Concrete Anchor Design for the International Building Code: Part 3

Specification of Concrete Anchors
The 2024 IBC and its Referenced Standard, ACI 318-19, is the first to mandate that contract documents specifically address installation, inspections and design parameters of concrete anchorage. For this reason, the specification of anchors in drawing details alone is impractical. To fully and effectively address these code mandates, concrete anchorage is more practically specified in both drawing detail(s) and the General Structural Notes or specifications of the contract documents. The drawing detail(s) would typically call out the anchor type, material specification, diameter, and embedment depth. The General Structural Notes or specifications would include the name of the qualified anchor(s) and address the installation, inspections and design parameter requirements of ACI 318-19.

The following provisions of ACI 318-19 Chapter 26 discuss the contract document requirements for concrete anchorage. Section 26.7.2 establishes compliance requirements for anchor installation, including proper positioning of cast-in anchors, consolidation of concrete around anchors, and installation of post-installed anchors in accordance with the Manufacturer’s Printed Installation Instructions (MPII). Section 26.7.2 further requires that post-installed anchors be installed by qualified installers, and that adhesive anchors installed in horizontal or upwardly inclined orientations to resist sustained tensile loads be installed by certified installers. Minimum concrete age requirements for adhesive anchors are also addressed.

The commentaries to Section 26.7.2, R26.7.2 (c)(e)(f) emphasize the sensitivity of anchor performance to proper installation and reinforces the importance of qualified personnel and strict compliance with MPII for post-installed anchors. Simpson Strong-Tie Co. Inc. provides free installer training conducted by experienced Technical Sales Representatives for adhesive, mechanical, and specialty anchors. For additional information regarding installer training opportunities, contact 1-800-999-5099.
Additional requirements related to inspection, installer qualification, and testing of adhesive anchors – including special inspection and proof loading where required – are addressed in ACI 318-19 Sections 26.7.1 and 26.7.2, as well as through the applicable inspection provisions of the general building code.

Per the section above, anchor installation requires inspection per ACI 318-19 Section 26.7.2. In addition, the design parameters for adhesive anchors are required to be specified in the contract documents. An explanation of the design parameters listed in ACI 318-19 Section 26.7.1 is provided below:

Proof loading where required in accordance with ACI 355.4. Proof loading is only required for adhesive anchors loaded in tension in which the inspection level chosen for the adhesive anchor design is “Continuous” (Ref. ACI 355.4 Section 10.4.6). Selecting “Continuous Inspection” can result in a higher “Anchor Category,” which in turn results in a higher strength reduction factor, φ. Reference Section 13.3.4 of ACI 355.4 for the minimum requirements of the proof loading program, where required. The Design Professional is responsible for performing the quantity, the duration of the applied load, and the proof load to which the anchors will be tested. These parameters will be specific to the anchor design conditions.
Minimum age of concrete at time of anchor installation. Per ACI 318-19 Section 26.7.2(f), adhesive anchors must be installed in concrete having a minimum age of 21 days at time of anchor installation unless otherwise permitted by the applicable evaluation report. Design professionals should refer to current manufacturer evaluation reports and published technical data for any product-specific allowances, limitations, or installation requirements.
Concrete temperature range. This is the in-service temperature of the concrete into which the adhesive anchor is installed. Temperature Ranges are categorized as 1, 2 or 3. Some manufacturers use A, B, or C as the category designations. Each Temperature Range category has a maximum short-term concrete temperature and a maximum long-term concrete temperature. Short-term concrete temperatures are those that occur over short intervals (diurnal cycling). Long-term concrete temperatures are constant temperatures over a significant time period.
Moisture condition of concrete at time of installation. Moisture conditions, as designated by ACI 355.4, are “dry,” or “water-saturated.” Moisture condition impacts the characteristic bond stress of an adhesive.
Type of lightweight concrete, if applicable.
Requirements for hole drilling and preparation. These requirements are specific to the adhesive, and are described in the Manufacturer’s Printed Installation Instructions (MPII). Reference to the MPII in the contract documents is sufficient.

