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.”

 

Part II: Tensile Performance of Simpson Strong-Tie® SET-XP® Adhesive in Reinforced Brick – Test Results

Guest blogger Jason Oakley, field engineer

Guest blogger Jason Oakley, field engineer

This week’s blog post is written by Jason Oakley. Jason is a California registered professional engineer who graduated from UCSD in 1997 with a degree in Structural Engineering and earned his MBA from Cal State Fullerton in 2013. He is a field engineer for Simpson Strong-Tie who has specialized in anchor systems for more than 12 years. He also covers concrete repair and Fiber-Reinforced Polymer (FRP) systems. His territory includes Southern California, Hawaii and Guam.

This post is the second of a two-part series on the results of research on anchorage in reinforced brick. The research was done to shed light on what tensile values can be expected for adhesive anchors. In last week’s post, we covered the test set-up. This week, we’re taking a look at our results and findings.

To briefly recap the test set up, it was conducted in September 2014, at an office building in Burbank, Calif. Slated for demolition, this building provided an opportunity for Simpson Strong-Tie to install and test 1/2-inch diameter anchors using Simpson Strong-Tie® SET-XP® anchoring adhesive in both the face and end of the 8-1/2 inch wide reinforced brick wall. A 12-ton rated pull rig at the face and end of the wall was used to pull test the anchors to failure.

Table 1 shows the results for both face and end of wall anchors. Each data set was limited to testing three anchors of the same diameter and embedment depth. The coefficient of variation (COV) showed that the spread of the data was fairly narrow (11% maximum) for the face of wall anchors, but much higher for the end of wall anchors (24%). There are a couple of things worth noting here.

Table 1 - Tensile Results of 1/2" Diameter Threaded Rods in Reinforced Brick

Table 1 – Tensile Results of 1/2″ Diameter Threaded Rods in Reinforced Brick

Anchors 4, 5 and 6 showed that reinforced brick is capable of achieving significant capacity for anchors embedded past the grouted portion of the wall to a depth of six inches. The threaded rods were a mix of F1554 Gr. 36 (newer specification) and A307 Gr. C (older specification – likely the anchors that failed at 14,000 lbs.), which might explain the observed variation in capacity for anchors 4, 5 and 6. At what point breakout would have been achieved if higher tensile strength steel had been used is unknown but it can be estimated. What is clear is a significant reduction – probably around 60% (relative to an estimated breakout capacity of around 17,000 lbs. for an anchor embedded six inches deep far away from an edge) – can be expected for near-edge conditions, despite the presence of two #4 bars running along the edge of the wall at the window. A near-edge failure is shown in Figure 6.

Figure 6 – Anchor 13 near edge at window (anchor 14 and 15 similar)

Figure 6 – Anchor 13 near edge at window (anchor 14 and 15 similar)

At a reduced embedment depth of 4-1/2 inches, Table 1 showed that anchor location (anchors 7 through 12) had little effect on performance whether anchors were installed in the middle of the brick or in the head joint mortar. The failure modes were largely a combination of breakout and pullout as shown in Figure 7 and 8.

Figure 7 – Failures of anchors 7 through 12 (white arrows point to anchor center)

Figure 7 – Failures of anchors 7 through 12 (white arrows point to anchor center)

Figure 8 – Anchor 10 failure at face of wall (anchors 7, 8, 9, 11 and 12 similar)

Figure 8 – Anchor 10 failure at face of wall (anchors 7, 8, 9, 11 and 12 similar)

The end of wall anchor results shown in Table 1 revealed a significant reduction in adhesive tensile capacity and greater variation (COV) relative to face of wall results. Two possible contributing factors for such a high COV could be:

1) The bond strength between the grout and surrounding brick wythes is variable, and

2) The size and quantity of the voids present in the grout is probably inconsistent along the height of the wall – some areas are better than others – leading to further variation of the test results.

Figure 9 shows evidence of a slip plane failure for anchors 1, 2 and 3. Looking at the brick top and bottom surface, referred to as the bed, a scored surface can be seen running perpendicular to the length of the brick (and hence the wall surface) as shown in Figure 10. Perhaps the intent of scores is to help improve the bond strength between the brick and mortar. But this assumed benefit is limited to the bed line. The face and side of the brick are smooth. Consequently, the bond strength between the grout and brick is low enough, combined with lack of grout confinement between the two wythes, to have an appreciable effect on the anchor ultimate tensile capacity.

