Seismic

1

Seismic Design of Steel Structures Amit H. Varma and Judy Liu

CE697R

Fall 2012

MWF 2:30 – 3:20 PM

CIVL 2123

2

Course Introduction

• Syllabus, Course Organization

• CE 697R Topics

• Introduction

• Basic Principles

3

Syllabus Review syllabus; make sure that you

understand all course policies (e.g.

grading, ethics, etc.) and procedures in

event of an emergency.

4

https://engineering.purdue.edu/Intranet/Groups/Administration/RPM/Safety/Classroom

EmergencyPlanning/CIVL

https://engineering.purdue.edu/Intranet/Groups/Administration/RPM/Safety/ClassroomEmergencyPlanning/CIVL

https://engineering.purdue.edu/Intranet/Groups/Administration/RPM/Safety/ClassroomEmergencyPlanning/CIVL

5

Required Book

• Bruneau, M., Uang, C., Sabelli, R. Ductile Design of Steel Structures, McGraw-Hill, New York, NY, 2011.

http://www.michelbruneau.com/Ductile

%20Design%202nd%20Ed%20-

%20Errata.pdf

Errata (8/8/12 file also

posted to CE697 Dropbox)

http://www.michelbruneau.com/Ductile Design 2nd Ed - Errata.pdf




6

Additional References

• Will be made available in shared folder on Dropbox or otherwise

Respond to e-mail with:

The e-mail address

associated with your

existing Dropbox account.

OR

E-mail address you’d like for

us to use in our invitation

to join Dropbox and shared

folder.

Please wait for

e-mail

invitation to

join Dropbox !!

7

Some References • 2010 Seismic Provisions for Structural Steel

Buildings, ANSI/AISC 341-10

• 2010 Prequalified Connections for Special and Intermediate Steel Moment Frames for Seismic Applications, with 2011 Supp. No. 1,ANSI/AISC 358-10, with ANSI/AISC 358s1-11

• Seismic Rehabilitation of Existing Buildings, ASCE/SEI 41-06

• NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures, FEMA 450, 2003

• Recommended Seismic Design Criteria for New Steel Moment-Frame Buildings, FEMA 350, 2000

• Minimum Design Loads for Buildings and Other Structures, ASCE 7-10

8

9

10

Course Project

Will also send e-mail requesting

information to help us form teams.

11

Homework / Reading Assignments

Files

12

13

CE697R Topics

• Introduction and Basic Principles

• Structural Steel, Properties, Plastic Behavior

• Moment Resisting Frames

• Steel Plate Shear Walls

• Braced Frames – Concentrically, Eccentrically Braced;

Buckling-Restrained

• Analysis for Performance Evaluation

• Special Topics / Innovative Systems

14

Acknowledgments

• Michael D. Engelhardt , Ph.D. – Professor, University of Texas at Austin

– Eccentrically Braced Frames, with Egor Popov, U.C. Berkeley

– T.R. Higgins Award for “Design of Reduced Beam Section Moment Connections.”

• AISC Educator Career Enhancement Award to develop Teaching Modules on Design of Seismic-Resistant Steel Buildings

Design of Seismic-

Resistant Steel

Building Structures

Prepared by:

Michael D. Engelhardt

University of Texas at Austin

with the support of the

American Institute of Steel Construction.

Version 1 - March 2007

16

Introduction 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 – General Requirements

17



Earthquakes





18

Causes of Earthquake Fatalities: 1900 to 1990

EERI slide series entitled: "Structural and Nonstructural Failures in Past Earthquakes."

Recent Earthquakes

• 2010 Haiti Earthquake

• 2010 Maule, Chile Earthquake

• 2010 -2011 Christchurch, New Zealand

• 2011 Tohoku, Japan

– Steel Reinforced

Concrete (SRC) buildings

- Tsunami damage

industrial steel buildings

and residences

19

http://www.aisc.org/uploadedcontent/2012

NASCCSessions/N9-1/

http://www.aisc.org/uploadedcontent/2012NASCCSessions/N9-1/




Recent Earthquakes

• 2010 -2011 Christchurch, New Zealand

– 6 damaging earthquakes

– Steel structures generally performed well

– Most steel buildings constructed from 1990s (modern seismic codes)

– A few EBF link fractures, CBF brace fracture (design/as-built detailing issues?)

