seismic-resistant steel design - intro

8/13/2019 Seismic-Resistant Steel Design - Intro

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Design of SeismicDesign of Seismic--Resistant SteelResistant Steel

Building StructuresBuilding Structures

Design of SeismicDesign of Seismic--ResistantResistant

Steel Building StructuresSteel Building Structures

1 - Introduction and Basic Principles

2 - Moment Resisting Frames

3 - Concentrically Braced Frames

4 - Eccentrically Braced Frames

11 -- Introduction and Basic PrinciplesIntroduction and Basic Principles

Performance of Steel Buildings in Past Earthquakes

Codes for Seismic Resistant Steel Buildings

Building Code Philosophy

Overview of AISC Seismic Provisions

AISC Seismic Provisions - General Requirements

Applicable to All Steel SystemsCauses of Earthquake Fatalities: 1900 to 1990Causes of Earthquake Fatalities: 1900 to 1990


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US Seismic Code ProvisionsUS Seismic Code Provisions

for Steelfor Steel

Structural Engineers Association of California (SEAOC) BlueBook 1988:First comprehensive detailing provisions for steel

Americ an Ins tit ute o f Steel Cons truction (A ISC) SeismicProvisions

1s t ed . 1990

2nd ed. 1992

3rd ed . 1997

Supplement No. 1: February 1999

Supplement No. 2: November 2000

4th ed . 2002

5th ed . 2005

Conventional Building Code PhilosophyConventional Building Code Philosophy

Objective: Prevent collapse in the extreme

earthquake likely to occur at a

building site.

Objectives are not to:

- limit damage

- maintain function

- provide for easy repair


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To Survive Strong Earthquake

without Collapse:

Design for Ductile BehaviorDesign for Ductile Behavior

H

HDuctility = Inelastic Deformation

HH

yield failure

Ductility Factor =failure

yield

HH

Strength

Reqd Ductility

MAX

Helastic

3/4 *Helastic

1/2 *Helastic

1/4 *Helastic


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HDuctility = Yielding

Failure =

Fracture

orInstability

Ductility in Steel Structures: Yielding

Nonductile Failure Modes: Fracture or Instability

Developing Ductile BehaviorDeveloping Ductile Behavior:

Choose frame elements ("fuses") that will yield in anearthquake; e.g. beams in moment resisting frames, braces

in concentrically braced frames, links in eccentrically braced

frames, etc.

Detail "fuses" to sustain large inelastic deformations prior tothe onset of fracture or instability (i.e. , detail fuses for

ductility).

Design all other frame elements to be stronger than the fuses,

i.e., design all other frame elements to develop the plasticcapacity of the fuses.

(a) (b)

Examples of:

(a) Less Ductile Behavior

(b) More Ductile Behavior

Key Elements of SeismicKey Elements of Seismic--Resistant DesignResistant Design

(1) Lateral Forces - Strength & Stiffness

ASCE-7 and NEHRP Provisions

(2) Detailing Requirements - Ductility

AISC Seismic Provisions


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DS D1sS S

CR I T R I

Design EQ Loads Base Shear per ASCE 7-05:

sV C W

R factors for Selected Steel Systems (ASCE 7):

SMF (Special Moment Resisting Frames): R = 8

IMF (Intermediate Moment Resisting Frames): R = 4.5

OMF (Ordinary Moment Resisting Frames): R = 3.5

EBF (Eccentrically Br aced Frames): R = 8 or 7

SCBF (Special Concentrically Braced Frames): R = 6

OCBF (Ordinary Concentrically B raced Frames): R = 3.25

BRBF (Buckling Restrained Braced Frame): R = 8 or 7SPSW (Special Plate Shear Walls): R = 7

Undetailed Steel Systems inSeismic Design Categories A, B or C R = 3(AISC Seismic Provisions not needed)

2005 AISC Seismic Provisions2005 AISC Seismic Provisions Organization of the

2005 AISC Seismic Provisions

Part I: Seismic design provisions for structural

steel buildings

Part II: Seismic design provis ions for composi te

struc tural steel and reinforced concrete

buildings


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AISC Seismic Provisions for

Structural Steel Buildings Part ISymbols

Glossary

1. Scope

2. Referenced Specifications, Codes and Standards

3. General Seismic Design Requirements

4. Loads, Load Combinations and Nominal Strengths

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

6. Mat er ial s

7. Connections, Joints and Fasteners

8. Member s

9. Special Moment Frames (SMF)

10. Intermediate Moment Frames (IMF)

11. Ordinary Moment Frames (OMF)

12. Special Truss Moment Frames (STMF)

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)

