NIGHT SCHOOL SEISMIC MANUAL AISC

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AISC Night SchoolNovember 23, 2015

Application of the AISC Seismic Design ManualSession 8: Buckling Restrained Braced Frames

and Quality Requirements

Copyright © 2015American Institute of Steel Construction

AISC Night School – Seismic Design Manual

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© The American Institute of Steel Construction 2015

The information presented herein is based on recognized engineering principles and is for generalinformation only. While it is believed to be accurate, this information should not be applied to anyspecific application without competent professional examination and verification by a licensedprofessional engineer. Anyone making use of this information assumes all liability arising fromsuch use.


Session 8: Buckling Restrained Braced Frames and Quality RequirementsNovember 23, 2015

Course Description

This session will define buckling-restrained braced frames and provide an overview of the design and test requirements per the AISC Seismic Provisions. A design example will be presented as part of the lecture. The session will then discuss Quality Control and Quality Assurance requirements per the Seismic Provisions.

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• Become familiar with the basis of buckling restrained brace frame(BRBF) design.• Gain an understanding of the differences between BRBF and typical concentrically

braced frames.• Gain an understanding of the BRBF system and connection requirements per the

AISC Seismic Provisions.• Become familiar with the Quality Control and Quality Assurance requirements per

chapter J of the AISC Seismic Provisions.

Learning Objectives

AISC Night School – Seismic Design Manual8

Presented by Thomas A. Sabol, Ph.D., S.E.Principal at Englekirk InstitutionalLos Angeles, CA

Application of the AISC Seismic Design ManualSession 8: Buckling Restrained Braced Braced Frames and Quality Requirements

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Chapter E

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Application of the

AISC Seismic Design Manual

Session 8


Last Session• Special Concentrically Braced Frames

• Examples from the Seismic Design Manual

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F4 Buckling-Restrained Braced Frame (BRBF)

Basis of Design

Specially fabricated braces connected concentrically to beams and columns

Eccentricities less than beam depth OK if considered in design and do not change source of inelastic deformation


Chapter F

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F4.2 BRBF – Basis of Design

BRBF are expected to withstand significant inelastic deformation (R = 8) in the links when subjected to the design earthquake.

Bracing members shall be composed of a structural core and a system that retrains steel core from buckling

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Braces shall be designed, tested and detailed to accommodate expected deformations

Expected deformations are minimum 2% story drift or 2 x (design story drift), whichever is larger, in addition to brace deformations

BRBF to be designed so that inelastic deformations under design earthquake occur as brace yielding in compression or tension



Typical brace behavior is asymmetric with respect to tension and compression and is subject to strength and stiffness degradation

Tension

CompressionPcr

Ry Ag Fy

P

Conventional brace behavior

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Balanced Hysteresis(Performance Definition)

Compression buckling of limited capacity tension member is resisted by “sleeve”

“Sleeve”

Compression and tension performance are nearly identical

P

Δ

β Ag Fy

Ag Fy



Advantages of BRBFBalanced Hysteresis

Slightly Stronger in Compression

Hysteretic Energy Dissipation

Hysteretic Stability Strength

Stiffness

Long Fracture Life

Ag Fy

β Ag Fy

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Chapter F

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Performance Advantages of BRBFStory Mechanisms Uncommon Fine-Tuning of Sizes

No Story Degradation

Distributed Yielding

Reduced Drift

No Chevron Problems


Chapter F

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Steel core shall be designed to resist entire axial force in the brace

Buckling-Restrained Brace Assembly

Core

Sleeve

+

=

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Chapter F

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Unbonded Brace Type

Decoupling

Yielding steel core

Unbonding material between steel core and mortar

BucklingRestraint

Encasing mortar

Steel tube


F4.2 BRBF – Basis of Design – Brace Design

Adjusted brace strength in compression: βωRyPysc

Where: β = compression adjustment factor

ω = strain hardening adjustment factor

Pysc = axial yield strength of steel core

Adjusted brace strength in tension: ωRyPysc

Note: Ry need not be applied if Pysc established using coupon tests

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Chapter F

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F4.2 BRBF – Basis of Design – Brace DesignCompression stress adjustment factor, β, shall be

calculated as ratio of maximum compression force to maximum tension force of test specimen per Section K3.4c

