Why You Should Specify Stainless-Steel Screw Anchors When Designing for Corrosive Environments

Figure 1. Spalled concrete below a concrete bridge.

I was driving under a concrete bridge one nice clear day in Chicago, and I happened to look up to see rusted rebar exposed below a concrete bridge. My beautiful wife, who is not a structural engineer, turned to me and asked, “What happened to that bridge?” I explained that there are many reasons why spalling occurs below a bridge. One common reason is the expansion of steel when it rusts or corrodes.

This week’s blog will briefly explain the corrosion process and why concrete spalls when the embedded metals corrode. Corrosion may be defined as the degradation of a material as a reaction to its environment.1. As described in our previous SE Blog post, “Corrosion: The Issues, Code Requirements, Research and Solutions” dated January 3, 2013, corrosion of metallic surfaces is an electrochemical process. Because of moisture evaporation, concrete is a porous material. Water and oxygen molecules enter the pores of the concrete, and an electrochemical process occurs with the carbon-steel bar. The iron in the steel is oxidized, which then produces rust. A buildup of rust products at the surface of the carbon-steel bar exerts an expansive force on the concrete. Based on the amount of oxidation, the rust products of steel can occupy more than six times the volume of the original steel.2 Over time, further rust occurs and surface cracks will form. Eventually spalling will occur, exposing the rusted carbon steel bar. (See figure 1.)

Figure 2. Stages of corrosion.

Just as with reinforcing bars below a concrete bridge, cracking and spalling can occur when a carbon-steel anchor is used adjacent to a concrete edge. Simpson Strong-Tie® has many anchorage products that can be used in these conditions to prevent cracking. One specific product is the new stainless-steel Titen HD® screw anchor. This new innovative screw anchor is made up of Type 316 stainless steel. As seen in Figure 3, Type 316 stainless steel has a high level of resistance. This makes the stainless-steel Titen HD an excellent choice when it comes to an anchorage solution in corrosive environments. These environments include wastewater treatment plants, exterior handrails, exterior ledger attachments, stadium seating, central utility plants, and kitchens just to name a few.

Figure 3. Simpson Strong-Tie level of corrosion by material/coating.

Unlike expansion anchors, screw anchors require the leading threads to cut into predrilled holes. This can be easily achieved with hardened carbon-steel cutting threads. Stainless steel is not hard enough to cut into concrete. The new innovative stainless-steel Titen HD solves the problem by brazing heat-treated carbon-steel cutting threads to the surface of the stainless-steel tips of the screw anchor. (See figure 4.) These carbon-steel threads are hard enough to cut grooves into the surface of a predrilled hole, allowing the anchor to be installed with ease. The volume of the carbon-steel cutting threads is less than 1% of the stainless steel, reducing the buildup of rust that eventually spalls the concrete edge. Other stainless-steel screw anchor manufacturers in the market have a bi-metal product that attaches a full carbon-steel tip. This bi-metal screw anchors contain up to 18% carbon steel. Such a large amount of carbon steel can expand up to six times its volume when it corrodes and can spall the concrete when used adjacent to an edge.

Figure 4. Carbon-steel cutting threads.

Figure 5. Graphic representation of spalling in concrete adjacent to an edge.

When designing an anchorage solution for your next job in a corrosive environment, the stainless-steel Titen HD will provide the best resistance for corrosion, and also give the ability to drive these anchors into the concrete with ease. More information about the product can be obtained by visiting strongtie.com/thdss.

  1. Corrosion Technology Laboratory (https://corrosion.ksc.nasa.gov/corr_fundamentals.htm).
  2. Galvanized Rebar (http://www.concreteconstruction.net/how-to/repair/galvanized-rebar_o).

Stainless-Steel Titen HD®

The Next Era of Stainless-Steel Screw Anchor For Concrete and Masonry.


Concrete Anchor Design for the International Building Code: Part 1

Purpose
The intent of this technical bulletin is to clarify code language and outline the correct path for the design of concrete anchors under the International Building Code (IBC). The reader will be able to clearly distinguish between “code anchors” and anchors that are considered “alternative materials,” as well as understand the logical sequence of code language for designing each type. The distinction between “cracked” concrete and “uncracked” concrete anchor design will be made. This technical bulletin will lend clarity to the qualification of post-installed anchors for use in concrete. Excerpts from the IBC and its Referenced Standards will be provided to facilitate the description of the design requirements.