18. Quality Assurance Plan

Appendix P: Prequalification of Beam-to-Column and

Link-to-Column Connections

Appendix Q: Quality Assurance Plan

Appendix R: Seismic Design Coefficients and

Approximate Period Parameters

Appendix S: Qualifying Cyclic Tests of Beam-to-

Column and Link-to-Column

Connections

Appendix T: Qualifying Cyclic Tests of Buckling

Restrained Braces

Appendix W: Welding Provisions

Appendix X: Weld Metal / Welding Procedure

Specification Toughness Verification

Test

2005 AISC Seismic Provisions2005 AISC Seismic Provisions

General Provisions Applicable

to All Systems

Highlights ofHighlights of

Glossary and Sections 1 to 8Glossary and Sections 1 to 8


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Use of Seismic Provisions is mandatory for

Seismic Design Category D, E or F.

Use of Seismic Provisions are mandatory for

Seismic Design Categories A, B or C; when using

R > 3

For Seismic Design Categories A, B or C: can

design using R=3, and provide no special detailing

(just design per main AISC Specification)

AISC Seismic Provis ions:AISC Seismic Provisions:

Section 1Section 1 -- Scope (cont)Scope (cont)AISC Seismic Provis ions:AISC Seismic Provisions:

Section 3Section 3 -- General Seismic Design RequirementsGeneral Seismic Design Requirements

Go to theAppl icable Building Code for:

Occupancy Category

Seismic Design Category

Limits on Height and Irregularity

Drift Limitations

Required Strength

Section 4Section 4

Loads, Load CombinationsLoads, Load Combinations

and Nominal Strengthsand Nominal Strengths

AISC Seismic Provisions:AISC Seismic Provisions:

4.1 Loads and Load Combinations

4.2 Nominal Strength

AISC Seismic Provis ions:

4.1 Loads and Load Combinations

Go to theAppl icable Bu ilding Code for Loads

and Load Combinations.


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Basic LRFD Load Combinations (ASCE-7):

1.4D

1.2D + 1.6L + 0.5(Lror S or R)

1.2D + 1.6(Lror S or R) + (0.5L or 0.8W)

1.2D + 1.6W + 0.5L + 0.5(Lror S or R)

0.9D + 1.6W

1.2D + 1.0E + 0.5L + 0.2S

0.9D + 1.0E

Load Combinations

Including E

Definition of E for use in basic load combinations:

For Load Combination: 1.2D + 1.0E + 0.5L + 0.2S

E = QE + 0.2 SDS D

For Load Combination: 0.9D + 1.0E

E = QE - 0.2 SDS D

E = QE0.2 SDS D

effect of horizontal forces effect of vertical forces

E = the effect of horizontal and vertical

earthquake-induced forces

QE = effect of horizontal earthquake-

induced forces

SDS = design spectral acceleration at short

periods

D = dead load effect

= reliability factor

(depends on extent of redundancy in the

seismic lateral resisting system;

varies from 1.0 to 1.5)

Substitute E into basic load combinations:


substitute: E = QE + 0.2 SDS D


substitute: E = QE - 0.2 SDS D

(1.2 + 0.2 SDS) D + 1.0 QE + 0.5L +0.2S

(0.9 - 0.2 SDS) D + 1.0 QE


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AISC Seismic Provisions:

4.1 Loads and Load Combinations (cont.)

Where amplified seismic loads are required by

theAISC Seismic Provisions:

The horizontal portion of the earthquake load E

shall be multiplied by the overstrength factor oprescribed by the applicable building code.