In no case shall β < 1.0


Chapter F

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F4.2 BRBF – Basis of Design – Brace DesignStrain hardening adjustment factor, ω, shall be

calculated as ratio of maximum tension force per Section K3.4c (for expected deformations) to measured yield force RyPysc

Where core material does not match prototype, ωshall be based on coupon test

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Chapter F

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F4.3 BRBF – AnalysisBRBF braces shall not be considered to resist

gravity forces

Required strength of columns based on load combinations including amplified seismic load

For amplified seismic load: effect of horizontal forces including overstrength Emh shall assume all braces achieve adjusted tension or compression strength


Chapter F

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F4.3 BRBF – AnalysisBraces shall be classified as in either tension or

compression ignoring gravity loads

Analyses shall consider both directions of seismic loading

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F4.3 BRBF – AnalysisExceptions:

May neglect flexural forces from seismic drift, but moment from load applied to the column between points of lateral support must be considered

Required strength of columns need not exceed lesser of

• Forces from foundation uplift

• Forces from nonlinear analysis per Section C3.


Chapter F

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F4.4 BRBF – System RequirementsV- and Inverted V-Braced Frames

Required strength of beams, connections and supporting members shall be determined assuming braces support no gravity loads

Vertical load effect on beam shall be determined using adjusted brace strength

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Chapter F

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F4.4 BRBF – System RequirementsV- and Inverted V-Braced Frames

Beams shall be continuous between columns and braced per moderately ductile requirements in Section D1.2(a)

See discussion of SCBF bracing requirements

For purposes of brace design, calculated maximum deformation of braces shall be increased to account for beam vertical deflection


Chapter F

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F4.4 BRBF – System RequirementsK-Braced Frames

Not permitted

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Chapter F

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F4.5 BRBF – Members – Basic RequirementsDiagonal Braces

Steel Core Plates used in steel core 2 in. thick or greater shall

satisfy minimum CVN requirements of Section A3.3

Splices in steel core are not permitted

Buckling-restraining system shall consist of casing of steel core

In stability calculations, beams, columns and gussets connecting the core shall be considered parts of the system


Typically, Fysc (min) = 38 ksi

F4.5 BRBF – Members – Basic RequirementsAvailable Strength

Steel core designed to resist entire axial force in the brace

The brace design axial force, φPysc, and brace allowable axial strength, Pysc/Ω, in tension and compression, according to limit state of yielding shall be:

Pysc = Fysc Asc

φ = 0.9 (LRFD) Ω = 1.67 (ASD)

whereAsc = cross-sectional area of yielding segment of steel coreFysc = specified minimum yield strength of steel core or actual yield stress of core as determined by coupon test

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F4.5 BRBF – Members – Basic RequirementsProtected Zones

Protected zones include steel core of braces and elements connecting core to beams and columns.


F4.6 BRBF – Members – ConnectionsDemand Critical Welds

Groove welds at column splices

Welds at the column-to-base plate connection unless column hinging at base can be shown to be precluded by conditions of restraint and absence of net tension (including amplified seismic loads)

Welds at beam-to-column connections per Section F4.6b(b)

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F4.6 BRBF – Members – ConnectionsBeam-to-Column Connections

Where brace or gusset connects to both members at beam-to-column connection:

Connection shall be “simple” per Specification Section B3.6a with required rotation of 0.025 rad…or…


F4.6 BRBF – Members – ConnectionsBeam-to-Column Connections

Where brace or gusset connects to both members at beam-to-column connection:

…or…Designed for moment to resist lesser of:

• 1.1 x (beam expected flexural strength = RyMp)

• Sum of 1.1 x (column expected flexural strength = Σ (RyFyZ)

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F4.6 BRBF – Members – ConnectionsDiagonal Brace Connections

Required strength of brace connections in tension and compression (including beam-to-column connections, if part of braced frame) shall be 1.1 x (adjusted brace strength in compression)

When oversized holes are used, required strength for bolt slip limit state need not exceed that from required load combinations, including amplified seismic load


F4.6 BRBF – Members – ConnectionsDiagonal Brace Connections

Connection design shall consider local and overall buckling. Provide lateral bracing consistent with applicable testing.

