Considerations for Designing Anchorage in Proximity to Abandoned Anchor Holes

danharmon.headshot.finalThis week’s post comes from Dan Harmon, an R&D engineer for Simpson Strong-Tie’s Infrastructure-Commercial-Industrial (ICI) group. Dan specializes in post-installed concrete anchor design and spent a decade managing Simpson’s anchor testing lab, where he developed extensive knowledge of anchor behavior and performance. He has a Bachelor of Science in mechanical engineering from the University of Illinois Urbana-Champaign.

Designers and engineers can spend hundreds of hours on detailed drawings of structures, but there are often conditions and coordination that can change well-planned details and drawings. As we all know, paper and reality don’t always agree. Anchorage locations can move as a result of unforeseen circumstances such as encountering reinforcing bars in an existing concrete slab or interference between different utility trades.

With post-installed anchors, one particular jobsite change may require abandoning a hole that has been drilled, leaving the final anchor location adjacent to the abandoned hole. When a hole for an anchor is drilled but never used, it essentially creates a large void in the concrete. Depending on where this void is located in relation to an installed anchor, there is potential for the capacity of that anchor to be reduced. To give guidance on this situation to specifiers, users and contractors, Simpson Strong-Tie conducted a large series of tests in their ISO 17025–accredited Anchor Systems Test Lab in Addison, Illinois.

To evaluate the effect of abandoned holes located adjacent to post-installed anchors, we performed tension tests meeting the requirements of ASTM E488-15 (see Figure 1). A variety of anchor types with common diameters were tested:

  • Drop-in anchors (1/2″ and 3/4″ diameter)
  • Wedge-type anchors (1/2″ and 3/4″ diameter)
  • Concrete screws (1/2″ diameter)
  • Adhesive anchors with threaded rod (1/2″ diameter)
Figure 1: Common Unconfined Tension Test Set-Up per ASTM E488-15

Figure 1: Common Unconfined Tension Test Set-Up per ASTM E488-15

Each anchor type and diameter was tested under five different conditions:

  • No abandoned hole near the installed anchor. This is considered the reference condition to which other tests are to be compared.
  • One abandoned hole at a distance of two times the hole diameter (2d) away from the installed anchor. See Figure 2.
  • One abandoned hole at a distance of four times the hole diameter (4d) away from the installed anchor.
  • Two abandoned holes, each at a distance of two times the hole diameter (2d) away from the installed anchor. In test conditions with two holes, the holes were located on either side of the installed anchor, approximately 180º from each other. See Figure 3.
  • Two abandoned holes, each at a distance of two times the hole diameter (2d) away from the installed anchor, with the holes refilled with a concrete anchoring adhesive that was allowed to cure fully prior to testing. See Figure 4.
Figure 2: Drop-In Anchor with a Single Hole at a Distance of 2d

Figure 2: Drop-In Anchor with a Single Hole at a Distance of 2d

Figure 3: Drop-In Anchor with Two Holes at a Distance of 2d

Figure 3: Drop-In Anchor with Two Holes at a Distance of 2d

Figure 4: Drop-In Anchor with Two Holes, Filled with Anchoring Adhesive, at a Distance of 2d

Figure 4: Drop-In Anchor with Two Holes, Filled with Anchoring Adhesive, at a Distance of 2d

This test program is summarized in Table 1. In all cases, the abandoned hole was of the same diameter and depth as the hole prescribed for the installed anchor.

Table 1. Summary of Test Program

Table 1. Summary of Test Program

Five tests for each anchor under each condition were tested, and the mean and coefficient of variance of each data set were calculated. These calculated values were used to compare the different conditions.

Across the different anchor types and diameters, the test results showed a number of general rules that held true.

Summary Results

Abandoned holes that are 2” or more away from the anchor have little to no effect on the tension performance of the anchor. Compared to the reference condition with no abandoned hole near the anchor, conditions where the abandoned hole was sufficiently far away were found to be essentially equivalent. This equivalence held true even for anchor types that create expansion forces (drop-in and wedge-type anchors) during their installation.

Two abandoned holes have the same effect on performances as one, regardless of distance from the anchor. This testing showed that adding a second abandoned hole near an installed anchor did not adversely affect tension performance in a significant way. Even within distances of 2 inches, performance did not drop substantially – if at all – in conditions involving two abandoned holes as compared to one.