20 Fractured EBF links Intact gusset plate and endplate

21

Why the good track record for steel?

• Little loss of life attributed to collapse of steel buildings in earthquakes

• Likely causes? Steel structures … – are generally lighter than masonry or RC. Lower

weight translates to lower seismic forces.

– typically show good ductility, even when not specifically designed or detailed for seismic resistance.

– have not been exposed as much to strong earthquakes. Highly destructive earthquakes around the world have generally occurred in areas where there are very few steel structures.

22

However ….

… modern welded steel buildings had shown an increasing number of problems in „recent‟ earthquakes.

Pino Suarez Complex

1985 Mexico City Earthquake

23 1994 Northridge Earthquake

24 1994 Northridge Earthquake

25 1995 Hyogoken-Nanbu (Kobe) Earthquake

26

1995 Hyogoken-Nanbu (Kobe) Earthquake

•Approximately 90 steel buildings collapsed

•Most heavily damaged steel buildings constructed before Japan‟s current design code adopted (1981)

•But, even modern steel buildings showed unexpected damage, including fractures at welded beam-to-column connections

28 1995 Hyogoken-Nanbu (Kobe) Earthquake

29

Good Track Record?

• „Recent‟ earthquakes (1985 Mexico City; 1994 Northridge; 1995 Hyogoken-Nanbu) have exposed problems with modern welded steel structures

• Care in the design, detailing, and construction of steel structures needed to assure satisfactory performance

• This has led to the development of building code regulations that specifically address seismic detailing of steel building structures.

30



Earthquakes





31

• Structural Engineers Association of California (SEAOC) Blue Book – 1988: First comprehensive detailing provisions for steel

• American Institute of Steel Construction (AISC) Seismic Provisions

– 1st ed. 1990

– 2nd ed. 1992

– 3rd ed. 1997

» Supplement No. 1: February 1999

» Supplement No. 2: November 2000

– 4th ed. 2002

– 5th ed. 2005

– 6th ed. 2010

US Seismic Code Provisions for Steel

Northridge & Kobe

research findings

32



Earthquakes





33

Conventional 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

Prevent loss of life

34

Maximum Considered Earthquake

• “extreme earthquake” = Maximum Considered Earthquake (MCE)

– In the western U.S., MCE based on the largest earthquake that can be generated by known faults

– In the rest of the U.S., MCE defined as an earthquake with a 2-percent probability of exceedance in 50 years

• recurrence interval of about 2500 years

• In MCE, can expect substantial and costly damage to the structure

35

Engelhardt’s Car Analogy

In the event of a major collision, the design goal is to protect the occupants of the car; not to protect the car itself.

In the event of a major earthquake, a building is used in a sacrificial manner to absorb the energy of the earthquake, in order to prevent collapse and protect the occupants.

36

The key to an economical design for a building which must withstand a very strong earthquake?

Design for Ductile Behavior

HIGH

STRENGTH? Let me know if you can find

“ductile burrito” video clip!

DUCTILITY?

37

H

H Ductility = Inelastic Deformation

38

H

Δyield Δfailure

Ductility Factor μ = Δfailure

Δyield

H

39

H

Strength

Required

Ductility

MAX

Helastic

3/4 *Helastic

1/2 *Helastic

1/4 *Helastic

H

40

Strength

Required

Ductility

H

MAX

Helastic

3/4 *Helastic

1/2 *Helastic

1/4 *Helastic

•Trade-off between strength and ductility

•Ductility means damage

•For a structure designed to yield in an earthquake, the maximum lateral force that the structure will see during the earthquake is defined by its own lateral strength

•A typical code-based design uses ductility

41

H Ductility = Yielding

Failure =

Fracture

or

Instability

Ductility in Steel Structures: Yielding

Nonductile Failure Modes: Fracture or Instability

WILL NOT COLLAPSE

42

• Choose frame elements ("fuses") that will

yield in an earthquake; e.g. beams in

moment resisting frames, braces in

concentrically braced frames, links in

eccentrically braced frames, etc.

