An Introduction to Helical Wall and Stitching Ties

This week’s post is written by Kevin Davenport, who works as a Field Engineer with Simpson Strong-Tie. Kevin is responsible for providing technical support on Simpson Strong-Tie products for Infrastructure, Commercial and Industrial market segments within his Southeastern territory. He is a registered professional engineer in Georgia and received his B.S. (’97) and M.S. (’98) from Clemson University. Kevin is a member of ICRI, ACI and various local chapters of SEA. 

What do you do when brickwork is in bad condition? Depending on what state the brickwork is in, a tear-down may be called for. However, often brickwork can be restored and strengthened using helical ties such as Simpson Strong-Tie® Heli-Tie™ wall ties and stitching ties. This post introduces these two types of helical ties, which might be just what you need for your next brick restoration project.

What is a helical tie? 

A helical tie is made by twisting a metal profile into the shape of a helix. The design of the Simpson Strong-Tie Heli-Tie wall tie also  incorporates a large core diameter in order to provide higher torsional capacity. The benefit of this feature is less axial deflection due to a propensity for normal helix shape to “uncoil” under tension load. Since helical ties are typically used in building façades, they are generally made from stainless steel in order provide the necessary corrosion resistance.  Helical ties can be used to retrofit and stabilize brickwork in two common applications: 1) Wall anchor applications, and 2) Stitching tie applications.

heli tie

What are common helical wall tie applications?

Application #1: Anchoring building façades to structural members

In a wall anchor application, the helical tie is used to stabilize the façade by transferring out of plane façade forces through the anchor into the backup material. The need for this type of reinforcement arises when pre-existing wall anchors were never installed, were inadequately spaced or have corroded away over time. Helical ties are an economical solution that can be installed directly through a brick façade into various backup materials such as solid concrete, CMU block, and even wood or metal studs.

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Image 1: Installing a helical tie.

A pilot hole is drilled through the existing brick wall and any air gap into the backup material. Then the helical tie is placed in an installation tool and driven into the pre-drilled hole. As it is driven, the fins of the helical tie tap into both the masonry and backup material and provide an expansion-free connection that will withstand tension and compression loads. Some helical wall ties, like the Heli-Tie, use an installation tool that countersinks the tie below the surface of the brickwork. This allows the hole to be patched and concealed with a color matching material.  Thereby, helical anchors allow the repair to be both efficient and inconspicuous when completed.

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There are presently no specific U.S. design standards for the use and qualification of helical wall ties. However, a rational calculation of required spacing given the demand load can be easily calculated using ACI 530 (Building Code Requirement and Specifications for Masonry Structures), Section 6.2, and test data with an appropriate factor of safety. In addition, many Designers also follow the detailing practice for prescriptive anchored veneer in Section 6.2.2.5.6 that prescribes the following:

  1. At least one anchor for each 3.5 ft2 of wall area, and
  2. A maximum anchor spacing of 32″ horizontal and 25″ vertical, and
  3. Around openings larger than 16″ in either dimension: Additional perimeter anchors at maximum 36″ spacing within 12″ of the opening.

Prescriptive anchors also require bed joints to be at least twice the thickness of the embedded anchor. However, this provision is not relevant for helical anchors since they are installed into a drilled hole, rather than embedded into a wet mortar joint.

Application #2: Stabilizing multiple-wythe brick walls

In this application, the wall tie is used to attach wythes of brick to one another in an effort to stabilize the wall. By intermittently alternating installation angles (0 o, 45 o, 0o, -45 o, etc.) the tie promotes more monolithic behavior of the wall.

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What are common helical stitching tie applications?

Unlike wall anchor applications, in a stitching tie application the helical tie is used to stabilize brickwork by transferring in- and out-of-plane shear and bending forces across an existing crack. Stitching ties are placed in the plane of the wall within the horizontal bed joint.

The existing bed joint is routed out deep enough to recess the helical tie and cleaned out. Then, the recess is filled about 2/3 deep with a repair mortar (such as Simpson Strong-Tie® FX-263 Rapid Hardening Vertical/Overhead Repair Mortar). The helical stitching tie is then pressed into the mortar, followed by a trowelling with encapsulating grout. The installation provides an inconspicuous repair and preserves the appearance of the structure.

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For red brick, we recommend placing stitching ties  at a minimum vertical spacing of 12″ and  extending the ties at least 20″ on either side of the crack.

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Helical ties are not something that you see on every jobsite, however, they can provide a fast and cost-effective solution for brickwork rehabilitation. Hopefully this post provided you some background about them and an insight into our Heli-Tie product offering. The recent launch of our Repair Protection Strengthening Systems product line complements the Heli-Tie with a wide array of other repair products.

Do you have any past experience with helical wall ties, or questions? Please share in the comments below.

 

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.