Filling abandoned holes with an anchoring adhesive prior to installation of the anchor improves performance. In all cases tested, filling abandoned holes with adhesives resulted in increased performance compared to leaving the holes empty. In a majority of cases, performance with filled holes was equivalent to performance in the reference condition regardless of the distance from the anchor.

When the abandoned hole is more than two times the drilled hole diameter but less than 2″from the anchor – and left unfilled – the testing showed a loss in performance. Not surprisingly, the degree of that loss was dependent on the type of anchor. Table 2 shows the capacity reduction compared to the reference condition in testing with expansion anchors. Table 3 shows the same results for concrete screws and adhesive anchors. Conservative suggested performance reductions in these conditions would be 20% for expansion anchors and 10% for concrete screws and adhesive anchors.

Table 2: Performance Reduction for Expansion Anchors

Table 2: Performance Reduction for Expansion Anchors

Table 3: Performance Reduction for Concrete Screws and Adhesive Anchors

Table 3: Performance Reduction for Concrete Screws and Adhesive Anchors

In an ideal world, the engineer’s designs could be followed at all times at the jobsite. But we don’t live in an ideal world. Good engineering judgment is needed in situations where variation is required, and having data to support those decisions is always helpful. In the case of abandoned holes near post-installed anchors, it’s Simpson Strong-Tie’s hope that this testing provides additional guidance for the designer, inspector, and jobsite worker.

 

Concrete Anchorage for ASD Designs

One of the first things I learned in school about using load combinations was that you had to pick either Load and Resistance Factor Design (LRFD)/Strength Design (SD) or Allowable Stress Design (ASD) for a building and stick with it, no mixing allowed! This worked for the most part since many material design standards were available in a dual format. So even though I may prefer to use LRFD for steel and ASD for wood, when a steel beam was needed at the bottom of a wood-framed building that was designed using ASD load combinations, the steel beam could easily be designed using the ASD loads that were already calculated for the wood framing above since AISC 360 is a dual- format material standard. And when the wood-framed building had to anchor to concrete, ASD anchor values were available in the IBC for cast-in-place anchors and from manufacturers for post-installed anchors in easy-to-use tables, even though ACI 318 was not a dual-format material standard. (Those were good times!)

Then along came ACI 318-02 and its introduction of Appendix D – Anchoring to Concrete, which requires the use of Strength Design. The 2003 IBC referenced Appendix D for Strength Design anchorage, but it also provided a table of ASD values for some cast-in-place headed anchors that did not resist earthquake loads or effects. This option to use ASD anchors for limited cases remained in the 2006, 2009 and 2012 codes. In the 2015 IBC, all references to the ASD anchor values have been removed, closing the book on the old way of designing anchors.

ICC-ES-equation-tensionSo what do you do now? Well, there is some guidance provided by ICC-ES for manufacturers to convert calculated SD capacities to ASD allowable load values. Since there is no conversion procedure stated in the IBC or referenced standards, designers may want to use this generally accepted method for converting anchor capacities designed using ACI 318. ICC-ES acceptance criteria for post-installed mechanical and adhesive anchors (AC193 and AC308) and cast-in-place steel connectors and proprietary bolts (AC398 and AC399) outline a procedure to convert LRFD capacities to ASD using a weighted average for the governing LRFD/SD load combination. So if the governing load combination for this anchor was 1.2D + 1.6L and the dead load was 1,000 pounds and the live load was 4,000, then the conversion factor would be (1.2)(0.2) + (1.6)(0.8) = 1.52 (keep in mind that the LRFD/SD capacity is divided by the conversion factor in the ICC-ES equation shown here for tension).

Right away, there are a few things that you may be thinking:

  1. What about load factors that may exist in ASD load combinations?
  2. It may just be easier to just recalculate my design loads using LRFD/SD combinations!
  3. The resulting allowable loads will vary based on the load type, or combination thereof.
  4. If the ACI 318 design strength is limited by the steel anchor, then the conversion will result in an allowable load that is different from the allowable load listed for the steel element in AISC 360.

Let’s take a look at these objections one by one.

