The buzz about cracked concrete

New code requirements are redefining anchor capacity

Editor’s Note: Powers Fasteners’ professional engineers Jake Olsen, director of product development, and Mark Ziegler, director of engineering, offer insight on the changing building code and how it will affect anchor selection.

Those of us in the concrete anchoring industry have been a little jealous of the fancy acronyms and slogans of the high-tech industry. But it appears that we have finally coined a new phrase of our own: cracked concrete.

It may be an exaggeration that cracked concrete issues are sweeping the construction industry, but there have been enough questions from vendors, specifiers and end-users to warrant some clarification.

The term “cracked concrete,” as it relates to concrete anchors, comes from the new building code that requires special design considerations and prequalification on anchors installed in concrete that has a possibility of cracking.

Determining which portions of concrete are susceptible to cracking is fairly complicated but, in general, cracking is expected where:

1. The concrete is in tension. A typical example of this is the underside of a suspended slab or ceiling. This is a common area of application for post-installed anchors used to support overhead bracing and hanging rod for suspended systems.

2. The concrete is located in a geographic region of moderate or high seismic risk. Structural concrete anchors in these regions are designed assuming “cracked concrete” conditions.

The new strength design method, often referred to as the Concrete Capacity Design (CCD) method, is required to be used for post-installed expansion anchors (anchors that are installed in a drilled hole) starting with the 2002 American Concrete Institute (ACI) 318, found in Appendix D of that document.

Prior to Appendix D, anchor design was performed using allowable stress design (ASD). Allowable load tables for anchors have been published in manufacturers’ literature and in load tables found in International Code Council, Evaluation Service, formerly ICBO, BOCA, SBCCI and NER evaluation reports. The testing for these reports did not investigate an anchor’s ability to function when installed in a region of cracking.

The testing criteria for the strength design method, which considers the possibility of concrete cracking, originally appeared in the United States as ACI 355.2. This was later modified to become a new ICC-ES acceptance criteria (AC193). This addresses testing of mechanical anchors, while AC308 followed shortly after to address the testing of adhesive anchors. Both documents provide options to approve anchors for uncracked concrete only or cracked and uncracked concrete.

For the latter, various tests on anchors installed directly in a crack are required. To put the crack size in perspective, the largest crack width required for testing is 0.020", about the thickness of a business card.

Old evaluations phasing out
In conjunction with the onset of these new methods for the design, testing and evaluation of post-installed anchors, ICC-ES is phasing out the old evaluation reports and test methods used for allowable stress design of anchors in concrete. This will require anchor manufacturers to retest products for cracked or uncracked concrete so they can be in compliance with jurisdictions that have adopted the most recent versions of the building code.

Some of the potential advantages of post-installed anchors qualified for the strength design method under the new code include:

1. Increased reliability. It is expected that specifying engineers will become more confident in post-installed anchors that have obtained the new approvals based on the more stringent testing requirements.

Design load values will now be based on statistical analysis of test results that will ensure a more consistent probability against anchor failure. This is in stark contrast to the blind safety factors used in the ASD method. Considering this, we believe that post-installed anchors will become more widely accepted and specified, especially in regions of “cracked concrete.”

2. Greater efficiency. One primary benefit of the strength design method is that it gives a designer the ability to control the failure mode of the anchor (whether the concrete fails, the steel breaks or other failure mode). By doing so, appropriate levels of safety can be applied using statistical analysis.

This usually results in greater capacity for an anchor designed using strength design compared with the same anchor design using the ASD method.

3. Accounting for reinforcement. The new strength design method allows the designer to increase the capacity of anchors that are near reinforcement for cracked and uncracked concrete. This previously was not an option.

Initial reaction of manufacturers, specifiers, and contractors to changes surrounding post-installed anchors frequently evokes the, “if it ain't broke, don’t fix it” sentiment. The costs associated with the new requirements include, but is by no means limited to:

• Anchor manufacturers must retest and reapprove their entire concrete anchor product lines. Some current anchor types may be abandoned in the future, requiring entirely new types of anchors.

• Specifying engineers must learn the new strength design procedure, which is fairly complex in contrast to the ASD method.

• Anchor sellers and distributors must become educated in the new requirements and adjust their inventories, sales and marketing strategies.

• Contractors using concrete anchors under the new code will likely be required to pay more attention to installation procedures and be held to a higher level of inspection by building officials.

The impact on commercial and residential construction projects will not likely be immediate. Now that the new code is complete, local jurisdictions must accept the code and begin enforcement, and design and specifications must be completed under the new requirements before construction takes place. This cycle often takes years.

Watch for new anchors
As the concrete anchoring industry adopts the new design methods, Powers Fasteners will keep pace with new products and approvals. It has established a training school and a field engineering program to help educate salespeople and design engineers on the new standards and products. Its Powers Training Vehicle (PTV) completes training on jobsites.

Finally, the Powers customer service department and engineering team can address questions related to this transition.

Published in the May/June 2006 issue of Contractor Tools and Supplies magazine.

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