Earthquake Performance

Earthquake Performance of Residential Masonry Veneer Construction

Following the 2010/2011 Canterbury earthquake swarm, a comprehensive literature review and detailed door to door assessments of residential brick veneer dwellings was undertaken by engineers from University of Auckland, and the University of Adelaide.

In total, 1084 residential dwellings were inspected throughout the wider Christchurch area during June-July 2011, with 24% of the inspected dwellings being constructed pre 1996 and 76% being constructed post 1996, and with 30% of all dwellings being of two storey construction.

Masonry Veneer Systems

The use of clay brick masonry as a veneer was first seen in late 18th century England and today is used in approximately 45% of new residential buildings in New Zealand. Clay brick veneer systems are favoured because of their durability, resistance to fire and moisture, as well as for aesthetic reasons.

Masonry veneer systems have a history of poor performance in earthquakes. However recent advances in construction techniques have improved the seismic performance of masonry veneer systems. These changes include reduction of the mass of the bricks, by reducing of the width to 70mm and the introduction of hollow cores.

Current Brick Veneer Construction Illustrations

These hollow cores have a double benefit of both reducing the wall mass and allowing mortar to set in these holes which results in a physical key and hence a better connectivity between bricks through dowel or shear key action. The performance of wall ties has also advanced in recent years with the requirement for ties to be screw fixed.

Illustrations of Tie Fixings

Modern veneer ties are also capable of transferring stresses and accommodating differential movement.

Laboratory Testing of Seismic Performance

Experimental investigations of the seismic performance of masonry veneer have shown positive results for clay brick veneer. Not only can clay brick veneer carry a considerable amount of its own seismic load, and act as an energy dissipater, but veneer also helps keep building deflections low due to its self weight and added stiffness, reducing internal damage.

Tests have also shown that seismic performance depends more on mortar bond strength than on mortar compressive strength and that horizontal bed joint reinforcement has been proven to not help seismic performance. It was also shown that screw fixed veneer ties worked better than nailed ties.

Project Scope

• Detailed door to door assessment of brick veneer construction in a variety of areas of Christchurch.
• The inspection survey included information such as: square metres of veneer, single or double storey, estimated age, extent of damage, was the veneer removed or did it fall off, type of foundation and type of wall ties.
• Compare veneers that were laid using older systems employing nail-on or wire ties, and the new system of screw fixed brick ties that was introduced in 1996.
• Compare concrete brick veneers and clay brick veneers.
• Perform statistical analysis of the data and interpretation of the results.

Conclusions

• Little shaking damage was observed following the 4th September 2010 Darfield earthquake. Rather, damage was mainly due to foundation settlement, soil liquefaction and lateral spreading. However, it was noted that newer, lighter clay brick veneer systems appeared to perform better than older, heavier systems.
• The severe and extreme damage to veneer clad dwellings was concentrated in the Port Hills and foothills suburbs (13% of inspected dwellings), due in part to the proximity to the epicentre as well as topography amplification.
• It is evident that the majority of cases of severe and moderate damage was concentrated close to the river banks (typically the residential ‘Red Zone’) mainly due to substantial liquefaction and lateral spreading. Map illustrating distribution of damage
• The level damage increased with an increasing level of acceleration. Strong shaking mainly resulted in no visible damage, but did generate some minor damage and a little moderate damage. Very strong shaking mainly resulted in no visible damage, but did generate some minor damage and a little moderate damage.
• Generally houses constructed since the 1990s tended to suffer lower levels of damage then those built earlier.

 

Damage Level By Decade of Construction

 

 

Note: 2010 - 55 houses, a majority of these were located in Port Hills and Foothill suburbs where severe and extreme damage was concentrated due to the proximity to the epicentre and topography amplification

• Overall screw fixed ties performed better than other types of ties, with majority of the dwellings having this type of fixing exhibiting no visible or minor damage.