Item 1: Since unfactored earthquake loads are determined at the ultimate level in the IBC, they have an LRFD/SD load factor of 1.0 and an ASD load factor less than 1.0, which is also true for wind loads in the 2012 and 2015 IBC (see graphic below). Using the LRFD/SD load factor of 1.0 obviously does not convert the capacity from LRFD to ASD so you must also account for ASD load factors when calculating the conversion factor. To do so, instead of just using the LRFD load factor, use the ratio of LRFD Factor over ASD Factor. So if the governing load combination for an anchor was 0.9D + 1.0E and the dead load was 1,000 pounds and the seismic load was 4,000, then the conversion factor would be (0.9)(0.2) + (1.0/0.7)(0.8) = 1.32.

ICC-ES-equations

Item 2: Even though the weighted average conversion requires you to go back and dissect the demand load into its various load types, often this can be simplified. ICC-ES acceptance criteria permit you to conservatively use the largest load factor. The most common application I run into is working with ASD-level tension loads for wood shearwall overturning that must be evaluated using SD-level capacities for the concrete anchorage. Since these loads almost always consist of wind or seismic loads, using the largest factor is not overly conservative. Depending on the direction in which you are converting the demand loads or resistance capacities, the adjustment factors are as shown in the figure below. Affected Simpson Strong-Tie products now have different allowable load tables for each load type. (For examples, see pp. 33-36 of our Wood Construction Connectors catalog for wind/seismic tables and pp. 28-30 of our Anchoring and Fastening Systems catalog for static/wind/seismic tables.)

IBC-ealier-later

Item 3: I am unsure whether there is any sound rationale for having allowable loads for an anchor resisting 10% dead load and 90% live load differ from those of an anchor that resists 20% dead load and 80% live load. Perhaps a reader could share some insight, but I just accept it as an expedience for constructing an ASD conversion method for a material design standard that was developed for SD methodology only.

Item 4: We have differing opinions within our engineering department on how to handle the steel strength component of the various SD failure modes listed in ACI 318. Some believe all SD failure modes in ACI 318 should be converted using the load factor conversion method. I side with others who believe that the ASD capacity of a steel element should be determined using AISC 360. So when converting SD anchor tension values for a headed anchor, I would apply the conversion factor to the concrete breakout and pullout failure modes from ACI 318, but use the ASD steel strength from AISC 360.

Finally, I wanted to point out that the seismic provisions in ACI 318, such as ductility and stretch length, must be considered when designing anchors and are not always apparent when simply converting to ASD. For this reason, I usually suggest converting ASD demand loads to SD levels so you can use our Anchor Designer™ software to check all of the ACI 318 provisions. But for some quick references, we now publish tabulated ASD values for our code-listed mechanical and adhesive anchors in our C-A-2016 catalog —  just be sure to read all of the footnotes!

Understanding and Meeting the ACI 318 – 11 App. D Ductility Requirements – A Design Example

If you’re one of the many engineers still confused by the ACI 318 – 11 Appendix D design provisions, this blog will help explain what’s required to achieve a ductile performing anchorage. Most building codes currently reference ACI 318 – 11 Appendix D as the required provision for designing a wide variety of anchor types that include expansion, undercut, adhesive and cast-in-place anchors in concrete base materials. This blog post will focus on section D.3.3.4.3(a) for an anchor located in a high seismic region. We’ll go over what these requirements are with a simple design example.

Ductility is a benefit in seismic design. A ductile anchor system is one that exhibits a meaningful degree of deformation before failure occurs. However, ductility is distinct from an equally important dimension called strength. Add strength, and a ductile steel element like the one shown in Figure 1 can now exhibit toughness. During a serious earthquake, a structural system with appreciable toughness (i.e., one that possesses both strength and ductility in sufficient degree) can be expected to absorb a tremendous amount of energy as the material plastically deforms and increases the likelihood that an outright failure won’t occur. Any visible deformations could help determine if repair is necessary.

Figure 1 – ½" mild steel threaded rod tensilely loaded to failure (starting stretch length = 8d)

Figure 1 – ½” mild steel threaded rod tensilely loaded to failure (starting stretch length = 8d)

Let’s start off with a simple example that will cover the essential requirements for achieving ductility and applies to any type of structural anchor used in concrete. We’ll arbitrarily choose a post-installed adhesive anchor. This type of anchor is very common in concrete construction and is used for making structural and nonstructural connections that include anchorage of sill plates and holdowns for shear walls, equipment, racks, architectural/mechanical/electrical components and, very frequently, rebar dowels for making section enlargements. We’ll assume the anchor is limited to resisting earthquake loading in tension only and is in seismic design category C – F. Section D.3.3.4.2 requires that if the strength-level earthquake force exceeds 20% of the total factored load, that the anchor be designed in accordance with section D.3.3.4.3 and D.3.3.4.4. We will focus on achieving the ductility option, (a), of D.3.3.4.3.

