STESSA 1 8 1 4 - 1 7 February 2 0 1 8 STESSA 1 8 Christchurch, - - PowerPoint PPT Presentation

STESSA ’1 8

STESSA ’1 8

1 4 - 1 7 February 2 0 1 8 Christchurch, New Zealand

Lessons from the Field; Steel Structure Performance in Earthquakes in New Zealand from 2010 to 2016

Paper by G Charles Clifton, University of Auckland and Gregory A MacRae, University of Canterbury Presentation by Charles Clifton

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Scope of Talk

• The earthquakes; location, intensity
• Special focus on Christchurch earthquake series
• Overview of building performance
• Damage observed and lessons learned
• Multi-storey steel buildings
• Light steel framed buildings
• Assessment of multi-storey steel framed

buildings damaged by earthquake

• Conclusions

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Geological Setting

Christchurch Wellington

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Christchurch – between the sea and the hills: NZ’s second largest city

Christchurch, population 400,000

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New Zealand Geology

Auckland eruption (artist’s impression) Mt Ruapehu 1995 eruption Christchurch Feb 2011 earthquake Lake Taupo, Rhyolite Caldera Volcano

STESSA ’1 8 How well do we learn from the past?

In 1876, following the 1848 and 1855 Wellington earthquakes, this seismic-resistant masterpiece was opened. (Old Govnmt Bldg.)

By the early 1900’s we were back to building unreinforced masonry

BUT:

We have done much better since the 1931 Napier earthquake that killed 285 people, but

• In life safety, not mitigating economic impact
• This is the lesson to be learned from Christchurch and Wellington

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1 4 - 1 7 February 2 0 1 8 Christchurch, New Zealand

Christchurch Earthquake Series

Timing, Intensity, Expected Building Performance

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The Earthquake Sequence: Impact

• n Christchurch CBD

Magnitude and Intensity of damaging events: 4 Sept 2010: M 7.1, MM 7, ≈ 0.7 x DLE* 26 Dec 2010: M 5.5?, MM 7 to 8 22 Feb 2011: M 6.3, MM 9 to 10, ≈ 1.8 x DLE* 13 June, 2011: M 5.4?, MM 7 to 8 13 June 2011: M 6.3, MM 8 to 9, ≈ 0.9 x DLE* 23 December 2011: M 5.5, MM 6 to 7, ≈ 0.6 x DLE*

DLE* = design level event for ultimate limit state (ie the design “big one”)

Cumulative effect ≡ close to maximum considered event

(step above DLE)

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Figure 1 NZS 1 1 7 0 .5 Spectra and Largest Horizontal Direction Recorded from the CBD Strong Motion Records Notes: 1 . The long dotted black line is the ULS design spectrum for norm al im portance buildings for the soft soil type, Class D, generally considered in the CBD, Z = 0 .2 2 2 . The short dotted black line is the Maxim um Considered Event design spectrum for norm al im portance buildings for Class D soil in the CBD, Z = 0 .2 2 3 . The solid thick black line is the average from the 4 recording stations all of w hich are w ithin 1 km of the CBD and in sim ilar ground conditions

22 February Earthquake – Intensity of Shaking and Duration

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Very good strong motion records

• Large number and good quality

PGA from February earthquake very intense

• 0.5g to 1.2g Hor PGA within CBD (cf: 0.22g for ULS DLE)
• Up to 1.8g Hor and Ver PGA in hill suburbs

Aftershocks more intense than main event

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Columbo St looking South, September 2010 earthquake Columbo St looking South, February 2011 earthquake

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• Worcester Street

September 2010 earthquake

• PGA ≈ 0.15 to 0.2g

Worcester Street Feb 2011 earthquake PGA ≈ 0.5 to 0.8g Note vulnerability of street corner building compared with adjacent buildings

Retrofitted buildings to 0.15g

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1 4 - 1 7 February 2 0 1 8 Christchurch, New Zealand

Seddon Earthquake 16 August 2013

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2013: Key Points Summary

• Intensity approx 50 to 70% ULS
• See details opposite
• Widespread minor damage in Wellington
• Some loss of masonry in older buildings
• Cracking of masonry and concrete in older

buildings

• Cracks of drywall construction in new

buildings

• Structural damage in some new buildings;

two subsequently demolished.

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Intensity of event in CBD and suburbs

• PSA up to 1g at Karori

and in hill suburbs

• PSA 0.5g at Te Papa
• PGA = 0.4PSA approx
• PGA = 0.26g recorded Te

Puni Village (65% 500 year RP value for Wellington)

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1 4 - 1 7 February 2 0 1 8 Christchurch, New Zealand

Kaikoura Earthquake 11 November 2016

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Details and Intensity

• M7.8; second most powerful

earthquake in NZ’s history

• Over 6 faults ruptured
• Approx 30 seconds strong

ground motion shaking

• Intensity under 50% ULS most

sites (500 year RP) but exceeding ULS in soft soil basins in Wellington city; soil periods of around 1.5 seconds.

