MRO Today - Meeting new standards — the evolution of anchoring in concrete

Meeting new standards
— the evolution of anchoring in concrete

by Bill Dubon
Engineering Manager
ITW Red Head

The testing of post-installed concrete anchors used in cracked or uncracked concrete continues to dramatically evolve. Building codes now require structural anchor design to address the effects of cracks on the anchors – a significant specification change for architects and engineers. The qualification and design of concrete anchors under these new criteria and requirements differ substantially from past practice, challenging designers and manufacturers to develop new products to meet the more stringent standards. As a result, concrete anchoring will be safer and end users will have a more reliable product, further preserving the structural integrity and longevity of buildings.

This important focus on the cracked concrete anchorage concept began with the introduction of the American Concrete Institute’s Standard ACI 355-2, which prescribes a comprehensive testing program to ascertain design parameters for post-installed concrete anchors used in cracked or un-cracked concrete.

Following ACI’s lead, the Evaluation Service of the International Code Council (ICC-ES) developed specific criteria for the testing of these structural products to meet ACI 355-2. The latest editions for post-installed anchors meeting the requirements of the 2003 and 2006 International Building Code (IBC) are AC193 for mechanical anchors and AC308 for adhesive anchors.

Both AC193 and AC308 permit evaluation of post-installed anchorage in cracked concrete, including seismic performance factors not previously addressed. As of January 2007, all evaluation reports on mechanical wedge anchors must comply with AC193, and by January 2008 adhesive concrete anchors must comply with AC308.

Cracking impacts anchor performance
Studies have revealed that cracking typically occurs in the tension zone of the concrete element, and that these cracks can significantly impact the performance of anchors. Cracks can originate on a concrete beam or slab in a variety of ways: creep, temperature settlement of the support or foundation, thermal expansion and contraction, stress overload, or from a natural disaster such as an earthquake or flood.

Logically, structures in high seismic zones of the country are especially affected because cracks in concrete tend to be developed or enlarged in the event of an earthquake. Therefore, the testing of post-installed concrete anchors used in cracked or un-cracked concrete becomes extremely critical in these regions.

With the adoption of new standards, particularly seismic considerations, concrete anchor manufacturers must now conduct accredited independent testing in conformance to AC193 and AC308, and evaluate the test results to establish the required design parameters for the anchors.

New testing criteria
In the past, the development of concrete anchors was largely based on allowable stress design concepts. Testers used a hydraulic system to pull an anchor out of the concrete and then calculated allowable shear and tension values by dividing the ultimate value by a safety factor – typically four.

The design value procedures posted on the new ICC reports, based on ACI 355-2, are more reliable and consistent with laboratory results. Factors now considered are the statistical analysis of test results from three interrelated categories: reference, reliability and service conditions.

The reference test is similar to the previously mentioned tension test. A reliability test considers factors related to the hole (i.e. oversized, undersized, cleaned, etc.) through proper function tests. The service condition test takes into account the anchor’s location in concrete, seismic considerations and other capacities.

The new criteria also list concrete failure in more detail, describing how the anchor must meet those requirements. Three different types - steel failure, concrete failure and pull-out failure - are observed during testing so that appropriate safety factors for the usage of a particular anchor can be applied for future reference by specifiers.

Additionally, test score deductions are considered for the projected area of failure, edge effects, cracking, post-installed anchor, and whether a single or multiple anchor is used. When tests are conducted, anchor designers must factor in the diameter of the anchor, installation torque, the effective tension area, yield of anchor, ultimate strength, embedment depth, and several other new criteria to determine the anchor’s actual strength.

Under the new, in-depth testing requirements, a product may perform well in one particular test, but poorly in others. For example, an anchor that functions suitably in a high density, deep embedment may not be as effective in a low density, shallow embedment. If an anchor is well engineered, it will have high values posted on the ESR report. If a fastener doesn’t perform satisfactorily in all testing, that product will receive a poor rating or will be disqualified. A Category One is the highest rank for an anchor, and means it achieves consistent results across all tests.

Designing new anchors
Based on all of these developments, it’s quite evident that the design of anchors under the new codes requires submitting a significant amount of new information. The ACI has developed a precise formula as detailed in ACI 318 Appendix D that requires specific calculations in determining the total strength of the anchor for the specified location. The anchor performance categories are used by ACI 318 to assign capacity reduction factors and other parameters.

This new design approach is more complicated for structural designers who must now become more familiar with varying reduction factors as outlined in Appendix D. They will have to adapt just as manufacturers have adapted in the development of these new products.

ITW Red Head, for example, developed a new carbon steel wedge anchor for cracked concrete that was tested in accordance with ACI 355-2 and ICC-ES AC 193. The fastener is designed for heavy-duty applications such as hanging pipe overhead and anchoring down heavy equipment. This new anchor meets cracked concrete standards (tension zone concrete) and also satisfies seismic considerations, making it ideal for use in earthquake prone regions of the country.

Most standards today only call for cracked concrete testing, meaning that during a seismic event, if the concrete cracks, the mechanical anchor or adhesive anchor will remain in place. To take into consideration seismic events, the new criteria developed by the ICC requires testing for shear and tension loads in a concrete element that has a crack as thick as 0.02 inches, or the thickness of a business card. Cycle pull-out tests are conducted to obtain those results.

However, Red Head’s product tests results, soon to be published, reveal that there isn’t any difference between cracked concrete and this kind of seismic simulation - the values are identical because the anchor was specifically designed for seismic considerations.

In summary
Professional engineers who specify anchors for use in commercial projects must now consider additional factors other than the anchor’s ultimate load. In order to determine an anchor’s load value, specifiers will need to address the different categories of testing (reference, reliability, service conditions) and failures such as steel failure, concrete failure and pull-out failure. Manufacturers can make it easier for professionals by providing their test data and creating tables within specific guidelines.

Because of new standards and criteria, anchoring in concrete is safer now. Well-engineered products that meet ACI requirements by the criteria of ICC will be in high demand. Manufacturers will be forced to spend more time in their labs creating new products, improving old products and revising formulas.

It will be time well-spent. As a result, professional engineers, architects and designers will have a safer, more reliable product.

Bill Dubon is engineering manager for ITW Red Head, a manufacturer of mechanical and adhesive anchoring systems used in commercial construction and industrial applications. For more information, contact ITW Red Head at 1-800-899-7890 or visit www.itw-redhead.com.