By Mark Harvinen, P.E.
In the last few years, Simpson Strong-Tie has heard from a number of structural engineers expressing frustration with the lack of performance data for shallowly embedded, post-installed anchors (shallow anchors). Engineers of Record (EOR) have identified a common application for shallow anchors as those related to attachment of sill plates for structural and nonstructural wall-to-podium slab connections. One dilemma faced by the EORs originates in their desire to prevent damage to concrete podium slab reinforcement, especially where reinforcement is located close to the slab’s top surface to resist negative bending moments. EORs further indicate that shallow anchors are frequently needed for the following attachments: hanging MEP fixtures; attaching nonstructural components associated with tenant improvements; and anchoring light equipment.
Addressing the problem
Problem solving is in the DNA of Simpson Strong-Tie. Our engineering team didn’t hesitate to see this as an opportunity to come up with a solution. We investigated how to address this apparent deficiency in post-installed anchor performance data. We reviewed relevant test and qualification standards for post-installed anchors and polled the specifier community for critical details related to the challenge. Questions such as “What is the limit of acceptable embedment depth?” “What type and magnitude of loading need be accommodated?” and “What type of post-installed anchor is most desired?” required answers.
Our team polled structural engineers across the United States for answers to these questions. The largest percentage of respondents indicated they most frequently encountered the need for shallow anchors in concrete-on-metal-deck and elevated, post-tensioned slab construction. Over 70% of respondents indicated they were comfortable specifying drill depths not greater than 1” and that they either preferred mechanical screw anchors or had no preference regarding post-installed anchor type. Fifty-three percent of respondents desired tension performance and shear performance within the range of 200–500 lb. per anchor. Finally, a significant majority of respondents believed that inclusion of shallow-anchor performance data in an Evaluation Report issued by an accredited agency was mandatory.
To fulfill EOR requests for inclusion of performance data into an Evaluation Report, we approached testing in accordance with ICC-ES AC193 (AC193), “Acceptance Criteria for Mechanical Anchors in Concrete Elements,” Appendix A5, “Requirements for the Qualification and Design of Shallow Anchors.” In addition to Appendix A5 requirements that shallow anchors be tested and assessed in accordance with Table 4.2 of AC193, and used and designed in accordance with ACI 318-14 (Chapter 17), Appendix A5 imposes the following additional conditions:
1. Installation of shallow anchors shall be limited to formed concrete surfaces (A.5.1.3).
2. Use of shallow anchors shall be limited to installations with dry, interior exposure (A.5.1.4).
3. Anchors qualified as shallow anchors shall be limited to support of nonstructural elements (A.5.1.5).
4. Anchors qualified as shallow anchors are limited to a minimum spacing distance of no less than 4Hef and a minimum edge distance of no less than 2Hef (A.5.1.6).
The first and third conditions appear to be the most restrictive when considering the use of shallow anchors for shearwall sill plate anchorage.
With the possible exception of precast concrete slab elements, it’s prudent for the EOR to give due consideration to the quality of concrete at a finished surface and its impact on the reliability of shallow anchor performance. The quality of unformed concrete surfaces is most certainly affected by concrete mix ratios, ambient conditions, the specified concrete finishing and curing procedures, and how strictly these specifications are executed in the field. Additionally, finished concrete surfaces may be the most susceptible to surface deficiencies such as cracking, crazing, blistering, delamination, and spalling. Interestingly, the preliminary test results indicated no discernible difference in performance between shallow anchors installed in formed surfaces and those installed in unformed or hand-troweled surfaces.
While consideration of shallow-anchor performance data for nonstructural components is clearly permitted by AC193 Annex A5, the attachment of shearwall sill plates is undeniably a structural anchorage. We requested an explanation of the basis of Condition 3 from ICC-ES and their response was that the restriction originates from the lack of test data related to tension performance of shallow anchors under sustained loading. One could argue that utilization of shallow anchors for in-plane, shear-force transfer from a sill plate to concrete is neither a tension nor a sustained loading application. Here, too, the EOR must apply his or her own engineering judgment as to the suitability of specifying shallow anchors based on performance obtained via testing per Annex A5 for any structural anchorage.
