STRC23 Lateral Forces Steel Seismic 0716

Structural Engineering Exam Review Course

Steel

Steel (Part 3) Structural Engineering Review Course

STRC ©2015 Professional Publications, Inc.

© 2015 Professional Publications, Inc.

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Steel

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Steel Part 3

Lesson Overview Seismic Design of Steel Structures •

seismic provisions

•

welded and bolted connections

•

ordinary moment frames

•

ordinary concentrically braced frames

•

intermediate moment frames

•

special concentrically braced frames

•

special moment frames

•

eccentrically braced frames

•

special moment frame connections

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Steel Part 3

Learning Objectives You will learn •

•

•

the difference between ordinary, intermediate, and special moment frames, and how to detail them the difference between ordinary and special concentrically braced frames, and how to detail them potential advantages of eccentrically braced frames

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Steel Part 3

Prerequisite Knowledge You should already be familiar with •

load combinations

•

design for flexure, shear, tension and compression

•

plastic design

•

bolted and welded connections

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Steel Part 3

Referenced Codes and Standards •

International Building Code (IBC, 2012)

•

Minimum Design Loads for Buildings and other Structures (ASCE/SEI7, 2010)

•

Prequalified Connections for Special and Intermediate Steel Moment Frames for Seismic Applications (ANSI/AISC 358, 2010)

•

Seismic Design Manual (AISC 327, 2012)

•

Seismic Provisions for Structural Steel Buildings (AISC 341, 2010)

•

Specification for Structural Steel Buildings (AISC 360, 2010)

•

Steel Construction Manual (AISC 325, 2011)

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Poll: Earthquake‐Resistant Structural Steel Systems Is the following statement true or false? Welded steel moment‐resisting frame systems demonstrated their superior performance during the magnitude 6 Northridge earthquake of 1994. (A) true (B) false

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Poll: Earthquake‐Resistant Structural Steel Systems Is the following statement true or false?

Solution

Welded steel moment‐resisting frame systems demonstrated their superior performance during the magnitude 6 Northridge earthquake of 1994.

Widespread and unanticipated brittle fractures occurred in welded steel beam‐ to‐column connections. The answer is (B).

(A) true (B) false

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Welded and Bolted Connections 1994 California Northridge earthquake •

•

damaged welded beam‐column connections in moment‐resisting frames cracking from beam bottom flange to column flange (sometimes propagated into column flange and/or beam web)

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Steel Part 3

Welded and Bolted Connections reasons for damage to welded steel moment frames in 1994 Northridge earthquake 1. limited number of frame bays

2. actual strengths did not meet code specifications 3. inappropriate detailing caused vast inelastic demands and stress concentrations at beam‐column connections 4. weak column panel zones 5. low ‐toughness weld material 6. inadequate welding quality control

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Steel Part 3

Welded and Bolted Connections response to 1994 Northridge earthquake •

•

•

Figure 10.5 Desired Plastic Hinge Behavior

Beam‐column connections must demonstrate (by cyclic load test or calculation) a 0.04 rad inelastic rotation capacity at special moment frames [AISC 341 Sec. E3.6b]. Effect of steel overstrength and strain hardening must be considered SMF plastic hinge locations should develop within the protected zone



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Steel Part 3

Design, Construction, and Seismic Provisions design, construction, and quality of steel components (IBC Chap. 22)

IBC refers to •

Buildings in seismic design categories A, B, and C need not be designed or detailed according to the structural steel seismic provisions [AISC 341 Sec. A1]. Use "Steel Systems Not Specifically Detailed for Seismic Resistance" per ASCE 7‐10 Table 12.2‐1.

•

•

ASCE/SEI7: load combinations and factors, application of seismic and wind requirements AISC 360: design and construction provisions AISC 341: seismic provisions

R = 3 (or less).



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Steel Frames SEIS Figure 10.1 Moment‐Resisting Joint

ductile steel frame •

•

most common structural system for tall buildings in seismically active areas

for New Construction Incorporating SidePlateTM Connection Technology.

moment‐resisting connections transmit column moments to beams and girders

special moment‐resisting frame (SMRF)

frames with beam‐column connections able to transmit moment = plastic capacity of the beam without failure SidePlateTM is the trademark of SidePlate Systems, Inc., of Long Beach, CA. STRC ©2015 Professional Publications, Inc.


