Full Paper: Exploring New Concrete Connection Developments
Product approvals in the United States typically are decided by local authorities as no national authority exists. This paper discusses the history of authorizing concrete connections in buildings along various means for approving proprietary and unique connection methods not previously authorized in building codes and ordinances.
Building codes tend to be brief and succinct in the stated provisions, which is somewhat unfortunate when one wishes to understand the intent and history of certain provisions to apply them to alternative materials. The model codes have made some improvements in providing commentary and references, but these items tend to lack detail and are difficult to locate. This is significant, as the codes obligate the local authorities to consider technologies that haven’t yet made their way into the codes as well as new technologies initiated by manufacturers that reflect a significant investment in resources.
Available concrete connections have expanded significantly due to new technologies. In some cases, the connection types from various producers have sufficient similarities to eventually enter the codes. Yet most products contain original content and have unique designs, installation methods and performance characteristics—justification for the performance characteristics consumes volumes of data. Designers need to gather sufficient knowledge on the products, decide whether the product is acceptable and then seek approval from the code official.
Although some large jurisdictions have procedures and staff dedicated to approving proprietary connections, most local authorities lack the time and expertise to dedicate to such an effort, so research or evaluation reports have become a viable option for streamlining the approval process. Many countries employ a government-sponsored agency for such a task, but in the United States, report issuers often are private concerns. In most cases, the agencies issuing the reports operate according to ISO/IEC 170651 and use the codes and other normative documents to base the content. To ensure continued conformance, the agencies arrange for follow-up inspections of the manufacturing facilities.
Most people would probably take building products and materials for granted, believing that if they didn’t function as intended, they wouldn’t be used. The most obvious materials are those readily seen: roofing; exterior wall cladding; interior walls; ceiling and floor finishes; and plumbing, electrical and ventilation fixtures. There would be a realization of certain unseen elements, such as the foundation and underlying structure of the walls, floors and roofs, which create usable space protected from weather, for human activities or as repositories for storing items. Then there are the unseen products that provide important functions—concrete connections are one such group of products. The International Building Code® (IBC)2, currently the basis of most construction regulations in the United States and other countries, is guilty of this lack of realization when it comes to testing structural elements. Although connections may be implied by in situ testing, for prototype testing, the IBC discusses performance of the building element—wall, floor or roof—without mentioning tests of connections.
Although IBC provides information on using certain concrete connections, many aren’t addressed, and a means for testing isn’t within its scope. Therefore, research reports issued by model code agencies and other organizations provide the validated information for regulators, designers, builders, and owners to reference in verifying and selecting the connecting elements.
Developments in U.S. Codes
One of the earliest mentions of connections in U.S. building codes pertains to anchors. Cast-in-place anchors were the first types to be described by the building codes, with anchorage using headed cast-in-place bolts first appearing in the 1943 edition of the Uniform Building Code™ (UBC)3. Here, permissible shear values for the anchor types were published, with tension capacities added in 1970. These values were reported in tabular form, with strengths predicated on bolt diameter and embedment.
In 1997, UBC ceased publication, and the International Building Code (IBC)1 was published in 2000 as a replacement. The 2000 IBC contained the first mention of cast-in-place anchors for use in cracked and uncracked concrete and allowed for strength design in addition to tabulated service or allowable loads taken from the IBC. In 2002, ACI 3184 added an Appendix D, with provisions similar to IBC for cast-in-place anchors as well as adding methods for use of mechanical (expansion and undercut) anchors.
This move by ACI was significant, as ACI 318 allowed for proprietary products to be code- complying by relying on proprietary testing. Unlike cast-in-anchors, where the justification is largely available on publicly available testing and research, ACI permitted the actual testing on mechanical anchors to be kept from the public, provided the testing was assessed by an Independent Testing and Evaluation Agency (ITEA), which provided a summary of information to be used by regulators, designers and builders.
Although the evaluation report was intended to summarize a significant amount of information, it was a case of “too much at once.” First, Appendix D represented a fairly significant increase in anchor design complexity, particularly since earlier codes provided the information in easy-to-follow tables. Later, in 2011, ACI 318 added adhesive anchors. Appendix D presented an extensive set of design information to independently assess failure modes—steel failure, concrete breakout, anchor pullout and anchor pryout—that would be computed for a selected installation condition, and the least of which would be the design value. The setup of the evaluation report didn’t relate clearly to Appendix D, but computer software soon made the task of calculating anchor strengths easier. Manufacturers found that most of their accumulated research was unusable, and retesting was needed. The resistance to Appendix D continues to this day, although ACI now makes anchor provisions part of the code (in Chapter 17).