Developing Ductile Behavior

43

• Detail "fuses" to sustain large inelastic

deformations prior to the onset of fracture

or instability (i.e. , detail fuses for ductility).


M

q

44

• Design frame elements to be stronger than

the fuses, i.e., design all other frame

elements to develop the plastic capacity of

the fuses.


CAPACITY DESIGN CONCEPT

45

(a) (b)

Less Ductile Behavior

Ductility of Steel Frames

More Ductile Behavior

46

Ductility of Steel Frames – “Backbone” Curve

47

Key Elements of Seismic-Resistant Design

Lateral Forces - Strength & Stiffness

ASCE-7 (Minimum Design Loads for Buildings and Other Structures)

National Earthquake Hazards Reduction Program (NEHRP) Provisions

Detailing Requirements - Ductility

AISC Seismic Provisions

H

H Ductility = Inelastic

Deformation

48

Design EQ Loads – Base Shear per ASCE 7-10:

V S I

R W

T R W

DS = S I D1

Strength

Required

Ductility

response modification coefficient

What does it mean if R = 1.0?

R> 1.0?

49

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

H

MAX

Helastic

3/4 *Helastic

1/2 *Helastic

1/4 *Helastic

50

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 Braced Frames): R = 8 SCBF (Special Concentrically Braced Frames): R = 6 OCBF (Ordinary Concentrically Braced Frames): R = 3.25 BRBF (Buckling Restrained Braced Frame): R = 8 SPSW (Special Plate Shear Walls): R = 7

51

R factors for Selected Steel Systems (ASCE 7): Undetailed Steel Systems in Seismic Design Categories A, or B or C with R = 3

This availability of this option reflects the

view that a steel structure, even without

special seismic detailing, will generally exhibit

some reasonable degree of ductility.

AISC Seismic Provisions not needed; follow main AISC specification

R-factors

• How were current R-factors determined?

• R-factors for new systems?

– ATC-63 project

52

http://peer.berkeley.edu/tbi/wp-content/uploads/2010/09/Heintz_ATC-63.pdf

Some background:






53



Earthquakes





54

2010 AISC

Seismic

Provisions

55

AISC Seismic Provisions for

Structural Steel Buildings

Symbols, Glossary, Acronyms

A. General Requirements

B. General Design Requirements

C. Analysis

D. General Member and Connection Design Requirements

E. Moment-Frame Systems

F. Braced-Frame and Shear-Wall Systems

G. Composite Moment-Frame Systems

H. Composite Braced-Frame and Shear-Wall Systems

cont’d

56

I. Fabrication and Erection

J. Quality Control and Quality Assurance

K. Prequalification and Cyclic Qualification Testing Provisions

Commentary A-K

References


Structural Steel Buildings, cont’d

57




A1. Scope

A2. Referenced Specifications, Codes and Standards

A3. Materials

A4. Structural Design Drawings and Specifications


B1. General Seismic Design Requirements

B2. Loads and Load Combinations

B3. Design Basis (Required Strength/Available Strength)

B4. System Type

58



C. Analysis

C1. General Requirements

C2. Additional Requirements

C3. Nonlinear Analysis

D. General Member and Connection Design Requirements

D1. Member Requirements

D2. Connections

D3. Deformation Compatibility of Non-SFRS Members and

Connections

D4. H-Piles

New chapter,

more of a

“pointer” to other

sections and

documents

59



Earthquakes





60

2010 AISC Seismic Provisions

General Provisions Applicable to All Systems

Highlights of Glossary

and Chapters A-D

61

AISC Seismic Provisions:

Glossary - Selected Terms

Applicable Building Code (ABC)

ABC = Building code under which the structure is

designed (the local building code that

governs the design of the structure)

Where there is no local building code - use ASCE 7

We will use ASCE 7 in this course.

(Int’l Bldg Code (IBC), referenced by

Indiana Building Code, takes seismic

design requirements from ASCE 7)

62

Seismic Force Resisting System (SFRS)

That part of the structural system that has been considered in

the design to provide the required resistance to the seismic

forces prescribed in ASCE/SEI 7.