Background
More than a decade after the introduction of the American Concrete Institute’s ACI 318, Appendix D design methodology for anchor design in 2002, many design professionals either do not fully understand or are unaware of the code requirements for the design of concrete anchors. Several factors contribute to the challenges associated with understanding the code mandates:
1. The incorrect notion that ACI 318, Appendix D is exclusively for anchors designed for “cracked concrete,” leading to regionally varying degrees of enforcement and implementation of the design requirements
2. Multiple Reference Standards for the design and qualification of different anchor types
3. The evolving scope of Reference Standards, which have reclassified some anchors as “Code Anchors” that were previously considered “Alternative Materials”
4. Confusing language in IBC sections that address concrete anchorage
5. Complexity of the anchor design methodology itself
6. Varying levels of special inspections enforcement

It is nevertheless incumbent upon the licensed design professional to design anchors in accordance with the minimum provisions of the code in order to protect public safety, reduce liability risk and fulfill professional responsibilities.

The International Building Code, beginning with the 2000 edition, describes the design methodology of concrete anchors by virtue of the language within the IBC itself, or through language in the Referenced Standard (ACI 318). In this technical bulletin, specific reference to the 2012 IBC and ACI 318-11 will be made, since this is currently the most widely adopted edition of the IBC.

cadp1.1

“Code Anchors” and “Alternative Materials”
Anchors can be divided into two major categories: 1) “Code Anchors”, which are those that are specifically addressed in the IBC or its Referenced Standards, and 2) “Alternative Materials”, the design and qualification of which are not addressed in the IBC or its Referenced Standards.
The following “Code Anchors” recognized by the 2012 IBC:

  • Headed studs
  • Headed bolts
  • Hooked (J- or L-) bolts
  • Expansion anchors(such as Simpson Strong-Tie® Strong-Bolt® 2)
  • Undercut anchors (such as Simpson Strong-Tie® Torq-Cut™)
  • Adhesive anchors (such as Simpson Strong-Tie® SET-XP®, AT-XP®, and ET-HP®)

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Anchor types not listed above are considered “Alternative Materials.”
The following are anchors qualified as such:

  • Screw anchors (such as Simpson Strong-Tie® Titen HD®)

Alternative materials also apply to anchor types specifically excluded from ACI 318-11 calculation and analysis requirements.

  • Specialty inserts (such as Simpson Strong-Tie® Blue Banger Hanger®)
  • Through-bolts
  • Multiple anchors connected to a single steel plate at the embedded end
  • Grouted anchors
  • Powder- or gas-actuated fasteners (such as Simpson Strong-Tie® PDPA)

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Designing “Code Anchors”
The starting point for the design of all anchors is Section 1908 of the 2012 IBC.

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Section 1908.1 states that only cast-in-place headed bolts and headed studs are permitted to be designed using “Allowable Stress Design,” provided that they are not used to resist earthquake loads or effects. For these anchors, Section 1908.2 references Table 1908.2 for the determination of the allowable service load. Section 1908.1 makes explicit reference to post-installed anchors (anchors installed into hardened concrete), stating that the provisions of “Allowable Stress Design” is not permitted. For the design professional, this means that determining anchor by means of “Allowable Load Tables” based on previous test criteria that used a safety factor of 4.0 to determine allowable loads, as in the example below, is not permitted under the IBC.

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Section 1909 of the 2012 IBC, “Anchorage to Concrete – Strength Design” makes explicit reference to Appendix D of ACI 318 as the required design standard for the anchors listed in this section.

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Cast-in-place headed bolts and headed studs used to resist earthquake loads or effects must be designed using “Strength Design” in accordance with ACI 318 Appendix D. Additionally, Section 1909 does not make reference to adhesive anchors, despite their status as “code anchors.” ACI 318-11 was the first edition to include adhesive anchors in its scope; however, the 2012 IBC was approved prior to the approval of ACI 318-11. This resulted in the omission of adhesive anchors from the language in Section 1909 of the 2012 IBC. Section 1901.3 of the 2015 IBC, entitled “Anchoring to Concrete” includes language for adhesive anchors and their applicability to the ACI 318-14 design and qualification requirements. The omission of adhesive anchors from Section 1909 of the 2012 IBC, however, does not exclude them from the design and qualification requirements of ACI 318-11 by virtue of their inclusion in ACI 318-11 Section D.2.2. The design professional must then reference Section D.2 of ACI 318-11, Appendix D to confirm that the anchors being designed fall within its scope.

cadp1.7

Note that anchors used for temporary construction means, such as tilt wall panel bracing, are not addressed in the IBC. As a result, they are not required to be designed in accordance with the provisions of ACI 318, Appendix D. Section D.2.2 lists anchor types that fall within its scope, and those that are excluded (considered “Alternative Materials”).

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Code Anchors are required to meet the ACI 318-11 Section D.2.3 qualification requirements described below.