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Steel Properties special moment frames •

use steel with Fy ≤ 50 ksi to ensure ductile behavior [AISC 341 Sec. A3.1]

•

Fy ≤ 50 ksi applies to components expected to yield

•

•

does not apply to •

grade 65 steel columns (columns not yielding elements)

•

grade D steel used for base plates

•

anchor bolts

additional Charpy V‐notch toughness requirements for hot‐rolled shapes with tf ≥ 1.5 in and plates with t ≥ 2 in [AISC 360 Sec. A3.1c and AISC 341 Sec. A3.3]



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Steel Properties ordinary moment frames

For ordinary moment frames and ordinary concentrically braced frames, Fy ≤ 55 ksi for components expected to yield to ensure ductile behavior.



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Expected Material Strength expected material strength •

•

AISC 341 Sec. A3.2 Table A3.1 Ry and Rt Values

use expected yield stress, RyFy, to calculate required strength [AISC 341 Sec. A3.2]

for Steel and Steel Reinforcement Materials

use expected tensile strength, RtFu, to calculate nominal strength, Rn [AISC 341 Sec. A3.2]



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Example: Ductility Rate the following moment and braced frames in order of increasing response modification coefficient, R. 1. ordinary moment frame (OMF) 2. intermediate moment frame (IMF) 3. special moment frame (SMF) 4. ordinary concentrically braced frame (OCBF) 5. special concentrically braced frame (SCBF) 6. eccentrically braced frame (EBF)



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Example: Ductility response modification coefficient, R*

frame type

4. ordinary concentrically braced frame (OCBF)

3.25

1. ordinary moment frame (OMF)

3.5

2. intermediate moment frame (IMF)

4.5

5. special concentrically braced frame (SCBF)

6.0

7. special truss moment frame (STMF)

7.0

3. special moment frame (SMF)

8.0

6. eccentrically braced frame (EBF)

8.0

*

from ASCE/SEI7 Table 12.2‐1



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Additional Seismic Force Resisting Systems Several additional seismic force resisting systems are not on the SE exam, but are acceptable according to AISC 341. •

special truss moment frames

•

buckling restrained braced frames

•

ordinary cantilever column systems

•

special cantilever column systems

•

special plate shearwalls

•

composite moment frames

•

composite braced frames

•

composite shear walls STRC ©2015 Professional Publications, Inc.


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Column Requirements AISC 341 Table D1.1 Limiting Width‐to‐Thickness

SFRS column requirements •

•

use amplified seismic loads (overstrength factor) combinations to determine Pr in the absence of column moments [AISC 341 Sec. D1.4a(2)].

Ratios For Compression Elements for Moderately Ductile and Highly Ductile Members

for moderately ductile members,

b/t ≤ λmd •

•

for highly ductile members, b/t ≤

λhd

for column splices, see AISC 341 Sec. D2



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Steel Part 3

Example: Steel Frames SEPPM Problem 3.1.9



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Steel Part 3

Example: Steel Frames SEPPM Problem 3.1.9



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Ordinary Moment Frames ordinary moment frames (AISC 341 Sec. E1) •

•

•

use is permitted, but discouraged by SEAOC Blue Book commentary resist seismic forces elastically, so likely to be heavier and costlier than SMRFs may have applications where seismic forces are low and design controlled by wind



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Ordinary Moment Frames basis of design (AISC 341 Sec. E1.2) •

OMFs designed in accordance with AISC 341 are expected to provide minimal inelastic deformation capacity in their members and connections.

•

no additional analysis requirements

•

no additional system requirements

•

not common due to limitations on use and tendency to be noneconomical



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Ordinary Moment Frames members (AISC 341 Sec. E1.5) •

no additional limitations on b/t ratios

•

no additional requirements for stability bracing of beams or joints

•

beams permitted to be composite with RC slab to resist gravity loads

•

no designated protected zones



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Ordinary Moment Frames connections (AISC 341 Sec. E1.6) •

•

filler metals

demand critical welds: complete‐joint‐ penetration (CJP) groove welds of beam flanges to columns [AISC 341 Sec. A3.4b and Sec. I2.3].

fully restrained (FR) and partially restrained (PR) moment connections permitted

•

•

classified using AWS A5 standards heat input envelope testing used to determine mechanical properties (yield strength, tensile strength, elongation, CVN toughness) specified in AWS D1.8/D1.8M



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Ordinary Moment Frames fully restrained moment connections (AISC 341 Sec. 1.6b) design requirement option (a) •

•

•

required flexural strength = expected beam flexural strength, RyMp, multiplied by 1.1 (LRFD) or by 1.1/1.5 (ASD).

Emh, horizontal force effects, amplified including overstrength Emh, including overstrength needed to calculate amplified seismic load and required shear strength is Emh = ((2)(1.1)RyMp)/Lcf.