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F4.6 BRBF – Members – ConnectionsColumn Splices

Comply with Section D2.5

When groove welds are used, they shall be CJP

Column splice shall develop at least 50% of the flexural strength of smaller member

Required shear strength shall be ΣMpc/Hc where Mpc = FycZc of spliced columns and Hc is clear height of column (including slab)


SDM Example 5.5.1Example 5.5.1: BRBF Brace Design

This example shows the information needed by and

produced by the engineer of record

working with a brace manufacturer

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Given:

Refer to Brace BRB-1 in Figure 5-70. Design a buckling-restrained brace to resist the resulting axial loading, PQE = 113 kips.

SDM Example 5.5.1

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Note: There are two of these frames in the direction under consideration

BRB-1


Note: Some printings of the SDM text incorrectly have

“1.5” on page 5-419

Frame configurations and preliminary loads have been sent to a BRB manufacturer

Elastic stiffness of the braces have been found to be 1.28 times higher than the stiffness of the yielding core area alone, if it were extended from work-point to work-point (KF = Kactual/Kcore= 1.28) .

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These stiffness factors may be used to determine the horizontal load distribution on each story.

The applicable building code specifies the use of ASCE 7 for calculation of loads. According to AISC Seismic Provisions Section F4.3, buckling-restrained braces should not be considered as resisting gravity forces.

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Allow for material variability of 42 ksi ± 4 ksi.

From an elastic analysis, the first-order interstorydrift is ΔH = 0.223 in.

Assume that the ends of the brace are pinned and braced against translation for both the x-x and y-y axes.

38 ksiyscF =min 46 ksiyscF =max

SDM Example 5.5.1

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Solution:The governing load combinations in ASCE 7

including seismic effects are:LRFD ASD

LRFD Load Combinations 5 and 6 from ASCE 7 Section 12.4.2.3 (including the 0.5 factor on L permitted in Section 12.4.2.3)

ASD Load Combinations 5 and 8 from ASCE 7 Section 12.4.2.3

( )1.2 0.2 ρ 0.5 0.2EDSS D Q L S+ + + +

( )0.9 0.2 ρ 1.6EDSS D Q H− + +

( )1.0 0.14 0.7ρDS ES D H F Q+ + + +

( )0.6 0.14 0.7ρ EDSS D Q H− + +

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The required compressive and tensile strengths of the brace are:

LRFD ASD

ρ

1.3(113 kips)

147 kips

E

u u

Q

P T

P

====

( )0.7ρ

0.7 1.3 (113 kips)

103 kips

E

a a

Q

P T

P

===

=

44

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Required Strength

Consider second-order effects

AISC Specification Appendix 8 is used to addresssecond-order effects. The required second-orderaxial strength is:

2r nt ltP P B P= + (Spec. Eq. A-8-2)

SDM Example 5.5.1

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For the calculation of B2:

To determine Pstory, use an area of 9,000 ft2 on each floor and the surface gravity loads given in the BRBF Design Example Plan and Elevation section. Use load combinations that include seismic effects.