Definition of Ampl if ied Seismic Load (ASCE-7)


E = o QE + 0.2 SDS D


Amplified Seismic Load:

E = o QE - 0.2 SDS DAmplified Seismic Load:

Basic load combinations incorporatingAmpl if ied

Seismic Load:


substitute: E = o QE + 0.2 SDS D


substitute: E = o QE - 0.2 SDS D

(1.2 + 0.2 SDS) D + o QE + 0.5L +0.2S

(0.9 - 0.2 SDS) D + o QE

Seismic Overstrength Factor: o

System o

Moment Frames (SMF, IMF, OMF)

Concentrically Braced Frames (SCBF, OCBF)

Eccentrically Braced Frames (EBF)

Special Plate Shear Walls (SPSW)

Buckling Restrained Braced Frames (BRBF)

- moment resisting beam-column

connections

- non-moment resisting beam-column

connections

3

2

2

2

2.5

2

Per ASCE-7:


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Ampl if ied Seismic Load

LateralSeismicForce

Frame Lateral Deflection

Qe

o Qe

Amplified Seismic Load, oQe, is intended to provide an

estimate of a frame's plastic lateral strength

Section 6Section 6

MaterialsMaterials


6.1 Material Specifications

6.2 Material Properties for Determination of

Required Strength of Members and

Connections

6.3 Heavy Section CVN Requirements


6.1 Material Specifi cations

For members in which inelastic behavior is

expected:

Specified minimum Fy 50 ksi

Exceptions:

Columns for which only expected yielding

is at the base;

Members in OMFs and OCBFs (permitted

to use up to Fy = 55 ksi)


6.2 Material Properties for Determination of Required

Strength of Members and Connections

Expected Yield Strength = Ry Fy

Expected Tensile Strength = Rt Fu

Fy = minimum specified yield strength

Fu = minimum specified tensile strength

Ry and Ru are based on statistical analysis of

mill data.


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Table I-6-1

Ry and Rt Values for Different Member Types

Hot-Rolled Shapes and Bars:

ASTM A36 1.5 1.2

ASTM A572 Gr 42 1.1 1.1

ASTM A992; A572 Gr 50 or Gr 55;

ASTM A913 Gr 50, 60 or 65; ASTM A588;

A1011 HSLAS Gr 50 1.1 1.1

ASTM A529 Gr 50 1.2 1.2

ASTM A529 Gr 55 1.1 1.2

Hollow Structural Sections (HSS):

ASTM A500 Gr B or Gr C; ASTM A501 1.4 1.3

Pipe:

ASTM A53 1.6 1.2

Plates:

ASTM A36 1.3 1.2

ASTM A572 Gr50; ASTM A588 1.1 1.2

Application Ry Rt

Example:A36 angles used for brace in an SCBF

Fy = 36 ksi

Fu = 58 ksi

Ry Fy = 1.5 36 ksi = 54 ksi

Rt Fu = 1.2 58 ksi = 70 ksi

Example:A992 wide flange used for beam in an SMF

Fy = 50 ksiFu = 65 ksi

Ry Fy = 1.1 50 ksi = 55 ksi

Rt Fu = 1.1 65 ksi = 72 ksi

Where specified in the Seismic Provisions, the

required strength of a member or connection shall

be based on the Expected Yield Strength, Ry Fy of

an adjoining member.

The Expected Tensile Strength, Rt Fu and the

Expected Yield Strength, Ry Fy may be used to

compute the nominal strength for rupture and

yielding limit states within the same member.

AISC Seismic Provis ions:

6.2 Material Propert ies for Determination of Required

Strength of Members and Connections (cont)

Example: SCBF Brace and Brace Connection

To size brace member:

Required Strength defined by code

specified forces (using ASCE-7 load

combinations)

Design Strength of member computed

using minimum specified Fy


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Example: SCBF Brace and Brace Connection (cont)

Required Axial Tension Strength of

brace connection is the expected yield

strength of bracing member = Ry FyAg

Ry FyAg


Ry FyAg

Gusset Plate:

Compute design strength using min

specified Fy and Fu of gusset platematerial


Ry FyAg

Bolts:

Compute design shear strength using

min specified Fu of bolt


Ry FyAg

Net Section Fracture and Block Shear

Fracture of Bracing Member:

Compute design strength using

expected yield strength, RyFy and

expected tensile strength, RtF

uof the

brace material.


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Section 7Section 7

Connections, Joint s and FastenersConnections, Join ts and Fasteners


7.1 Scope

7.2 Bolted Joints

7.3 Welded Joints

7.4 Protected Zone

7.5 Continuity Plates and Stiffeners


7.Connections, Joint s and Fasteners

7.1 Scope

Connections, joints and fasteners that are part of the seismic

load resisting system (SLRS) shall comply with the AISC

Specification Chapter J, and with the additional requirements in

this section.