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Ordinary Moment Frames fully restrained moment connections

‐

SEIS11 Fig. P 1 Connection No Longer Used

(AISC 341 Sec. 1.6b) design requirement option (b)

maximum moment and corresponding shear that can be transferred to the connection by the system

Continuity plates are needed for both options (a) and (b) [AISC Sec. J10.2‐J10.3].



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Ordinary Moment Frames fully restrained moment connections (AISC 341 Sec. 1.6b) design requirement option (c)

between wide flange beams and flange of wide flange, columns must satisfy AISC 341 Sec. E2.6, Sec. E3.6, or all of the following requirements •

all welds at beam‐to‐column connection satisfy ANSI/AISC 358 Chap. 3

•

beam flanges connected to column flanges using CJP groove welds

•

weld access holes in accordance with AWS D1.8/D1.8M

•

continuity plates satisfy AISC 341 Sec. 3.6f

•

beam web connected to column flange using either a CJP groove weld extending between weld access holes, or a bolted single plate shear connection designed for required shear strength per AISC 341 Eq. E1‐1. STRC ©2015 Professional Publications, Inc.


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Ordinary Moment Frames partially restrained moment connections (AISC 341 Sec. E1.6c) •

•

designed for maximum moment and shear from load combinations [AISC 341 Sec. B2 and B3]. The stiffness, strength, and deformation capacity of PR moment connections shall be considered in the design, including the effect on overall frame stability.

•

nominal flexural strength of the connection Mn,PR ≥ 0.5Mp of the connected beam. (for one‐story structures, Mn,PR ≥ 0.5Mp of the connected column)

•

Vu or Va determined per ASCE 341 Sec. E1.6b(a) with Mp in Eq. E1‐1 taken as Mn,PR.



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Intermediate Moment Frames basis of design (AISC 341 Sec. E2.2) •

•

•

IMFs expected to provide limited inelastic deformation capacity through flexural yielding of IMF beams and columns, and shear yielding of column panel zones design of connections of beams to columns, including panel zones and continuity plates, based on connection tests (i.e., connections designed per AISC 358, prequalified connections, etc.) [AISC 341 Sec. E2.6b and E2.6c] no additional analysis requirements



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Intermediate Moment Frames stability bracing of beams (AISC 341 Sec. E2.4)

Beams shall be braced to satisfy the requirements for moderately ductile members [AISC 341 Sec. D1.2a]. •

•

•

Both flanges of beams shall be laterally braced or the beam cross section shall be torsionally braced. beam bracing requirements [AISC 360 App. 6] beam bracing maximum spacing, Lb, is Lb = 0.17ryE/Fy

Beam braces shall be placed near concentrated forces, changes in cross section, and other locations where a plastic hinge is expected to form [AISC 341 Sec. D1.2c].



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Intermediate Moment Frames member requirements (AISC 341 Sec. E2.5) •

Beams and columns are moderately ductile members [AISC 341 Sec. D1].

•


•

abrupt changes in beam flange area not permitted in plastic hinge regions



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Welded and Bolted Connections SMF protected zones (AISC 341 Sec. E3) •

•

Figure 10.6 Connection Reinforcement Configurations

generally extends from end of stiffening to one‐half beam depth past expected location of plastic hinge. ensure that plastic hinge location develops within protected zone •

•

reinforce beam at connection decrease beam cross section at half beam depth from column face



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Welded and Bolted Connections SMF girder‐column joint restraints (AISC 341 Sec. E3.4)

Restrictions ensure that moment frames are capable of reaching seismic design capacity. •

•

•

If columns remain elastic, column flanges require lateral support only at the level of the top girder flange. column assumed to remain elastic if

M

* pc

M

* pb

 2.0

If a column does not remain elastic, column flange supports are required at tops and bottoms of girder flanges.



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Intermediate Moment Frames protected zones (AISC 341 Sec. E2.5c) •

•

•

region at end of each beam subject to inelastic strain [AISC 341 Sec. D1.3]

•

for IMF beams, plastic hinging zones at the beam ends are protected welded, bolted, screwed, and shot‐in connections not permitted in protected zones (includes shear studs or repair pins that penetrate the beam flange)

•

should be established as part of a prequalification or qualification program for the connection, per AISC 341 Sec E2.6c for unreinforced connections, extend from the face of the column to one half of the beam depth beyond the plastic hinge point



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Intermediate Moment Frames connections (AISC 341 Sec. E2.6) •

IMF connections designed in accordance with AISC 341 Sec. E2.6c

•

Demand critical welds must satisfy AISC 341 Sec. A3.4b and Sec. I2.3.