Assembly of structural elements in the building that resists

seismic loads, including struts, collectors, chords,

diaphragms and trusses



www.atcouncil.org/pdfs/bp1d.pdf

63

Use or Occupancy of Buildings and Structures

Risk Category

Essential facilities

(Hospitals, fire and police stations, emergency shelters, etc)

Structures containing extremely hazardous materials

IV

Structures that could pose a substantial hazard to human

life, substantial economic impact, and/or mass disruption

of day-to-day civilian life in the event of failure

(previously defined as buildings with large assembly areas,

etc., could include facilities with hazardous materials)

III

Buildings not in Risk Categories I, III, or IV

(most buildings) II

Buildings that represent a low risk to human life in the

event of failure

(agricultural facilities, temporary facilities, minor storage

facilities)

I

Risk Category – classification as specified by

applicable building code (ASCE 7)

64

Seismic Design

Category (SDC):

ASCE 7

Classification

assigned to a

building by the

applicable building

code based upon its

risk category and the

design spectral

response

acceleration

coefficients.


Glossary

65

Seismic Design Category (SDC)

SDCs:

A

B

C

D

E

F



Increasing seismic

risk

and

Increasingly

stringent seismic

design and detailing

requirements

66

To Determine the Seismic Design Category (ASCE 7-10):

Determine Risk Category

Determine SS and S1 SS = spectral response acceleration for maximum considered earthquake at short periods

S1 = spectral response acceleration for maximum considered earthquake at 1-sec period Ss and S1 are read from maps

Determine Site Class Site Class depends on soils conditions - classified according to shear wave velocity

Determine SMS and SM1 Spectral response accelerations for maximum considered earthquake

adjusted for the Site Class;

SMS = Fa Ss SM1 = Fv S1

Fa and Fv depend on Site Class and on Ss and S1

Determine SDS and SD1 Design spectral response accelerations

SDS = 2/3 x SMS SD1 = 2/3 x SM1

67 Map for S1 (ASCE 7)

Seismic Hazard Maps

• Interactive program available from USGS website.

– Seismic design values for buildings

– Input longitude and latitude at site, or zip code

– Output SS and S1

• http://earthquake.usgs.gov/research/hazmaps/design/

To Determine the Seismic Design Category (ASCE 7-10):

Evaluate Seismic Design Category according to

Tables 11.6-1 and 11.6-2;

The Seismic Design Category is the more severe value based on

both Tables.

For sites with S1 ≥ 0.75g: Seismic Design Category = E for I, II, or III

Seismic Design Category = F for IV

71




A1. Scope

A2. Referenced Specifications, Codes and Standards

A3. Materials

A4. Structural Design Drawings and Specifications


B1. General Seismic Design Requirements

B2. Loads and Load Combinations

B3. Design Basis (Required Strength/Available Strength)

B4. System Type

72


Section A1. Scope

The Seismic Provisions shall govern the design,

fabrication and erection of structural steel members and

connections in the seismic force resisting systems

(SFRS), and splices and bases of columns in gravity

framing systems of buildings and other structures with

moment frames, braced frames and shear walls.

The Seismic Provisions are used in conjunction

with the AISC Specification for Structural Steel

Buildings

Both are in Unified LRFD-ASD format

73

Use of Seismic Provisions is mandatory for

Seismic Design Category D, E or F.

Use of Seismic Provisions are mandatory for

Seismic Design Categories B or C, when using

R > 3

For Seismic Design Categories B or C: can design

using R=3 and provide no special detailing (just

design per main AISC Specification)

SDC A designed following ASCE 7 Section 1.4; AISC

Seismic Provisions do not apply.


Section A1. Scope (cont’d.)

74


Section B1. General Seismic Design Requirements

Go to the Applicable Building Code for:

• Seismic Design Category

• Risk Categories

• Limits on Height and Irregularity

• Drift Limitations

• Required Strength

75


Section B2. Loads and Load Combinations

Go to the Applicable Building Code

Section B3.1 Required Strength

Greater of

1) as determined by analysis, or

2) as determined by AISC Seismic Provisions

Chapter C. Analysis

Follow requirements of Applicable Building Code,

AISC Seismic Provisions, AISC Specification;

nonlinear analysis per Chapter 16 of ASCE 7

76

Basic LRFD Load Combinations (ASCE-7):