Adhesive anchors installed in a horizontal or upwardly inclined orientation that resist sustained tension loads require a “certified” installer according to ACI 318-19 Section 26.7.1(l).

According to the commentary to Section 26.7.1, R26.7.1(l), certiﬁcation may also be appropriate for other safety-related applications. Installers can become certiﬁed through testing and training programs that include written and performance examinations as deﬁned by the ACI Adhesive Anchor Installer Certiﬁcation program (ACI CPP 680.1-17) or similar programs with equivalent requirements. The acceptability of certiﬁcation other than the ACI Adhesive Anchor Installer Certiﬁcation should be determined by the Licensed Design Professional. In addition, installers should obtain instruction through productspeciﬁc training oﬀered by manufacturers of qualiﬁed adhesive anchor systems.
An equivalent certiﬁed installer program should test the adhesive anchor installer’s knowledge and skill by an objectively fair and unbiased administration and grading of a written and performance exam. Programs should reﬂect the knowledge and skill required to install available commercial anchor systems. The eﬀectiveness of a written exam should be veriﬁed through statistical analysis of the questions and answers. An equivalent program should provide a responsive and accurate mechanism to verify credentials, which are renewed on a periodic basis.

The installation of adhesive anchors in a horizontal or upwardly inclined orientation presents unique challenges to the installer. Simply put, the effects of gravity for these applications make it difficult to prevent air bubbles and voids, which can limit full adhesive coverage of the insert (threaded rod or reinforcing bar). Due to the increased installation difficulty of these anchors, they are required to be continuously inspected by a certified special inspector according to ACI 318-19 Section 26.13.3.2(e).

Suggested General Structural Notes or specifications for post-installed anchors can be viewed and downloaded at here, or contact a Simpson Strong-Tie® representative for help with your post-installed General Structural Notes or specifications.

Simpson Strong-Tie Suggested General Note for Anchor Products

Post-Installed Anchors into Concrete, Masonry and
Steel and Cast-in-Place Anchors into Concrete

The below products are the design basis for this project. Substitution requests for products other than those listed below may be submitted by the contractor to the Engineer-of-Record (EOR) for review. Substitutions will only be considered for products having a code Report recognizing the product for the appropriate application and project building code. Substitution requests shall include calculations that demonstrate the substituted product is capable of achieving the equivalent performance values of the
design basis product. Contractor shall contact manufacturer’s representative (800-999-5099) for product installation training and a letter shall be submitted to the EOR indicating training has taken place. Refer to the building code and/or evaluation report for special inspections and proof load requirements.

For anchoring into cracked and uncracked concrete

a) Mechanical anchors shall have been tested in accordance with ACI 355.2 and/or ICC-ES AC193 for cracked concrete and seismic applications. Pre-approved products include:

Simpson Strong-Tie® Strong-Bolt® 2 (ICC-ES ESR-3037)
Simpson Strong-Tie® Titen HD® (ICC-ES ESR-2713)
Simpson Strong-Tie® Titen HD® Rod Hanger (ICC-ES ESR-2713)

b) Adhesive anchors shall have been tested in accordance with ACI 355.4 and/or ICC-ES
AC308 for cracked concrete and seismic applications. Adhesive anchors shall be installed
by a certified adhesive anchor installer where designated on the contract documents.
Pre-approved products include:

Simpson Strong-Tie® AT-3G™ (ICC-ES ESR-5026)
Simpson Strong-Tie® SET-3G™ (ICC-ES ESR-4057)
Simpson Strong-Tie® ET-3G™ (ICC-ES ESR-5334)

Concrete Anchor Design for the International Building Code: Part 2

Designing “Alternative Materials”

Concrete anchor types whose designs are not addressed in the IBC or its referenced standards, or are specifically excluded from the scope of the referenced standard (ACI 318-19 Chapter 17), may be recognized as Alternative Materials. Section 1901.3 of the 2024 IBC requires anchoring to concrete to be designed in accordance with ACI 318-19 Chapter 17, as supplemented by Section 1905, for anchor types that fall within its scope.

