B_BELEV_Weimar_2011_Part_2.pdf

Dr. Dr. BorislavBorislav BelevBelev

Dept. of Steel and Timber StructuresDept. of Steel and Timber Structures

UACEG, Sofia, BulgariaUACEG, Sofia, Bulgaria

Model Validation and SimulationModel Validation and Simulation

BAUHAUS Summer SchoolBAUHAUS Summer School

August 2011August 2011

Implementation of capacity design Implementation of capacity design

rules to steel structures in seismic rules to steel structures in seismic

regions (Part 2)regions (Part 2)

22Model Validation and Simulation, Model Validation and Simulation,

BAUHAUS Summer School, Aug. 2011BAUHAUS Summer School, Aug. 2011

Lecture overviewLecture overview

1.1. IntroductionIntroduction

2.2. Basic concepts in seismic designBasic concepts in seismic design

3.3. Capacity design principles (CDP) Capacity design principles (CDP)

4.4. Major seismicMajor seismic--resisting systems in steelresisting systems in steel

5.5. Application of CDP to Application of CDP to MRFsMRFs

6.6. Application of CDP to Application of CDP to CBFsCBFs

7.7. Evolution of capacity design philosophyEvolution of capacity design philosophy

8.8. Concluding remarksConcluding remarks



Application of CDP to Application of CDP to MRFsMRFs



MRF: Major points to be discussedMRF: Major points to be discussed

Basic Basic behaviourbehaviour

Factors influencing the cyclic responseFactors influencing the cyclic response

Capacity design statementCapacity design statement

Design options for achieving SCWBDesign options for achieving SCWB--actionaction

Basic design rules for ductilityBasic design rules for ductility

Basic capacity design rulesBasic capacity design rules

Potential problems & issuesPotential problems & issues



MRF: Basic MRF: Basic behaviourbehaviour

Beams and columns connected in moment resisting joints

Lateral forces resisted mainly by flexural action of the beams and columns

High-ductility system if properly designed

Low elastic stiffness (possible P-∆ effects and overall instability)

Complex (3-D) stress-and-strain state in the frame joints – detailing and exectuion is crusial

“Hidden” interaction with the RC floor slabs



Factors influencing the cyclic Factors influencing the cyclic

responseresponse

Local bucklingLocal buckling

LateralLateral--torsionaltorsional bucklingbuckling

Fracture (esp. at welded zones)Fracture (esp. at welded zones)



Importance of plate slendernessImportance of plate slenderness

y

pl

y

yuR

θ

θ

θ

θθ

θ=

−=

(1) Cross-section Class 1




Note: Classification to

Eurocode 3



Capacity design statement (EC8)Capacity design statement (EC8)

1.1. MRFsMRFs shall be designed so shall be designed so that plastic hinges form in that plastic hinges form in the beams OR in the beamthe beams OR in the beam--toto--column connections, but column connections, but not in the columns;not in the columns;

2.2. This requirement is waived This requirement is waived at the base of the frame, at at the base of the frame, at the top level of multithe top level of multi--storey storey buildings and for singlebuildings and for single--storey buildings;storey buildings;

3.3. Typical target plastic Typical target plastic mechanism: “strongmechanism: “strong--column column weakweak--beam” patternbeam” pattern



Design options for SCWBDesign options for SCWB--actionaction

FEMA 350 guidelines:FEMA 350 guidelines:

Plastic hinges to be Plastic hinges to be

formed close to the formed close to the

column faces, but not at column faces, but not at

the connectionsthe connections

Conventional approach Conventional approach ––

local strengthening local strengthening

(haunches)(haunches)

Novel approach Novel approach –– local local

weakingweaking (RBS)(RBS)



Typical response of MRF Typical response of MRF

to increasing lateral loadsto increasing lateral loads



Column web panel: the third Column web panel: the third

component of MRFcomponent of MRF

Web panels subject to distortion,

which increases the lateral frame

displacements and drifts;

Estimation of horizontal shear force:



Basic design rules for beamsBasic design rules for beams

CrossCross--section class allowed: section class allowed:

Resistance and local ductility Resistance and local ductility

checks:checks:

LTLT--Buckling check Buckling check

(not required if the beam is (not required if the beam is

properly braced outproperly braced out--ofof--plane)plane)



Basic design rules for columnsBasic design rules for columns

General SCWB requirement: General SCWB requirement:

Resistance and stability checks for “Resistance and stability checks for “ampliifiedampliified” internal ” internal

forces accounting for the beam forces accounting for the beam overstrengthoverstrength::



Design rules for column web panelsDesign rules for column web panels

Resistance in shear: Resistance in shear:

Shear buckling check (EC 3, Part 1Shear buckling check (EC 3, Part 1--5)5)

Strengthening optionsStrengthening options



Design rules for connections Design rules for connections

(bolted/welded)(bolted/welded)

Capacity design approach compulsory if the Capacity design approach compulsory if the

connections are not dissipative zonesconnections are not dissipative zones

General design rule for adding General design rule for adding overstrengthoverstrength to to

connections:connections:

RRdd = = design resistance of connectiondesign resistance of connection

RRfyfy = design plastic resistance of connected dissipative member = design plastic resistance of connected dissipative member

(frame beam/girder)(frame beam/girder)

γγovov = material = material overstrengthoverstrength factor = 1,25factor = 1,25




The generic SCWB criterion does not The generic SCWB criterion does not specify the distribution of the column specify the distribution of the column bending moments above and below the bending moments above and below the jointjoint

The The ΩΩ--approach may underestimate the approach may underestimate the actual actual overstrengthoverstrength of beams when large of beams when large gravity loading is presentgravity loading is present

The max. bending moments and shears in The max. bending moments and shears in the columns are not well predicted due to:the columns are not well predicted due to:

(a)(a) Dynamic amplification from higher mode Dynamic amplification from higher mode effects after beam hingingeffects after beam hinging

(b)(b) Plastic hinges forming nonPlastic hinges forming non--simultaneously simultaneously at all floor levelsat all floor levels

(c)(c) Columns deformations not similar to those Columns deformations not similar to those predicted by elastic analysis and static predicted by elastic analysis and static pushoverpushover



Application of CDP to Application of CDP to CBFsCBFs



CBF: Major points to be discussedCBF: Major points to be discussed

Basic Basic behaviourbehaviour

Factors influencing the cyclic responseFactors influencing the cyclic response

Capacity design statementCapacity design statement

Basic design rules for brace membersBasic design rules for brace members

Basic capacity design rulesBasic capacity design rules




CBF: Basic CBF: Basic behaviourbehaviour

Beams, columns and braces arranged to form vertical cantilever trusses

Lateral forces resisted mainly by truss action (tension and compression in the CBF members)

Lower ductility than MRFs and EBFs

High elastic stiffness (cost-effective for wind loads)

Dissipation provided mainly by the brace members yielding in tension

Seismic response depends strongly on the brace configurations (X, V, K, etc.) and slenderness

Possible degradation under cyclic loading due to repeated brace yielding and buckling



Cyclic response of brace memberCyclic response of brace member

(Plot from: M.D. Engelhardt, AISC Teaching Module)

F

δδδδ3

2

Cu

1

4

Tu

5

6

7

F

δδδδ

TENSION

COMPRESSION



Factors influencing the brace cyclic Factors influencing the brace cyclic

response and ductilityresponse and ductility

Slenderness Slenderness λλ = max = max LLcr,ycr,y / / iiyy, , LLcr,zcr,z / / iizz

CrossCross--sectional shapesectional shape

Slenderness of crossSlenderness of cross--section parts section parts b/tb/t and and d/td/t

Support type at member ends Support type at member ends –– pinned or otherpinned or other



Inelastic response of inverted VInelastic response of inverted V--bracesbraces

Compressed brace members buckle first

Tensile brace members yield next

Unbalanced vertical force appears at the joint

The beam flexural stiffness and strength very important for the force redistribution



Capacity design statement (EC8)Capacity design statement (EC8)

(Images from Rogers & Tremblay)

• CBFs shall be designed so that yielding of the diagonals in tension will take place before failure of the connections and

before yielding or buckling of the beams or columns



Basic design rules for brace membersBasic design rules for brace members

Slenderness limits of EC8: Slenderness limits of EC8:

(a) for X(a) for X--braces:braces:

(b) for V(b) for V--braces and inverted Vbraces and inverted V--braces: braces:

(c) for 1(c) for 1-- and 2and 2--storey storey bldgsbldgs –– no slenderness limitno slenderness limit

Limitation on brace crossLimitation on brace cross--section class for Vsection class for V--braces:braces:



Basic design rules for braces (cont.)Basic design rules for braces (cont.)