•

beam‐to‐column connections must •

•

accommodate story drift ≥ 0.02 rad measured flexural resistance ≥ 0.80Mp of connected beam at story drift of 0.02 rad



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Intermediate Moment Frames connections (AISC 341 Sec. E2.6) •

required shear strength based on amplified seismic load where

Emh = (2)(1.1)RyMp/Lh •

AISC 341 Eq. E2‐1

no additional panel zone requirements (but check panel zone shear strength) [AISC 341 Sec. E3.6e]

•

continuity plates required [AISC 341 Sec. E3.6f]

•

column splices comply with AISC 341 Sec. D2.5 [AISC 341 Sec. E2.6g]



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Special Moment Frames special moment frames (SMFs) •

•

•

ensure that joints and members behave in a ductile manner panel zone is normally the column web central to the beam‐column connection With specific limitations, inelastic behavior in these areas is permitted.



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Special Moment Frames For special moment‐resisting frames, the adequacy of the beam‐column connections is crucial to the integrity of the entire system. New guidelines adopted after 1994 Northridge earthquake •

•

•

doubler plates welded across the plate width top and bottom used to reduce panel zone shear stress or web depth/thickness ratio (minimum fillet weld is 3/16 in) continuity plates, column stiffeners at top and bottom of the panel zone, match prequalified connection details or the thickness of the column flange satisfies AISC 341 Sec. E3.6f.

minimum strength ratio given to prevent column failures at joints when beams are “stronger” than columns (“strong column‐weak beam” test; some exceptions permitted [AISC 341 Sec. E3.4a]) STRC ©2015 Professional Publications, Inc.


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Special Moment Frames basis of design (AISC 341 Sec. E3.2) •

SMFs provide significant inelastic deformation capacity through flexural yielding of the SMF beams and limited yielding of column panel zones.

•

• •

•

in general, columns stronger than fully yielded and strain‐hardened beams and girders

design of connections (including panel zones and continuity plates) based on cyclic test results no additional analysis requirements [AISC 341 Sec. E3.3]

flexural yielding of columns permitted at base



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Special Moment Frames moment ratio (AISC 341 Sec. E3.4) •

beam‐to‐column connection ratio AISC 341 Eq. E3‐1

•

•

sum of projections of beams’ expected flexural strengths at plastic hinge locations to column centerline

sum of projections of columns’ nominal flexural strengths AISC 341 Eq. E3‐2a

AISC 341 Eq. E3‐3a

AISC 341 Eq. E3‐3b

AISC 341 Eq. E3‐2b STRC ©2015 Professional Publications, Inc.


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Special Moment Frames moment ratio (AISC 341 Sec. E3.4)

permitted to determine ΣM*pc •

consistent with a prequalified connection design as designated in ANSI/AISI 358

•

as determined in a connection prequalification in accordance with AISC 341 Sec. K1

•

as determined in a program qualification testing in accordance with AISC 341 Sec. K2



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Special Moment Frames moment ratio (AISC 341 Sec. E3.4)

When connections with reduced beam sections are used, it is permitted to determine ΣM*pc as AISC 341 Eq. E3‐4a AISC 341 Eq. E3‐4b



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Special Moment Frames moment ratio exception (AISC 341 Sec. E3.4)

The beam‐to‐column connection ratio shall not apply to columns for which one of the following is true. •

•

Prc < 0.3Pc, and one of the following is true •

the building is one story or this is the top story of a building

•

either the exempted columns are < 20% of the story’s shear capacity

•

the exempted columns are < 33% of the shear capacity of that column line

The ratio of available shear strength to required shear strength is 50% greater in the story with the column in question than that of the story above it.



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Special Moment Frames stability bracing of beams (AISC 341 Sec. E3.4b) •

beams braced to satisfy requirements for highly ductile members [AISC 341 Sec. D1.2b]

•

beam cross section torsionally braced or both flanges of beams laterally braced

•

•

•

braces placed near concentrated forces, changes in cross section, and other locations where a plastic hinge will form during inelastic deformations [AISC 341 Sec. D1.2c] beam bracing requirements [AISC 360 App. 6] beam bracing maximum spacing, Lb = 0.086ryE/Fy



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Special Moment Frames stability bracing of beam‐to‐column connections (AISC 341 Sec. E3.4c) braced connections •

•

•

If columns are elastic, stability bracing is only necessary at the elevation of the top beam flanges (otherwise, column flanges braced at top and bottom beam flanges). If the beam‐to‐column connection ratio > 2.0 then the column may be assumed to be elastic outside of the panel zone. Each brace is designed for required strength equal to 2% of available beam flange strength, Fybf tbf (LRFD) or Fybf tbf /1.5 (ASD). Column flange bracing can be provided by continuity plates and a full‐depth shear plate. This occurs between the continuity plates at the connection of the girder framing into the weak axis of the column. STRC ©2015 Professional Publications, Inc.


