Concrete Anchor Design for the International Building Code: Part 2

Designing “Alternative Materials”
Concrete anchor types whose designs are not addressed in the IBC or its Referenced Standards, or are specifically excluded from the scope of the Referenced Standard (ACI 318-11), may be recognized as Alternative Materials. Section 1909 of the 2012 IBC requires that “The strength design of anchors that are not within the scope of Appendix D of ACI 318, shall be in accordance with an approved procedure.” Section D.2.2 of ACI 318-11 lists some concrete anchor types that are considered “Alternative Materials” and specifically excludes these anchors from its scope. The list of “Alternative Material” anchors provided in this section is not, however, a comprehensive list.

Section 104.11 of the 2012 IBC describes how the design professional must approach the design of Alternative Materials.

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Section 104.11 provides the design professional with two options for the substantiation of the acceptable performance of an Alternative Material:

a.
Research Reports. As described in the previous section (Design of Code Anchors), Research Reports are referenced as the primary source for the design and qualification of Alternative Materials. Research Reports for anchors are published by IAPMO UES or ICC-ES, both ANSI ISO 17065 accredited agencies. Publicly developed, majority-approved acceptance criteria are used to establish the test program and minimum performance requirements for an anchor type. Some Alternative Material anchor types have established acceptance criteria to which a product can be evaluated:

  • Screw Anchors in Concrete (such as Simpson Strong-Tie® Titen HD®): ICC-ES AC193
  • Headed Cast-in Specialty Inserts (such as Simpson Strong-Tie® Blue Banger Hanger®): ICC-ES AC446
  • Powder- or Gas-Actuated Fasteners (such as Simpson Strong-Tie® PDPA and GDP): ICC-ES AC70

If Research Reports are used to substantiate an anchor’s performance, the design professional is bound by the design methodology and product limitations described in the Research Report.

b.
Tests. If a Research Report is not available, and no acceptance criteria exists for a given anchor type, IBC Section 104.11 permits the use of tests performed in accordance with “recognized and accepted test methods” by an “approved agency” to substantiate performance. One example of an anchor type for which no acceptance criteria exists is:

  • Helical Wall Ties (such as Simpson Strong-Tie® Heli-Tie™)

Cracked Concrete Determination
One of the many design considerations that the design professional must determine when designing either “Code Anchors” or anchors qualified as “Alternative Materials” is whether to consider the state of the concrete “cracked” or “uncracked.” The concrete state can significantly influence the anchor’s capacity. Neither the IBC nor ACI 318, Appendix D explicitly defines which applications should be categorized as “cracked” or “uncracked” concrete. The design professional must determine by analysis whether cracking will occur in the region of the concrete member where the anchors are installed. Absent an analysis to determine whether cracking will occur, the design professional may conservatively assume that the concrete state is “cracked.” With that said, there are two circumstances that require the design professional to design for “cracked” concrete:

a) Anchors in structures assigned to Seismic Design Categories C, D, E, or F (per 2012 IBC, Chapter 16) are required to be designed for “cracked” concrete unless the design professional can demonstrate that cracking does not occur at the anchor locations. The prequalification requirements of ACI 355.2 for mechanical anchors and ACI 355.4 for adhesive anchors include a test program that evaluates the performance of anchors in cracked concrete. Only anchors that have been tested and have passed the cracked concrete test program qualify for use in “cracked” concrete. The Research Report for a post-installed anchor (mechanical or adhesive) will clearly indicate whether it qualifies for use in “cracked concrete.”
b) Anchors located in a region of the concrete element where analysis indicates cracking at service level loading must be designed for “cracked” concrete (e.g. fr ≥ 7.5λ√f’c, ACI 318-11 eq. 9-10).

The design professional must consider additional factors that have the potential to result in concrete cracking in the region of anchorage. These factors include restrained shrinkage, temperature changes, soil pressure, and differential settlement. If no cracking is assumed in the region of the anchorage, the design professional should be able to justify that assumption.