Resistance checks:Resistance checks:

(a) for X(a) for X--bracingsbracings NNeded ≤≤ NNpl,Rdpl,Rd (tensile strength)(tensile strength)

(b) for V(b) for V--bracingsbracings NNeded ≤≤ NNb,Rdb,Rd (buckling check)(buckling check)

Local ductility checkLocal ductility check



Basic design rules for braces (cont.)Basic design rules for braces (cont.)

Provide homogeneous dissipative behaviour of the diagonalsIt should be checked that the maximum member overstrength Ωi does not differ from the minimum value Ω by more than 25%

Conclusion: the brace cross-sections shall be gradually reduced along the height of the building.Assuming equal cross-sections of the brace members in all storeys will adversely concentrate the inelastic response in the ground storey only !!!

Ωi = member overstrength factor, Ω = system overstrength factor



Capacity design rules Capacity design rules

for the beams and columnsfor the beams and columnsResistance and stability checks for the following Resistance and stability checks for the following

“amplified” internal forces:“amplified” internal forces:

Beams of inverted VBeams of inverted V--bracings: postbracings: post--buckling scenariobuckling scenario

NNpl,Rdpl,Rd

0,3NNpl,Rdpl,Rdθθθθ

( 0,7 NNpl,Rdpl,Rd ) sin θθθθ



Design rules for connections Design rules for connections

(bolted/welded)(bolted/welded)

Capacity design concept compulsory if the Capacity design concept compulsory if the

connections are not dissipative zonesconnections are not dissipative zones

General design rule of EC8 for adding extraGeneral design rule of EC8 for adding extra--

strength to connections:strength to connections:RRdd = = design resistance of connectiondesign resistance of connection

RRfyfy = design plastic resistance of connected dissipative member= design plastic resistance of connected dissipative member

For brace members For brace members RRfyfy = = NNpl,Rdpl,Rd γγovov =1,25=1,25

1,1γovNpl,Rd1,1γovNpl,Rd



Desired sequence of connection Desired sequence of connection

failure modesfailure modes

(Source: Prof. Astaneh-Asl, Steel Tips)

Capacity design rule for bolts:

shear resistance ≥ 1.2 x bearing resistance

Conclusion: high strength bolts (grades 8.8 and 10.9) preferred



Gusset plate failuresGusset plate failures

OutOut--ofof--plane buckling mode of braces “protects” gusset plates and plane buckling mode of braces “protects” gusset plates and

frame joints from fractureframe joints from fracture

“Fold“Fold--line” ductile cyclic response if flexible portion is availableline” ductile cyclic response if flexible portion is available

NetNet--section fracture, block shear and local buckling shall be section fracture, block shear and local buckling shall be

avoided by proper design checks + detailingavoided by proper design checks + detailing

Brittle fracture Ductile response 2t-rule (AISC)



Innovative solutions for brace end Innovative solutions for brace end

connectionsconnections

Standardized high-strength connectors for seismic applications with CHS




Major parameter for brace Major parameter for brace overstrengthoverstrength –– the the slenderness ratio: what limits of slenderness ratio: what limits of λλ to use?to use?

Buckling resistance drops significantly after Buckling resistance drops significantly after several loading cyclesseveral loading cycles

Uniform yielding along height difficult to achieveUniform yielding along height difficult to achieve

XX--braces: braces: modellingmodelling issues; tensionissues; tension--based based design or compressiondesign or compression--based design ?based design ?