The codes also recognize power-driven fasteners, which resemble nails and are produced from hardened steel. AISI S1005 includes provisions for power-driven fastener placement into steel and concrete. The specification doesn’t provide for a clear discussion on pullout from concrete, however, limiting its usefulness as a design reference.
Other types of available fasteners include screw anchors, anchor channels, and specialty cast-in-place devices suited for connecting steel and wood light frame to concrete. These connectors currently aren’t included into codes; usable information relies on criteria and research reports. Table 1 summarizes the connectors and the relevant codes, standards and supplemental references for each.
IBC Section 104.11 discusses alternative materials, granting the code official broad authority to approve these substitutions for compliant materials. Unlike most other countries, the U.S. federal government doesn’t mandate a national set of codes, but rather defers to local rule, which results in the states determining code use. Or the state itself can defer to local cities and other jurisdictions to make a determination.
Most states and local jurisdictions, however, can’t afford to develop, educate and train employees on code writing. In the early 20th century, code associations were formed, which eventually developed codes that were adopted with minor to significant revisions by political entities. Aside from publishing codes, these associations took on the responsibilities of providing educational services and research reports.
One of the earliest building codes with widespread use in the United States was the 1927 Uniform Building Code (UBC)3, which was 46 chapters and 256 pages in length. Specific references to building materials were included, along with testing and specifications standards by the American Society of Testing and Materials.
Specific mention of proprietary materials wasn’t included, but Section 302 allowed for alternate materials and types of construction. A building inspector could accept an alternate if there was proof in support of the claims made and requested approval, but there wasn’t much guidance in the UBC for determining acceptance. If a building inspector denied approval, the applicant could appeal the decision to a board of examiners and appeals, and this language essentially remains to this day in the IBC.
The codes assume all involved had immense knowledge and experience, and will arrive at a proper decision. If the building inspector couldn’t render an approval, and no further support was available, a board could step in and arrive at a final decision.
Table 1. Approvals of Concrete Connections
|Basis Document for Approval||Anchor Type|
|Cast-in Bolts||Expansion Anchors||Undercut Anchors||Screw Anchors||Adhesive Anchors||Anchor Channels||Power-Actuated Fasteners|
|Code||ACI 318||ACI 318||ACI 318||None||ACI 318||None||AISI S100|
|Standard||ASME B1.1, ASME B18.2.1, ASME B18.2.6||ACI 355.2||ASME 355.2||None||ACI 355.4||None||None|
|Supplemental Documents||ASTM F1554, ASTM F593||AC193||AC193||AC193||AC308||AC232||AC70|
So building inspectors could either take on the task and review the supporting materials before rendering approval or accomplish the same result with less effort. As one can imagine, this could be daunting, because it’s one thing to know the code content, another to understand the reasoning and background information, and yet another to use this accumulated knowledge to determine the viability of an alternate material.
In some cases, larger jurisdictions were up to the task. Should the alternate be totally new, such as using adhesives as a structural material, then one would have to allow rational judgements for determining acceptance under the codes. There’s also an implied objective to do the same thing consistently, so if Company B came in seeking approval, knowing that Company A had already received approval, both companies should have the comfort of knowing that the bar was set at the same level for each. If no ruling existed—often the case for new products—the code official should apply the rules consistently, but the premise is that each permit is treated differently, and different individuals would likely interpret the code based primarily on the information available.
Early on, manufacturers found this to be true and petitioned the jurisdictions to do something about it. Larger building departments formed departments to determine acceptability of products within its jurisdiction, but these were few and far between. Around 1931, one of the early code agencies, the Pacific Coast Building Officials, formed committees to review manufacturers’ data and issue reports summarizing their conclusions for interested parties.
This was a great service for all parties interested in using a product and needing some assurance that the proper questions were asked and the proper data were submitted to ensure the full intent of the code is met. Eventually, the committees found themselves unable to devote resources needed to meet demand, so it was necessary to bring on full- and part-time staff to keep up with the flow of work.