Connections in the SLRS shall be configured such that a ductile

limit state in either the connection or in the connected membercontrols the design.



7.2 Bolted Joints

Requirements for bolted joints:

All bolts must be high strength (A325 or A490)

Bolted joints may be designed as bearing type connections, but must

be constructed as slip critical

- bolts must be pretensioned- faying surfaces must satisfy Class A surface requirements

Holes: standard size or short-slots perpendicular to load

(exception: oversize holes are permitted for diagonal brace

connections, but the connection must be designed as slip-critical and

the oversize hole is permitted in one ply only)

Nominal bearing strength at bolt holes cannot exceed 2.4 d t Fu



7.2 Bolted Joints (cont)

Bolts and welds shall not be designed to share force in a joint, or the

same force component in a connection.


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Bolts and welds sharing same

force:

Not Permitted

Fig. C-I-7.1a. Desirable details that avoid shared forces between welds and bolts.



7.3 Welded Join ts

Welding shall be performed in accordance

with Appendix W

Welding shall be performed in accordance with a

welding procedure specification (WPS) as required

in AWS D1.1 and approved by the engineer of

record.

WPS variables (voltage, current, wire feed speed,

etc) shall be within the limits recommended by the

filler metal manufacturer.



7.3a Welded Joints - General Requirements

All welds in the SLRS shall have a minimum

Charpy V-Notch (CVN) toughness of:

20 ft-lbs at 0 oF

CVN rating of filler metal may be determined using

AWS classification test methods.


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7.3b Welded Joints - Demand Criti cal Welds

Welds designated as Demand Critical shall have a

minimum Charpy V-Notch (CVN) toughness of:

20 ft-lbs at -20 oF (per AWS test methods)

AND

40 ft-lbs at 70 oF (per AISC SeismicProvisions - Appendix X)



7.4 Protected ZonePortions of the SLRS designated as a Protected Zone,

shall comply with the following:

No welded shear studs are permitted.

No decking attachments that penetrate the beam

flange are permitted (no powder actuated fasteners);

but, decking arc spot welds are permitted.

No welded, bolted, screwed, or shot-in attachments

for edge angles, exterior facades, partitions, duct

work, piping, etc are permitted.

Discontinuities from fabrication or erection operations

(such as tack welds, erection aids, etc) shall be

repaired.

Examples of Protected Zones: SMF

Protected Zones

Examples of Protected Zones: SCBF

Protected Zones


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Examples of Protected Zones: EBF

Protected Zones

Section 8Section 8

MembersMembers


8.1 Scope

8.2 Classification of Sections for Local

Buckling

8.3 Column Strength

8.4 Column Splices

8.5 Column Bases


8.2 Classification of Sections for Local Buckling

Local buckling of members can significantly affect both strength and

ductility of the member.

Members of the SLRS that are expected to experience significant

inelastic action (e.g. beams in SMF, braces in SCBF, links in EBF,

etc), must satisfy strict width-thickness limits to assure adequateductility can be developed prior to local buckling.

Such members must be seismically compact.

For seismically compact sections, the width-thickness ratios of the

elements of the cross-section cannot exceed ps, as specified in

Table I-8-1.

Local buckling of a moment frame beam.....


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Local buckling of an EBF link..... Local buckling of an HSS column....

Local buckling of an HSS brace.....

M

Mp

Increasing b / t

Effect of Local Buckling on Flexural Strength and Ductility

M


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8.3 Column Strength (cont)

Exception:

The required axial compressive and tensile strength of

a column need not exceed:

a) The maximum load transferred to the column

considering 1.1Ry times the nominal strengths of

the connecting beam or brace elements

b) The limit as determined from the resistance of thefoundation to overturning uplift.

11 -- Introduction and Basic PrinciplesIntroduction and Basic Principles

Performance of Steel Buildings in Past Earthquakes

Codes for Seismic Resistant Steel Buildings

Building Code Philosophy

Overview of AISC Seismic Provisions

AISC Seismic Provisions - General Requirements

Applicable to All Steel Systems

seismic-resistant steel design - intro

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