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ACI 355.2 (Qualification standard for expansion and undercut anchors) and ACI 355.4 (Qualification standard for adhesive anchors) are referenced here as the qualification criteria for specific types of postinstalled anchors. For the design professional it can be difficult to determine, without fully investigating these Referenced Standards, whether a specific proprietary anchor has been tested and is qualified for use in concrete. A simpler means by which to identify whether a proprietary anchor has been qualified to the Referenced Standard is a current Research Report (e.g., Evaluation or Code Report) which provides third-party review and verification that the product has been tested to and meets the qualification standard. There are two primary Research Report providers: IAPMO UES (International Association of Plumbing & Mechanical Officials Uniform Evaluation Service) and ICC-ES (International Code Council Evaluation Service).
These agencies are ANSI ISO 17065 accredited. They review independent laboratory test data, witnessed or conducted by an accredited third party, for a product and verify its conformance to publicly developed and majority-approved qualification criteria (or acceptance criteria) established for a given anchor type. Research Reports are an invaluable tool to the design professional and building official as evidence of conformance with the IBC.

There are two acceptance criteria that apply to post-installed “Code Anchors”:

  • ICC-ES AC193 – Acceptance Criteria for Post-Installed Mechanical Anchors in Concrete Elements
  • ICC-ES AC308 – Acceptance Criteria for Post-Installed Adhesive Anchors in Concrete Elements

These acceptance criteria reference ACI 355.2 and ACI 355.4, respectively, as the foundation for the test program by which the anchor is evaluated, and establish minimum performance standards for qualification. A Research Report is issued for an anchor that meets these minimum standards.

Newest Connector to Satisfy Code

“Does Simpson Strong-Tie write the building code?”

If you work at Simpson Strong-Tie, you get asked this question from time to time when you’re in the field. Over the years, I’ve heard it dozens of times, and because the answer is obviously “no,” it makes you wonder why this belief persists with so many people in the industry. Well, here is my theory: We develop and test products for new code provisions faster than it takes states to adopt the newest codes. So a designer, contractor or building official will often hear about a new Simpson Strong-Tie product or tested application that fills a need before their state building code even defines what that need is. Here are some recent examples:

  • The FWAZ foundation anchor released in 2007 for a 2006 IRC provision that addresses soil pressure loads on basement walls
  • Strong-Drive® SDS screw testing for deck ledgers published in 2008 as alternates to bolts and lags that weren’t prescribed in the IRC until the 2009 edition
  • The DTT2 deck tension tie released in 2009 is used for a 2009 IRC provision that addresses lateral loads on decks
  • BPS ½ -6 bearing plate released in 2011 to address new provisions for shear wall bearing plates in the 2008 SDPWS, which is referenced in the 2009 and 2012 IBC

The latest example is the DTT1Z deck tension tie. Two of our engineers, Randy Shackelford and David Finkenbinder, attended the ICC hearings that resulted in the new 2015 IRC. As soon as a new provision was passed to provide an alternate 750-pound deck lateral load connection (submitted by Washington Assoc. of Building Officials, not Simpson Strong-Tie) we began working on a connector designed to do the job. After several months of R&D, field trials and new tooling, our presses began to stamp out the first production run of the DTT1Z to meet the 2015 IRC provision on December 30, 2014.

DTT1Z Production Run

DTT1Z Production Run

2015 IRC Detail

2015 IRC Detail

The IRC detail shows an ideal condition where the bottom of the deck joist lines up with the wall plates in the house. We tested this application, but we also wanted to support variations that may come up in the field. The results of this testing appear in our T-C-DECKLAT15 technical bulletin. We also tested the DTT1Z with our Strong-Drive® SDWH Timber-Hex HDG screw and our Titen HD® concrete screw anchor so it can be used in a variety of applications, including prescriptive wall bracing and (very) light shear walls. Many of these applications are covered in the code report (ER130) that was completed just this past week.

2015 IRC Test Setup

Test setup: 2015 IRC detail

Joist Scab Test Setup

Test setup: Blocking attached to side of joist

Joist Blocking Test Setup

Test setup: Blocking running between joists

 

 

 

 

 

 

 

If you are interested in reading more about the new IRC deck provisions, Randy wrote about them in his Code Corner column in the current Structural Report and David wrote about them in this blog last August.

In case you are wondering how I respond when asked if we write the code, lately I have been answering it with another question: No, but do you know who is responsible for writing the code? My answer to this is “all of us.” If you don’t like what is in there now, work with an association that represents your interests (NCSEA for a lot of us) to submit a code-change proposal, or even submit one yourself. There is no guarantee it will get in, but if it involves a connection, I can guarantee we will get working on it right away!

Let us know if you see a need for a new connection product. If you already have a product idea and would like to work with us to develop it, you can more-formally submit it here.