Re-thinking 22.5° Angled Adhesive Anchors in URM Retrofits
Comprehensive research challenges long-held assumptions in masonry anchoring practice.
By Dmytro Dizhur, Ph.D., IntPE
A joint US-New Zealand study* has revisited the longstanding use of 22.5° angled adhesive anchors in unreinforced masonry (URM) retrofits. Over 500+ tension tests were conducted on varied masonry substrates, with findings that question accepted global practice.
Load Transfer Mechanism
Straight adhesive anchors mobilize uniform bond stresses and sustain tensile loads. Anchors installed at 22.5° tended to straighten under demand, causing local masonry crushing and progressive bond deterioration. These findings have been further validated in large-scale site test programs, including some of North America’s most significant retrofit projects.
Capacity, Stiffness, and Failure Modes
Angled anchors exhibited up to 25% lower tensile capacity and stiffness than straight anchors. Failures were dominated by bar straightening and crushing of the surrounding masonry. Post earthquake surveys and supplementary pull-out tests further confirmed pre-mature pull-out behavior.
Engineering Practice Implications
• 25% tension capacity reduction angled vs. straight anchors.
In addition to impacts of angled installation, this study highlighted the significance of more well-known adhesive anchoring risks:
• 45% reduction due to poorly cleaned holes.
• 74% reduction due to insufficient adhesive volume.
• Larger Ø1” anchors performed worse than Ø5/8”, primarily due to brick splitting caused by larger diameter.
The Way Forward: Mechanical Anchors
These results highlight the need for rethinking retrofit anchoring strategies in URM buildings.
Mechanical anchoring systems provide a reliable alternative for seismic retrofits and other structural applications, avoiding the variability of bond-dependent adhesives.
Straight-installed, seismically rated anchors—such as Ø5/16” PYTHON MT Fasteners (ICC-evaluated, available in lengths up to 4 feet) deliver dependable capacity without reliance on bond quality, adhesive volume, or substrate preparation. This shift offers engineers a robust solution for URM retrofits where reliability and seismic performance are critical.