To understand anchor ductility we need to first identify the possible failure modes of an anchor. Figure 2 shows the three types of failure modes we can expect for an adhesive anchor located away from a free edge. These three failure modes generically apply to virtually any type of anchor (expansion, screw, cast-in-place or undercut). Breakout (Nb) and pullout (Na) are not considered ductile failure modes. Breakout failure (Nb) can occur very suddenly and behaves mostly linear elastic and consequently absorbs a relatively small amount of energy. After pullout failure (Na) has been initiated, the load/displacement behavior of the anchor can be unpredictable, and furthermore, no reliable mechanism exists for plastic deformation to take place. So we’re left with steel (Nsa). To achieve ductility, not only does the steel need to be made of a ductile material but the steel must govern out of the three failure modes. Additionally, the anchor system must be designed so that steel failure governs by a comfortable margin. Breakout and pullout can never control while the steel yields and plastically deforms. This is what is meant by meeting the ductility requirements of Appendix D.

Figure 2 –Three possible failure modes for an adhesive anchor loaded in tension

Figure 2 – Three possible failure modes for an adhesive anchor loaded in tension

Getting back to our design example, we have a single post-installed 5/8” diameter ASTM F1554 Gr. 36 threaded rod that’s embedded 12” deep, in a dry hole, in a concrete element that has a compressive strength of 2,500 psi. The concrete is 18” thick and we assume that the edge distance is large enough to be irrelevant. For this size anchor, the published characteristic bond strength is 743 psi. Anchor software calculations will produce the following information:

ductility4

The governing design strength is compared to a demand or load combination that’s defined elsewhere in the code.

Here’s the question: Before proceeding with the remainder of this blog, judging by the design strength values shown above, should we consider this anchorage ductile? Your intuition might tell you that it’s not ductile. Why? Pullout clearly governs (i.e., steel does not). So it might come as a surprise to learn that this adhesive anchor actually is ductile!

To understand why, we need to look at the nominal strength (not the design strength) of the different anchor failure modes. But first let’s examine the equations used to determine the design strength values above:

ductility5

The above values incorporate the notation φ (“phi”) and a mandatory 0.75 reduction factor for nonductile failure modes (Ncb ,Na) for applications located in high seismic areas (seismic design category C–F). The φ factor is defined in section D.4. However, manufacturers will list factors specific to their adhesive based on anchor testing. The mandatory 0.75 reduction comes from section D.3.3.4.4 and is meant to account for any reduction associated with concrete damage during earthquake loading. The important thing to remember is that the nominal strength provides a better representation of the relative capacity of the different failure modes. Remove these reduction factors and we get the following:

ductility6

Now steel governs since it has the lowest strength. But we’re not done yet. Section D.3.3.4.3.(a).1 of Appendix D requires that the expected steel strength be used in design when checking for ductility. This is done by increasing the specified steel strength by 20%. This is to account for the fact that F1554 Gr. 36 threaded rod, for example, will probably have an ultimate tensile strength greater than the specified 58,000 psi. (Interestingly, the ultimate strength of the ½” threaded rod tested in Figure 1 is roughly 74 ksi, which is about 27% greater than 58,000 psi.) With this in mind, the next step would be to additionally meet section D.3.3.4.3.(a).2 such that the following is met:

ductility7

By increasing the steel strength by 20%, the nominal strength of the nonductile failure modes (Ncb ,Na) must be at least that much greater to help ensure that a ductile anchor system can be achieved. The values to compare finally become:

 

ductility8Now steel governs, but one more thing is required. As shown in Figure 3, Section D.3.3.4.3.(a).3 of Appendix D also requires that the rod be made of ductile steel and have a stretch length of at least eight times the insert diameter (8d). Appendix D defines a ductile steel element as exhibiting an elongation of at least 14% and a reduction in area of at least 30%. ASTM F1554 meets this requirement for all three grades of steel (Grade 36, 55 and 105) with the exception of Grade 55 for anchor nominal sizes greater than 2”. Research has shown that a sufficient stretch length helps ensure that an anchor can experience significant yielding and plastic deformation during tensile loading. The threaded rod shown in Figure 1 was tested using a stretch length of 4” (8d). Lastly, section D.3.3.4.3.(a).4 requires that the anchor be engineered to protect against buckling.