Research reports provide a means by which code officials can rely on a third party to review tests and other data to convey information to permit use of a product as meeting principles and specific items in the IBC. The International Building Code Section 104.11.1 refers to research reports as a source of information on alternative building materials. This reference is fairly new, first appearing in the 2003 IBC2. It’s also brief, reading as follows: “Supporting data, where necessary to assist in the approval of materials or assemblies not specifically provided for in this code, shall consist of valid research reports from approved sources.”
The IBC also doesn’t define what a research report should be, so this obligation is left to the approved source, shaped by the code official’s needs. Approved source is defined as: “An independent person, firm or corporation, approved by the building official, who is competent and experienced in the application of engineering principles to materials, methods or systems analyses.” By this definition, anyone who may be knowledgeable in engineering would conform. As written, it could narrowly mean an engineering report issued by a person found by the code official to be competent in the subject.
In retrospect, early attempts at research reports focused on available information from each producer of the product, as the issuing organization made efforts to demand such information conform to the requirements of the codes. As late as 2002, model code agencies issuing reports didn’t require any inspections to verify ongoing production of products conformed to the specifics given in the research report. Rather, the norm was that the manufacturer was on the “honor system” and would inform the research report issuer whether the product had changed and determine whether retesting would be needed or not.
Currently, most research reports are developed by organizations accredited to ISO/IEC 17065 as certification bodies. Accreditation is provided by accreditation bodies operating under mutual recognition associations, such the International Laboratory Accreditation Council (ILAC) and the International Accreditation Forum (IAF). These organizations tend to be model code agencies, standards development organizations, testing laboratories and inspection agencies.
One of the mandates of ISO/17065, which is not implicit in the IBC, is ongoing inspection of the products being certified in the report. Inspections are intended to provide a form of guarantee by the report issuer that the product will perform as indicated in the research report. Tests and engineering services are fairly expensive but only provide results for the products being investigated at that point in time. By inspections, the certification body can verify the products being reported will continue to perform as well as expected.
Research Reports vs. Evaluation Reports
For the purposes of this paper, research reports are documents issued by an entity independent of the manufacturer that documents conformance to the IBC and other building codes. These reports typically address all relevant aspects of the codes, including structural use, installation and durability against weather exposure, fire resistance, and inspections for manufacture and field installation.
Evaluation reports are documents specified by ACI 355.26, ACI 355.47 and acceptance criteria that describe details for design and installation for use under ACI 318. These evaluation reports are usually done by one of the testing agencies. In some cases, the evaluation report may mention the relevant ICC acceptance criteria. Table 1 notes the availability of research and evaluation reports for each connector type.
General Notes on Anchors under ACI 318
The maximum anchor diameter is 4 inches, which coincides with ASTM F15548 (ACI 318 184.108.40.206). No minimum diameter is stated by ACI 318, although ASTM F1554 relies on a ½ inch (12.7 mm) diameter as a minimum. Post-installed anchors (mechanical, adhesive) have been qualified at diameters as small as ¼ inch (6.4 mm).
With respect to anchor steel strengths, ACI 318 offers no direct statements, though ASTM F1554 gives a minimum expected tensile strength of 58 ksi (400 MPa). The maximum permitted by the strength calculation, however, is the lesser of 1.9 fy or 125 ksi (861.8 MPa).
ACI 318 doesn’t explain codes for stainless steels or zinc-coated carbon steels, but Zinc-coated carbon steels are covered in ASTM F1554, while stainless-steel products can be selected by reliance on ASTM A1939 and ASTM F59310. Section 1704.5 of the IBC requires special inspection during installation. ACI 318 is somewhat vague on inspection requirements, although adhesive types are discussed in fair detail.
The scope of discussion on cast-in anchors focuses on circular bolts with a bearing element in the form of a manufactured head or nut and washer at the embedded end. Bolts with a bent end into an “L” or “J” shaped hook also provide a bearing element and are usable here. Table 2 summarizes data to be considered in order to comply with the IBC and ACI 318, because these anchor types don’t possess research reports.
