Adoption of American Concrete Institute’s Standard ACI 355-2, prescribing a comprehensive testing program to ascertain design parameters for post-installed
Adoption of American Concrete Institute’s Standard ACI 355-2, prescribing a comprehensive testing program to ascertain design parameters for post-installed concrete anchors used in cracked or uncracked concrete, has spawned further development of anchoring technology. As building codes now require structural anchor design to address the effects of cracks on the anchors Û a significant change in specifications for architects and engineers Û post-installation testing of anchors used in cracked or uncracked concrete evolves. Accordingly, designers and manufacturers are challenged to develop new products to meet the more stringent standards whose criteria and requirements constitute a significant departure from previous practice. The result will be safer concrete anchoring, a more reliable product for end users, and enhanced structural integrity and longevity.
Following ACI’s lead, the Evaluation Service of the International Code Council (ICC-ES) developed specific criteria for testing of these structural products that meets ACI 355-2 standards. Latest editions, compliant with 2003 and 2006 International Building Code (IBC) requirements for post-installed anchors, are AC 193 for mechanical anchors and AC 308 for adhesive anchors. Both AC 193 and AC 308 stipulate evaluation of post-installed anchorage in cracked concrete, including seismic performance factors not previously addressed. As of January 2007, compliance with AC 193 was required of all evaluation reports on mechanical wedge anchors; and, by January 2008, adhesive concrete anchors must comply with AC 308.
CRACKING IMPACT ON ANCHORS
Studies have revealed that (1) cracking typically occurs in a concrete element’s tension zone, and (2) these cracks can significantly impact anchor performance. On a concrete beam or slab, the various causes of cracking include creep; settlement of the support or foundation with changing temperature; thermal expansion and contraction; stress overload; or, the effects of a natural disaster, such as an earthquake or flood.
Predictably, concrete structures in high-seismic zones are especially prone to cracks developed or enlarged during earthquake tremors. Testing of post-installed concrete anchors used in cracked or uncracked concrete, therefore, becomes critical in such regions. With the adoption of new standards emphasizing seismic considerations, concrete anchor manufacturers now must conduct accredited independent, AC 193- and AC 308-compliant testing and evaluate the results to establish required anchor design parameters.
NEW TESTING CRITERIA
Formerly, concrete anchor development was based largely on Îallowable stressÌ design criteria. Testers used a hydraulic system to pull an anchor out of the concrete, calculating allowable shear and tension values by dividing the ultimate figure by a safety factor (typically four).
Design value procedures cited in the new ICC reports, based on ACI 355-2, provide greater reliability and consistency with laboratory results. Factors now considered include a 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 examinations. The service condition test takes into account the anchor’s location in concrete, among other characteristics, and seismic factors.
Also cited extensively in the new criteria are modes of concrete failure, as well as how the anchor must meet relevant requirements. Three types Û steel failure, concrete failure and pull-out failure Û are observed during testing in order that appropriate safety factors for the use of a particular anchor can be applied and referenced by specifiers.
Additionally, test score deductions are considered for the projected area of failure, edge effects, cracking, post-installed anchors, and use of single or multiple anchors. As tests are conducted, anchorage system designers must factor in anchor diameter, installation torque, effective tension area, anchor yield, ultimate strength, embedment depth, and several other new criteria to determine the anchor’s actual strength.
Under such in-depth examination, a product may perform well in a 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 show high values on the ESR report. Conversely, a fastener’s failure to perform satisfactorily in all testing areas will reflect in a poor rating or disqualification. The top rank of Category 1 for an anchor indicates consistent results obtained across all tests.
DESIGNING NEW ANCHORS
Evident in view of such regulatory developments is the necessity of submitting considerable additional information for the design of anchors under the new codes. ACI has developed a precise formula, detailed in ACI 318 Appendix D, that requires specific calculations in determining total anchor strength for a specified location. Anchor performance categories are used by ACI 318 to assign capacity-reduction factors and other parameters.
The more stringent design approach thus introduces additional complications for structural designers, who now must become familiar with varying reduction factors outlined in Appendix D, adapting to code as manufacturers have in the development of new products. ITW Red Head, for example, developed a carbon steel wedge anchor for cracked concrete that was tested in accordance with ACI 355-2 and ICC-ES AC 193. Designed for heavy-duty applications such as hanging pipe overhead or anchoring massive equipment, the fastener meets cracked-concrete standards (tension-zone concrete) and satisfies seismic considerations, making it suitable for use in earthquake-prone regions.
Currently, most standards require only cracked concrete testing, i.e., confirmation that the mechanical or adhesive anchor will remain in place if the concrete cracks during a seismic event. In further consideration of performance under seismic stress, new ICC criteria mandate testing for shear and tension loads in a concrete element that has a 0.02-in.-thick crack (the thickness of a business card). Cycle pull-out tests are conducted to obtain such results. In some instances, as in the case of soon-to-be-published ITW Red Head product test results, no difference between cracked-concrete and seismic-simulation test results is revealed, because the anchor was specifically designed for seismic conditions.
PRESENT AND FUTURE PROSPECTS
Engineers who specify anchors for commercial projects must now consider factors beyond the anchor’s ultimate load. To determine an anchor’s load value, specifiers will need to address various testing categories, i.e., reference, reliability, service conditions, as well as modes of failure, including steel, concrete and pull-out. Manufacturers can assist by providing their test data and creating tables within specific guidelines.
Due to new standards and criteria, anchoring in concrete is now safer. Increasingly in demand will be well-engineered products that meet ACI requirements via ICC criteria. Manufacturers will be challenged to spend more time developing new products, improving old ones, and revising formulas.
This article was adapted from a report by Bill Dubon, engineering manager for ITW Red Head, a manufacturer of mechanical and adhesive anchoring systems used in commercial construction and industrial applications. More information can be obtained by contacting ITW Red Head at 800/899-7890 or visiting www.itw-redhead.com