Anchor types that are excluded from the scope of ACI 318-19 Chapter 17 are therefore not considered code-prescribed anchors and must be evaluated as alternative materials, designs, or methods of construction. While ACI 318-19 Chapter 17 identifies certain anchor types that are excluded from its scope, the list of excluded anchor types is not intended to be comprehensive.

Section 104.2.3 of the 2024 IBC describes how the design professional must approach the design and approval of Alternative Materials, including demonstrating that the proposed system is equivalent to that prescribed by the code in terms of quality, strength, effectiveness, durability, and safety, subject to approval by the building official.

Section 104.2.3 of the 2024 IBC provides the design professional with two options for the substantiation of the acceptable performance of an Alternative Material:

a. Evaluation Reports. As described in the previous section (Design of Code Anchors), Evaluation Reports are referenced as the primary source for the design and qualification of Alternative Materials. In accordance with IBC Section 104.2.3.6, supporting data submitted to assist in the approval of materials or assemblies not specifically provided for in the code shall comply with Sections 104.2.3.6.1 and 104.2.3.6.2. Evaluation reports, as addressed in Section 104.2.3.6.1, are issued by an approved agency and require approval by the building official for the installation. Evaluation Reports for anchors are published by IAPMO UES or ICC-ES, both ANSI ISO 17065 accredited agencies. Publicly developed, majority-approved acceptance criteria are used to establish the test program and minimum performance requirements for an anchor type. Some anchor types historically evaluated as Alternative Materials have established acceptance criteria. However, under the 2024 IBC, screw anchors in concrete are recognized as code anchors when designed in accordance with ACI 318-19 Chapter 17 and therefore are no longer considered Alternative Materials. Examples of anchor types for which acceptance criteria have been developed include:

Headed Cast-in Specialty Inserts: ICC-ES AC446
Powder- or Gas-Actuated Fasteners (such as Simpson Strong-Tie® PDPA and GDP): ICC-ES AC70

If Evaluation Reports are used to substantiate an anchor’s performance, the design professional is bound by the design methodology and product limitations described in the Evaluation Report.

b. Tests

If an evaluation report is not available and no acceptance criteria exist for a given anchor type, IBC Section 104.2.2.4 permits the use of tests to demonstrate compliance where there is insufficient evidence of code conformity. Tests must be performed using recognized test standards, or other testing procedures approved by the building official, and conducted by a party acceptable to the building official. One example of an anchor type for which no published acceptance criteria currently exist is:

Helical Wall Ties (such as Simpson Strong-Tie® Heli-Tie™)

Cracked Concrete Determination
One of the many design considerations that the design professional must determine when designing either “Code Anchors” or anchors qualified as “Alternative Materials” is whether to consider the state of the concrete “cracked” or “uncracked.” The concrete state can significantly influence the anchor’s capacity. Neither the 2024 IBC nor ACI 318-19 Chapter 17 explicitly defines which applications should be categorized as “cracked” or “uncracked” concrete. The design professional must determine by analysis whether cracking will occur in the region of the concrete member where the anchors are installed. Absent an analysis to determine whether cracking will occur, the design professional may conservatively assume that the concrete state is “cracked.” With that said, there are two circumstances that require the design professional to design for “cracked” concrete:

a) Anchors in structures assigned to Seismic Design Categories C, D, E, or F (per 2024 IBC Chapter 16, referencing ASCE/SEI 7) are required to be designed for “cracked” concrete unless the design professional can demonstrate that cracking does not occur at the anchor locations. The prequalification requirements of ACI 355.2 for mechanical anchors and ACI 355.4 for adhesive anchors include a test program that evaluates the performance of anchors in cracked concrete. Only anchors that have been tested and have passed the cracked concrete test program qualify for use in “cracked” concrete. The Evaluation or Research Report for a post-installed anchor (mechanical or adhesive) will clearly indicate whether it qualifies for use in “cracked concrete.”
b) Anchors located in a region of the concrete element where analysis indicates cracking at service level loading must be designed for “cracked” concrete (ACI 318-19 Eq. (19.2.3.1)).

