Push Pier System – Technical Information
The FSI Push Pier System utilizes high-strength round steel tube and a load transfer bracket (retrofit foundation repair bracket) to stabilize and/or lift sinking or settling foundations.
The foundation bracket is secured against the existing footing and pier sections are driven hydraulically through the foundation bracket and into the soil below using the combined structural weight and any contributory soil load as resistance.
Pier sections are continuously driven until a suitable load-bearing stratum is encountered. At that point, the structure either begins to lift or the target pressure/load is achieved. The weight of the structure is then transferred from the unstable soil, to the foundation brackets, through the piers, and to firm load-bearing soil or bedrock.
The first pier section advanced into the ground includes a larger-diameter “friction reducing collar” welded to the lead end. This collar, being larger in diameter than the pier tube, effectively creates annular space around the pier as it is advanced through most clayey soils. In soft clay or clean sand and gravel, an annular space may only temporarily be created.
However, the larger diameter collar causes soil disturbance or remolding to occur, which also significantly reduces frictional resistance on the outside surface of the pier during driving.
The result is a driven pier that generates most of its capacity in end-bearing. Over time, the soils surrounding the pier relax back into the annular space and against the pier shaft. This provides an additional frictional component to the pier’s capacity. Even though this frictional capacity may be significant, it is conservatively ignored in the determination of the pier’s factor of safety against pier settlement.
The FSI Push Pier System develops a factor of safety against pier settlement by the pier installation methods used and the sequence with which multiple piers are driven and then re-loaded. Piers are first driven individually using the maximum weight of the structure and any contributory soil load.
After all of the piers are driven, the piers are re-loaded simultaneously, and the total reaction load is distributed over the multiple pier locations.
The average load on each pier during the load transfer operation is generally less than 75 percent of the load during pier installation/driving, for a factor of safety of at least 1.3. Typical factors of safety against pier settlement range from about 1.5 to 3.0, with higher values generally achieved for structures with greater rigidity.
Again, these factors of safety conservatively ignore any additional frictional component to the pier’s capacity.
The FSI Push Pier System was designed by licensed professional structural and geotechnical engineers (PE) on staff at FSI.
Currently, there are no standards that dictate how push pier systems are to be designed and tested. Therefore, many manufacturers of push pier systems simply fabricate a concept and then test according to their own self-approved methods. This often results in pier system capacities which are inappropriate and not representative of actual field applications.
FSI chose a different design approach. In June 2007, the International Code Council Evaluation Service, Inc. (ICC-ES) adopted AC358, Acceptance Criteria for Helical Foundation Systems and Devices. Sections of AC358 that discuss design and testing of pier shafts and side-load (retrofit foundation repair) brackets intuitively apply to push pier systems as well. FSI is confident that any future design and testing procedures adopted by the ICC-ES for push piers will closely follow the respective guidelines of AC358. Interested parties may review AC358 on-line at www.icc-es.org.
The FSI 288 Push Pier System was the very first in the industry to be designed and tested in accordance with an accepted standard, AC358.
The pier tubes of a push pier system are not located directly under the structure’s footing. Therefore, these systems are eccentrically loaded and in turn need to resist the bending forces generated by this loading condition. See Figure 2.
Overall dimensions of a pier cross section are typically less than four inches in most applications. These sections are therefore very sensitive to the bending moments introduced by this eccentricity, thereby reducing the capacity of the pier to carry axial load.
This sensitivity can be demonstrated with the following example. A given pier section with a 3.50-inch diameter, 0.30-inch wall thickness, and a yield strength of 36 ksi has a maximum allowable compressive capacity of 63.3 kips according to Allowable Stress Design. When a bending moment of 30 kip-in is applied to the same section, its allowable compressive capacity drops to 26.3 kips. This is a reduction of nearly 60 percent of the section’s full axial capacity. What’s more, this moment would equate to an equivalent eccentricity of only 1.14 inches, which is a seemingly small eccentricity and is well within the envelope of a typical pier cross section.
Fortunately, since the pier tubes are confined by the earth, these bending moments dissipate rather quickly into the surrounding soils within the first few feet. Softer soils will require these bending forces to dissipate over a greater length than stiffer soils. Foundation Supportworks has developed a unique, patent-pending method to address this issue (see next section).
Other methods exist to reinforce or partially reinforce this region of bending. However, with a critical eye and some understanding about how sensitive piers are to bending, one might question some capacities published by various manufacturers. It would appear that the effects of eccentric loading have been underestimated.
Figure 2. Eccentrically-loaded push pier system.
The FSI Push Pier System External Sleeve
The FSI 288 Push Pier System incorporates an external sleeve to resist the bending forces generated by the eccentric loading on the bracket, thereby preserving the axial compressive capacity of the pier. The external sleeve is hydraulically driven with and around the pier starter tube section to extend through and below the bracket. The effect of the sleeve essentially creates a bracket that is 48 inches long without any additional excavation. A 30-inch-long sleeve is available for use in limited headroom or crawl space applications.
