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About this event

This webinar is part of the "Seismic Retrofit Strategies for Existing Structures," a technical series designed for structural engineers and professionals working in seismic zones, jointly hosted by ClearCalcs and PYTHON Fixings. 

In this New Zealand–focused session, join Robert Hudson, PYTHON Fixings VP of Engineering, on Wednesday, August 13, 2025, from 10 am to 11 am New Zealand Time (NZT) to explore how you can identify common structural issues in existing masonry buildings and apply modern, cost-effective remediation techniques. 

With insights drawn from large-scale dynamic testing and field practice, this webinar will provide practical guidance for engineers working to enhance the seismic resilience of existing building stock in Aotearoa.

This session is ideal for engineers involved in retrofit design, peer reviews, or structural condition assessments under NZ standards.

All registered attendees will receive a recording of the webinar after the event.

In this webinar, we will cover:

• Earthquake reconnaissance observations on the performance of different URM retrofit and restoration techniques.
• The performance of common anchor types used in existing URM structures, assessing their effectiveness and resilience to different loading scenarios.
• The experimental validation of cost-effective retrofit techniques for URM structures.
• Introduction of newly developed retrofit techniques for URM parapets and in-plane loading of URM walls.

The session will emphasize cost-effectiveness, constructibility, and code-informed design decisions tailored to New Zealand’s seismic context.

Webinar Resources

• PYTHON Fixings Product Guide
• Retrofitted URM cavity walls experimentally validated and a simplified out-of-plane assessment
• Seismic retrofit of URM parapets using vertical screw reinforcing

Summary

This on-demand webinar, hosted by ClearCalcs in partnership with Python Fixings, explored seismic retrofit strategies for unreinforced masonry (URM) buildings in New Zealand. The session was designed for structural engineers working in earthquake-prone regions and focused on practical retrofit techniques, common failure mechanisms, and lessons learned from both field reconnaissance and laboratory testing.

The webinar began with an overview of retrofit systems for parapets, chimneys, wall-to-diaphragm connections, and cavity walls. Robert discussed the relative performance of different methods, such as steel bracing, post-installed rebar, mechanical anchors, and post-tensioning, as well as the advantages and limitations of each. Special attention was given to Christchurch earthquake data, which revealed why certain parapet retrofits—like concrete bond beams—underperformed compared to steel bracing.

The session also reviewed the role of timber strongbacks, cross-laminated timber (CLT) panels, and plywood shear walls as alternatives to traditional concrete shear walls, particularly in heritage-sensitive contexts. Robert shared findings from shake table tests and semi-static airbag tests that measured the performance of these systems under high seismic loads. Additional topics included diaphragm strengthening, cavity wall composite action using semi-rigid veneer ties, and combining post-tensioning with composite action for greater out-of-plane capacity.

A significant portion of the webinar focused on anchor performance in existing masonry. Robert explained that while epoxy anchors can work well when installed perfectly, they have a high risk of failure due to poor hole cleaning, insufficient epoxy, or premature loading. Mechanical anchors, when long enough to engage all wythes, often provide more reliable results in historic masonry. He emphasized that any anchoring product used for seismic retrofits should be tested for seismic performance rather than relying solely on static load ratings.

The presentation closed with practical inspection advice for cavity wall ties, guidance on when to use through-bolts or plates, and strategies for working within cost constraints. Attendees were encouraged to treat each retrofit method as a tool in the “engineering toolkit” and select solutions based on site-specific conditions, structural behavior, and heritage considerations.

Questions and Answers

1. To what extent have the findings and failure modes discussed been incorporated into the current NZSEE C8 guidelines?

Answer: Many of the observed behaviors and retrofit performance findings are being incorporated into the upcoming NZSEE C8 updates, including new provisions for gable wall calculations, improved modeling of out-of-plane behavior, and guidance on composite action. All related research is publicly available, and engineers can request specific datasets and design examples directly from Python Fixings.

2. What spacing of post-tensioning bars is typical for solid and cavity walls?

Answer: Spacing is highly project-specific and often determined by the location of openings and wall geometry. For façades with windows, a common approach is to place a post-tensioning bar at the center of each pier, typically around 1.5 m wide. For longer solid walls, spacing should be informed by stress distribution patterns from the top anchorage point, ensuring no unreinforced “dead zones” remain between bars.

3. What plywood thickness was used in the New Zealand in-plane testing following CLT panel trials?

Answer: Testing was conducted on various thicknesses, including one, two, and three sheets of standard plywood. The required thickness depends on the seismic demand for the specific site, with higher seismic zones such as Wellington often requiring more material to achieve the necessary capacity.

4. Are screw anchors for masonry required to comply with NZ C2 provisions?

Answer: No. NZ C2 relates to reinforced concrete, not masonry, and there is no code anywhere in the world that requires C2 compliance for masonry anchors. While seismic performance testing in reinforced masonry is becoming more common internationally, these requirements have not yet been adopted in New Zealand for existing masonry retrofits.

5. How should step cracking in mortar joints be addressed before installing vertical MT screws in parapets?

Answer: The first step is to determine the cause of the cracking. Minor cracks caused by thermal movement may not require structural intervention, while cracks from settlement or ongoing movement should be addressed before retrofit. Where cracks are minor and in the middle of a wall, crack stitching may be effective. For cracks along edges or parapets, structural pinning with anchors like MT screws is often the best solution.