Design requirements are provided in ACI 318. The IBC Section 1905.1.8 also allows an alternative design method for cast-in bolts when attaching. These cast-in devices must be proven to develop apparent pullout strength at least 1.4Np (ACI 318 17.1.3), where Np is the calculated pullout capacity based on the geometry of the bearing element. ACI 318 doesn’t elaborate on how the requisite pullout strength would be determined, but most likely a set of tests of the anchor set in concrete would be appropriate.
ACI 318 also doesn’t specify testing methods. One test procedure often used is ASTM E48811, but ACI 318 doesn’t provide guidance on the number of test specimens for the basis of comparison. Would the basis be the mean result, the least result or the 5 percent fractile value? The design professional and code authority would have to reach agreement here.
The commentary to ACI 318 indicates that headed bolts complying with ASME B1.112, B18.2.113 or B18.2.614 would acceptable by calculation, so no tests are necessary. ASME B18.2.6 is titled “Fasteners for Use in Structural Applications” and encompasses dimensional and structural properties for heavy hex-head bolts. ASME B18.2.1 only addresses dimensional requirements for square and hex-head bolts, while ASMR B2.1 discusses screw threads. Neither ASME B2.1 nor B18.2.1 addresses structural properties. Structural Properties are based on ASTM F1554, which is available in Grades 36, 55 and 105. If the design professional wished to check pullout values, the AISC Design Guide 115 provides a table using Eq. 220.127.116.11 of ACI 318.
Table 2. Checklist for Cast-In Anchors
|Property||Cast-in Anchor Type|
|Headed||Hooked||Threaded Nut and Washer||Stud|
|Dimensions||ASME B1.1, B18.2.1, B18.2.6||ASME B1.1||ASME B1.1||AWS D1.1
|Materials||ASTM F1554 (carbon)
|ASTM F1554 (carbon)
ASTM A193 B8 (stainless)
|ASTM F1554, ASTM A193 B7 (carbon); B8 (stainless)||AWS D1.1 Clause 7|
|Size||ACI 318 18.104.22.168||ACI 318 22.214.171.124||ACI 318 126.96.36.199||ACI 318 188.8.131.52|
|Ductile/Brittle Steel||ACI 318 2.3||ACI 318 2.3||ACI 318 2.3||ACI 318 2.3|
|Pullout Data||Headed bolts conforming to ASME B1.1, B18.2.1, or B18.2.6 can be accepted without pullout data||SP 292-2||—||79-77|
Headed bolts can be forged onto the end of the bars to shapes conforming to ASME B18.2.1 or B18.2.6. For larger diameters, it may not be possible to obtain a forged head, so the option to use nuts and either plates or washers to form the bearing area may need to be pursued. Substituting a forged head bolt with threaded rod and nut may result in a lesser strength connection, because the stress area of the threaded section near the nut is less than the full cross section provided by a forged head bolt.
Although not mentioned in ACI 318, headed studs conforming to AWS D1.116 may be a suitable alternative to headed bolts. These anchors are not threaded and are suitable for anchorage for weld plates. The head dimensions for these connectors suggest the head width to thickness ratio is less stiff than what’s specified for heads in ASME B18.2.1 and B18.2.6. Some research on tensile strength (79-77)17 suggests the studs are superior to Grade 36 bolts.
For hooked bolts, ACI 318 provides no guidance on pullout. A recent publication (296-2)18 suggests pull-out strengths for L-shaped hooked bars could be increased beyond that currently in ACI 318.
Hooked bars likewise are produced from steel conforming to ASTM F1554. The standard specifies that the bend section zone not be so deformed that cracking occurs under 10x magnifications. Also, the cross-sectional area must be no less than 90 percent of the straight section. Bars that exhibit a notch were likely bent at a small radius and should be avoided, because the notch is a stress riser, and the bend is where the stresses are concentrated. Bends in the threaded zone should be avoided.
Expansion anchors transfer loads in friction by setting a component that wedges against the concrete hole. The holes need to be relatively precise and clean to ensure proper wedge action. Torque-set anchors need to be set by the using the manufacturers’ recommended torque values and not the installer’s “turn-of-the-nut” method. In some cases, the hole may intersect the reinforcing steel. The installer may try to elongate the hole to avoid the steel, but this will likely reduce the anchor capacity. Design data are based on ACI 318, and qualification is taken from ACI 355.2 and AC19319. Information on design and installation can be found in a research report or an evaluation report.
