sy,jose - shanghai 2012

8/12/2019 Sy,Jose - Shanghai 2012

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Application of Buckling Restrained

Braces in a 50-Story BuildingJose A. Sy, CEO, Sy^2 + Associates, Inc.


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Located in Makati City,

Philippines

50-story residential building

with 3 - story below gradeparking

Total height of 166.8 m above

ground level

Standing on one-level podium

Tower plan area (34.5 x 26 m)

Building Description


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Reinforced concrete

bearing wall

Gravity columns Post-tensioned (PT) flat

slabs

Gravity Load Resisting System


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BRBs (43rd 47thfloor)

BRBs (19th 23rdfloor)

Reinforced concrete

bearing wall coupled with

outrigger columns

Buckling restrained brace

(BRB) system

Lateral Load ResistingSystem


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BRBs in Plan


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Seismic energy is absorbed by formation of plastic

hinges at specifically designed regions

- Example: base of shear wall and end of coupling

beams

Able to deform into an inelastic range and sustain

reversible cycles of plastic deformation

Structure may suffer structural and non-structural

damage to an extent that it may not be economically

repairable

Conventional EarthquakeResistant System


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Incorporates energy dissipating devices in the structural

system

Reduce the inelastic energy dissipation demand on the

framing system

Structural components may remain elastic during an

earthquake

Structural and non-structural damage may be

considerably reduced

Alternate Earthquake ResistantSystem


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One of the promising

options to use as the

energy dissipating devices

Widely used in seismicdesign and retrofitting of

buildings

Especially used in United

States and Japan

Application of BRBs


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Core steel cross-shape or flat bar member

Steel tube casing

Confined by infill concrete

Core and infill concrete are decoupled by the un-bonding

material

Components of BRB


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Axial load is transmitted

by the steel core only

Casing through its flexural

rigidity provides theproper lateral support

against flexural buckling

of the core

Steel core member resiststhe axial forces with a full

tensile or compressive

yield capacity without the

local or global flexural

buckling failure

How BRBs Work?


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Level of Earthquake Seismic Performance Objective

Frequent/Service: 50% probability of

exceedance in 30 years (43-yearreturn period), 2.5% damping

Serviceability: Structure to remain

essentially elastic with minor damageto structural and non-structural

elements

Design Basis Earthquake (DBE): As

defined by ASCE 7, Section 11.4, 5%

damping

Code Level: Moderate structural

damage; extensive repairs may be

required

Maximum Considered Earthquake

(MCE): 2% probability of exceedance

in 50 years (2475-year return period),

2 to 3% damping

Collapse Prevention: Extensive

structural damage; repairs are

required and may not be

economically feasible

Performance Objectives


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Preliminary Design

(Code based design

approach)

Service Level Evaluation

(43-year return period)

Collapse Prevention LevelEvaluation

(2475-year return period)

Overall Design Procedure


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Elastic response spectrum analysis in accordance with

the code.

Site specific response spectrum for DBE level is used.

Appropriate initial sizes of BRBs are used.

Structural components to be remained elastic are

designed by applying the appropriate amplification

factors.

Preliminary Design


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Site specific service level response spectrum with 43-

year return period (50% of probability of exceedance in

30 years) is used.

Story drift, coupling beam and shear wall demand tocapacity ratios are checked.

Required capacities of buckling restrained braces are

determined so that they remain elastic under the service

level earthquakes.

Service Level Evaluation


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Nonlinear response history analysis is conducted.

Seven pairs of site specific ground motions, with 2475-year return

period (2% of probability of exceedance in 50 years) is used.

Average of demands from seven ground motions approach isused.

The preliminary design is modified as required in order to meet

the acceptance criteria.

Coupling beams, core wall flexural response and bucklingrestrained braces are checked in anticipation of nonlinear

response while core wall shear.

Diaphragms, basement walls, foundations and columns are

checked to remain essentially elastic during the nonlinear

response history analysis.

Collapse Prevention Level Evaluation


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Item Limit

Story Drift 0.5 percent

Coupling Beams Shear strength to remain essentially elastic

Core Wall Flexure Remain essentially elastic

Core Wall Shear Remain essentially elastic

Columns Remain essentially elastic

BRB Remain elastic (no yielding permitted)

Essentially elastic behavior is defined as no more than 20% of the

elements with ductile actions having a D/C between 1.0 and 1.5. No

elements will be allowed to have a D/C >1.5

Brittle actions are limited to D/C of 1.0

Acceptance Criteria (Service Level)


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Item Limit

Story Drift 3 percent

Coupling Beam Rotation (Diagonal

Reinforcement) 0.06 radian rotation limitCoupling Beam Rotation

(Conventional reinforcement)0.025 radian rotation limit

Core Wall Reinforcement Axial

Strain

Rebar strain = 0.05 in tension and

0.02 in compression

Core Wall Concrete Axial Strain Concrete Compression Strain= 0.004 + 0.1 (fy / fc)

BRB 9 times yield strain

Core Wall Shear and Basement

WallsAverage shear demand times 1.3

Acceptance Criteria (MCE Level)


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Design Analysis Type Software Used

Preliminary design Response spectrum analysis ETABS V9.5

Serviceability check Response spectrum analysis ETABS V9.5

Collapse prevention

check

Nonlinear time history

analysis

Perform3D

V4.0.4

Finite Element ModelingAnalysis Tools


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Three-dimensional finite element model, including the tower

and the whole podium is used.

Flexural response of core wall, slender coupling beams and

slab outrigger beam; the shear response of deep coupling

beams and the axial load response of BRBs are modeled with

nonlinear force-deformation behavior.

Finite Element Modeling for MCElevel


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PERFORM-3D-BRB compound component is used.