Design Calculations

The design methodology in ACI 318 Appendix D is cumbersome. Calculations can be performed by hand using the design equations in Appendix D, inserting the substantiated data from an anchor manufacturer’s data tables or Research Reports to design with post-installed anchors. Designing with cast-in-place “Code Anchors” does not require additional data beyond what is included in ACI 318, Appendix D since these are “standard” anchors with standard design characteristics.

Performing hand calculations can be time-consuming, and for most design professionals is impractical due to the complexity of the design equations associated with multiple failure modes required to be considered. Design software, such as Simpson Strong-Tie® Anchor Designer™ Software for ACI 318, ETAG and CSA provides a fast, reliable method of calculating anchor performance for both cast-in-place and post-installed anchors. This software designs both “Code Anchors” and “Alternative Materials” for which an acceptance criteria exists.

Simpson Strong-Tie® Anchor Designer™ Software for ACI 318, ETAG and CSA is free and can be downloaded here.

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How Heavy Are Your Calculations?

Designing my first building was truly a learning experience. I remember one event in particular when I determined the required thickness for a steel column base plate. That day I wrote “1.5-inch thk. min.” on my calc pad and months later while out walking the job, I got to see that 1.5-inch thick plate in the flesh. Let me tell you, it was much thicker and heavier in-person than on my calc pad. This eye-opening experience – the realization that what you’re designing isn’t just a word or a number, but rather a physical object with width, height, length and weight – is something every structural engineer goes through early in their career. Designing something on paper doesn’t convey those physical properties very well.

Although there is no replacement for hands-on experience, technology has certainly come a long way in bridging the gap. Building Information Modeling (BIM) has grown significantly and is pushing to become the industry standard. It allows all parties involved to generate a 3D virtual model with objects drawn to scale and precisely placed. By collaborating and designing within a digital environment, multiple groups like contractors, builders, and structural engineers can coordinate project changes on the fly and head off any potential conflict.

Engineers commonly use 2D and 3D analysis software such as ETABS®STAAD.Pro or RISA, to model, design, and export structural components’ actual dimensions to commonly used BIM software, like Revit®Simpson Strong-Tie provides a number of 2D component details and 3D model families for Revit, allowing for accurate representation of each connector, Strong Frame® moment frame or ATS run within the building model. This type of visual representation provides users with an immediate sense of scale and understanding of how it will look and fit within the structure. It’s as close to the real thing as you’ll get.

That’s all fine and dandy for large commercial and multi-family residential projects with big budgets and complicated structural, mechanical and electrical systems, but what about single-family residential and smaller budget projects where a full building model might be cost prohibitive or simply unnecessary? Engineers may still use one of the analysis programs mentioned earlier, but may only model one particular frame or wall within the building. For simpler beam or column calculations, Excel spreadsheets and programs like Enercalc and CFS can provide quick solutions, but without an accurate visual representation.

Simpson Strong-Tie has developed several free stand-alone and web-based programs that help designers translate from calculation to physical product. The Anchor Designer™ Software has a visual interface that automatically updates when the user changes anchor configuration, base plate size or footing dimensions. The Strong Frame® Selector can provide an ordinary or special moment frame design in minutes, then export the design to AutoCAD® and generate an accurate elevation with the click of a button.

Paul mentioned in an earlier blog post how not that long ago he upgraded one of his PCs to a whopping 8 megabytes of RAM. With many computers now coming with 8 gigabytes out of the box, the hardware to run these resource-intensive programs is readily available. As technology continues to evolve, I see BIM and similar software becoming the benchmark for all types of design. When design software can convey tangible properties in a process that was formerly very intangible, it becomes an invaluable tool, especially to structural engineers just starting out.

What tools or software do you use to help convey your design’s physical properties?

*Please note*

Upcoming Web Seminar – Frame of Reference: Steel Moment Frames Explained S.K. Ghosh Associates, Inc. is offering a seminar April 22, 2014 from 9:00 am to 11:30 am PST. The seminar will address design and detailing requirements for ordinary and special moment frames designed per the 2012 International Building Code. The presenter is Simpson Strong-Tie Southwest Branch Engineer Damon Ho, M.S., P.E. Details about the seminar and complete speaker biography can be found here.