Undercut anchors transfer loads in bearing via an enlarged portion of the hole at the end into which the anchor expands. These fasteners are grouped separately in ACI 318, but qualification requirements in ACI 355.2 and AC193 don’t distinguish (for the most part) undercuts from expansion-type devices. These anchors are more suitable for dynamic loads than other post-installed types. Design data are based on ACI 318; information on design and installation can be found in a research report or an evaluation report.
Adhesive anchors transfer loads by a mechanically bonding a steel rod or bar to the concrete hole using an organic adhesive. The holes don’t need to be as precise as expansion anchors but need to be cleaned according to the manufacturer’s instructions using a sequence of brushing and blowing or vacuuming to remove debris. Adhesives are sensitive to temperature effects during installation and in service. Installations into horizontal and overhead orientations need to observe manufacturers’ instructions for placement and retention until cure is complete.
For these reasons, IBC and ACI 318 prescribe special inspection, either continuous or periodic. The conditions stipulating special inspection are examined in research and evaluation reports. The research report also provides conditions where the adhesive anchor could be used in fire-resistive construction. There are fire test methods in ASTM E151220, but most research reports haven’t provided this information. These products are qualified in accordance with ACI 355.4 and AC30821. Design details are given in ACI 318, while information on design and installation can be found in a research report or an evaluation report.
Screw anchors mechanically bond to concrete along the entire length. Neither ACI 318 nor ACI 355.2 currently includes screw anchors; AC193 is the most-specific resource for the qualification and design of these anchors, which are produced from hardened steel to thread into the concrete. This hardened steel is subject to brittle failure, which is examined by AC193. Users are cautioned to investigate this failure mode if choosing an anchor without a research report. Design provisions are taken from ACI 318, but pullout resistance needs to be taken from test results. Steel tension and shear values typically are tabulated, as these fasteners often have unique dimensions.
Anchor channels transfer loads by way of cast-in elements bearing to concrete. The cast-in element is in turn connected to the channel, which receives a channel bolt. AC23222 is the main reference for qualifying the overall design and installation of these devices. The channel bolts can be set along the channel length, allowing flexibility in positioning a connection.
One challenging aspect of anchor channels is that the slotted geometry doesn’t allow for superior performance when loaded in shear along the channel axis. In particular, solutions for dynamic loads such as seismic sources have been limited. Design provisions for each element are determined separately to arrive at a connection strength. Research reports on these connectors are typically quite comprehensive in addressing pertinent design considerations.
Headed Specialty Cast-in Inserts
Headed specialty cast-in inserts transfer loads to concrete in bearing similar to a headed anchor bolt. These connectors typically have an internally threaded element to receive bolts or rods. The embedment into the concrete is around 1½ to 2 inches (38 to 51 mm) and may be placed through the underside of concrete-filled steel deck panels. As such, the resulting capacities are appropriate for support of non-structural elements. Qualification of these inserts isn’t addressed in either the IBC or ACI 318, but AC44623 is available for this task. The anchorage design is according to ACI 318, although pullout often isn’t limiting due to the shallow embedment. Research reports provide extensive detail on steel strengths, due to the unique configurations available.
Other Concrete Connections
These fasteners resemble nails in appearance and function by transferring loads by friction to the concrete. Formed from hardened steel, power-driven fasteners also are suitable connecting attachments to steel. Unlike anchors, these fasteners aren’t suited for installation where cracks are present or expected by design. Design details are given in AISI S100, although most research reports currently are qualified by a series of tension and shear tests as stipulated by AC7024.
Other connectors available aren’t entirely subject to design according to ACI 318, and structural use largely is supported by tests in concrete. Research reports are available for many of these types, include the following:
1. Sill plate strap anchors, which attach light-frame wood or cold-formed steel members to concrete. These devices are formed from cold-formed steel and don’t need deep embedments, becasue overall capacity would be limited by connection to the attached wood or steel member.
2. Deformed bar anchors are deformed bar or wire that serve as anchorage. These products serve as a dowel or may be welded to steel plates.
3. Specialty bolts are bent into unique configurations and cast into concrete. As such, the bolts can be embedded fairly deep into concrete. These anchors currently are only used for tension loadings.