Figure 3 – Stretch length

Figure 3 – Stretch length

Appendix D doesn’t require that an anchor system behave ductilely. Three additional options exist for Designers in section D3.3.4.3. Option (b) allows for the design of an alternate failure mechanism that behaves ductilely. Designing a base plate (or support) that plastically hinges to exhibit ductile performance is one example. Option (c) involves a case where there’s a limit to how much load can be delivered to the anchor. Although option (c) under D.3.3.4.3 falls under the tensile loading section of Appendix D, the best example would apply to anchorage used to secure a wood sill plate or cold-formed steel track. We know from experiments that the wood crushes or the steel yields and locally buckles at a force less than the capacity of the concrete anchorage. Clearly energy is absorbed in the process. The most commonly used option is (d), which amplifies the earthquake load by Ωo. Ωo can be found in ASCE 7 – 10 for both structural and nonstructural components. The value of Ωo is typically taken to be equal to 2.5 (2.0 for storage racks) and is intended to make the anchor system behave linear elastically for the expected design-level earthquake demand.

These same options exist for shear loading cases. However, achieving system ductility through anchor steel is no longer an option for shear loading according to ACI 318 – 11, because the material probably won’t deform appreciably enough to be considered ductile.

While factors such as edge-distance and embedment-depth restrictions make achieving ductility difficult for post-installed anchors, it should come as some consolation that in many cases the Designer can achieve ductile performance for cast-in-place anchors loaded in tension through creative detailing of reinforcing steel (section D.5.2.9) to eliminate breakout as a possible failure mode. This has been explored in some detail in two previous Simpson Strong-Tie blogs titled “Anchor Reinforcement for Concrete Podium Slabs” and “Steel Strong Wall Footings Just Got a Little Slimmer.”

 

Anchor Reinforcement for Concrete Podium Slabs

Guest blogger Scott Fischer, R&D Engineer

Guest blogger Scott Fischer, R&D Engineer

This week’s post comes from Scott Fischer who is an R&D Engineer at our home office. Since joining Simpson Strong-Tie in 2006, Scott has worked on cast-in-place connectors to concrete. He helped develop the current testing criteria for cast-in-place concrete products and also performed the testing and code report requirements needed for these product lines. Prior to joining Simpson Strong-Tie, Scott worked for nine years as a consulting engineer. His experience includes the design and analysis of concrete structures, including post-tensioned slab design, concrete lateral systems and foundations. Scott is a licensed professional engineer in the state of California and received his bachelor’s degree in Architectural Engineering from Cal Poly San Luis Obispo.

How often do you get the opportunity to high five a co-worker in the office? Maybe it’s when you just worked through a really complex calculation, or finally figured out that tough detail. Whatever it might be, there are times when we should raise a hand and celebrate the hard work that we do. So when we recently relaunched the Simpson Strong-Tie Strong-Rod™ Systems website, which includes a link to our new Shallow Podium Anchorage Solutions, there were a few high fives going around the office. With that in mind, we want to share the latest developments and continue our anchorage-to-concrete blog discussions that began in May 2012, continued with a March 2014 post referencing the Structure magazine article on anchor testing, and more recently one discussing our release of anchor reinforcement solutions for Steel Strong-Wall® shearwall to grade beams.

Anchor Reinforcement Testing and Research Program

Simpson Strong-Tie has been studying cast-in-place tension anchorage and anchor reinforcement concepts extensively over the past several years. Designing an anchor solution in a thin concrete slab for a high anchor demand load while meeting the ductility provisions of ACI 318-11, D.3.3.4.3 is extremely challenging. A strong industry-need for a safe, logical, yet economical design solution, led to a cooperative research program between Structural Engineers Association of Northern California (SEAONC) members and Simpson Strong-Tie. Testing was initiated by a Special Project Initiative grant from SEAONC to members Andy Fennell, P.E. (principal at SCL) and Gary Mochizuki, S.E. (principal at Structural Solutions at the time, now with Simpson Strong-Tie). We completed the program with continued involvement of SEAONC members. This is the second time we have partnered with SEAONC on non-proprietary anchor bolt testing. The first partnership, on sill plate anchor bolts in shear, resulted in successful code change provisions (led by SEAONC) restoring the capacity of these connections to pre-ACI 318 Appendix D values.