1.4D

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

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

1.2D + 1.0W + L + 0.5(Lr or S or R)

0.9D + 1.0W

1.2D + 1.0E + L + 0.2S

0.9D + 1.0E Load Combinations

Including E

77

Definition of E for use in basic load combinations:

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

E = ρ QE + 0.2 SDS D

For Load Combination: 0.9D + 1.0E

E = ρ QE - 0.2 SDS D

78

E = ρ QE 0.2 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

ρ = reliability factor

(depends on extent of redundancy in the

seismic lateral resisting system;

ρ varies from 1.0 to 1.3)

79

Substitute E into basic load combinations:


substitute: E = ρ QE + 0.2 SDS D


substitute: E = ρ QE - 0.2 SDS D

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

(1.2 - 0.2 SDS) D + 1.0 ρ QE

80


B2. Loads and Load Combinations (cont’d.)

Where amplified seismic loads are required by

the AISC Seismic Provisions:

The horizontal portion of the earthquake load E

shall be multiplied by the overstrength factor o

prescribed by the applicable building code.

81

Definition of Amplified Seismic Load (ASCE-7)


E = Ωo QE + 0.2 SDS D


Amplified Seismic Load:

E = Ωo QE - 0.2 SDS D Amplified Seismic Load:

82

Basic load combinations incorporating Amplified

Seismic Load:


substitute: E = Ωo QE + 0.2 SDS D


substitute: E = Ωo QE - 0.2 SDS D

(1.2 + 0.2 SDS) D + Ωo QE + L +0.2S

(0.9 - 0.2 SDS) D + Ωo QE

83

Seismic Overstrength Factor: Ωo

System Ωo

Moment Frames (SMF, IMF, OMF)

Concentrically Braced Frames (SCBF, OCBF)

Eccentrically Braced Frames (EBF)

Special Plate Shear Walls (SPSW)

Buckling Restrained Braced Frames (BRBF)

3

2

2

2

2.5

Per ASCE-7:

84

Amplified Seismic Load

Late

ral S

eis

mic

Fo

rce

Frame Lateral Deflection

Qe

Ωo Qe

Amplified Seismic Load, ΩoQe, is intended to provide an

estimate of a frame's plastic lateral strength

85

Amplified Seismic Load, cont’d

Late

ral S

eis

mic

Fo

rce

Frame Lateral Deflection

Qe

Ωo Qe

•Reasons for overstrength

– Use of resistance factors

– Actual yield stress

– Members sized to satisfy drift limits

– Members sized to simplify design and construction

– Increase in strength in going from 1st plastic hinge to plastic mechanism

86


Section A3.1 Material Specifications

Limits and ASTM Specifications

Section A3.2 Expected Material Strength

For determining required strength as applicable

Section A3.3 Heavy Sections

Toughness requirements

Section A3.4 Consumables for Welding

SFRS, Demand Critical welds (discuss more later)

87


A3.1 Material Specifications

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, OCBFs , C-OMFs, C-

OBFs, C-OSWs (permitted to use up to Fy

= 55 ksi)

Grade 65 can be advantageous

To accommodate

materials commonly used

in metal building systems

88


A3.1 Material Specifications

For members in which inelastic behavior is

expected:

Specified minimum Fy ≤ 50 ksi

WHY?

Majority of experiments conducted

on seismic frame elements has

been for steels with specified yield

stress of 50 ksi and less.

Higher strength steels tend to be

more brittle.

89


A3.2 Expected Material Strength

Expected Yield Strength = Ry Fy

Expected Tensile Strength = Rt Fu

Fy = minimum specified yield strength

Fu = minimum specified tensile strength

Ry and Rt are based on statistical analysis of

mill data.

90



Ry

Rt

Added to Seismic Provisions after 1994

Northridge Earthquake

Added to Seismic Provisions more recently

for checks of fracture limit states in same

member for which expected yield stress is

used (motivated by Braced Frame design)

connections

1.1RyFyZ

connections

RyFyAg

91

92

93

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 ksi

Fu = 65 ksi

Ry Fy = 1.1 50 ksi = 55 ksi

Rt Fu = 1.1 65 ksi = 72 ksi

94

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.