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Special Moment Frames stability bracing of beam‐to‐column connections (AISC 341 Sec. E3.4c) unbraced connections •

•

•

If beam‐to‐column connection is unbraced, design per AISC 360 Chap. H, except as modified below. Design using distance between adjacent member braces as the column height for buckling transverse to the seismic frame. For amplified seismic load, Emh need not exceed 125% of the frame available strength based upon either the beam available flexural strength or panel zone available shear strength.

•

slenderness, L/r ≤ 60

•

Column required flexural strength includes amplified seismic load and second‐order load.



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Special Moment Frames member requirements (AISC 341 Sec. E3.5) •

Beams and columns are highly ductile members [AISC 341 Sec. D1.1].

•


•

abrupt changes in beam flange area not permitted in plastic hinge regions

•

region at each beam end is a protected zone subject to inelastic straining [AISC 341 Sec. D1.3]



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Special Moment Frame Connections demand critical welds (AISC 341 Sec. E3.6a) •

groove welds at column splices

•

welds at column‐to‐base plate connections

•

complete‐joint‐penetration groove welds of beam flanges and beam webs to columns

Demand critical welds must satisfy the requirements of AISC 341 Sec. A3.4b and I2.3.



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Special Moment Frame Connections beam‐to‐column connections (AISC 341 Sec. E3.6b) •

•

accommodate story drift angle of ≥ 0.04 rad measured flexural resistance of connection at column face ≥ 0.80Mp of the connected beam at a story drift angle of 0.04 rad



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Special Moment Frame Connections conformance demonstration (AISC 341 Sec. E3.6c)

Beam‐to‐column connections satisfy AISC 341 Sec. E3.6b by meeting one of the following conditions. •

SMF connections designed per ANSI/AISC 358.

•

connections prequalified for SMF per AISC 341 Sec. K1

•

connections pass at least two qualifying cyclic tests per AISC 341 Sec. K2 1. research literature or documented tests performed for other projects that represent the project conditions 2. tests conducted specifically for the project



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Special Moment Frame Connections required shear strength of connections (AISC 341 Sec. E3.6d) •

required shear strength based on amplified seismic load where

Emh = (2)(1.1)RyMp/Lh

AISC 341 Eq. E2‐1

exceptions •

specified in ANSI/AISC 358

•

determined in connection prequalification in AISC 341 Sec. K1

•

determined in program of qualification testing per AISC 341 Sec. K2



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Special Moment Frame Connections panel zones (AISC 341 Sec. E3.6e) •

•

•

SEIS Figure 10.2 Panel Zone

column web central to the beam‐ column connection (normally) required shear strength determined from summation of moments at column faces, by projecting expected moments at plastic hinge points to column faces individual thicknesses of column webs and doubler plates, t ≥ (dz + wz)/90



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Special Moment Frame Connections panel zone doubler plates (AISC 341 Sec. E3.6e(3))

If t < (dz + wz)/90, doubler plates are applied directly to column web (otherwise, can be spaced away from web) •

•

Doubler plates in contact with web are welded to column flanges to develop available strength of full doubler plate thickness, using CJP groove welded or fillet welded joint. Spaced doubler plates placed symmetrically in pairs and welded to column flanges to develop available strength of full doubler plate thickness, using a CJP groove welded joint. Doubler plates shall be and located between 1/3 and 2/3 of the distance between the beam flange tip and column centerline.



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Special Moment Frame Connections continuity plates (AISC 341 Sec. E3.6f)

If any of the following are true, continuity plates are not required. •

•

•

connection prequalification per AISC 341 Sec. K1 program of qualification testing per AISC 341 Sec. K2 if beam flange is welded to column flange, tcf  0.41 t b .8bfbf

t

R b yb F

and cf

Ryc Fyc

bf 

•

if beam flange is welded to column flange of a boxed wide‐flange column,



tcf  0.4 1  



and tcf 

b

bbf 12

bbf 

R F

yb yb  (1.8)bf b t b  R yc yc F 2 cf

AISC 341 Eq. E3‐10 AISC 341 Eq. E3‐11

6

AISC 341 Eq. E3‐8 and Eq. E3‐9 STRC ©2015 Professional Publications, Inc.