Building Codes in the United States have long acknowledged the need for regulators to allow for alternative materials. Yet the codes don’t provide much beyond general provisions to ensure full consideration is given toward determining acceptability. With respect to concrete connections, several types (e.g., cast-in bolts, expansion anchors, undercut anchors, and adhesive anchors) are permitted by ACI 318.
ACI 318, however, focuses on structural performance and does not completely guide users on other aspects such as fire exposure, durability and inspection, which are under the scope of the IBC. Therefore specifiers, users and regulators would benefit from mutually acceptable resources documenting these characteristics.
Research reports offer a comprehensive reference for assessing code compliance. The strength of available research reports is even greater for connections that aren’t described in the IBC or its reference standards, such screw anchors, anchor channels and headed specialty cast-in inserts. Research reports on these products are, in most cases, based on criteria documents, which provide direction as to comprehensiveness and consistency among reports on similar products from different manufacturers.
Cast-in anchors currently lack support in the form of research reports, and neither IBC nor ACI 318 reference requirements in sufficient detail. Information presented in this paper will provide guidance toward acquiring data that satisfy the codes.
1. ISO/IEC 17065, Conformity assessment—Requirements for bodies certifying products, processes and services, International Organization for Standardization (ISO), 2012
2. International Building Code® (IBC), International Code Council, 2003, 2006, 2015
3. Uniform Building Code™ (UBC), International Conference of Building Officials, 1927, 1997
4. ACI 318, Building Code Requirements for Structural Concrete, American Concrete Institute, 2014
5. AISI S100, North American Specification for the Design of Cold-Formed Steel Structural Members, American Iron and Steel Institute and CSA Group, 2012
6. ACI 355.2, Qualification of Post-Installed Mechanical Anchors in Concrete and Commentary, American Concrete Institute, 2007
7. ACI 355.4, Qualification of Post-Installed Adhesive Anchors in Concrete, American Concrete Institute, 2011
8. ASTM F1554, Standard Specification for Anchor Bolts, Steel, 36, 55, and 105-ksi Yield Strength, ASTM International, 2015
9. ASTM A193, Standard Specification for Alloy-Steel and Stainless Steel Bolting for High Temperature or High Pressure Service and Other Special Purpose Applications, ASTM International, 2016
10. ASTM F593, Standard Specification for Stainless Steel Bolts, Hex Cap Screws, and Studs, ASTM International, 2013
11. ASTM E488, Standard Test Methods for Strength of Anchors in Concrete Elements, ASTM International, 2015
12. ASME B1.1, Unified Inch Screw Threads, (UN and UNR Thread Form), American Society of Mechanical Engineers, 2003
13. ASME B18.2.1, Square, Hex, Heavy Hex, and Askew Head Bolts and Hex, Heavy Hex, Hex Flange, Lobed Head, and Lag Screws (Inch Series), American Society of Mechanical Engineers, 2012
14. ASME B18.2.6, Fasteners for Use in Structural Applications, American Society of Mechanical Engineers, 2010
15. Design Guide 1: Base Plate and Anchor Rod Design (Second Edition), American Institute of Steel Construction, 2006
16. AWS D1.1, Structural Welding Code—Steel, 23rd Edition, American Welding Society, 2015
17. Klingner, R.E. and Mendonca, J.A., Tensile Capacity of Short Anchor Bolts and Welded Studs: A Literature Review, No. 79-27, 270-279, ACI Journal, July-August 1982
18. Meinheit, D.F., Osborn A.E.N., and Krueger, M.R., Pullout Strength of L-Bolt Anchors – Revisiting Design Equations, SP-296-2, American Concrete Institute, 2014
19. AC193, Mechanical Anchors in Concrete Elements, ICC Evaluation Service, 2015
20. ASTM E1512, Standard Test Methods for Testing Bond Performance of Bonded Anchors, ASTM International, 2015
21. AC308, Post-Installed Adhesive Anchors in Concrete Elements, ICC Evaluation Service, 2016
22. AC232, Anchor Channels in Concrete Elements, ICC Evaluation Service, 2016
23. AC446, Headed Cast-in Specialty Inserts in Concrete, ICC Evaluation Service, 2016
24. AC70, Power-driven Fasteners Driven into Concrete, Steel and Masonry Elements, ICC Evaluation Service, 2016