The design professional must consider additional factors that have the potential to result in concrete cracking in the region of anchorage. These factors include restrained shrinkage, temperature changes, soil pressure, and differential settlement. If no cracking is assumed in the region of the anchorage, the design professional should be able to justify that assumption.

Design Calculations

The design methodology in ACI 318-19 Chapter 17 is detailed and can be cumbersome. Calculations can be performed by hand using the design equations in Chapter 17, inserting the substantiated data from an anchor manufacturer’s data tables or applicable evaluation or research reports to design with post-installed anchors. Designing with cast-in-place “Code Anchors” does not require additional data beyond what is included in ACI 318-19 Chapter 17 since these are “standard” anchors with standard design characteristics.

Performing hand calculations can be time-consuming, and for most design professionals is impractical due to the complexity of the design equations associated with multiple failure modes required to be considered. Design software, such as Simpson Strong-Tie® Anchor Designer™ Software for ACI 318, ETAG and CSA provides a fast, reliable method of calculating anchor performance for both cast-in-place and post-installed anchors. This software designs both “Code Anchors” and “Alternative Materials” for which an acceptance criteria exists.

Simpson Strong-Tie® Anchor Designer™ Software for ACI 318, ETAG and CSA is free and can be downloaded here.

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 2024 IBC and ACI 318-19 will be made, since this is currently the most widely adopted edition of the IBC.

“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 2024 IBC:

Headed studs
Headed bolts
Hooked (J- or L-) bolts
Expansion anchors (such as Simpson Strong-Tie® Strong-Bolt® 2)
Undercut anchors
Screw anchors (such as Simpson Strong-Tie® Titen HD®)
Adhesive anchors (such as Simpson Strong-Tie® ET-3G™, AT-3G™ and SET-3G®)

Anchor types not listed above are considered “Alternative Materials.”

Alternative materials also apply to anchor types specifically excluded from ACI 318-19 calculation and analysis requirements.

Specialty inserts
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)

Designing “Code Anchors”
The starting point for the design of all anchors is Section 1901.3 and Section 1905 of the 2024 IBC, which reference ACI 318-19 Chapter 17 for anchorage design requirements.

Under the 2024 IBC, anchor design is governed by Section 1901.3 and Section 1905, which reference ACI 318-19 Chapter 17. Chapter 17 establishes a comprehensive strength design (LRFD) approach for anchoring to concrete, rather than traditional “Allowable Stress Design” (ASD). It provides detailed equations for steel strength, concrete breakout, pullout, and pryout failure modes for both cast-in and post-installed anchors, applying strength reduction factors (ϕ) and load factors (γ). For general calculations, the compressive strength of concrete (f’c) is limited to 8,000 psi.
While Chapter 17 is fundamentally strength design, some anchor product evaluations – such as ICC-ES Evaluation Reports (ESRs) or Technical Engineering Bulletins (TEBs) – include ASD tables derived from ACI 318 strength design equations. These tables use specific footnotes and conversion factors to present allowable service loads, and may incorporate additional safety factors or seismic overstrength factors (Ω0). Designers must carefully review these footnotes to confirm whether values represent strength design capacities or manufacturer-provided allowable loads, and ensure compliance with the applicable code edition.
In essence, ACI 318-19 Chapter 17 sets the governing rules for strength design, while manufacturers bridge the gap for projects requiring allowable loads through ESRs or TEBs. Regardless of format, all anchors – cast-in or post-installed – must meet the qualification standards in ACI 355.2 (mechanical anchors) and ACI 355.4 (adhesive anchors), and comply with the performance requirements referenced by the IBC.

Anchors resisting earthquake loads or effects must be designed using strength design in accordance with ACI 318-19 Chapter 17, as referenced by 2024 IBC Section 1901.3 and Section 1905. This includes cast-in headed bolts, headed studs, hooked bolts, and post-installed anchors such as mechanical and adhesive anchors. Adhesive anchors are explicitly included in ACI 318-19 and must meet installation and qualification requirements defined in ACI 355.4, with special provisions for sustained tension and seismic applications. The design professional must confirm that anchors fall within the scope of ACI 318-19 Chapter 17 and meet all seismic and load combination provisions. Post-installed anchors must also satisfy qualification standards and be supported by ICC-ES Evaluation Reports (ESRs) or equivalent documentation to confirm compliance.

