfcf bt <

II-108


No requirements.

Seismic Provisions



II-109




11.8. Lateral Bracing of Beams


Seismic Provisions

III-111

Prequalified Connection Standard


ANSI/AISC 358 Prequalified Connections for Special and Intermediate

Moment Frames for Seismic Applications

Supplement 1 issued June 2009

www.aisc.org/freepubs

III-112

1.1 Scope

Provide design, detailing, fabrication, and quality criteria for special and intermediate moment frames

To be used as prequalified connections with Seismic Provisions

Not intended to preclude use of other connections tested per Seismic Provisions Appendix S




III-113

5. Reduced Beam Section (RBS) Moment Connection

Prequalified Connection StandardIII-114

5.1 General

In reduced beam section (RBS), portions of beam flanges are selectively trimmed in a region adjacent to beam-to-column connection

Yielding and hinge formation are intended to occur primarily within the RBS Trimmed

(reduced) flange

Reduced Beam Section

Yielding in RBS


III-115

5.3.1 Beam Limitations

Beams shall satisfy the following limitationsBeams shall be rolled wide-flange or built-up I-shaped members conforming to Section 2.3Beam depth is limited to W36 (and equivalent for built-up shapes)Beam weight is limited to 300 lbs/ft

Reduced beam section

Depth: W36 x max or equivalent for built-up member Weight: 300 plf max


5.3.1 Beam Limitations

Beams shall satisfy the following limitationsBeam flange thickness is limited to 1.75 in.Clear span-to-depth ratio is limited to

7 or greater for SMF and 5 or greater for IMF

Clear span

Dep

th

For same drift angle, greater beam depth requires larger extreme fiber strain




III-117

5.5 Beam Flange to Column Flange Weld Limitations

Weld access hole geometry shall conform to requirements of Section J1.6 of AISC Specification(i.e. not the special weld access hole)


5.6 Beam Web to Column Connection Limitations

For SMF:Beam web shall be connected to column flange with a CJP weld extending between weld access holesSingle plate shear connection, with minimum thickness of 3/8 in., may be used as backing


III-119

5.6 Beam Web to Column Connection Limitations

For IMF:Beam web shall be connected to column flange per requirements for SMFException:

Bolted web connection using single shear plate is permittedBolts shall be designed as slip-criticalNominal bearing strength at bolt holes per Section J3.8 of AISC Specification


5.8 Design Procedures

Procedures outline steps to design RBS connectionNote that currently there is no HSS or weak-axis

wide flange RBS connection that has been prequalified


c

b a

RBS Dimensions



IV-121

13. Special Concentrically Braced Frames (SCBF)

Seismic ProvisionsIV-122

13. Special Concentrically Braced Frames (SCBF)

Seismic Provisions

IV-123

13.1. Scope

SCBF are expected to withstand significant inelastic deformations (R = 6) when subjected to design earthquake.

SCBF are expected to have increased ductility compared to OCBF because negative consequences caused by strength degradation in buckled OCBF compression braces is minimized

Seismic Provisions

Preferred mode of behavior: tension brace yielding

IV-124

13.1. Scope

Seismic Provisions

Δ

F

Consider maximum effects due to brace force (RyFyAg)

RyFyAg



Preferred mode of behavior: compression brace buckling

IV-125

13.1. Scope

Seismic Provisions

Δ

F

Consider maximum effects due to brace force (sometimes P = RyPn, sometimes P = 0.3Pn)

RyPn,

0.3Pn

IV-126

13.1. Scope

Seismic Provisions

Unfavorable modes of behaviorConnection fractureColumn bucklingBeam failure

IV-127

13.1. Scope

Seismic Provisions

Basic Design ProcedureCalculate demands based on building codeAnalyze frameSize “fuses” (i.e. braces)Size other members so fuses will govern

IV-128

13.2b. Required Strength

Where effective net area of bracing is less than gross area, required tensile strength of brace based on limit state of fracture in the net sectionshall be greater than the lesser of:

Expected yield strength, in tension, of bracing member: RyFyAg (LRFD) RyFyAg/1.5 (ASD)

Maximum load effect indicated by analysis that can be transferred to brace by the system

Typical example:

slotted HSS

Seismic Provisions

ΩoQE does not satisfy this requirement



IV-129

132b. Required Strength

Where effective net area of bracing is less than gross area…

Objective is to yield gross section of brace prior to fracture of net section


132b. Required Strength

Where effective net area of bracing is less than gross area…often requires local strengthening of the brace

Typical example:

slotted HSS

Seismic Provisions

Slot needs to be neatly radiused to avoid brittle fracture

Plate added to each side to compensate for slot

IV-131

13.2c. Lateral Force Distribution

Along any line of braces, braces shall be deployed in alternate directions such that, for either direction of force parallel to bracing, at least 30% but no more than 70% of total horizontal force is resisted by tension braces unless…


13.2c. Lateral Force Distribution

Seismic Provisions

Δ

F

Δ

F

F

F

Braces oriented in same direction

Braces oriented in alternate directions



IV-133

13.2d. Width-Thickness Limitations

Column and brace members shall meet requirements of Section 8.2b (i.e. seismically compact per Table I-8-1)

Seismic Provisions

For rectangular HSS (A500 Gr B steel) there are many sections that will not satisfy Table I-8-1:

290000 64 0 64 16 146y

b E ksi. . .t F ksi

≤ = =

IV-134Seismic Provisions

Examples of brace buckling shows local buckling (and fracture) at the mid-length

of the brace


IV-135


Seismic Provisions

(There aren’t a lot of them)

16 1b .t

≤

IV-136

13.3. Required Strength of Bracing Connections

Seismic Provisions



IV-137

13.3a. Required Tensile Strength

Required tensile strength of bracing connections(including beam-to column connections if part of bracing system) shall be lesser of:

Expected yield strength of bracing member, RyFyAg (LRFD) RyFyAg/1.5 (ASD)

Maximum load effect, indicated by analysis, that can be transferred to brace by the system

Seismic Provisions

ΩoQE does not satisfy this requirement

IV-138

13.3b. Required Flexural Strength

In direction brace will buckle, required flexural strength of connection shall be equal to 1.1RyMp (LRFD) or 1.1RyMp/1.5 (ASD) of brace about critical axis.

