In the world of engineering design, it’s not difficult to find yourself lost amidst a confusing landscape of standards, codes, and references. For decades, the design community has been left with multiple options that,when pieced together, incompletely cover the holistic steel element of a post-installed adhesive anchor. The purpose of this article is to outline the differences between standards specifying the material properties of steel elements to be used with post-installed adhesive anchors and to discuss the benefits of designing, specifying, and using American Society for Testing and Materials (ASTM) F1554 compliant rods. These design standards, although valid for their intended applications, could lack acceptable ranges of steel strength for post-installed adhesive anchor applications. Consequently, this may allow for significant variability in the actual steel strength in comparison to the design values used for steel strength. For post-installed adhesive anchors intended for ductile failure, variability in the steel that is manufactured, ordered, and installed on a jobsite may not represent the engineer’s intended design and may be susceptible to brittle and unpredictable failure. Despite the multiple standards that may apply to post-installed anchors, specifiers need clarity of the material properties from which they design and reliability in the products being manufactured and inspected on the jobsite. Although initially introduced in 1994, ASTM F1554 compliant steel rods have not always been widely available. ASTM F1554 specifies the acceptable ranges of mechanical properties for Grade 36, Grade 55, and Grade 105 anchor rods, as well as compatible nuts and washers, providing the clarity and reliability long needed in fastener design. With the ASTM F1554 standards, post-installed adhesive anchors may be less vulnerable to inconsistent material properties and increased likelihood of unpredictable failure in seismic design. In Load and Resistance Factor Design (LRFD), an engineer juggles the capacities of three possible failure modes: breakout of the concrete, bond stress between the steel anchor element and the concrete, and fracturing/yielding of the steel element. Engineers calculate the capacities of each failure mode and determine the overall anchor capacity by the governing failure mode, or lowest of the three capacities. Unlike bond or concrete failures, steel offers a ductile failure mode in which the steel slowly but visibly demonstrates sign of strain through plastic deformation. When standards provide only a minimum steel strength to which manufacturers must adhere, manufacturing processes may result in much higher actual steel resistance of an anchor rod element. When an engineer designs for ductile failure of the steel element they assume the steel strength to be the specified minimum value. If a manufactured steel product results in a higher steel strength above that of the concrete and bond capacities, the resulting failure mode may be unpredictable regardless of design intention.
With the adoption of ASTM F1554, design strength and actual steel strength is within a specified range and failure of the fastening point can be governed by the resistance of steel in the fastening point rather than the concrete or bond capacity. This helps increase the probability that the connection is predictable and ductile – two characteristics engineers may generally desire in a fastening point. It is important to note that with ASTM F1554 ductility characteristics, concrete or bond strengths are more likely to govern in cracked concrete, groups of anchors, shallow embedment depths, or high service temperatures.
Although it may be relatively simple to design and select the products with which you design in the office, proper selection and identification of product on the jobsite is often unclear. Currently, an engineer’s generic specification of an anchor rod is often vulnerable to an array of interpretations on the jobsite. An installer may misinterpret specifications or opt for “value-engineered” materials such as “commercial” or “industrial grade” steel, often with no detailed information about mechanical and chemical properties or steel quality. Once the steel rods have been delivered to the jobsite, there is typically no identification markers on the material to indicate the rod type, length, or lot number, hindering the facilitation of thorough inspection. Additionally, these commercially available rods usually come without nuts and washers to properly fasten down the fixture. The final combination of rods, nuts, and washers on the jobsite may be of dissimilar strength, geometry, or material properties that can result in stripping of threads and/or corrosion issues. Lastly, when installers choose to buy bulk threaded rod and use labor in a shop or on-site to cut the rods to the specified length, the final product may be of poor finish. The cutting or grinding processes by which the rods are cut leave steel burrs and residual oils on the surface of the steel element and can remove corrosion protection coatings. These factors can affect performance of the fastening once installed in the injected adhesive.
By specifying in accordance with ASTM F1554, these variables have been removed to help ensure the anchor system which was designed meets the performance required. This standard:
- Specifies mechanical properties that meet the ductility requirements of ACI 318 and acceptable ranges of steel strength. For those designing for seismic applications or a ductile failure, this allows for predictable steel failure of a post-installed adhesive anchor.
- Applies to the holistic steel element of a post-installed adhesive anchor, including the anchor rod, nut, and washer to reduce material and dimensional incompatibility.
- Helps ensure the ability to verify the correct material has been procured and installed upon inspection by requiring specific head markings on manufactured pre-cut anchor rods.
Hilti introduces their new carbon steel threaded rod portfolio, fully compliant with ASTM F1554. These new rods are manufactured to perform within the prescribed ranges of acceptable steel strength and incorporate a restructured nomenclature and identification for simplicity and clarity throughout the entire anchor life cycle, from selection to inspection. When ordering from this new portfolio, rods are delivered to the jobsite with the appropriate nuts and washers for a fully compliant steel element. In addition to F1554 compliance, the Hilti anchor rod portfolio offers chamfered and oil-free cut rods that meet standard dimensions while protecting installation integrity. The portfolio boasts a range of coatings and material properties for effective corrosion resistance as well as a cut-to-length program for customized connections that demand reliability.
Designers, specifiers, and contractors continue to trust Hilti to consistently deliver some of the industry’s leading anchoring systems. Hilti’s
Anchor Design Centeris a complete source for fastening selection, design, and specification.
To assist you in designing with various steel elements for use with post-installed adhesives, Hilti offers best-in-class design software, PROFIS Engineering.Hilti PROFISEngineering is comprehensive design software for anchor design in concrete, masonry, and concrete-over-metal-deck base materials and includes a versatile load engine and various options for base plate analysis. PROFISEngineering includes the Anchoring to Concrete provisions of the ACI 318 Building Code and the ACI 349 Nuclear Code and the Anchorage provisions of CSA A23.3 Annex D.
Included within PROFISEngineering software are a number of powerful features, exclusive to Hilti, that greatly enhance the user's choice and efficiency. Users can design with Hilti mechanical and adhesive anchor systems as well as cast-in-place headed studs and headed bolts. Tutorials explain how to navigate within PROFIS Engineering. The PROFISEngineering Design Guide is an innovative, interactive tool that explains ACI 318 Chapter 17 strength design calculations and PROFISEngineering design assumptions.