Anchors used for temporary construction purposes, such as tilt-up wall panel bracing, are not addressed by the anchorage provisions of the IBC and therefore are not required to be designed in accordance with ACI 318-19 Chapter 17. Chapter 17 clearly defines the scope of anchors covered by the code, including cast-in and post-installed anchors (mechanical and adhesive), and identifies anchor types that fall outside its requirements. Anchors excluded from Chapter 17 are considered alternative materials and must be evaluated and approved under the provisions of 2024 IBC Section 104.2.3, which allows alternative materials and methods when they provide equivalent performance to code requirements.

Code Anchors are required to meet the ACI 318-19 Section 17.1.2 qualification requirements described below.

ACI 355.2 (Qualification standard for expansion, undercut, and screw anchors) and ACI 355.4 (Qualification standard for adhesive anchors) are referenced here as the qualification criteria for specific types of post-installed 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.

Wood-framed Deck Guard Post Resources and Residential Details

A deck and porch study reported that 33% of deck failure-related injuries over the 5-year study period were attributed to guard or railing failures. While the importance of a deck guard is widely known, there was a significant omission from my May 2014 post on Wood-framed Deck Design Resources for Engineers regarding the design of deck guards.

A good starting point for information about wood-framed guard posts is a two-part article published in the October 2014 and January 2015 issues of Civil + Structural Engineer magazine. “Building Strong Guards, Part 1” provides an overview of typical wood-framed decks, the related code requirements and several examples that aim to demonstrate code-compliance through an analysis approach. The article discusses the difficulties in making an adequate connection at the bottom of a guard post, which involve countering the moment generated by the live load being applied at the top of the post. Other connections in a typical guard are not as difficult to design through analysis. This is due to common component geometries resulting in the rails and balusters/in-fill being simple-supported rather than cantilevered. “Building Strong Guards, Part 2” provides information on the testing approach to demonstrate code-compliance. Information about code requirements and testing criteria are included in the article as well.

Research and commentary from Virginia Tech on the performance of several tested guard post details for residential applications (36” guard height above decking) is featured in an article titled “Tested Guardrail Post Connections for Residential Decks” in the July 2007 issue of Structure magazine. Research showed that the common construction practice of attaching a 4×4 guard post to a 2x band joist with either ½” diameter lag screws or bolts, fell significantly below the 500 pound horizontal load target due to inadequate load transfer from the band joist into the surrounding deck floor framing. Ultimately, the research found that anchoring the post with a holdown installed horizontally provided enough leverage to meet the target load. The article also discussed the importance of testing to 500 pounds (which provides a safety factor of 2.5 over the 200-pound code live load), and the testing with a horizontal outward load to represent the worst-case safety scenario of a person falling away from the deck surface.

Simpson Strong-Tie has tested several connection options for a guard post at the typical 36” height, subjected to a horizontal outward load. Holdown solutions are included in our T-GRDRLPST22 technical bulletin. In response to recent industry interest, guard post details utilizing blocking and Strong-Drive® SDWS TIMBER screws have been developed (see picture below for a test view) and recently released in the engineering letter L-F-SDWSGRD15. The number of screws and the blocking shown are a reflection of the issue previously identified by the Virginia Tech researchers – an adequate load path must be provided to have sufficient support.

SDWS Detail C: Interior Post on Rim Joist between Joists, at 500-Pound Horizontal Test Target Load

Have you found any other resources that have been helpful in your guard post designs? Let us know by posting a comment.