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Special Moment Frame Connections continuity plate thickness

continuity plate welding

•

for one‐sided connections, tcp ≥ 0.5tbf

•

for two‐sided connections, tcp ≥ tbf,max

•

conform to AISC 360 Sec. J10

•

•

welded to column flanges using CJP groove welds welded to column webs using groove welds or fillet welds



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Special Moment Frame Connections continuity plate weld strength (AISC 341 Sec. E3.6f)

required strength of the sum of the welded joints is the smallest of •

sum of the design strengths in tension of the contact areas of the continuity plates to the column flanges that have attached beam flanges

•

design strength in shear of the contact area of the plate with the column web

•

design strength in shear of the column panel zone

•

sum of the expected yield strengths of the beam flanges transmitting force to the continuity plates



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Example: Continuity Plates In a special moment frame, a W10x60 grade 50 beam flange is welded to the flange of a W24x250 grade 50 column. Are continuity plates are required?



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Example: Continuity Plates In a special moment frame, a W10x60 grade 50 beam flange is welded to the flange of a W24x250 grade 50 column. Are continuity plates are required?

Use AISC 341 Eq. E3‐8 and 3‐9 to determine whether a continuity plate is required. Since grade 50 steel is used for both the beam and the column, the R and F factors cancel out. 0.4 1.8bbf tbf  0.4 (1.8)(10.1)(0.680)

Solution

For grade 50 steel, the length of the beam flange, bbf, is 10.1 in. The thickness of the beam flange, tbf, is 0.680 in. The thickness of the column flange, tcf, is 1.89 in.

 1.27 [  tcf 1.89; satisfactory] bbf 6

10.1 6

 1.68 [  tcf 1.89; satisfactory] Both conditions are met, so continuity plates are not required.





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Special Moment Frame Connections column splices (AISC 341 Sec. E3.6g) •

must comply with AISC 341 Sec. D2.5

•

Welds used to make splice must be CJP groove welds.

•

•

for bolted column splices, required flexural strength ≥ RyFyZx (LRFD) or RyFyZx/1.5 (ASD). required shear strength of column web splices ≥ ΣMpc /H (LRFD) or ΣMpc /1.5H (ASD).



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Steel Part 3

Example: Special Moment Frames SXST Problem 23



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Steel

Steel Part 3

Example: Special Moment Frames SXST Problem 23



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Steel Part 3

Example: Special Moment Frames



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Steel Part 3

Braced Frames diagonal bracing diagonals connect the joints in adjacent levels

Figure 10.7 Types of Braced Frames*

chevron bracing a pair of braces terminate at a single point within the clear beam span V‐bracing form of chevron bracing that intersects a beam from above

K‐braced frames are not permitted for OCBFs or SCBFs.

*



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Braced Frames inverted V‐bracing form of chevron bracing that intersects a beam from below

Figure 10.7 Types of Braced Frames*

K‐bracing pair of braces located on one side of a column terminate at a single point within the clear column height X‐bracing pair of diagonal braces cross near midlength of the bracing members

K‐braced frames are not permitted for OCBFs or SCBFs.

*



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Braced Frames Figure 10.7 Types of Braced Frames*

ordinary concentrically braced frames (OCBF) •

•

R = 3.25 [ASCE/SEI7 Table 12.2‐1] Chevron‐braced frames must satisfy AISC 341 Sec. F1.4a for beam design.

special concentrically braced frames (SCBF) K‐braced frames are not permitted for OCBFs or SCBFs.

* •

•

R = 6 [ASCE/SEI7 Table 12.2‐1] tension‐only frames not permitted [AISC 341 Sec. F1.4b, Sec. F2.4c, and Sec. F2.4d].



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Ordinary Concentrically Braced Frames basis of design (AISC 341 Sec. F1) •

•

•

expected to provide limited inelastic deformation capacity in members and connections Eccentricities less than the beam depth are permitted if accounted for in member design by determination of eccentric moments using amplified seismic load. no additional analysis requirements



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Ordinary Concentrically Braced Frames system requirements (AISC 341 Sec. F1.4) •

•

•

required beam strength must account for the least of

Beams in V‐braced and inverted‐V‐ braced OCBFs are continuous at brace connections away from the beam‐ column connection. Assume a compression force of 0.3Pn in braces.

•

•

•

expected yield strength of brace in tension, RyFyAg amplified seismic load maximum force developed by the system

K‐braced frames are not permitted for OCBFs.