2

11 ( . Eq. A-8-6)

α1 story

e story

B SpecP

P

= ≥−

46

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LRFD ASD[ ]

( )( )( )

( )

[ ]( )( )

2

1.2 0.2(1.0)

68 psf 3 85 psf9,000 ft

0 psf 0.5 3 50 psf

0.2 20 psf

(1 kip/1,000 lb)

1.2 0.2(1.0)

+ 175 lb/ft 4 390 ft

(1 kip/1,000 lb)

5,160 kips

storyP

+

× + = + +

+ ×

+

× ×

=

[ ]( )

[ ]( )( )

2

1.0 0.14(1.0)

9,000 ft 68 psf 3 85 psf

0 psf 0 psf 0 psf

(1 kip/1,000 lb)

1.0 0.14(1.0)

+ 175 lb/ft 4 390 ft

(1 kip/1,000 lb)

3,630 kips

storyP

+

= × + + + +

×

+

× ×

=

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The total story shear, H, with two bays of bracing in the direction under consideration where each braced frame is designed to resist the seismic loads shown in Figure 5-70.

SDM Example 5.5.1

2(54.0 kips 49.0 kips 32.0 kips 16.0 kips)

302 kips

14.0 ft

1.0 for braced framesM

H

L

R

= + + +===

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54k x 2

49k x 2

32k x 2

16k x 2Note: There are two of these frames in the

direction under consideration

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Δe story M

H

HLP R= (Spec. Eq. A-8-7)

( )( )

302 kips 14.0 ft1.0

0.223 in. 1 ft/12 in.

228,000 kips

=

=

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From an elastic analysis, the first-order interstorydrift is ΔH = 0.223 in.


Using AISC Specification Equation A-8-6:

LRFD ASD

2

α 1.00

11

α1

11.00(5,160 kips)

1228,000 kips

1.02

story

e story

BP

P

=

= ≥−

=−

=

2

α 1.60

11

α1

11.60(3,630 kips)

1228,000 kips

1.03

story

e story

BP

P

=

= ≥−

=−

=

50

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Considering second-order effects, the required compressive and tensile strengths of the brace are:

Determination of the brace area required to resist the required brace strength must use the minimum yield of the core material, Fysc min.

LRFD ASD

( )1.02 147 kips

150 kips

u uP T===

( )1.03 103 kips

106 kips

a aP T===

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LRFD ASD

( )2

150 kips0.90 38 ksi

4.39 in.

usc

ysc

PA

F=

=

=

minminφ

( )

2

Ω

1.67 106 kips

38 ksi 4.66 in.

asc

ysc

PA

F=

=

=

minmin

For the limit state of tensile or compressive yielding, set the required strength equal to AISC Seismic Provisions Equation F4-1 and solve for Asc min:

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In design practice, either LRFD or ASD design should be used consistently. The two methods give slightly different results here.

In order not to show two separate designs, the LRFD result will be used.

Try a BRB with a core area, Asc, of 4.50 in.2

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Note that while BRB manufacturers can fabricate a BRB with the accuracy to which the core can be cut (generally ± 1/8 in. in width) it is common to round the required core area up to standard increments.

Generally, it is good practice to specify core areas in:

• 0.25 in.² increments for 0 in.² < Asc ≤ 5.00 in.²

• 0.50 in.² increments for 5.00 in.² < Asc ≤ 10.0 in.²

• 1.00 in.² increments for 10.0 in.² < Asc ≤ 20.0 in.²,

• 2.00 in.² increments for Asc > 20.0 in.²

• (or maintaining increment amounts in the range of 5% to 10% of the total amount).

SDM Example 5.5.1

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When specifying BRB area greater than required, the EOR must account for the increased demand that the specified area will place on the structure, because the beams and columns are designed to be stronger than the adjusted brace strength.

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For LRFD, the available axial strength for the limit state of tensile or compression yielding is:

n ysc scP F A=min minφ φ

( )( )20.90 38 ksi 4.50 in.

154 kips >150 kips o.k.

=

=

(Spec. Eq. D2-1)

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Verify with the brace manufacturer that the stiffness factor KF = 1.28 is acceptable for a 4.50 in.2 brace of this length.

The remainder of the brace design is performed by the BRB manufacturer.

Overstrength factors, β and ω, along with available stroke, the maximum deformation capability of the brace, must be provided by the brace manufacturer in order to design the columns and beams of the BRBF and to determine the BRB applicability to the design.