The moment or bending force is localized within a relatively short distance below the bracket. Although the bending force is dissipated quickly by the pier bearing against the confining soil, it is significant and cannot be ignored. The depth or length of sleeve and pier over which the bending force dissipates is a function of the soil stiffness. The depth is greater in soft clay and loose sand, and less in stiff clay and dense sand. In soft or loose soils, a small portion of the bending force may be transferred to the pier below the sleeve, thereby reducing the pier’s allowable axial compressive capacity.
Finite element software was used to analyze how the external sleeve and pier interact with soils of various properties. Bracket rotation is resisted not only by the rigidity of the pier system, but also by the passive pressure of the soil surrounding the external sleeve and the pier. Therefore, the capacity of the pier system is in part governed by the stiffness of the confining soils. Refer to Figure 4 for the allowable capacities of standard (48-inch sleeve) and crawl space (30-inch sleeve) 288 push pier systems in varying soil conditions.
Additional benefits of the external sleeve include:
- Easy to install. The sleeve is driven at the same time as the starter tube. No additional steps.
- Extra steel where it needs to be. Much more efficient than using thicker-walled pier tube sections over the entire pier length. The sleeve is a local solution to a local issue.
- Sleeve is in place while driving pier tubes, which is when the system experiences maximum load.
- No cumbersome internal reinforcement to install after driving. Internal reinforcement can be of inconsistent length and difficult to install properly. It may not be possible to install internal reinforcement if the pier yields or bends under load.
- Sleeve penetration into the soil supports the bracket against kinking back and forth between each cylinder stroke.
- Sleeve acts to guide pier sections into the ground at the recommended installation angle.
- Sleeve length reduces friction loss between the pier bracket and pier tube, resulting in reduced hydraulic pressures during driving and lifting operations. This allows the hydraulic pressures to give a more accurate estimate of applied force.
The allowable capacities for the FSI 288 Push Pier System presented in Figure 4 consider a loss in steel thickness due to corrosion over a period of 50 years. The design period and corrosion loss rates are in accordance with ICC-ES AC358.
Bolting the Bracket to the Foundation
FSI does not require nor recommend bolting of the bracket to a concrete foundation with expansion or epoxy anchors. Experience has shown that bolting routinely causes concrete to crack and spall while drilling for and installing the anchors, or during the repeated loading/unloading procedure of driving piers. At best, bolting provides little benefit to the pier capacity and stability while introducing the potential to weaken the system by damaging the footing. Holes are included in the bracket to be used at the discretion of the installer or if a project engineer or building official requires that the piering system be positively attached to the structure.
Actually, the manner in which a push pier system is loaded and supported would tend to cause the bracket to push against the structure, not pull away from it. At the same time, however, while the bracket is pushing against the structure, it also tends to rotate toward the structure. If the pier system does not have adequate stiffness, then the tendency for excessive bracket rotation will be evidenced by the bearing plate being pried away from beneath the structure. This phenomenon does not mean that the overall pier system is translating away from the structure. Instead, it means the pier system needs to be much stiffer. The stiffness of the FSI 288 Push Pier System greatly reduces this rotational tendency and precludes the need to positively attach the bracket to the structure. When such an attachment is made due to preference or local requirements, FSI recommends the anchors be installed after completion of the piering operations.
Expansion or epoxy anchors to connect the bracket to the concrete foundation were not considered in the calculation of the allowable capacities noted in Figure 4. Anchors were also not used when the pier system was tested in accordance with AC358.
The FSI Push Pier System was designed using the guidelines presented in ICC-ES AC358 for corrosion loss rates and design period (50 years). Corrosion loss rates are provided in AC358 for both bare steel and zinc-coated steel.
The pier tube used for the FSI Push Pier System is manufactured with a triple-layer, in-line galvanized coating. This coating process consists of: (1) a uniform hot-dip zinc galvanizing layer; (2) an intermediate conversion coating to inhibit the formation of white rust and enhance corrosion resistance; and (3) a clear organic top coating to further enhance appearance and durability. The inside of the pier tube also has a zinc-rich coating.
The pier system bracket, external sleeve, and pier cap are available standard as black steel or optional with a hot-dip zinc coating for galvanic protection. The bracket, sleeve, and pier cap analyzed for the determination of allowable capacity, as presented in Figure 4, were black steel. The hot-dip galvanization process is in accordance with ASTM A123, “Standard Specification for Zinc (Hot—Dip Galvanized) Coatings on Iron and Steel Products”. The bracket and pier cap with steel plate thickness of at least 1/4-inch have an average zinc coating thickness of at least 3.9 mils (0.0039 inch) or 2.3 oz/ft2. The external sleeve with a wall thickness between 3/16-inch and 1/4-inch has an average zinc coating thickness of at least 3.0 mils (0.003 inch) or 1.7 oz/ft2.
The all-thread rod and heavy hex nuts come standard as zinc-plated in accordance with ASTM B633, “Standard Specification for Electrodeposited Coatings of Zinc on Iron and Steel”.