Anchor Testing for Light-Frame Construction

I started off doing a four-part series on how connectors, fasteners, concrete anchors and cold-formed steel products are tested and load rated. I realized that holdown testing and evaluation is quite a bit different than wood connector testing, so there was an additional post on holdowns. We have done several posts on concrete anchor testing (here and here), but I realize I never did a proper post about how we test and load rate concrete products per ICC-ES AC398 and AC399.

AC398 – Cast-in-place Cold-formed Steel Connectors in Concrete for Light-frame Construction and AC399 – Cast-in-place Proprietary Bolts in concrete for Light-frame Construction are two acceptance criteria related to cast-in-place concrete products.

Cold-formed steel connectors embedded in concrete are not considered in ACI 318 Appendix D, so it was necessary to create criteria for evaluating those types of connectors. Some examples of products covered by AC398 are the MASA mudsill anchor, CBSQ post base, and the STHD holdown.

ACI 318 Appendix D addresses the design of cast-in-place anchors. However, the design methodology is limited to several standard bolt types.

There are a number of anchor bolt products that have proprietary features that fall outside the scope of ACI 318, so AC399 fills in that gap by establishing test procedures to evaluate cast-in-place specialty anchors. Simpson Strong-Tie SB and SSTB anchor bolts are two families of anchors we have tested in accordance with AC399.

SB and SSTB anchors have a sweep geometry which increases the concrete cover at the anchored end of the bolt, allowing them to achieve higher loads with a 1¾” edge distance. The SSTB is anchored with a double bend, whereas the SB utilizes a plate washer and double nut.

AC398 (concrete connectors) and AC399 (proprietary bolts) are similar in their test and evaluation methodology. AC398 addressing both tension and shear loads, whereas AC399 is limited to tension loads. Testing requires a minimum of 5 test specimens. These are the allowable load equations for AC398 and AC399:

AC398 - Allowable Load Equation — AC398 – Allowable Load Equation

AC399 - Allowable Load Equation — AC399 – Allowable Load Equation

For comparison, here is the standard AC13 allowable load equation for joist hangers:

Allowable Load = Lowest Ultimate / 3

Without getting into Greek letter overload, what are these terms doing?

N_u (or V_u) is the average maximum tested load. Calculating averages is something I actually remember from statistics class. Everything else I have to look up when we do these calculations.

(1 – K x COV) uses K as a statistical one-sided tolerance factor used to establish the 5 percent fractile value with 90% confidence. This term is to ensure that 95% of the actual tested strengths will exceed the 5% fractile value with 90% confidence. COV is the coefficient of variation, which is a measure of how variable your test results are. For the same average ultimate load, a higher COV will result in a lower allowable load.

The K value is 3.4 for the minimum required 5 tests, and it reduces as you run more tests. As K decreases, the allowable load increases. In practice, we usually run 7 to 10 tests for each installation we are evaluating.

Table A2.1 – K values for evaluating the characteristic capacity at 90% confidence

R_d is seismic reduction factor, 1.0 for seismic design category A or B, and 0.75 for all others. This is similar to what you would do in an Appendix D anchor calculation, where anchor capacities in higher seismic regions are reduced by 0.75.

R_s and R_c are reduction factors to account for the tested steel or concrete strength being higher than specified. There are some differences in how the two acceptance criteria apply these factors, which aren’t critical to this discussion. Φ is a strength reduction factor, which varies by failure mode and construction details. Brittle steel failure, ductile steel failure, concrete failure and the presence of supplemental reinforcement.

The α factor is used to convert LRFD values to ASD values. So α = 1.0 for LRFD and α = 1.4 for seismic and 1.6 for wind. Both criteria also allow you to calculate alpha based on a weighted average of your controlling load combinations. This has never made a lot of sense to me in practice. If you are going to work through the LRFD equations to get a different alpha value, you might as well do LRFD design.

R_se is a reduction factor for cyclic loading, which is applied to proprietary anchor bolts covered under AC399, such as the SSTB or SB anchors. A comparison of static load and cyclic load is required for qualification in Seismic Design Category C through F. Unlike the cracked reduction factor, manufacturers cannot take a default reduction if they want recognition for high seismic.