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Ordinary Concentrically Braced Frames members (AISC 341 Sec. F1.5) •

•

Braces meet requirements for moderately ductile members [ASCE 341 Sec. D1.1]. For V‐braced or inverted‐V‐braced frames, KL E 4 r F



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Ordinary Concentrically Braced Frames diagonal brace connections (AISC 341 Sec. F1.6a)

required strength = load effect based upon amplified seismic load required strength need not exceed •

•

•

in tension, RyFyAg (LRFD) or RyFyAg/1.5 (ASD) in compression, lesser of RyFyAg and 1.14FcreAg multiplied by 1.0 (LRFD) or divided by 1.5 (ASD) For oversized holes, do not include amplified seismic load for the limit state of bolt slip.



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Ordinary Concentrically Braced Frames OCBF above seismic isolation systems (AISC Sec. F1.7) •

•

beams in V‐braced and inverted‐V‐ braced frames continuous between columns braces have a slenderness ratio of KL E 4 r F



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Special Concentrically Braced Frames basis of design (AISC 341 Sec. F2) •

•

expected to provide significant inelastic deformation capacity primarily through brace buckling and yielding of brace in tension eccentricities < beam depth permitted if resulting member and connection forces •

are addressed in design, and

•

do not change expected source of inelastic deformation capacity



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Special Concentrically Braced Frames required strength of columns, beams, and connections •

•

based on the load combinations in the applicable building code that include the amplified seismic load When determining amplified seismic load, the effect of horizontal forces including overstrength, Emh, is taken as the larger force determined from the following two analyses. 1. All braces are assumed to resist forces corresponding to their expected strength in compression or in tension. 2. All braces in tension are assumed to resist forces corresponding to their expected strength and all braces in compression are assumed to resist their expected post‐buckling strength. STRC ©2015 Professional Publications, Inc.


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Special Concentrically Braced Frames analysis (AISC 341 Sec. F2.3) •

consider both loading directions

•

determine tension/compression in brace neglecting effect of gravity loads

•

•

•

expected brace strength in tension = RyFyAg expected brace strength in compression permitted to be taken as the lesser of RyFyAg and 1.14FcreAg [AISC 360 Chap. E] expected post‐buckling brace strength ≤ (0.3)(expected brace strength in compression)



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Special Concentrically Braced Frames analysis (AISC 341 Sec. F2.3)

required strength of columns needs not exceed •

amplified forces calculated with all compression braces removed

•

forces corresponding to resistance of foundation to overturning uplift

•

forces from nonlinear analysis [AISC 341 Sec. C3]



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Special Concentrically Braced Frames members (AISC 341 Sec. F2.5) •

Columns and braces are highly ductile members [AISC 341 Sec. D1.1].

•

Beams are moderately ductile members [AISC 341 Sec. D1.1].

•

protected zone satisfies AISC 341 Sec. D1.3 and includes •

•

for braces, the center one‐quarter of the brace length and a zone adjacent to each connection equal to the brace depth in the plane of buckling elements that connect braces to beams and columns



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Special Concentrically Braced Frames diagonal braces (AISC 341 Sec. F2.5b) •

slenderness ratio KL/r ≤ 200

•

for built‐up braces •

•

slenderness ratio, a/ri, of individual elements between connectors ≤ (0.4)(governing slenderness ratio of built‐up member) sum of available shear strengths of connectors ≥ available tensile strength of each element

•

uniform connector spacing

•

connectors not within middle one‐fourth of clear brace length



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Special Concentrically Braced Frames diagonal braces (AISC 341 Sec. F2.5b) •

brace effective net area ≥ brace gross area

•

where reinforcement on braces is used, •

•

specified minimum yield strength of the reinforcement ≥ specified minimum yield strength of the brace connections of reinforcement to the brace must have sufficient strength to develop the expected reinforcement strength on each side of a reduced section



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Special Concentrically Braced Frames

SEIS11 Figure 10.8 Angle Sections

SEIS11 Figure 10.9 Circular Sections



SEIS11 Figure 10.10 Rectangular Tubes

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Special Concentrically Braced Frames demand critical welds

The following welds are demand critical welds [AISC 341 Sec. A3.4b and I2.3]. •

groove welds at column splices

•

welds at column‐to‐base plate connections

•

welds at beam‐to‐column connections conforming to AISC 341 Sec. F2.6b(b)



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Special Concentrically Braced Frames beam‐to‐column connections

Where a brace or gusset plate connects to both beam and column, the connection must conform to one of the following. •

•

It is a simple connection per AISC 360 Sec. B3.6a (required rotation = 0.025 rad). It is designed to resist a moment equal to the lesser of the following moments (considered in combination with required strength of brace connection and beam connection, including amplified diaphragm collector forces). •

1.1RyMp (LRFD) or 1.1/1.5RyMp (ASD)

