Welcome to our Structural Engineering Blog! I’m Paul McEntee, Engineering R&D Manager at Simpson Strong-Tie. We’ll cover a variety of structural engineering topics here that I hope interest you and help with your projects and work. Social media is “uncharted territory” for a lot of us (me included!), but we here at Simpson Strong-Tie think this is a good way to connect and even start useful discussions among our peers in a way that’s easy to use and doesn’t take up too much of your time. Continue reading
As many of you know, LinkedIn is a social networking website specifically aimed at business professionals and is designed to help you connect and network with people you know and trust. You can add colleagues, peers and others as contacts and send them messages. You can create and update your personal profile to let your contacts know about your professional activities, and both recommend or endorse your contacts and get recommended or endorsed by your contacts for your professional skills. In addition, you can join groups to communicate with other professionals within the same sector or industry. You are also able to ask and answer industry-related questions, and to learn about and apply for job openings.
A basic membership on LinkedIn is free, but you can also upgrade your account in order to have access to professionals outside of your network.
To help guide you, here are some best practices for how to set up and optimize your LinkedIn account.
Update and Complete Your Profile
Having a complete and updated profile on LinkedIn allows you to put your best face forward. Make sure to summarize your role and responsibilities and current and past work experience, highlighting the details you think will make a prospective customer want to work with you. Include a professional-looking headshot and your current contact information. LinkedIn will even tell you your profile strength on the right-hand rail.
Join Industry Specific Groups
Joining groups that are relevant to our industry will allow you to participate in online industry discussions. Answering questions related to your field of expertise within these discussions is an excellent way to position yourself as an authority and build your professional reputation. Here are some structural engineering groups that you can start with:
Connect with people you already know using your email contacts. This will help you maintain your existing relationships as well as branch out to connect with industry-related people your contacts may know. Another great feature of LinkedIn is that it will tell you “People You May Know” based on where you work or are already linked to. This feature will help you find meaningful connections.
Follow the Company Page and Share Posts
Simpson Strong-Tie has a company LinkedIn page to connect with customers. Company pages are a way to keep up to date on trends in design and building materials, code changes, product launches and other industry news. Make sure to follow the Simpson Strong-Tie company page so that you can stay informed about our latest news and updates.
Make sure to review and manage your privacy settings to help you control how many people can view your activities and personal information. You can do this by hovering your mouse on your thumbnail image on the far right- hand side of your home page.You should see an Account & Settings drop-down menu appear with an option that says “Privacy & Settings: Manage.” Click this option. Once you are there, you can manage all of your privacy settings.
How do you use LinkedIn as part of your engineering career? Let us know in the comments below.
Specification of Concrete Anchors
The 2012 IBC and its Referenced Standard, ACI 318-11, 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 318-11.
The following sections of ACI 318-11 discuss the contract document requirements for concrete anchorage:
The commentary in ACI 318-11, RD.9.1 discusses the sensitivity of anchor performance to proper installation. It emphasizes the importance of qualified installers for all anchors, and compliance with the Manufacturer’s Printed Installation Instructions (MPII) for post-installed anchors. Training is required for adhesive anchor installers per ACI 318-11 D.9.1. Simpson Strong-Tie Co. Inc. provides free installer training by experienced Technical Sales Representatives for our adhesive, mechanical and specialty anchors. Contact us at 1-800-999-5099. Special inspection and proof loading are addressed in ACI 318-11 D.9.2
Per the section above, anchor installation requires inspection per Section D.9.2. 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 318-11 D.9.2.1 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 D.2.2, adhesive anchors must be installed in concrete having a minimum age of 21 days at time of anchor installation. Simpson Strong-Tie® has performed in-house testing of SET-XP®, AT-XP®, and ET-HP® adhesive anchors installed in 7-day- and 14-day-old concrete. The results of testing are published in an engineering letter (L-A-ADHGRNCON15.pdf), which can be viewed and downloaded at www.strongtie.com.
- 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.
A certification program has been established by ACI/CRSI. Installers can obtain certification by successful completion of this program. Contact your local ACI or CRSI chapter for more information. Other means of certification are permitted, and are the responsibility of the licensed design professional.
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.