Product Testing Procedures
As stated in an earlier section of this document, there are currently no standards that dictate how push pier systems are to be designed and tested. Therefore, many manufacturers of push pier systems simply fabricate a concept and then test according to their own self-approved methods. This often results in pier system capacities which are inappropriate and not representative of actual field applications. By compromising on test methods, manufacturers and installers are not aware of actual product limitations. Potential, unexpected issues are also not identified. Ultimately, compromised testing methods inhibit creativity and innovation in the industry. Simply designing a bracket to attain higher and higher test results by these inappropriate methods inhibits innovation for the design of pier systems that perform better in actual field conditions.
In June 2007, the International Code Council Evaluation Service, Inc. (ICC-ES) approved AC358, Acceptance Criteria for Helical Foundation Systems and Devices. Sections of AC358 that discuss design and testing of pier shafts and side-load (retrofit foundation repair) brackets intuitively apply to push pier systems as well. FSI is confident that any future design and testing procedures approved by the ICC-ES for push piers will closely follow the respective guidelines of AC358. Interested parties may review AC358 on-line at www.icc-es.org.
The FSI 288 Push Pier System was the very first in the industry to be designed and tested in accordance with an accepted standard, AC358.
Testing according to AC358 is much more demanding than many methods commonly practiced and considered acceptable across the industry. In many respects, results from tests completed in accordance with AC358 may be considered conservative. The criteria, as compared to other test procedures, more accurately determines failure loads. It also more appropriately identifies failure mechanisms of both the components of the pier system and the interface with the concrete structure. Refer to Figures 5, 6, and 7 for additional commentary regarding the differences in test procedures.
Figure 5. Included in AC358 are specific criteria for the testing of side-load retrofit piering systems. The sketch is as it appears in the acceptance criteria illustrating an example of an appropriate laboratory test set up. The photo is of a FSI 288 Push Pier System being tested accordingly. As outlined in the test procedure, the exposed, unsupported length of pier from the bearing surface of the bracket to the point of fixity on the test frame is 60 inches. The bracket is mounted to a concrete block of known strength. The test sample is loaded until failure occurs at either the concrete interface or within the pier system’s steel components. This test will not show failure loads in excess of those you would expect to see in actual application. Actually, one might consider the results of this test to be conservative since the component which contributes most to the systems strength, the external sleeve, is not supported by confining soils. Although the external sleeve makes a significant contribution to the strength of this test sample, the full benefit it provides is only realized when it’s in the ground. This test method is very demanding and despite considering the results conservative, it is a very appropriate test method.
Figure 6. Additional testing of the FSI 288 Push Pier System illustrates how a manufacturer who does not wish to conform to AC358 may test their product. The test sample is made as short as possible to limit the exposed length of the pier. The bracket is mounted to a steel fixture which will prevent the system capacity from being limited by any potential concrete failure. The bracket is bolted to the fixture with high strength bolts which provide a far greater benefit than the concrete expansion anchors for which these holes are intended. The ultimate failure load for this test was 83,000 pounds. Since all of the pier system components are in place, this may be considered by some to be a fair test. If FSI did not have a long external sleeve as part of the system, the test might look different still.”
Figure 7. A similar test arrangement as in Figure 6 is shown here. This time the external sleeve was trimmed down so it only extends slightly below the bracket. The test sample is again made as short as possible and the bracket is bolted to the steel fixture. Since this test sample is expected to exceed the results of the previous test, double cap plates are used to prevent the mode of failure from being bending of the cap plate. The ultimate failure load for this sample was 113,200 pounds. Manufacturers of systems without a similar reinforcing mechanism as the FSI External Sleeve may indeed test their products this way. Similarly, all of the pier system components are in place and, again, this may be considered by some to be a fair test. In addition to all the benefits enjoyed by the sample in the previous photo, this sample gets the added benefit of the lateral stiffness of the large hydraulic test apparatus. In contrast to the comments about the test sample in the Figure 6, having this specific arrangement confined by earth would prove to be a detriment and not a benefit since no soil could provide the lateral rigidity provided by the test apparatus. Having such a point of fixity certainly produces much higher test results.
Even non-technically minded individuals would agree that the FSI External Sleeve contributes a great deal to the stiffness and the strength of the pier system. They would be right. It actually increases the strength of the system by up to 40 percent. Yet when looking at the raw data of the tests just outlined, the sample without the sleeve tested 35 percent higher than the sample which included the sleeve. This is completely counterintuitive and illustrates perfectly the pitfalls of self-approved test methods. It inhibits progress and creativity since good designs may not test as well as poor ones. Tests conducted in such a manner have little basis in reality and make it impossible to make any direct comparisons between products.
One can see why manufacturers are not motivated to change their test procedures and why an appropriate standard for design and testing is so important. Foundation Supportworks has chosen to change the rules of the game. Our mission is to bring superior products to the marketplace, engineered to perform and tested in accordance with documented criteria. An accepted standard for push pier systems will bring a level playing field to the entire industry. In the meantime, there is AC358.