Performance of buildings in the February 2011 Christchurch

Earthquake

Associate Prof Rajesh DhakalUniversity of Canterbury

Christchurch, NZ

Sixth International Conference on Seismology and Earthquake Engineering

16-18 May 2011, Tehran, Iran

URM Buildings: General Observations

Extensive damage to URM buildings in general Many URM buildings in the city flattened Most of the remaining buildings very severely

damaged Few well constructed URM buildings in the

western suburbs were subjected to moderate shakings and suffered repairable damage

In the CBD, very few (unretrofitted) URM buildings will exist in future.

Some examples of typical URM building damage/collapse follow

Many buildings were about to collapse (short duration effect)

In-plane wall/pier failure

Gable wall failure

Parapet Failures

Anchorage Failure

Out-of-plane wall failure

Vulnerability of cavity construction

Out-of-plane failures (Cavity walls)

Poor quality of mortar

Many tested samples 1.0-1.5 MPa compression strengthΤ = C + µ N

Poor quality of diaphragm timber

Inadequate Cavity Wall Ties

Pounding of URM Buildings

Acknowledgement: Several slides in this section are provided by Gregory Cole, University of Canterbury

Pounding Survey statistics

≥ ≥ ≥ ≥≥

Typical masonry pounding damage

URM pounding damage mechanism

Diagonal damage path due to pounding

Buildings in a row with little separation

Example: Pounding damage

Example: Pounding damage

Example: Pounding damage

Example: Pounding damage

Inadequate building flashing details

Performance of Retrofitted URM

Buildings

Acknowledgement: Several slides in this section are provided by A/P Jason Ingham, University of Auckland

Some well-anchored walls did well

Damage to anchored walls

Steel strong backs generally performed well

Steel Frames: Generally did well

Steel Frames: Some suffered damage

Failed frame to wall connections mounted perpendicular to wall

Wall confined by steel plates

(cracks visible, but not wide)

Shotcrete

In general shotcreted masonry walls performed well.

Minor cracking seen in some walls.

Floor Diaphragms Retrofit

Successful parapet strengthening

Unsuccessful Parapet Strengthening

(require protection at corners)

Unsuccessful Parapet Strengthening

Wall detached from struts

Performance of Old RC buildings

Designed for smaller strength (compared to now) Subjected to large acceleration (higher than

current design level) Lacked ductility (specially the pre-1980 buildings) Mostly not retrofitted As expected, damaged severely

CTV Building (117 dead)

Hotel Grand Chancellor (Demolished)

Performance of Modern RC buildings

Subjected to large acceleration (higher than current design level)

Inherent ductility As expected, most buildings damaged But no collapse (post 1990) Most buildings can be reused after repair

(bonus?) In general, performance better than expected

Example: Clarendon Tower

Dislodging of precast stair from landing

Dislodging of precast stair from landing

Vertical acceleration effect

Issues related to performance of modern buildings

Staircase in many buildings collapsed (change of current practice needed)

Precast floor (issues with interaction between floor and beam elongation)

Irregularity of buildings (irregular buildings performed poorly)

Foundation: Not adequate for the soft soil underneath

Compression failure of columns: high vertical acceleration

Non-structural damage

Non-structural performance

Structural performance: no surprises Non-structural performance: DISAPPOINTING Ceiling: Very few buildings with ceilings intact Facade/Partition: Damaged severely in most buildings Parapets: Most unrestrained parapets fell September earthquake: Minor structural damage (modern

buildings); severe non-structural damage February earthquake: Moderate-severe damage to modern

buildings; Collapse of non-structural elements Clearly, a mismatch between the structural and non-structural

performance Need more focus in future

Non-structural damage could have killed more people

Falling objects could have, too

It is time that we start explicitly aiming for minimisation of NON-STRUCTURAL DAMAGE and DOWNTIME in seismic

design.

Relevance to Iran

1. There are many unreinforced masonry (URM) buildings which can suffer severe damage in moderate shakings and collapse in strong shakings. The brittle failure of URM building components can be fatal.

2. Systematically strengthened/retrofitted URM buildings perform noticeably better. Although they may suffer damage in large earthquakes, they are unlikely to collapse completely; thereby saving lives of inhabitants.

3. Hence, if you want to reduce the life safety threat from these URM buildings, you MUST retrofit them.

4. Be careful, there is a difference between strengthening and retrofitting.

Thank You!

Acknowledgements: Jason Ingham, and Gregory Colefor providing some photographs and slides