This current research program has focused on non-proprietary anchor reinforcement detailing that increases the nominal breakout capacity of concrete slabs. The anchor design satisfies the seismic ductility requirements of ACI 318-11 Appendix D and also significantly increases design capacity for wind applications. The project goal was to provide a design solution for the industry with independently witnessed proof testing of the anchor reinforcement detailing and application of ACI 318-11 Appendix D design procedures. (Note: Appendix D is moved into a new Chapter 17 in ACI 318-14.)

Test picture of anchor reinforcement

Figure 1: Test picture of anchor reinforcement

Anchor reinforcement detail at mid-slab

Figure 2: Anchor reinforcement detail at mid-slab

Significant Test Findings and Design Concepts

Anchoring to relatively thin concrete slabs introduces many unique challenges, so testing was bound to reveal some unique findings. The goal was to increase concrete breakout capacities and also satisfy the ACI 318 anchor ductility requirements with anchor reinforcement detailing. Here are some of the significant findings:

  • Relatively thin concrete slabs do not allow the placement of anchor reinforcement to drag the load down into a larger mass of concrete as shown in RD.5.2.9. Modified anchor reinforcement was required (Figure 2). The required area of anchor reinforcement is based on D.3.3.4.3(a) where the required area of anchor reinforcement exceeds the anchor steel strength, or 1.2Nsa < (nAsfy x 0.707). The 0.707 is for the 45 degree slope of the bars. The proof testing showed the horizontal leg development outside the cone and continuity through the cone adequately developed the anchor reinforcement.
  • ACI 318-11, D.4.2.1 states that when anchor reinforcement is provided, calculation of concrete breakout strength is not required. You know we love load path discussions, so where does the load go once it gets into the anchor reinforcement? The tests indicated that when the anchor reinforcement is provided, the concrete breakout area increases. This limit state is an extended breakout area past the anchor reinforcement bends that will form when that reinforcement is properly quantified and configured. The extended breakout is similar to multiple anchors loading the slab at each bottom bend of the anchor reinforcement. We have applied this concept to the calculations to evaluate extended breakout past the anchor reinforcement bends.
Initial breakout surface from head of anchor

Figure 3: Initial breakout surface from head of anchor

Breakout surface from bends in anchor reinforcement

Figure 4: Breakout surface from bends in anchor reinforcement

  • Let’s follow the load path some more. Once the anchor is connected to the slab, what is the slab bending capacity? The testing showed that to achieve the anchorage capacity, the slab must have an adequate amount of flexural reinforcement with anchorage forces corresponding to ACI 318-11, Section D.3.3.4.3 applied to the slab. Guidance from this section says to apply the anchor tension loads obtained from either design load combinations that include E, with E increased by Omega, or 1.2 x Nominal steel strength of the anchor (Nsa). If the anchors are not oversized, designing for 1.2Nsa should be the most economical solution. For wind applications, the slab Designer should consider the project specified design loads.
  • A vertical concrete block shear forming at the anchor bearing plate is possible if the anchor embedment is shallow and the anchor reinforcement is working to resist the initial anchor bolt breakout area. Our testing showed that this block shear is separate from Appendix D Pullout and is dependent on embedment depth, perimeter of the bearing surface and concrete strength.
  • For anchors with shallow embedment that have a double nut and washer, the concrete breakout can begin from the top nut, thereby reducing the effective embedment depth. To address this, we eliminated the top nut from our specified anchor assembly kit to insure the breakout begins from the top of the fixed-in-place plate washer.
  • The testing and modeling also allowed us to re-examine the appropriate bearing area for the plate washer, Abrg. The flat top surface of a nut is typically circular due to the chamfer at the points, so the resulting bearing area of the plate washer is circular extending out the thickness of the plate from the flat-to-flat dimension of the nut. For near-edge conditions, the side-face blowout capacity can be the controlling limit state and where the plate washer bearing area becomes more important.
  • Edge testing with anchor reinforcement details showed that the breakout area will spread out and begin from the anchor reinforcement bends, like the mid-slab. In a mid-slab condition, the breakout slope follows the 1.5:1 or 35 degree slope used in Appendix D. However for the near edge, we found that 1.5 x effective embedment (hef) from anchor reinforcement bends only holds true parallel to the edge. Due to eccentricities, the breakout angle from the anchor reinforcement bends into the slab (perpendicular to the edge) is steeper and a steeper 45 degree slope should be used.
Edge test with anchor reinforcement