Figure 9 – Anchor 1, 2, and 3 failures at end of wall (1/2 inch x 6” emb.)

Figure 9 – Anchor 1, 2, and 3 failures at end of wall (1/2 inch x 6” emb.)

Figure 10 – Reinforced brick bed profile

Figure 10 – Reinforced brick bed profile

To summarize, this test program discovered that the tensile performance of 1/2-inch adhesive anchors in the face of the wall can be substantial for cases where anchors are located far enough away from a free edge. Performance is similar for anchors placed in the center of the brick or in the mortar joint, suggesting it doesn’t matter where the anchors are placed on the wall (obviously this isn’t true for anchors near a free edge). Special precautions should be taken especially for anchors located near an edge where small intermittent voids may exist in the grout. Anchor installation should ensure that sufficient quantity of adhesive has been injected into the hole. Figure 11 proves that this is possible. However, screen tubes should be considered if large voids are present, although large voids are expected to be rare in reinforced brick. End of wall anchorage applications should be designed carefully especially if significant tensile capacity is a design requirement.

Figure 11 – Anchor 2 end of wall voids filled with SET-XP® adhesive

Figure 11 – Anchor 2 end of wall voids filled with SET-XP® adhesive

Determining what the allowable load should be can be a little tricky. ICC-ES AC 58, the criteria used for adhesive anchors in masonry base material, lists several safety factors depending on whether creep and/or seismic tests have been performed. Conducting creep and seismic tests on an outdated building material like reinforced brick would be difficult because replicating 60- year-old construction accurately in the laboratory will probably be difficult. Reinforced brick has been largely replaced by grout-filled CMU as the preferred masonry building material — at least in Southern California. What safety factor should be used that would permit seismic loading of anchors in a relatively antiquated building material like reinforced brick is debatable.  Perhaps AC 60, the criteria used for assessing adhesive anchor performance in unreinforced masonry elements (URM), would serve as the best guide. It requires a minimum safety factor of five against failure and limits adhesive anchors to resisting earthquake loads only. But AC 60 also requires that the average ultimate load used not exceed an axial displacement of 1/8″ and limits the allowable load to no more than 1,200 lbs.

Despite the obvious structural dissimilarity between URM and reinforced brick and additional AC 60 requirements, Table 2 shows what the allowable loads would look like for the results of this test program if a safety factor of five was chosen. These loads are based on a wall of unknown material properties (compressive strength, tensile strength and bond, etc.) for a specific building, and may not apply to other reinforced brick buildings.

Table 2 – Allowable loads of 1/2-inch diameter threaded rods in reinforced brick using AC58

Table 2 – Allowable loads of 1/2-inch diameter threaded rods in reinforced brick using AC58

Many factors were not investigated in this test program, such as shear, creep, the simulated seismic test, just to name a few. While the evidence so far suggests that an adhesive anchor in reinforced brick performs similarly to grout-filled CMU, more testing would be necessary to substantiate this claim fully. What is very clear is the tensile tests performed on the 60-year-old Burbank office building showed that reinforced brick is a material capable of resisting appreciable anchorage forces. Of course, while a major effort is made by manufacturers to provide engineers with lab tested “code values” for design use, it can’t be ignored that the material properties of any structural element can be variable. Additional factors such as material deterioration, workmanship, etc., can all have an effect on anchorage capacity. This means that it’s never a bad idea to assess anchor performance through site-specific pull tests if gauging strength accurately is important to the anchor system design.

What have your experiences been with reinforced brick? Have you called for pull tests in this material? What were the results? Please feel free to share your experiences in the comments below.