Overestimation of column compressive forces in Overestimation of column compressive forces in highhigh--rise buildings and expensive designrise buildings and expensive design

ΩΩ--factor approach may result in severe errorsfactor approach may result in severe errors



ModellingModelling of Xof X--braces for analysisbraces for analysis

• If linear elastic analysis is used (pseudostatic or response

spectrum analyses) – EC8

requires compressive brace members not to be included in

the model (suitable for low-rise)

• If nonlinear nonlinear analysis

is used – both brace members can be included if their pre- and

post-buckling behaviour is accounted for in the model



Potential problems & issues (cont’d)Potential problems & issues (cont’d)

How to calculate force demands for capacity design of How to calculate force demands for capacity design of

beams and columns: “local” versus “global” approach ?beams and columns: “local” versus “global” approach ?

ΩF3

ΩF2

ΩF1

NEd,E ≈ 0Global approach

Uplift !!!Local approach

Npl

Npl

0.3Npl

0.3Npl



Evolution of capacity design Evolution of capacity design

philosophyphilosophy

Structural fuse conceptStructural fuse concept

DamageDamage--tolerant structures (Prof. A. Wada)tolerant structures (Prof. A. Wada)

PerformancePerformance--based design with:based design with:

-- Explicit performance objectives for at least two Explicit performance objectives for at least two seismic intensity levelsseismic intensity levels

-- Direct comparison of seismic demands vs. Direct comparison of seismic demands vs. capacities via nonlinear analysescapacities via nonlinear analyses

-- Damage limitation not only to structure, but Damage limitation not only to structure, but also to nonstructural components and also to nonstructural components and equipmentequipment



Structural fuse concept (SFC)Structural fuse concept (SFC)

SFC uses the idea of protecting the electric circuits by SFC uses the idea of protecting the electric circuits by inserting fuses inserting fuses -- sacrificeablesacrificeable and replaceable (relative and replaceable (relative cheap) components that limit the damage in extreme cheap) components that limit the damage in extreme situations.Thesituations.The fuses are the fuses are the weakestweakest links of the system;links of the system;

Structural fuses have the following functions:Structural fuses have the following functions:

-- dissipate a major part of seismic input energydissipate a major part of seismic input energy

-- keep primary structure deformations in elastic rangekeep primary structure deformations in elastic range

-- provide a predictable response of the systemprovide a predictable response of the system

First implementation: the EBFFirst implementation: the EBF--systemsystem

Further developments:Further developments:

BucklingBuckling--restrained braces (BRB)restrained braces (BRB)

Rocking systemsRocking systems

Passive energy dissipation systemsPassive energy dissipation systems



Eccentrically braced frames (EBF)Eccentrically braced frames (EBF)



Eccentrically braces frames (EBF)Eccentrically braces frames (EBF)

A hybrid system which combines the strong points of the A hybrid system which combines the strong points of the MRF (high ductility) and CBF (high elastic stiffness)MRF (high ductility) and CBF (high elastic stiffness)

The inelastic action is restricted within special beam The inelastic action is restricted within special beam segments (links)segments (links)

The brace members are not dissipative zones anymoreThe brace members are not dissipative zones anymore

Deformation capacity of the links strongly depends on Deformation capacity of the links strongly depends on their length and crosstheir length and cross--section resistances to shear and section resistances to shear and bendingbending

Link elements could be made replaceable by using endLink elements could be made replaceable by using end--plate bolted connections, but this worsens the beam plate bolted connections, but this worsens the beam continuitycontinuity



EBF link elementsEBF link elements

Plastic link rotation angle (demand) : γp = (L/e) θp

The eccentric brace configuration “amplifies” the The eccentric brace configuration “amplifies” the interstoreyinterstorey drift: drift:

e.g. for e = 0.2L e.g. for e = 0.2L γp = 5 θp

“Short” links (e≤1.6Mpl / Vpl) preferred: θp,c = 0.08 Rad (capacity)

Closely spaced web stiffeners to suppress the web shear buckling



BucklingBuckling--restrained braces (BRB)restrained braces (BRB)

Also known as “Also known as “UnbondedUnbonded brace”brace”

Symmetrical response in tension and compression Symmetrical response in tension and compression due to avoided bucklingdue to avoided buckling

Enhanced energy dissipating capacityEnhanced energy dissipating capacity

Capacity design approach compulsory due to brace Capacity design approach compulsory due to brace overstrengthoverstrength (strain hardening)(strain hardening)



Rocking systems (fuses at base)Rocking systems (fuses at base)

(Image from Matt Eatherton et. al paper)



Passive energy dissipation systemsPassive energy dissipation systems

Classification of FEMA 450

(Chapter 15: Structures with damping systems)

The damping system (DS) may be external or internal to the structure

and may have no shared elements, some shared elements, or all

elements in common with the seismic-force-resisting system (SFRS).