To date, our shallow-anchor performance data has focused on shear performance. The results of the Simpson Strong-Tie laboratory testing program on 1/4”- and 3/8”-diameter Titen HD® (THD) shallow anchors may be found on pages 9 and 10 of TEB-A-THD20_R2, “Titen HD Design Information – Concrete.” [TEB-A-THD20_R2 (TEB) may be found at strongtie.com.] The Shear Strength Design Data table on page 9 of TEB is limited to shear performance data with the exception that the Effectiveness Factors (Kuncr and Kcr), needed for tension concrete breakout calculations, are included to permit shear concrete pryout calculations. As indicated in footnote 5, this data is presently not included in the Titen HD Evaluation Report (ESR-2713).
Shear performance data for a few selected combinations of edge distance and spacing may be found on Page 10 of TEB. The tabulated data has been calculated for 2,500 psi normal-weight concrete, and, accordingly, higher-strength concrete will produce increased design strength values. Performance for the 3/8”-diameter THD is generally lesser than that for the 1/4”-diameter Titen HD due to its shorter effective embedment depth (hef).
When considering use of shallow anchors for structural or nonstructural walls with stud spacing of 16” on center, a quantity of five total anchors between studs is a feasible, maximum anchor quantity (approximately 3” spacing). Based on performance tabulated in the TEB, the shear design strength of the 1/4”- diameter THD at such spacing could perhaps produce as much as 770 lb. per linear ft. and 995 lb. per linear ft. of in-plane shear force transfer in cracked and uncracked concrete service conditions, respectively. (Note: parallel-to-grain sill/sole plate or CFS track strength may limit in-plane shear performance and must be calculated separately.)
Shear Design Strength Data Tables
The tables provide shear performance data for single 1/4”- and 3/8”-diameter THD anchors at higher concrete compressive strengths (3,000 psi to 8,000 psi). The tables are developed with in-plane, sill plate or CFS track anchorage in mind (shear parallel to edge) but are also valid as shear-parallel-to-edge values even at each anchor’s minimum qualified edge distance of 1.33” (1/4” THD) and 0.98” (3/8” THD). Shear design strengths perpendicular to edge should be calculated and considered when resistance to out-of-place loading, acting towards a concrete edge, is required.
Tabulated shear design strengths are governed by the shear concrete pryout failure mode, and this will be true in most instances with the following exceptions:
• Steel strength in shear in cracked concrete governs 3/8”-diameter THD shear performance at concrete compressive strengths above 6,000 psi (highlighted in blue).
• Shear concrete breakout failure mode (perpendicular to concrete edge) may govern design strength for shear applied perpendicular to the concrete edge as anchor edge distance and spacing approach minimum qualified values.
Tabulated shear performance data from the tables may be used to calculate per-linear-foot shear strength values, such as for the following shallow-anchor parameters: 1/4”- diameter THD, spaced at 4” on center, in cracked 4,000 psi, NWC…..φVuw = 3 x 261 lb. = 783 lb. per ln. ft.
Where……. φVuw = Shear strength per linear foot of wall: 1/4” THD-to-concrete connection.
An ASD-level equivalent may be calculated by assuming 100% wind load and referencing performance values in the table’s ASD-Level-Wind Column: 3 x 157 lb. = 471 lb. per ln. ft.
Assuming uncracked concrete service conditions raises the ultimate performance by approximately 30% to 3 x 338 lb. = 1,014 lbs. per ln. ft., and an equivalent ASD-Level-Wind allowable capacity: 3 x 203 lb. = 609 lb. per ln. ft.
Simpson Strong-Tie shallow-anchor shear performance for 1/4” and 3/8” Titen HD anchor screws provides design professionals with shear design strengths that may be considered for a variety of nonstructural anchorage applications, including transferring wood sole plate and cold-formed steel track shear forces to concrete in the absence of other options. Tabulated values should be viewed as guide performance data that may vary depending on concrete surface curing and finishing operations insofar as they affect the concrete quality within the top 1” of the concrete’s surface. We’re eager to receive comments and feedback related to the shallow-anchor performance data included in TEB-A-THD20_R2 and this article before seeking inclusion of the data into ICC ES ESR-2713.
Mark Jarvinen, P.E. (MA) is Field Engineer at Simpson Strong-Tie, New Tripoli, Pennsylvania. You can contact him at [email protected], (781) 775-2346.