 

Part I: Tensile Performance of Simpson Strong-Tie® SET-XP Adhesive in Reinforced Brick: Test Set Up

Guest blogger Jason Oakley, field engineer

Guest blogger Jason Oakley, field engineer

This week’s blog post is written by Jason Oakley. Jason is a California registered professional engineer who graduated from UCSD in 1997 with a degree in Structural Engineering and earned his MBA from Cal State Fullerton in 2013. He is a field engineer for Simpson Strong-Tie who has specialized in anchor systems for more than 12 years. He also covers concrete repair and Fiber-Reinforced Polymer (FRP) systems. His territory includes Southern California, Hawaii and Guam.

 

This post is Part I of a two-part series. In this post, we’ll cover the test set-up and next week in Part II, we’ll take a look at our results and findings.

More than half a century ago, reinforced brick was a fairly common construction material used in buildings located in Southern California and probably elsewhere in the U.S. Reinforced brick can be found in schools, universities, and office buildings that still stand today. This material should not be confused with unreinforced brick masonry (URM) that is also composed of bricks but is structurally inferior to reinforced brick. Engineers are often called to look at existing reinforced brick structures to recommend retrofit schemes that, for example, might strengthen the out-of-plane wall anchorage between the roof (or floor) and wall to improve building performance during an earthquake. Yet, limited or no information exists on the performance of adhesive anchors in this base material. This series of posts shares the results of research on anchorage in reinforced brick in hopes of shedding light on what tensile values can be expected for adhesive anchors, including any important findings encountered during installation and testing.

Reinforced brick sample

Reinforced brick sample

In September 2014, one wall of an office building in Burbank, CA, was slated for demolition. This presented an opportunity for Simpson Strong-Tie to install and test 1/2-inch diameter anchors using Simpson Strong-Tie® SET-XP® anchoring adhesive in both the face and end of the 8-1/2 inch wide reinforced brick wall. The building is shown in Figure 1 and the wall cross section is shown in Figure 2. The bricks measured 3 inches wide by 3-1/2 inches tall by 11-1/2 inches long and the drawings required that the bricks conform to ASTM C62-50, a standard that still exists today. According to the drawings, the walls were reinforced with #4 vertical bars spaced 24 inches on center. Mortar was specified as “1 part plastic cement and 3 parts sand.” The grout used to fill the 2-1/2 inch gap between the two brick wythes is identical to the mortar except “add sufficient water to pour.” The engineer’s drawings specified two #4 bars running parallel to the edge at all wall openings including windows. Although the actual material properties of the mortar, grout, brick, and bond between these components are unknown, the results and findings of this research should serve as a reasonable but rough indicator as to the material quality and workmanship of the wall. Anchor identification numbers and locations are shown in Figures 3 and 4.

Figure 1 – Reinforced brick building

Figure 1 – Reinforced brick building

Figure 2 – Reinforced brick section

Figure 2 – Reinforced brick section

Figure 3 – Anchor identification at inside face of wall (anchor diameter ½”)

Figure 3 – Anchor identification at inside face of wall (anchor diameter ½”)

Figure 4 – Anchor identification at end of wall (anchor diameter ½”)

Figure 4 – Anchor identification at end of wall (anchor diameter ½”)

While the brick base material was mostly solid, in some cases it was necessary to inject more adhesive in the hole due to the presence of small intermittent voids in the grout that were doubtlessly air pockets trapped during the grouting process. To resolve this problem, enough adhesive was injected such that excess adhesive could be observed coming out of the hole during insertion of the ½ inch diameter all-thread rod. This condition was limited to anchors located near the window edge (anchors 13, 14 and 15) and the end of wall (anchors 1, 2 and 3). The base material was solid at all other locations. No screen tubes were used for any holes.

Figure 5 shows a 12-ton rated pull rig at the face and end of the wall used to pull test the anchors to failure. The pull rig reaction bridge has a clearance of 12 inches between supports to allow breakout as a possible failure mode. Using a reaction bridge extension increases the clear span to 18 inches. ASTM 488 requires a free span clearance of four times the embedment depth. This standard was not followed because exceeding the flexural bending capacity of the wall was a concern. In most cases a minimum clear span of at least three times the anchor embedment depth was met.

Figure 5 – Pull rig without (left) and with (right) reaction bridge extension

Figure 5 – Pull rig without (left) and with (right) reaction bridge extension

With the testing parameters in place, next week I’ll share the results of the tests.