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:
i. Simpson Strong-Tie® Strong-Bolt® 2 (ICC-ES ESR-3037)
ii. Simpson Strong-Tie® Titen HD® (ICC-ES ESR-2713)
iii. Simpson Strong-Tie® Torq-Cut® (ICC-ES ESR-2705)
iv. Simpson Strong-Tie® Titen HD® Rod Hanger (ICC-ES ESR-2713)
v. Simpson Strong-Tie® Blue Banger Hanger® (ICC-ES ESR-3707, except roof deck insert)
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:
i. Simpson Strong-Tie® AT-XP® (IAPMO-UES ER-263)
ii. Simpson Strong-Tie® SET-XP® (ICC-ES ESR-2508)
III. Simpson Strong-Tie® ET-HP® (ICC-ES ESR-3372)
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 318-11), may be recognized as Alternative Materials. Section 1909 of the 2012 IBC requires that “The strength design of anchors that are not within the scope of Appendix D of ACI 318, shall be in accordance with an approved procedure.” Section D.2.2 of ACI 318-11 lists some concrete anchor types that are considered “Alternative Materials” and specifically excludes these anchors from its scope. The list of “Alternative Material” anchors provided in this section is not, however, a comprehensive list.
Section 104.11 of the 2012 IBC describes how the design professional must approach the design of Alternative Materials.
Section 104.11 provides the design professional with two options for the substantiation of the acceptable performance of an Alternative Material:
a. Research Reports. As described in the previous section (Design of Code Anchors), Research Reports are referenced as the primary source for the design and qualification of Alternative Materials. Research 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 Alternative Material anchor types have established acceptance criteria to which a product can be evaluated:
- Screw Anchors in Concrete (such as Simpson Strong-Tie® Titen HD®): ICC-ES AC193
- Headed Cast-in Specialty Inserts (such as Simpson Strong-Tie® Blue Banger Hanger®): ICC-ES AC446
- Powder- or Gas-Actuated Fasteners (such as Simpson Strong-Tie® PDPA and GDP): ICC-ES AC70
If Research Reports are used to substantiate an anchor’s performance, the design professional is bound by the design methodology and product limitations described in the Research Report.
b. Tests. If a Research Report is not available, and no acceptance criteria exists for a given anchor type, IBC Section 104.11 permits the use of tests performed in accordance with “recognized and accepted test methods” by an “approved agency” to substantiate performance. One example of an anchor type for which no acceptance criteria exists 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 IBC nor ACI 318, Appendix D 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 2012 IBC, Chapter 16) 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 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 (e.g. fr ≥ 7.5λ√f’c, ACI 318-11 eq. 9-10).
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.
The design methodology in ACI 318 Appendix D is cumbersome. Calculations can be performed by hand using the design equations in Appendix D, inserting the substantiated data from an anchor manufacturer’s data tables 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, Appendix D 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.
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.
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.
“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®)
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®)
- 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 1908 of the 2012 IBC.
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.
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.
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.
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”).
Code Anchors are required to meet the ACI 318-11 Section D.2.3 qualification requirements described below.
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.
The use of high-strength steels in anchors and fasteners is not unusual in the construction world. For example, high-strength threaded rods are often used to reduce the number of mild-steel rods that would be required to meet a design load for an adhesive anchor. High-strength steel is essential to the function of some fasteners, such as self-drilling and self-tapping screws.
However, for anchors and fasteners, there are limits to what can be accomplished by increasing steel strengths due to a phenomenon known as hydrogen embrittlement. Hydrogen embrittlement is well known by anchor and fastener manufacturers, but it is not widely known by structural engineers. The purpose of this blog post is to provide an overview of this important subject and some insight into one reason why steels used in construction anchors and fasteners have upper strength limits.
What is hydrogen embrittlement?
Hydrogen embrittlement is a significant permanent loss of strength that can occur in some steels when hydrogen atoms are present in the steel and stress is applied. Embrittlement occurs as hydrogen atoms migrate to the region of highest stress and cause microcracks. When a crack forms, the hydrogen then migrates to the tip of the crack (Figure 1) and causes continued crack growth until the effective fastener cross-section is so reduced that the remaining cross-section is overloaded and the fastener fails. Figures 2, 3 and 4 show a failure plane and coarse granular morphology and intergranular cracks (dark lines in Figures 3 and 4). These failures occur suddenly, without warning, after the fastener or rod has been loaded for a period of time.
All three of the aforementioned conditions must be present for hydrogen embrittlement to occur:
- A steel that is susceptible to hydrogen embrittlement
- Atomic hydrogen (H+ ions, not H2 gas)
- Stress, such as from tightening or applied loads
If an application involves these, then there is a possibility of hydrogen embrittlement and fastener failure. The possibility of and time to failure depend on the degree of each of these conditions. Therefore, the level of concern should depend on the degree to which the Designer expects all three of these conditions to be present in the application.