95

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

96

Example: SCBF Brace and Brace Connection (cont)

Required Axial Tension Strength of

brace connection is the expected

yield strength of bracing member =

Ry Fy Ag

Ry Fy Ag

Note: no 1.1 multiplier for strain

hardening (used for moment

connections); braces exhibit little

strain hardening

97


Ry Fy Ag

Gusset Plate:

Compute design strength

using minimum specified Fy

and Fu of gusset plate material

Design strength

should exceed

Required Axial Tension

Strength of brace

98


Ry Fy Ag

Bolts:

Compute design shear

strength using minimum

specified Fu of bolt

Design strength

should exceed

Required Axial Tension

Strength of brace

99


Ry Fy Ag

Net Section Fracture and

Block Shear Fracture of

Bracing Member:

Compute design strength

using expected yield

strength, RyFy and expected

tensile strength, Rt Fu of the

brace material.

100


Ry Fy Ag

For example:

The required design

strength for limit states

of net section fracture

and block shear is

RyFyAg.

Block shear fracture:

[Ant RtFu + 0.6 AnvRtFu] ≤ [Ant RtFu + 0.6AgvRyFy]

Net section fracture:

AeRtFu

Whenever the required strength is based on the expected yield strength of

an element, then the design strength of that same element can be computed

using expected yield and tensile strength.

101


Section D1.3 Member Requirements: Protected Zones

Section D2.1 Connections: General

Section D2.2 Bolted Joints

Section D2.4 Continuity Plates and Stiffeners

Section D2.5 Column Splices

Section D2.6 Column Bases

Section D2.3 Welded Joints

Start here

and then

discuss

D1.3

Revisit Section A3.4 here

Discuss

with

“Members”

102


D2.1 Connections: General

Connections, joints and fasteners that are

part of the seismic force resisting system

(SFRS) shall comply with the AISC

Specification Chapter J, and with the

additional requirements in this section.

103


D2.2 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 reqs.

• 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 shall not be taken as

greater than 2.4 d t Fu

104


D2.2 Bolted Joints

Bolts and welds shall not be

designed to share force in a

joint, or the same force

component in a connection.

Not Permitted

105

Fig. C-D2.1. Desirable details that avoid shared forces between welds and bolts.

106

Fig. C-D2.1. Desirable details that avoid shared forces between welds and bolts.

107


D2.3 Welded Joints

Welded joints shall be designed in accordance with

Chapter J of the Specification.

All welds used in members and connections in the

SFRS shall be made with filler metals meeting the

requirements specified in clause 6.3 of Structural

Welding Code—Seismic Supplement (AWS

D1.8/D1.8M).

A3.4a Seismic Force Resisting System Welds

108


A3.4a Seismic Force Resisting System Welds

109


A3.4b Demand Critical Welds

Demand Critical – subjected to very high demands;

specifically identified in the Provisions in section

applicable to designated SFRS

Must ALSO satisfy:

110


D1.3 Protected Zone

Discontinuities specified in Section I2.1 resulting from

fabrication and erection procedures and from other

attachments are prohibited in the area of a member or a

connection element designated as a protected zone by

these Provisions or ANSI/AISC 358.

Exception: Welded steel headed stud anchors and other

connections are permitted in protected zones when

designated in ANSI/AISC 358, or as otherwise determined

with a connection prequalification in accordance with

Section K1, or as determined in a program of qualification

testing in accordance with Sections K2 and K3.

Attachments in the highly strained protected zones may

serve as fracture initiation sites

111


D1.3 Protected Zone

Some examples of prohibited attachments/

discontinuities:

• 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.

112

Examples of Protected Zones: SMF

Protected Zones

113

Examples of Protected Zones: SCBF

Protected Zones

114

Examples of Protected Zones: EBF

Protected Zones

115


Section D1.1 Member Requirements: Classification of

Sections for Ductility

Section D1.1a Section Requirements for Ductile

Members

Section D1.1b Width-to-Thickness Limitations of Steel

and Composite Sections

Section D2.5 Column Splices

Section D2.6 Column Bases

Section D1.4 Columns

Go back to:

116


Section D1.1 Member Requirements: Classification of

Sections for Ductility

Local buckling of members can significantly affect both

strength and ductility of the member.