BRB compound component has three parts

1) BRB basic component, which incorporate the nonlinear

behavior (inelastic deformation) of BRB2) Elastic bar basic component, which corresponds to the BRB

steel outside the main inelastic zone

3) Stiff end zone, with a specified length and a cross section

area that is a multiple of the elastic bar area. The end zone

accounts for the gusset plates at both ends of the member.

The coefficients Ry, , and are estimated based on the

properties of the BRBs provided by Star Seismic Inc.

70% of the pin to pin BRB length is used as effective length of

the brace for inelastic behavior.

Buckling Restrained Braces


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Backbone Curve of BRB


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Fiber modeling technique is

used

Perform 3D shear wall

element is used. Shear behavior is modeled

as elastic.

Shear Wall


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With diagonal reinforcement

(span/depth < 4)

Nonlinear shear hinge at

mid span of element

With conventional

reinforcement (span/depth > 4)

Moment curvature hingesat the ends of element

Coupling Beam


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Use rigid diaphragm.

Equivalent slab outrigger

beams connected the core

and columns. Moment curvature hinges

are used at the ends.

Slab Outrigger Beams


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Soil-structure interaction effect is considered.

Retaining walls in the basement are modeled with elastic

linear shell element.

Surrounding soil is modeled with nonlinear springs. Damping of the soil on the sides of the basement wall was

neglected in the study.

Base of the core wall is modeled as pin connection (i.e.

without rotational restrained)

Base of the columns are modeled as fixed.

Foundation


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Element Action Type ClassificationExpected

Behavior

RC columnAxial-flexure

Shear

Ductile

Brittle

Linear

Linear

RC shear wallFlexure

Shear

Ductile

Brittle

Nonlinear

Linear

RC coupling beams

(Deep beam, ln/d


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Initial sizing of the BRB is based on the wind and seismic

loading, with the worse condition from seismic loading for DBE

level earthquake.

After MCE level evaluation, the size of the BRBs is adjustedfor the optimal performance and to control the story drift.

The stiffness is adjusted in such a way that the stiffness of

BRB at the floor is higher than the story stiffness in that floor.

Relatively high stiffness of bracing leads to attract higher

seismic demand on the BRB caused to early yielding than

reinforced concrete core wall.

Design of BRBs


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BRB Details


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In accordance with AISC 360-05 and ACI 318-08.

Gusset plates are modeled separately and the maximum

design yield force of BRB is applied. Stresses are checked

against the expected yield stress of the plate. The buckling ofthe gusset plate under the compressive loading is also

checked.

Steel studs that restrained the gusset plate embedded in the

concrete are designed against the shear demand. Steel

strength of the studs, concrete breakout strength and concrete

pryout strength are checked.

Detailed Connection Design


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4500 kN

4500 kN

Case 3

Stresses in Gusset Plate


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Total number of studs (2

sides) = 120 x 2 = 240

36'

4500 kN

4500 kN

Resultant Force

Shear Stud Design


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Load path of net vertical force from the gusset plate to the

steel column (embedded in the reinforced concrete outrigger

column) and then from the steel column into the concrete

column is checked. The axial capacity of the drag element is also checked to

transfer the horizontal component of the force from the BRB.

Detailed Connection Design


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BRB Connection at Site


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BRB System


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BRBs at Site


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Drag Element at Site


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Mode Natural Period (s) Mode Shape

1 5.75 Translation in minor dir.

2 4.86 Translation in major dir.

3 3.77 Torsion

Mode 1 Mode 2 Mode 3

Analysis ResultsModal Analysis


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Load CasesBase Shear

(KN)

% of Seismic

Weight

DBE level (In major dir.) 21,012 3.56

DBE level (In minor dir.) 22,691 3.84

MCE level (In major dir.) 47,892 7.76

MCE level (In minor dir.) 46,462 7.53

Design base shear is larger than minimum limit of 3% set by LATBSDC-2008

Nonlinear dynamic base shear is approximately 2 times higher than design

base shear

Base Shear


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Story Shear

Story Shear in Principal

Major Direction

Story Shear in Principal

Minor Direction


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Story Moment

Story Moment about

Principal Minor Axis

Story Moment about

Principal Major Axis


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Story Drift

Story Drift in Principal

Major Direction

Story Drift in Principal

Minor Direction


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Wall Axial Strain

Wall Compressive Axial Strain Wall Tensile Axial Strain

-10

0

10

20

30

40

50

60

-0.005 -0.004 -0.003 -0.002 -0.001 0

Story

Axial Strain

TAB MIN

ARC MIN

CHY MIN

DAY MIN

ERZ MIN

LCN MIN

ROS MIN

Average MIN-10

0

10

20

30

40

50

60

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007

Story

Axial Strain

TAB MAX

ARC MAX

CHY MAX

DAY MAX

ERZ MAX

LCN MAX

ROS MAX

Average MAX


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Strain for each BRB is extracted from each analysis and

average ductility demand is calculated.

Yielded significantly at MCE level.

All BRBs have average ductility demand less than 9, which isthe maximum allowable ductility demand for primary braces

components mentioned in ASCE 41-06.

Ductility of BRBs


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Design base shear is reduced compared to the building

without BRB system.

Moment and shear in the core wall is reduced remarkably due

to the utilization of BRB system. Story drifts in the principal minor direction are improved after

the application of BRB system.

Effectiveness of BRB System


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Used as first time in the primary lateral force system of the

high-rise building in Philippines.

Design and application of BRBs are initiated into local

structural engineering practice. Lead to a better performance for tall buildings for reducing

base shear and controlling deformation.

Potentials of the application of BRB system in the future high-

rise building projects in Philippines.

Conclusion


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Thank You

sy,jose - shanghai 2012

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