Figure 5: Edge test with anchor reinforcement

Edge anchor reinforcement configuration

Figure 6: Edge anchor reinforcement configuration

  • The testing showed that even though we had cracks intersecting the anchor as it was loaded, the capacities exceeded uncracked assumptions. This is likely due to the flexural reinforcement running through the breakout cone providing continuity across the cracks. We’re still studying the effect of the flexural reinforcement, so for now we recommend assuming cracked concrete and providing a minimum of 4 – #5 flexural bars each way at anchor locations that require anchor reinforcement. Your slab design may require more flexural bars or they may already be there to meet other slab design requirements.

What Solutions Are Now Being Offered and How Do I Get Them?

With our newly launched Strong-Rod Systems webpages, you now go to the Shallow Podium Anchor link to find anchorage solutions. On the website, you will find anchor reinforcement detail drawings and design load tables with slab design recommendations. We’ll also be adding sample calculations, a guideline for selecting your anchor solutions, 3-D anchor reinforcement graphics and guidelines for addressing your condition if your installation is outside the scope of the current solutions that we offer. The anchor reinforcement is non-proprietary and is fabricated by the rebar supplier, but the configuration and placement is described in the details. In addition, you will see detailed information about the Simpson Strong-Tie Shallow Anchor Rod and Anchor Bolt Locator that is specified as a kit in the load tables. Note the absence of the top nut for reasons described above.

Shallow Anchor Kit

Figure 7: Shallow Anchor Kit

How Do We Specify It and Use It?

So now you know what is on the website but how do you put all these pieces together and apply them to a specific design? As a design professional, you will drive the bus on applying these details and design tables onto your drawings. Similar to specifying Strong-Wall® shearwalls or Strong Frame® moment frames, it just takes a little upfront coordination on your drawings. Typically, you’ll start with a slab key plan that shows the anchor bolt locations. You will need to know the design uplift forces from the light-frame structure above the slab and some basic project variables like specified concrete strength, slab thickness(es) and whether the structure is in high seismic or wind-controlled areas. Now you can choose the necessary design tables from the website by clicking on the individual design tables tab. Your key variables will help you select your specific table based on slab thickness, concrete strength, near edge, etc., and of course wind or seismic.

Sample Shallow Anchor design table

Figure 8: Sample Shallow Anchor design table

Once you have selected your design table, just match your project demand loads or your project-specified anchor bolt with the tabulated ASD or LRFD capacity to select the appropriate shallow anchor callout and reference detail that you can identify on the key plan. The detail callout from the table will send you to sheet SA1 where you will find the anchor reinforcement details and Shallow Anchor Kit recommended for your condition. As mentioned previously, the anchor reinforcement would be fabricated and bent by the rebar guys, but would follow these details. You can download the details shown on sheet SA1, place them on your construction documents and then coordinate them with your plans or schedules similar to how you might provide a shear wall schedule. The footnotes that accompany the tables provide important slab design information and other design and installation recommendations. You’ll soon be able to download the sample design calculations, use them as a tool to help follow the design procedure of the recommended details and submit with your project.

What if My Situation Does Not Fit the Details on the Website?

Though a great number of installations will be covered by these details and tables, there will be conditions that currently cannot be addressed with anchor reinforcement solutions. Installs that may be outside of the current scope could include: a demand uplift that’s too large, a slab that’s too thin, lower concrete strength, corner installs, double wood frame shear walls or two close anchors in tension. To address these conditions, we suggest alternatives like slight adjustment or reconfiguration of the shear walls, thickening slab edges, adding downturn concrete beams or extending the anchorage from above down into a cast-in-place wall or further extended down into the footing at ground level.

The joint SEAONC-Simpson Strong-Tie testing project has shown that anchor reinforcement details can greatly increase the breakout strength of concrete to support cast-in-place anchor bolts in concrete slabs. The testing also showed that the design provisions in ACI 318-11 Appendix D can be rationally applied to these anchor reinforcement details. The testing and Appendix D calculation approach are the basis of the details, load tables, graphics and application guides that can be found on our new Strong-Rod Systems webpages. We’re updating these pages as we create more content, so check back frequently. We look forward to working with you on your anchorage installation challenges and hope that some of these solutions will help you with projects you are working on today. How about a high five?

What do you think about these new anchorage solutions? Let us know by posting a comment below.