When required for the systems defined in Chapters E, F,

G, H and Section D4, members designated as

moderately ductile members or highly ductile members

shall comply with this section.

Plastic rotation

0.02 rad or less

Plastic rotation

0.04 rad or more

117


Section D1.1a Section Requirements for Ductile Members

Structural steel sections for both moderately ductile

members and highly ductile members shall have

flanges continuously connected to the web or webs.

Section D1.1b Width-to-Thickness Limitations of Steel and

Composite Sections

For members designated as moderately ductile

members, the width-to-thickness ratios of compression

elements shall not exceed the limiting width-to-thickness

ratios, λmd, from Table D1.1.

For members designated as highly ductile members, the

width-to-thickness ratios of compression elements shall

not exceed the limiting width-to-thickness ratios, λhd,

from Table D1.1.

118

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

119

Local buckling of an EBF link.....

120

Local buckling of an HSS column....

121

Local buckling of an HSS brace.....

122

M

q

Mp

Increasing b / t

Effect of Local Buckling on Flexural Strength and Ductility

M

q

123

Mr

Mo

men

t C

apac

ity

md r

Width-Thickness Ratio

Mp

Plastic Buckling

Inelastic Buckling

Elastic Buckling

hd

Du

ctili

ty

Effect of Local Buckling on Flexural Strength and Ductility

124

125

126


D1.4a Columns: Required Strength

The required strength of columns in the SFRS shall be

determined from the following:

(1) The load effect resulting from the analysis

requirements for the applicable system

(2) The compressive axial strength and tensile strength as

determined using the load combinations stipulated in

the applicable building code including the amplified

seismic load. It is permitted to neglect applied moments

in this determination unless the moment results from a

load applied to the column between points of lateral

support. (1.2 + 0.2 SDS) D + Ωo QE + L +0.2S

(1.2 - 0.2 SDS) D + Ωo QE

127


D1.4a Columns: Required Strength

The required axial compressive strength and tensile

strength need not exceed either of the following:

(a) The maximum load transferred to the column by the

system, including the effects of material overstrength and

strain hardening in those members where

yielding is expected.

(b) The forces corresponding to the resistance of the

foundation to overturning uplift.


D2.5 Column Splices


D2.5 Column Splices

Column splices in any SFRS frame must satisfy

requirements of Section D1.4a (Required

Strength for Columns)

Additional requirements for columns splices are

specified for:

- Moment Frames (Chapter E)

- Braced Frames and Shear Walls (Chapter F)

- Composite Braced-Frame and Shear-Wall Systems

(Chapter H)


D2.5 Column Splices

The required strength

determined using the load

combinations stipulated in

the applicable building

code including the

amplified seismic load.

The required strength

need not exceed the

maximum loads that can

be transferred to the splice

by the system.

Pu - splice

Mu - splice

Vu - splice


D2.5 Column Splices

Welded column splices subjected to net

tension when subjected to amplified

seismic loads, shall satisfy both of the

following requirements:

1. If partial joint penetration (PJP) groove

welded joints are used, the design strength of

the PJP welds shall be at least 200-percent of

the required strength.

And....

2. The design strength of each flange splice

shall be at least 0.5 Ry Fy Af for the smaller

flange


D2.5 Column Splices

PJP Groove Weld

Stress concentration:

Fracture initiation

point.

Design PJP groove

weld for 200 % of

required strength

( PJP Groove welds not permitted in column splices

for Special and Intermediate Moment Frames)


D2.5 Column Splices

For all building columns including those

not designated as part of the SFRS, the

required shear strength of column splices

with respect to both orthogonal axes of the

column shall be Mpc/H (LRFD), where Mpc is

the lesser nominal plastic flexural strength

of the column sections for the direction

in question, and H is the height of the story.

The required shear strength of splices of

columns in the SFRS shall be the greater of

the above requirement or the required

shear strength determined per Section

D2.5b(a) and (b).


D2.5 Column Splices

4 ft. min

Splices made with fillet

welds or PJP welds shall

be located at least 4-ft.

from beam-to-column

connections

Seismic

Documents

basic principles

good track

aisc seismic

amplified

basic load

load combinations

structural

ry fy ag