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About this event

Explore the unique challenges of retrofitting unreinforced masonry (URM) buildings across the US and Canada with ClearCalcs and PYTHON Fasteners. This session is part of the Seismic Retrofit Strategies for Existing Structures series.

You'll gain practical insights from field reconnaissance, experimental testing, and evolving seismic codes that influence design decisions for anchoring systems and overall retrofit performance.

Join ClearCalcs Head of Engineering Content, Laurent Gérin, in conversation with Robert Hudson, PYTHON Fasteners VP of Engineering, on Tuesday, August 5, 2025, from 1 pm to 2 pm Eastern Time (ET).

This session is ideal for structural engineers, seismic consultants, and building officials working with existing masonry buildings in moderate to high seismic zones.

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 designed to enhance the safety and longevity of existing URM structures.
• Updates to ICC and IEBC requirements governing the use of post-installed anchors in existing masonry.

Each insight is grounded in real-world research and will support better-informed retrofit designs that meet code and enhance safety.

Webinar resources:

• PYTHON Fasteners Product Guide
• PYTHON C and PYTHON MT URM Screw Anchors ICC-ES Evaluation Report
• Technical Study: Adhesive Wall-to-Diaphragm Connections in the Canterbury Earthquakes
• Reconnaissance Report: Retrofitted URM Parapet Performance

Summary

This on-demand webinar, hosted by ClearCalcs in partnership with Python Fasteners, provides a deep technical dive into seismic retrofit strategies for unreinforced masonry (URM) structures. The session is tailored for structural engineers working in seismically active regions of the United States and Canada, with a focus on code compliance, anchor systems, retrofit design methods, and field-tested lessons learned from recent earthquakes.

Presented by Laurent Carron (Head of Engineering Content at ClearCalcs) and Robert Hudson (Seismic Retrofit Specialist at Python Fasteners), the webinar walks through both the theoretical and practical considerations of retrofitting existing masonry buildings.

URM retrofit systems and practical design techniques

The webinar covered a wide range of retrofit methods, from parapet stabilization and chimney reinforcement to wall-to-diaphragm connections and full building bracing systems. Various techniques were compared, including concrete overlays, steel and timber bracing, post-installed mechanical anchors, and post-tensioning strategies. Special emphasis was given to common failure mechanisms observed during earthquakes, such as rocking and sliding in parapets or out-of-plane wall failures due to weak connections.

Anchor performance and quality assurance risks

A significant portion of the webinar addressed the limitations and failure risks associated with epoxy anchors, particularly in older brick construction. Based on findings from the Christchurch (2011) and Napa (2014) earthquakes, epoxy anchors were shown to frequently underperform due to installation challenges such as dust contamination, premature loading, and inconsistent embedment. The presenters highlighted why mechanical anchors that are long, thin, and fully threaded are often better suited to the unpredictable nature of historic masonry. They also discussed best practices for anchor QA and testing during design and construction.

Laboratory testing and retrofit research insights

Robert shared extensive findings from laboratory testing programs and post-earthquake field reconnaissance. Tests covered retrofitted parapets, chimneys, and masonry walls, and explored the effects of vertical accelerations, uplift, and the interaction between flexible diaphragms and walls. The session also introduced new retrofit methods, including cross-laminated timber (CLT) shear panels and timber strongbacks, and reviewed their performance compared to conventional concrete shear walls and moment frames.

Updates to ICC anchor certification and code requirements

The webinar provided an overview of recent improvements in ICC certification requirements for mechanical anchors in existing masonry. These updates include reduced minimum embedment and edge distance requirements, horizontal installation allowances (departing from legacy angled installation assumptions), and the inclusion of additional material types such as hollow clay tile. The presenters explained how these changes open up new options for engineers working on URM projects in both the U.S. and Canada.

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Questions and Answers

1. Do angled epoxy anchors improve performance in seismic applications?

Answer: No. While it might seem intuitive that angled anchors add mechanical resistance, testing has shown that bent epoxy anchors (typically installed at 22.5 degrees) often reduce performance in existing masonry. They introduce localized crushing and premature failure, and are generally less reliable than horizontally installed anchors—especially when epoxy installation is imperfect.

2. Can low-pressure grouting improve epoxy anchor performance?

Answer: In theory, grouting voids within multi-wythe masonry can improve epoxy bond quality. However, it is often expensive and may not be necessary. The recommended approach is to perform site-specific anchor testing to understand the actual behavior of the anchors in the field. This provides more reliable data at a lower cost.

3. What are the main reasons epoxy anchors fail in retrofit projects?

Answer: The three most common causes of epoxy anchor failure are: dust remaining in the hole during installation, insufficient epoxy volume, and loading the anchor before the epoxy has fully cured. In existing masonry, using a vacuum rather than compressed air is essential, as pressurized air can degrade the surrounding material.

4. When are through-bolts a good solution for URM wall connections?

Answer: Through-bolts offer excellent strength and are especially useful in rubble masonry or when wall quality is uncertain. However, they are visually intrusive and may not be appropriate for historic façades or heritage-listed buildings. They also require access to both sides of the wall and are more labor-intensive to install.

5. Does post-tensioning a chimney render it unusable?

Answer: Yes. Post-tensioning with a steel plate and concrete base is an effective way to stabilize chimneys, but it typically means the chimney will no longer be functional. This method is best used in cases where the architectural appearance must be preserved but active use of the chimney is not required.

6. Are there low-carbon or moisture-permeable alternatives to concrete shear walls?

Answer: Cross-laminated timber (CLT) panels and plywood shear walls offer effective seismic performance with lower weight and embodied carbon. These systems also allow for easier detailing to maintain breathability, which is important in older buildings with moisture-sensitive materials. The main design challenge lies in the hold-down connections, which must resist significant overturning forces.

7. Do local US building departments require anchor testing?

Answer: Yes. Many jurisdictions, including Portland, Oregon, now require pre-construction anchor testing as part of the seismic retrofit permitting process. Early testing also helps optimize design by preventing over-conservatism and reducing the number of anchors required, saving time and cost during construction.

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