What steels are sensitive to hydrogen embrittlement?
A fastener’s susceptibility to hydrogen embrittlement increases with elevated tensile strength or hardness. It is generally accepted that good-quality fasteners with an actual (not specified) Rockwell C scale hardness of less than 38 (tensile strength of less than approximately 170,000 psi) are not ordinarily susceptible to hydrogen embrittlement. Fastener manufacturers often establish lower hardness limits as an extra margin of safety for variability that may occur in production.
When are hydrogen atoms (H+) present?
Hydrogen atoms can be introduced into a fastener from manufacturing or from the service environment.
Sources of hydrogen from manufacturing:
Hydrogen can be present in the steel-making process, but the amount present in good-quality steel is below the level that causes problems. The most common way that hydrogen is introduced during anchor and fastener manufacturing is from the cleaning and coating processes. These processes often utilize acids that produce hydrogen. Compounding this problem is that many popular coatings like zinc plating (ASTM B633) and hot-dip galvanizing (ASTM A153) create a barrier around the fastener that does not allow hydrogen to easily diffuse out of the fastener. Some coatings, like mechanical galvanizing (ASTM B695), do have sufficient porosity that hydrogen can diffuse through the coating at room temperature.
Manufacturers most commonly manage internal hydrogen sources by minimizing cleaning time and by baking plated fasteners after coating for a number of hours to help diffuse hydrogen through the coating and out of the fastener. In some cases, mechanical cleaning and alkaline cleaning are used to prevent hydrogen from being introduced.
In cases where internal hydrogen is not properly managed in the manufacturing process, failure usually occurs quickly, within 48 hours of fastener installation.
External sources of hydrogen:
Hydrogen can also be introduced from the service environment. Zinc coatings galvanically protect steel from corrosion in damp or wet service environments. However, this process results in an electric current being passed through the water (H2O) to produce hydrogen (H+) and hydroxide (OH–) ions as shown in Figure 5. Since this process requires corrosion to generate the hydrogen, failures from externally generated hydrogen usually take much longer to occur than when internal hydrogen is the cause. Steel failure from externally generated hydrogen can take anywhere from weeks to years.
How much stress is too much stress?
Fortunately, for products that require very high-strength steels, hydrogen embrittlement risk diminishes, even for sensitive steels, as applied loads are reduced. For such products there are a number of complex tests that fastener manufacturers utilize in development and quality control to verify that the risk of hydrogen embrittlement is controlled for the intended application at the rated loads.
As illustrated in the discussion above, hydrogen embrittlement can be a serious concern for high-strength anchors and fasteners. A Designer should exercise great care to guard against the sudden brittle steel failure that this phenomenon can produce. There are a number of good practices to follow in this regard:
- Do not utilize exotic high-strength steels with actual tensile strengths greater than 170,000 psi (Rockwell C-scale hardness of 38) without carefully considering hydrogen embrittlement risk. Note that actual tensile strength may be higher than specified tensile strengths.
- Ensure that quality sources of high-strength fasteners are specified. High-quality manufacturers have design and manufacturing practices in place to guard against hydrogen embrittlement.
- Use discretion in selecting very high-strength fasteners for corrosive applications since these conditions often produce hydrogen through the galvanic process.
This post is an update to David Finkenbinder’s post on Guard Post Resources from August 13.
As David explained, the requirements in the IRC and IBC for guards are intended to prevent people from falling off of raised surfaces. The failure of this guard is a common source of injuries caused by failures of deck components.
Section R312.1.1 of the 2012 International Residential Code (IRC) states that “Guards shall be located along open-sided walking surfaces, including stairs, ramps and landings, that are located more than 30 inches measured vertically to the floor or grade below at any point within 36 inches horizontally to the edge of the open side.”
Table 301.5 of the 2012 IRC requires that guards and handrails be designed for “[a] single concentrated load” of 200 pounds “applied in any direction at any point along the top.”
David mentioned the article Tested Guardrail Post Connections for Residential Decks, which described a testing program at Virginia Tech that examined the ability of various assemblies to resist this concentrated load at the top of the guard post. But rather than test in any direction, the researchers decided to test in what they considered the most critical direction: outward away from the deck.
