Seismic Assessment - Risks Associated with Unreinforced Masonry (URM) buildings in New Zealand, and how to deal with URM buildings which are classified as Heritage Buildings.

Unreinforced Masonry (URM) buildings are structures characterized by load-bearing walls made from brick, stone, or concrete blocks without reinforcement. A substantial number of these buildings were erected in the early to mid-20th century in New Zealand and continue to serve as residences, offices, commercial shops, and heritage buildings.

While URM buildings typically exhibit satisfactory performance under normal gravitational loads, appearing robust and durable, their behavior under earthquake or lateral loads is markedly different. This shortcoming poses significant risks, especially in seismically active areas like New Zealand.

Why Are URM Buildings Vulnerable?

Unreinforced masonry structures are susceptible to various issues, particularly in seismically active areas such as New Zealand, where earthquakes and ground tremors frequently occur.

1. Poor Tensile strength:

Masonry structures exhibit strong performance under compressive forces but are significantly weaker under tensile forces. During seismic events, buildings experience cyclic tension and compression forces. Unreinforced masonry (URM) structures lack the capacity to withstand tensile stresses, which can result in abrupt cracking and collapse with minimal warning.

2. Brittle Structural Behavior

In contrast to the contemporary reinforced buildings, unreinforced masonry (URM) structures exhibit minimal capacity for deformation without failure. This brittle characteristic results in the potential for sudden damage, offering limited warning prior to collapse, thereby significantly elevating the risk to life safety.

3. Poor Connection Between Structural Elements

In our experience, the buildings with Unreinforced masonry (URM) walls frequently exhibit inadequate connections to floors and roof members. In the event of a significant seismic event, these walls may detach from the structural elements and may collapse outward, with external walls, parapets, and gable ends being especially vulnerable. Such structural failures present a significant risk to both occupants and individuals outside the building.

4. Age and Deterioration

Numerous Unreinforced Masonry (URM) buildings have been in use for several decades. The materials used in these structures, such as mortar, are prone to deterioration over time. Additionally, previous seismic events may have compromised the integrity of these buildings. These factors collectively diminish the buildings' capacity to endure future seismic activities.

With early structural intervention, these shortcomings can be addressed, enhancing the building's lifespan and also reducing the life safety risks.

Heritage Implications

Many unreinforced masonry (URM) buildings in New Zealand are heritage-listed, which adds complexity to the decision-making process. While preserving heritage value is important, earthquake-prone URM buildings pose significant life-safety risks. The key challenges associated with these structures include:

·       Balancing public safety with heritage preservation

·       Limitations on visible strengthening measures

·       Approval processes with Heritage New Zealand Pauhere Taonga

Implementing sensitive and well-designed strengthening solutions can maintain the heritage character of these buildings while significantly enhancing their seismic performance, ensuring they remain safe and functional for future generations.

Why This Matters to Building Owners and Engineers?

1. Building owners face the following URM-related risks:

·       Heightened risk to occupants and the public due to structural deterioration

·       Potential obligations under earthquake-prone building regulations

·       Insurance constraints and increased repair costs following seismic events

2. Engineers dealing with URM buildings must:

·       Conduct thorough seismic assessments

·       Identify critical failure mechanisms

·       Develop practical designs and appropriate strengthening solutions

Reducing the Risk

The risks associated with unreinforced masonry (URM) buildings can be significantly mitigated through the following measures:

·       Conducting a seismic assessment (either Initial Seismic Assessment or Detailed Seismic Assessment) by a qualified structural engineer experienced in evaluating unreinforced masonry structures.

·       Strengthening the buildings that are identified as earthquake-prone.

·       Securing parapets and facades.

It is important to note that not all URM buildings require comprehensive strengthening. In many instances, targeted strengthening can effectively reduce life-safety risks while managing costs.

Conclusion

Unreinforced masonry (URM) buildings are a significant component of New Zealand's architectural landscape, yet their susceptibility to earthquakes is a critical concern. Despite their seemingly robust appearance, the absence of reinforcement and their brittle nature render them particularly dangerous during the seismic events.

Proactive evaluation and strengthening can:

·       Mitigate the risk of injury or fatalities

·       Assist building owners in fulfilling legal responsibilities

·     Preserve the historical significance of these buildings to ensure their protection for future generations.

Proactively addressing issues enables risk management in a controlled and cost-effective manner, ensuring that URM buildings remain safe, compliant, and resilient, while continuing to enhance the character and heritage of the communities.

AB Consulting Engineers has conducted seismic assessments on numerous unreinforced masonry (URM) buildings and possesses experience working on a Grade 2 listed heritage building in Ngaruawahia, Waikato. The building has obtained building consent for strengthening and has also received a grant from Heritage New Zealand. Construction is due to start mid 2026. Please feel free to contact us for further information.

Unreinforced Masonry Building(URM) - Heritage Listed