•

1.1Σ(RyFyZ) (LRFD) or 1.1/1.5Σ (RyFyZ) (ASD)



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Special Concentrically Braced Frames required strength of brace connections (AISC 341 Sec. F2.6) •

•

•

•

required tensile strength = lesser of RyFyAg (LRFD) or RyFyAg /1.5 (ASD), or maximum load effect transferred to brace by system required compressive strength based on buckling limit states brace connections designed to withstand flexural forces and rotations imposed by brace buckling inelastic rotation typically accommodated using a single gusset plate with the brace terminating before the line of restraint distance from brace to line of restraint ≥ 2t [AISC 341 Fig. C‐F2.9]



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Special Concentrically Braced Frames column splices (AISC 341 Sec. F2.6d) •

must comply with AISC 341 Sec. D2.5

•

CJP groove welds used to make the splice

•

designed to develop ≥ 50% of lesser flexural strength of connected members

•

required shear strength is ΣMpc /Hc (LRFD) or ΣMpc /(1.5Hc) (ASD)



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Steel Part 3

Example: Special Concentrically Braced Frames SEIS Practice Problem 44



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Steel Part 3

Example: Special Concentrically Braced Frames SEIS Practice Problem 44



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Steel Part 3

Example: Doubler Plates SEIS Practice Problem 73



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Steel

Steel Part 3

Example: Doubler Plates SEIS Practice Problem 73



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Eccentrically Braced Frames eccentrically braced frames (EBFs) (AISC 341 Sec. F3) low to moderate ground shaking: frame performs as a braced frame, small drifts, no structural damage

SEIS Figure 10.11 Types of Eccentrically Braced Frames

link beam •

•

short section of girder between brace end and column, often designed to be replaced following a seismic event dissipate seismic loads through significant inelastic deformation STRC ©2015 Professional Publications, Inc.


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Eccentrically Braced Frames eccentrically braced frames (EBFs) (AISC 341 Sec. F3) •

•

SEIS Figure 10.12 Detail of Original Eccentric Link Design

major seismic event: link beam designed to yield, absorbing seismic energy, preventing buckling of other bracing members

link beam requires attention to detail at the connection to limit yielding



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Eccentrically Braced Frames beam‐to‐column connections (AISC 341 Sec. F3) •

•

•

•

should be at lower end of diagonal rather than at top

SEIS Figure 10.13 Typical Deflections of Eccentric Links

high ends stressed to yielding; links at low end remain elastic shear stress in link beam usually limited to (0.90)(0.60)Fy=0.54Fy shear force on link beam = (story shear)(story height)/(bay length)



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Eccentrically Braced Frames beam‐to‐column connections (AISC 341 Sec. F3) •

•

For a given story height and distance between columns, frames with the same story shear will have the same link beam shear, regardless of their geometries

SEIS Figure 10.13 Typical Deflections of Eccentric Links

load in column = link beam shear SEIS Eq. 10.4 SEIS Eq. 10.5



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Eccentrically Braced Frames end link beam •

•

location of a link beam’s inflection point initially assumed until all member sizes are determined

SEIS Figure 10.14 End Link Beam

location at mid‐length of a link beam is a reasonable initial assumption SEIS Eq. 10.6

SEIS Eq. 10.7



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Steel Part 3

Eccentrically Braced Frames center link beam •

•

beam‐to‐column connection can be “pinned” or can be a moment connection designed per AISC 341 Sec. F4.6b.

SEIS Figure 10.15 Center Link Beam

link beam point of inflection at the vertical centerline of bay

SEIS Eq. 10.8

SEIS Eq. 10.9 STRC ©2015 Professional Publications, Inc.


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Steel Part 3

Example: Eccentrically Braced Frames SEIS Practice Problem 74



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Steel Part 3

Example: Eccentrically Braced Frames SEIS Practice Problem 74



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Steel Part 3

Example: Eccentrically Braced Frames



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Steel Part 3

Learning Objectives You have learned •

•

the difference between ordinary, intermediate, and special moment frames, and how to detail them the difference between ordinary and special concentrically braced frames, and how to detail them

•

special truss moment frames and how to design them

•

potential advantages of eccentrically braced frames



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Steel Part 3

Lesson Overview Seismic Design of Steel Structures •

seismic provisions

•

welded and bolted connections

•

ordinary moment frames

•

ordinary concentrically braced frames

•

intermediate moment frames

•

special concentrically braced frames

•

special moment frames

•

eccentrically braced frames

•

special moment frame connections

•

steel stud wall systems

•

special truss moment frames



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STRC23 Lateral Forces Steel Seismic 0716

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