Exception: Brace connections are permitted that:Satisfy Section 13.3a, Can accommodate inelastic rotations associated with post-buckling deformations

Seismic Provisions

Brace tensile capacity

IV-139


Seismic Provisions

P

MM

Plastic Hinges

1.1 Ry Mp-brace

Plastic hinges form at ends and mid-length of brace. Brace imposes moments on connections and adjacent members

Fixed-End Braces 1.1RyMp-brace = 1.1RyFyZbrace

IV-140


Seismic Provisions

Pin-ended Braces

P

P

P

P Plastic Hinge

For "pinned" end braces: flexural plastic hinge will form at mid-length only. Brace will impose no bending moment on connections and adjoining members.

Must design brace connection to behave like a "pin"



IV-141


Seismic Provisions

Fold line

2t

Fold line

2tIV-142


Seismic Provisions

Fold line is free to form: OK

Fold line is NOT free to form: NG

>2t

IV-143

13.4. Special Bracing Configuration Requirements


13.4a. V-Type and Inverted V-Type Bracing

V-Type and Inverted-V-Type braced frames

Seismic Provisions

Undesirable behavior Desired behavior

Weak beam member neutralizes tension brace once compression brace buckles

Tension

Mem

ber

Strong beam member mobilizes tension brace once compression brace buckles

Tension

Mem

ber



IV-145


V-Type and Inverted-V-Type braced frames

Seismic Provisions

Two-story braces

Two-story braces eliminate the need to design this beam to support the unbalanced vertical brace load

IV-146


V-Type and Inverted-V-Type braced frames shall meet following requirements:

For load combinations that include earthquake effect on beam, E shall be determined as follows

Forces in tension braces shall be assumed to equal RyFyAg

Forces in all adjoining braces in compressionshall be assumed equal to 0.3Pn

Seismic Provisions

V-Type and Inverted-V-Type braced frames shall meet following requirements:

IV-147

Wgravtity = 1.2D + 0.5L

Ry Fy Ag

0.3 Pn

θ


Seismic Provisions

( Ry Fy Ag - 0.3 Pn ) sin θ

( Ry Fy Ag + 0.3 Pn ) cos θ

Beam is designed to support gravity load, horizontal axial load, and unbalanced vertical load without relying on braces

IV-148

13.4b. K-Type Bracing

K-Type braced frames are not permitted for SCBF.

Seismic Provisions

K-type braced frame (not permitted)



IV-149

13.5. Column Splices

In addition to meeting requirements of Section 8.4, column splices in SCBF shall:

50% of available flexural strength of smaller connected section. Required shear strength shall be ΣMpc/H (LRFD)or ΣMpc/(1.5H) (ASD)

where ΣMpc is sum of nominal plastic flexural strengths of columns above and below the splice


13.6. Protected Zone

Seismic Provisionsd

d

LL/

4

Protected zone atgussets

Protected zone on braces atexpected hinges

Miscellaneous attachments (cladding, plumbing, etc.) not permitted in the Protected Zone

IV-151

14. Ordinary Concentrically Braced Frames (OCBF)


14.1. Scope

OCBF are expected to withstand limited inelastic deformation (R = 3.25) in their members when subjected to the forces from the design earthquake.

Seismic Provisions



IV-153

14.2. Bracing Members

Seismic Provisions

Basically the same as SCBF

Bracing shall meet the requirements of Section 8.2b (i.e. seismically compact)Exception: braces filled with concrete need not comply with this

provision

Braces with Kl/r greater than 4√(Es/Fy) shall not be used in V-type or inverted-V-type configurations.

IV-154Seismic Provisions


V-Type, Inverted-V-Type and K-type braced frames shall meet following requirements:

Beam that is intersected by braces shall be continuousbetween columns (V-Type, Inverted-V-Type)Column that is intersected by braces shall be continuous between beams (K-Type)

Unique to OCBF


IV-155



Required strength of beam intersected by braces, their connections and supporting members shall be determined based on load combinations of building code assuming braces support no dead and live loads.

Seismic Provisions


IV-156


Seismic Provisions



For load combinations that include earthquake effect on beam, E shall be determined as follows

Forces in tension braces shall be assumed to equal RyFyAg

For V-type and Inverted V-type, brace tension forces need not exceed maximum force developed by systemForces in compression braces shall be assumed equal to 0.3Pn



IV-157


Seismic Provisions



Both flanges of beam shall be laterally braced with maximum spacing of Lb = Lpd per Equation A-1-7 and A-1-8 of Appendix 1 of the Specification. Braces need to possess sufficient strength and stiffness (See notes on Section 9.8 of Seismic Provisions and Appendix 6 of Specification for example requirements)

IV-158

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Documents

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