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The final part of the brace design is establishing the expected deformation of the brace and using this deformation to determine forces that the brace imposes on the columns, beams and connections.

AISC Seismic Provisions Section F4.2 requires consideration of deformations at the greater of 2% drift or two times the design story drift.

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The design story drift is defined in the AISC Seismic Provisions Glossary as the calculated story drift including the effect of expected inelastic action.

As given, the first-order interstory drift is ΔH = 0.223 in. This drift does not include the redundancy factor, ρ.

Note that ASCE 7 Section 12.3.4.1 permits ρ to be taken equal to 1 for drift calculations.

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The design story drift including inelastic action is:

Twice the story drift including inelastic action is:

( )

ΔΔ

5.0 0.223 in.

1.01.12 in.

d H

e

CI

=

=

=

(ASCE 7 Eq. 12.8-15)

( )2Δ 2 1.12 in.

2.24 in.

=

=

60

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2% drift corresponds to a deflection of:

( )

Δ 0.02

0.02(14.0 ft)

0.280 ft

Δ 0.280 ft 12 in./1 ft

3.36 in.

H====

=

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In this case, 2% drift governs. The brace spans 14.0 ft vertically and 12.5 ft horizontally. The brace deformation can be calculated to be:

( ) ( )

( ) ( )( )

2 2

2 2

14.0 ft 12.5 ft 0.280 ftΔ 12 in./1 ft

14.0 ft 12.5 ft

2.25 in.

br

+ + − = +

=

62

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Consulting with the brace manufacturer, the yield length for this brace is determined to be 70% of the work-point length.

The yield length is the length over which the core is expected to yield, and is typically equal to the length of casing.

( ) ( ) ( )2 2

0.7

0.7 14.0 ft 12.5 ft 12 in./1 ft

=158 in.

yL L≥

= +

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The strain is therefore:

Determination of the strain and the yield length is typically performed by the brace manufacturer and is shown here for illustrative purposes only.

Δ 2.25 in.ε

158 in.

1.42%

br

yL= =

=

64

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Consulting with the brace manufacturer, the ω andβ factors corresponding to this level of strain aredetermined to be: ω = 1.36 and β = 1.1

Alternatively, according to AISC SeismicProvisions Section F4.3 and ASCE 7 Chapter 16,brace deformation is permitted to be determinedfrom a nonlinear analysis in lieu of the expecteddeformation requirements in AISC SeismicProvisions Section F4.2 illustrated here.

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End of SDM Example 5.5.1

End of Example

Example 5.5.1: BRBF Brace Design

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J1 Scope

Quality Control and Quality Assurance

AISC emphasizes reliance on visual inspection rather than an over-reliance on nondestructive testing (NDT)

There are no QA/QC examples in the SDM, but if the built structure doesn’t

follow the design requirements, all the examples in the world won’t make any

difference


J1 Scope

Quality Control

Provided by the fabricator/erector

Quality assurance

Provided by others (e.g., third-party deputy inspectors)

NDT shall be provided by QA agency except as permitted by Specification Section N7.

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J1 Scope

Specification Chapter N (new for AISC 360-10) contains universal QA/QC requirements

Seismic Provisions Chapter J contains “seismic only” QA/QC requirements

The expectation is that will result in more appropriate inspection programs (e.g., engineers won’t invoke Seismic Provisions Chapter J requirements for gravity systems)


J5 Inspection Tasks

Tasks identified as:

• Observe (O): observe on a random, daily basis

• Perform (P): inspection shall be performed prior to final acceptance of the item

• Document (D): prepare reports indicating work has been performed in accordance with contract documents

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J6 (J7) Welding (Bolting) Inspection and NDT

Seismic Provisions Chapter J contains tables listing inspection/NDT tasks for welding and bolting for different stages of construction:

Prior to welding (bolting)

During welding (bolting)

After welding (bolting)

Distinguishes inspection work by QA and QC personnel

Also has a list of other inspection tasks (e.g., contour and finish on the RBS cut)



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NIGHT SCHOOL SEISMIC MANUAL AISC