Due to the differences in AC398 and AC399 products, the load tables are a little different. AC398 products end up with 4 different loads – wind cracked, wind uncracked, seismic cracked and seismic uncracked.

AC399 products are a little simpler, having just wind and seismic values to deal with.

What are your thoughts? Let us know in the comments below.

Snow Loads vs. Top Chord Live Loads – A Historical Look at Snow Loading for Trusses

In my former life working as a consulting engineer, I reviewed many truss submittal packages. I remember during my reviews wondering how it was possible to get so much information on to an 8½ inch by 11 inch piece of paper. I also remember how a lot of what was being reported was difficult to understand without some help interpreting the information.

As many of you may know, Simpson Strong-Tie has ventured into the truss industry and we are now offering truss connector plates and software to component manufacturers around the country. So given my past experiences, I figure some of you might appreciate some insight into the engineering that goes on behind those truss submittal packages. So I have asked one of our great truss engineers, Kelly Sias, to put together some blog posts on the topic that we can share our knowledge with you. Kelly has worked in the truss industry for years and spent time as the Technical Director at the Truss Plate Institute. I am sure her blog posts are going to help all of us have a better appreciation for trusses.

Have you ever been involved in a discussion with someone on a project that ended with “but that’s the way we’ve always done it!”? I heard those words spoken by a contractor in my first engineering job when I tried explaining why his single stud would not work at a particular location. When he said something about his grandfather having always done it that way, I knew I could explain the calculations all day and it wouldn’t do much good.Continue Reading

So, What’s Behind A Structural Connector’s Allowable Load? (Holdown Edition)

This is Part 1A of a four-part series I’ll be doing on how connectors, fasteners, anchors and cold-formed steel systems are load rated.
I envisioned doing a four-part series on how connectors, fasteners, anchors, and cold-formed steel are load rated. After writing the first installment on connectors, I realized that connectors are a bit more complicated, since the testing and evaluation for joist hangers (or similar devices) is different than testing for holdown devices. And I wanted to discuss holdowns. So without belaboring the apology for my numbering system, this will be part 1A of the series – still discussing wood connectors, but focusing on holdowns and some of the unique requirements in their load rating.
AC155, Acceptance Criteria for Hold-Downs (Tie-Downs) Attached to Wood Members, was first developed in 2005 to better address boundary conditions, deflection limits, and wood post limits. Prior to AC155, holdowns were evaluated based on testing on a steel jig with a safety factor of 3.0 and an NDS bolt calculation. Deflection at the allowable load was simply reported so that it was available for use in design, but there was not a deflection limit that affected the load rating.

Bolted Holdown - Steel Jig Test — Bolted Holdown – Steel Jig Test

In the steel jig setup shown, the jig keeps the holdowns stationary while the rectangular bar underneath the holdowns is pushed downward to simulate an uplift force. This was (and still is) an effective method of testing the capacity of the steel body of a holdown, but it does not tell you a lot about the deflection of the holdown when installed on a wood member. Since story-drift is such a critical component to shearwall performance and the deflection of holdowns has a significant effect on the total drift, this needed to be address in the holdown test standard.
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So, What’s Behind A Structural Connector’s Allowable Load?

This is Part 1 of a four-part series I’ll be doing on how connectors, fasteners, anchors and cold-formed steel systems are load rated.
Today I did my presentation for the WoodWorks webinar on Testing and Product Evaluation of Products for Wood-framed Construction. We covered a lot of material regarding code requirements for using alternate materials or construction methods, how testing and evaluation criteria are developed, and some specifics on several Acceptance Criteria (AC’s) that are commonly used for connector evaluations. We also discussed some specific testing requirements, so I thought it would be timely to discuss some of those issues in this week’s blog post.
So, how are structural connectors for light frame wood construction load rated? What’s behind the allowable loads information published in Simpson Strong-Tie literature or wood connector evaluation reports? These are things that you might find yourself wondering while driving to the office or jobsite, or on a Sunday afternoon while enjoying your favorite iced tea or barley-based beverage.
The short answer is: testing, calculations, and of course, sound engineering judgment.
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