Designing structural anchorage to concrete

Figure-4
Load-displacement behaviour requirements of post-installed mechanical anchors. Image courtesy American Concrete Institute

The 2004 edition of CSA A23.3’s Annex D specifically references ACI 355.2 for the prequalification of post-installed mechanical anchors for use with LSD methodology. Under ACI 355.2, there are four types of tests performed: identification, reference, reliability, and service condition. The last three assess performance.

Reference tests
Reference tests establish both a baseline for performance and the anchor category. Reference static tension tests are conducted in low- and high-strength uncracked or cracked concrete. The crack width in the reference tests is a minimum of 0.3 mm (0.01 in.). The results of the reference tests are used to determine the effectiveness factor (k), which helps determine concrete breakout strength.

Reliability tests
Reliability tests assess anchor behaviour and performance under normal and adverse conditions, both during installation and in service. Figure 4 (right) depicts acceptable and unacceptable load-displacement curves that represent part of the evaluation process of post-installed mechanical anchors.

These analyses include static tension tests in cracked or uncracked concrete with reduced installation effort or installations in small- or large-hole diameters. The crack width in the reliability tests is 0.3 mm (0.01 in.) for the reduced installation effort tests; for the sensitivity to hole diameter tests it is 0.5 mm (0.02 in.). Procedures are conducted in low- or high-strength concrete as specified in the criteria. One of the more demanding reliability tests requires the anchor be subjected to a sustained tension load, with the crack width cycled between 0.1 mm (0.004 in.) and 0.3 mm for 1000 cycles.

Figure-6
These two wedge-type expansion anchors look similar, but the one on the right—a redesign—is the only one suitable for cracked concrete. The anchor on the left is unacceptable.

The results of the reference and reliability tests establish the anchor category for each post-installed anchor by diameter. This indicates an anchor’s sensitivity to installation and reliability, which is then used to determine the appropriate resistance modification factor.

Service condition tests
Service condition tests are conducted to establish the data that then  predicts anchor performance under service conditions. The service condition tests include:
• single-anchor installations at close proximity to a corner;
• minimum edge distance and spacing to preclude splitting;
• static shear in cracked concrete;
• simulated seismic tension in cracked concrete; and
• simulated seismic shear in cracked concrete.
Crack width in the simulated seismic tension and shear tests is 0.5 mm (0.02 in.). All service condition tests are run in low-strength concrete.

Anchors meeting Annex D
The change to Annex D has prompted development of a new generation of post-installed anchors. Figure 5 shows two types of post-installed mechanical anchors pre-qualified for use in both cracked and uncracked concrete. In some instances, products have been redesigned to satisfy the new criteria.

In Figure 6, two wedge-type expansion anchors appear similar in design. The anchor on the left is not qualified for use in cracked concrete. The anchor on the right (a redesign of the product on the left) is qualified for use in cracked concrete due to a change in materials and expansion mechanism design.

From the previous discussion of the testing requirements, it is apparent post-installed mechanical anchors are subject to a much more demanding test regimen and assessment requirements for use under Annex D. Consequently, not all post-installed anchor products are suitable for use in cracked or uncracked concrete. It is therefore important the design professional specify only those products pre-qualified under ACI 355.2 when the anchorage to concrete connection has been designed under the Annex D methodology.

J. Bret Turley, PE, is the manager of technical services for Simpson Strong-Tie Anchor Systems. He has been active in the anchoring and fastening industry for more than 20 years. Turley is the secretary of the American Concrete Institute (ACI) Committee 355, Anchorage to Concrete, and is also a member of the committees 349-C, Nuclear Structures–Anchorage, and 408, Development & Splicing of Deformed Bars. He serves on the Fédération Internationale du Béton Special Activities Group 4 on Fastenings to Structural Concrete and Masonry. Turley may be contacted at bturley@strongtie.com.

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