Basic components of damping Basic components of damping

systemssystems

1 = Primary frame; 2 = Damper device; 3 = Supporting member



Basic types of damper devicesBasic types of damper devices

1. Displacement-dependent devices (metallic dampers, friction dampers)

2. Velocity-dependent devices (fluid viscous dampers, solid visco-elastic dampers, etc.)

3. Other types (shape-memory alloys, self-centering devices, etc.)



Expected benefitsExpected benefitsAdded damping (viscous dampers)Added damping (viscous dampers)

Added stiffness and damping (Added stiffness and damping (viscovisco--elastic, metallic, friction elastic, metallic, friction dampers)dampers)

As a result, enhanced control of the As a result, enhanced control of the interstoreyinterstorey driftsdrifts

The capacity design is not abandoned, but the sources of The capacity design is not abandoned, but the sources of overstrengthoverstrength in the dissipative zones (dampers) are essentially in the dissipative zones (dampers) are essentially reducedreduced

Seismic response is much more predictable than in conventional Seismic response is much more predictable than in conventional structuresstructures

------------------------------------------------------------------------------------

In new structures:In new structures:

Enhanced performance (reduced damage)Enhanced performance (reduced damage)

Less stringent detailing for ductilityLess stringent detailing for ductility

In existing structures:In existing structures:

Alternative solution to new shear walls (speedAlternative solution to new shear walls (speed--up retrofit works)up retrofit works)

Correction of irregularitiesCorrection of irregularities

SupressionSupression of of torsionaltorsional responseresponse



Example of capacity design with Example of capacity design with

dampersdampers

Seismic protection of industrial facilitySeismic protection of industrial facility

Design PGA=0.24g, I=1.0, Soil type=B (stiff soil)Design PGA=0.24g, I=1.0, Soil type=B (stiff soil)

Seismic weight W=7800 Seismic weight W=7800 kNkN

Design objective: To reduce the base shear to levels Design objective: To reduce the base shear to levels below 1100 below 1100 kNkN, for which the existing supporting RC, for which the existing supporting RC--structure was originally designedstructure was originally designed

Conventional design of the steel structure as CBF Conventional design of the steel structure as CBF system with chevron braces was inappropriate due to system with chevron braces was inappropriate due to higher base shear levelhigher base shear level

Design solution: use friction dampers with slip capacity Design solution: use friction dampers with slip capacity of 50of 50--60 60 kNkN per device (total slip capacity per direction per device (total slip capacity per direction ~~600 600 kNkN) to protect the foundations) to protect the foundations



Check of the energy dissipating Check of the energy dissipating

capacitycapacity



Under construction…Under construction…



Concluding remarksConcluding remarks

Steel structures cannot be considered ductile by defaultSteel structures cannot be considered ductile by default

The capacity design approach helps for providing more The capacity design approach helps for providing more predictable structural response to unpredictable seismic predictable structural response to unpredictable seismic actionsactions

The capacity design philosophy has evolved to new The capacity design philosophy has evolved to new structural systems based on structural fuse conceptstructural systems based on structural fuse concept

The practical application of the CDP cannot be covered The practical application of the CDP cannot be covered in detail by the design codes and requires engineering in detail by the design codes and requires engineering judgementjudgement for each particular project for each particular project

Nonlinear analyses to verify the system Nonlinear analyses to verify the system overstrengthoverstrength and and target plastic mechanism are target plastic mechanism are strongly recommendedstrongly recommended

The passive energy dissipation systems which make The passive energy dissipation systems which make best use of structural fuse concept are now a mature and best use of structural fuse concept are now a mature and reliable technology for seismic protectionreliable technology for seismic protection



End of Part 2End of Part 2

Thank you for your attention !Thank you for your attention !

Questions or comments?Questions or comments?

Documents

B_BELEV_Weimar_2011_Part_2.pdf