Podium Anchorage – Structure Magazine

It is hard to believe it has been almost two years since I posted The Anchorage to Concrete Challenge – How Do You Meet It? That post gave a summary of the challenges engineers face when designing anchorage to concrete. Challenges include just doing the calculations (software helps), developing a high enough load, satisfying ductility requirements or designing for overstrength. Over the past several years, Simpson Strong-Tie has worked closely with the Structural Engineers Association of Northern California (SEAONC) to help create more workable concrete anchorage solutions for light-frame construction.

Anchor FEA

Anchor FEA

Anchor Breakout

Anchor Breakout

Anchor Close Up

Anchor Close Up

This month’s issue of Structure magazine has an article, Testing Tension-Only Steel Anchor Rods Embedded in Reinforced Concrete Slabs, which provides an update on the ongoing work of SEAONC and Simpson Strong-Tie. The goal of the testing program is to create a useful design methodology that will allow structural engineers to develop the full tensile capacity of high-strength anchor rods in relatively thin (10” to 14”) podium slabs.

Anchor capacity is limited by steel strength, concrete strength, embedment depth, and edge distances. One way to achieve higher anchor strengths is to design anchor reinforcement per ACI 318-11 Appendix D.

ACI318-11 Figure RD.5.2.9

ACI318-11 Figure RD.5.2.9

ACI318-11 D.5.2.9

ACI318-11 D.5.2.9

Section D.5.2.9 requires anchor reinforcing to be developed on both sides of the breakout surface. Since this is not practical in thin podium slabs, alternate details using inclined reinforcing perpendicular to the breakout plane were developed and tested.

Anchor Reinforcing Drawing

Anchor Reinforcing Drawing

Anchor Reinforcing Layout

Anchor Reinforcing Layout

This month’s Structure magazine article summarizes the test results for anchors located at the interior of the slab, away from edges. Additional testing is needed for anchor solutions at the edge of slab. The anchor reinforcement concepts are similar, yet additional detailing is required to prevent side-face blowout failure modes. This testing is in progress at the Tyrell Gilb Research Laboratory and will be completed later this year.

Did you read the Structure article? What are your thoughts?

New Simpson Strong-Tie Anchor Designer software

Remember back to the days when you used allowable stress design for designing anchorage to concrete? Once you had your design loads, selecting an anchor was quick and easy. The 1997 UBC covered the anchorage to concrete in less than two pages, so the calculation was painless. Post-installed anchors were even easier, since allowable loads were tabulated and you just needed to apply a couple of edge distance and spacing reductions.

Since the introduction of strength design provisions and the adoption of ACI 318 Appendix D, first in the 2000 IBC, designing code-compliant anchorage to concrete has become much more complex. At least once (and probably not more) armed with a pencil,  calculator, and an eraser, most of us have set out to design a ‘simple’ anchorage to concrete connection using the  Appendix D provisions. Several pages of calculations later (and hopefully with a solution to the problem), most of us, I imagine, came to realize that designing anchorages to concrete by hand required much more time and effort than we anticipated or could allocate time for. As a result, many of us probably created an Excel template to speed up the design process using built-in functions and some Visual Basic programming.

I'm never going for looks on my spreadsheets.

I’m never going for looks on my spreadsheets.

The question is: are you still using the template?

For me, the answer is an emphatic “NO”, mainly because the spreadsheet I created has limited capability given the complexity in adapting the design methodology to complex anchor layouts, changes to the design provisions with each new code edition, and the need to add/modify data each time a new post-installed anchor product is introduced.

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The Anchorage to Concrete Challenge – How Do You Meet It?

We structural engineers here at Simpson Strong-Tie have a love/hate relationship with anchorage to concrete. Ever since the introduction of the strength design provisions in the 2000 IBC and ACI 318 Appendix D, anchorage to concrete has been a challenge for designers, building officials and manufacturers. SEAONC’s recent testing and the resulting code changes offer some relief to wood-frame designers for sill-plate anchor design at the edge of concrete, but many challenges remain.

Concrete Breakout in a Shallow Slab

With the increasing demand for high-density housing and urban infill projects, designers are now faced with anchoring multi-story wood-framed shear walls to relatively thin elevated concrete slabs (typically referred to as podium slabs).  Overturning tension anchorage forces at the ends of shear walls in these projects can routinely be in the 40 kip range and even get as high as 60 kips or more.

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