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1 4 - 1 7 February 2 0 1 8 Christchurch, New Zealand

Overview of building performance

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Performance requirements of modern buildings in > DLE event

For normal importance buildings to conventional ductile design, they:

• Shall remain standing under

DLE, should also under MCE

• Structural and non

structural damage will occur

• Building will probably

require replacement

1995 Reinforced Concrete Building

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What Types of Damage Occurred?

All possible types, singularly and in combination:

• Structural damage or collapse
• Ground instability:

liquefaction, lateral spreading

• Damage to external cladding

and internal wall linings

• Collapsed suspended ceilings

, shelving and contents

• Damage from landslides,

slope instability and rockfalls

STESSA ’1 8 Structural Damage

STESSA ’1 8 Structural Damage

STESSA ’1 8 Ground instability, liquefaction

STESSA ’1 8 Damage to suspended ceilings, shelving, contents

STESSA ’1 8 Damage from landslides and rockfalls

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Building Performance; Christchurch Feb 2011

• Houses performed well for

life safety

• Multi-storey buildings did

not collapse

• Old buildings did not kill
• ccupants but rather those
• utside
• Newer buildings that

collapsed killed occupants

• Fire suppression systems

worked extremely well

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Building Performance; Wellington 2016

• Houses performed well for life

safety including near epicentre, PGA> 1g

• Old buildings Wellington performed

well (low PGAH and PGAV)

• Damage most evident in ductile

modern buildings on soft ground

• One new building partial collapse;

building unoccupied at time of earthquake

• Fire suppression systems worked

extremely well

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1 4 - 1 7 February 2 0 1 8 Christchurch, New Zealand

Damage to Multi-Storey Steel Framed Buildings and Subsequent Studies into Their Behaviour

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Strength and Stiffness: Actual versus Predicted

• Steel buildings typically 2 to 2.6

times stronger and stiffer than the models predicted: why: we are working on reasons – slab, non structural elements, SFSI

• This determined from extent of
• bserved response versus

predicted response from model

• Most steel buildings effectively

self-centred without need for specific devices to ensure this

HSBC Tower:

• Open plan office building
• Design drift 1.3% under

DLE

• Actual drift ≅ 1% under 2.0

DLE

• Ratio of stiffness

real/ model = 2.6

Source: measurement of scuff marks on stairs; details from Design Engineer

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Damage and Disruption to Contents and Non-Structural Components

• Minimal in buildings that

performed well

• most contents still in place
• Proportional to observed drift
• more effects in buildings with

higher drift (compare PWC and HSBC tower)

• EBFs showed less damage than

MRFs

• Some effects of vertical

acceleration seen, eg

• doors off hinges

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Contribution of Composite Floor Slab to Steel Concrete Buildings’ Strength and Stiffness

• Excellent diaphragm action
• Ability to resist beam

elongation

• Out of plane resistance of

some 20 kN/ mm and 25mm elastic threshold

• Assists with self centering

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Contribution of composite slab to stiffness

Active link was omitted from the models in Vulcan, to only capture the out of plane stiffness of the concrete slab Also investigate the effects of:

• Secondary beam spacing; 2 beams & 4 beams
• Different mesh reinforcement; A142, A193, A252, A393

Slab spans perpendicular to active link Slab spans parallel to active link

Active Link Active Link

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2 secondary beam s 4 secondary beam s

More secondary beams Higher out of plane stiffness

Inelastic behaviour of of composite slab

• B. Slab parallel to active link

this case provides m uch higher out of plane stiffness

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EBF building model + out of plane stiffness of floor slab

Nonlinear tim e history analysis Of 1 0 -story EBF building in Ruaum oko

• Arcelik
• Duzce
• Elcentro
• Hokaido
• LaUnion
• Lucerne
• Tabas

Ground m otioned scaled to NZS1 1 7 0 .5

Typical floor plan Typical elevation

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SFSI effect on response of buildings on shallow foundation

by: Luke Storie

There are a number of multi-storey buildings on shallow foundations in the Christchurch CBD that performed well in the Christchurch Earthquake; SFSI has potentially reduced the forces transmitted to multi-storey buildings on shallow foundations in the CBD; Only small foundation rotations can result in large extents

• f uplift and significant effects on structural response;

Integrated numerical modelling of soil and structure is important to generate accurate demands on the superstructure

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Residual Displacement

B. Slab parallel to active link

• Decrease in residual displacem ent, m ore than for perpendicular case
• Even m ore decrease w hen SFSI is considered

No SLAB 142mm 2/ m mesh 393mm 2/ m mesh

w ithout SFSI Effect

No SLAB 142mm 2/ m mesh 393mm 2/ m mesh

w ith SFSI Effect

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Connections

• When well designed and detailed,

performed as intended

• No changes to design procedures required
• Some failures due to poor design or

detailing

• Gusset plate connections: out of plane

movement in endplates to column as intended

• No damage to splices or movement in

them

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Failures Due to Poor Design or Detailing

• Most important part of connection design
• as illustrated from the Christchurch

earthquake series

• Inadequate load paths
• Undersized welds

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Effects of Vertical Acceleration: Christchurch

Generally relatively minor, eg:

• Permanent deformation in long span floors

(HSBC Tower)

• Cracking of cantilever bay windows (HSBC

Tower)

• Contribution to cracking in composite floors
• Doors lifted out of supports

Sometimes much more serious, eg:

• Transfer beams end compression

failure (Crowne Plaza Hotel)

• Contribution to shear wall and concrete column failures

Vertical acceleration effects on structures not noticed in Wellington

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Adequacy of the capacity design procedure

• Defined plastic collapse mechanism with

inelastic demand concentrated into primary elements and suppressed in the secondary elements

• eg active links in EBFs
• Mechanism forced by overstrength design,

but upper limit actions on secondary elements apply to all systems

• Some systems’ secondary elements

governed by system overstrength, others by upper limit actions

• All kept inelastic demand in the primary

elements

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Effect of Non-Structural Components

• n Building Response

HSBC Tower:

• 10 storeys open plan
• ffices on massive pad

foundation

• Minimal non structural

components

• Inelastic demand

in all levels

Pacific Tower:

• 22 storeys very mixed use:
• Building on piles
• 6 levels car stacker very open

plan

• Two levels of offices
• 10 levels hotel rooms
• 6 levels apartments
• Top 16 levels large number

fire and acoustic rated walls

• Inelastic demand

concentrated into car stacker levels

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Redundancy

• Most EBFs comprised two

braced bays in each direction

• Royal Commission identified

this as concern in the event

• f active link failure
• Solution has been developed

mobilising the contribution

• f the gravity system with

continuous columns

But is it a problem in practice?.....

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1 4 - 1 7 February 2 0 1 8 Christchurch, New Zealand

Behaviour of Light Steel Framed Buildings

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Light Steel Framing: Excellent performance

• Around 50 houses in

strongly shaken areas

• New construction (most

within last 10 to 15 years)

• Typical LSF frame on

concrete slab with brick veneer

• No to minimal damage on

sites with good ground

Dislodged brick

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February 22nd 2011 Earthquake

• No damage to

brick veneer wall with ties in City Centre

• PGAH ≅ 0.5g

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Seismic Performance of Light Steel Framing With Brick Veneer

• Excellent performance observed from

tests

• no damage under SLS
• hairline cracking under ULS
• no loss of bricks at MCE
• some brick loss at 1.6xMCE (2.7x El

Centro or 0.95g PGA)

• Performance in the first earthquake

(Sept 2010) consistent with these tests

• minor cracking and few bricks loose was

the worst damage

• most houses show no damage including

no damage at corners

But worse was to come…

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2 Storey House on 22 Feb Fault Line

• PGAH and PGAV = 1.8g
• Oamaru stone veneer damaged
• Stones slid on mortar lines
• Up to 8 stones dislodged
• Minor cracking internal gypsum board linings in

places

• Longest 1m crack
• Minor misalignment of one internal wall
• Foundation bolt may have partially pulled out
• Client is very pleased
• His house is repairable; stone veneer only significant

damage

• Similar houses close by destroyed

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1 4 - 1 7 February 2 0 1 8 Christchurch, New Zealand

Assessment and Repair of Multi- Storey Steel Framed Buildings

3 examples given

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Tuam Street Car Parking Building CDHB

• 3 storey precast

concrete gravity frame

• Infill EBF units into

selected bays each storey

• High redundancy
• Minimal gravity frame

damage

• Active links yielded

and one tearing failure

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CDHB Building: Fractured Links Due to Misalignment of Braces with Stiffeners

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CDHB Building Repaired

• EBFs with fractures
• r significant

yielding replaced

• Lateral restraints for

frames parallel to floor improved

• Building back in

service

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Pacific Tower

• 42 active links cut out and

replaced

• 60m of cracks in floor slab

with crack widths > 0.75mm were epoxy grouted

Left in place Replaced

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Active Link Fracture and Bolted Endplate Link Replacement

• 38 links replaced using bolted endplates and

bolted in active links

• Eliminates locked-in weld residual stresses

into building

• Typical details shown below

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HSBC Tower: Detailed Evaluation No Structural Repair Needed

• Extensive yielding of active links;

no yield elsewhere

• Minimal cracking of floor slab
• All connections performed as

expected

• Building self centred to 0.14%

drift at roof

• Increase in mid-span deflection of

long span floors

• Detailed assessment of active link

residual capacity based on field hardness measurements

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Determination of Post Earthquake Capacity

• f EBF System

Procedure developed and applied to HSBC Tower: Included effect of increased zone factor from 0.22 to 0.3 % NBS for frame = 83% % NBS for building = 87% Building has not required structural repair; except Lift shaft guide rails realigned

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Lessons from the field; conclusions

The earthquakes ranged from 0.5xULS to MCE level. Findings are:

• 1. Buildings over 2x stiffer and stronger than predicted
• 2. SFSI is major contributor
• 3. Capacity design works well
• 4. Composite slabs assist in robustness and self centering
• 5. Current design, detailing procedures are adequate
• 6. Steel framed buildings with specific damaged components

are readily repairable

• 7. Element of good luck in the excellent response observed