Simpson Strong-Tie subsequently developed a new tension tie, the DTT2Z, to make an economical connection from the top bolt in a deck post back into the framing of the deck to resist the high tension forces that develop in the top bolt when the top of the post is pushed outward. Several details were developed to try to address the various orientations of the post and deck framing.
To allow evaluation of assemblies used to resist this deck guardrail force, ICC-ES developed AC273, Acceptance Criteria for Handrails and Guards. AC273 is available for purchase through the ICC bookstore.
Even with the connectors being readily available, deck builders have asked for guard post connection details that do not involve the use of connection hardware. So Simpson Strong-Tie again tested several framing configurations according to the AC273 criteria, using our Strong-Drive® SDWS TIMBER screws and additional blocking to try to prevent the post from rotating. These details are shown in the engineering letter L-F-SDWSGRD15.
That brings us to the update part.
A committee made up of building officials, manufacturers, deck builders, designers and other interested parties is currently developing a set of code proposals on deck construction for inclusion in the 2018 International Residential Code (IRC). Even though more and more deck information has been incorporated into the last few editions of the IRC, there is still insufficient information in the code to be able to completely build a deck prescriptively. One area of interest is this guard connection. There is a desire to develop prescriptive details for both connection of a 4×4 post to deck framing with blocking and fasteners and for connecting the deck band joist back to the deck framing so that pre-manufactured guard rails can simply be fastened to the deck band with the knowledge that the connection is secure.
The problem is that, with the current requirement, the guard must resist the 200-pound load in ANY direction. All current testing, including AC273, only uses testing in the outward direction away from the floor of the deck. If the post were really required to resist a 200 pound load in the inward direction as well, then two hardware connectors would be required, one on each bolt. However, the belief of the committee is that resistance of 200 pounds in the outward and downward direction is primarily what is needed to ensure the safety of the occupants of the deck.
So they are working on a code proposal to change Table R301.5 of the IRC to require that the guard only resist the 200 pounds in the outward and downward direction and reduce the load to 50 pounds in the inward and upward direction.
The committee recognizes that while this is not necessarily a departure from current practice, it is a departure from current loading requirements in the IRC, IBC, and ASCE 7. So representatives of Simpson Strong-Tie met on September 30 with the NCSEA Code Advisory Committee – General Requirements Subcommittee to get the opinions of this group of active structural engineers. They provided valuable input, including the consideration that at some locations near landings and other changes in elevation, resistance to 200 pounds in the inward direction could be important.
Prior to incorporation of NCSEA’s input, the committee thought the code change might look as shown below.
We are interested in getting additional comments on this code proposal. What do you think? Let us know in the comments below.
d) A single concentrated load applied in any direction at any point along the top, in pounds.
f) Guard in-fill components (all those except the handrail), balusters and panel fillers shall be designed to withstand a horizontally applied normal load of 50 pounds on an area equal to 1 square foot. This load need not be assumed to act concurrently with any other live load requirement.
h) Glazing used in handrail assemblies and guards shall be designed with a safety factor of 4. The safety factor shall be applied to each of the concentrated loads applied to the top of the rail, and to the load on the in-fill components. These loads shall be determined independent of one another, and loads are assumed not to occur with any other live load.
j) A single concentrated load applied at any point along the top, in pounds. The 200-pound load is required to be applied in either the outward or downward direction, and it is permitted to be reduced to 50 pounds in either the inward or upward direction. The guard is not required to resist these loads applied concurrently with each other.
They say you never forget your first love. Well, I remember my first earthquake, too. My elementary school had earthquake and fire drills often, but the Livermore Earthquake in January, 1980 was the first time we had to drop and cover during an actual earthquake. The earthquake occurred along the Greenville fault and over 20 years later, I was the project engineer for an event center not far from this fault. I don’t think that earthquake that led me on the path to become a structural engineer. I was only seven and was more focused on basketball and Atari games than future fields of study.
My favorite part about the Livermore Earthquake was the 9-day sleepover we managed to negotiate with my parents. I have a big family, so we had a large, sturdy dinner table. My brother Neil and I convinced my parents it would be better if we slept under the table, in case there was an aftershock. And, of course, we should invite our friends, the Stevensons, to sleepover because they don’t have as large a dinner table to sleep under at their house. And it worked! In our defense, there were a lot of aftershocks and an additional earthquake a few days later.
Each year, an earthquake preparedness event known as the Great ShakeOut Earthquake Drill takes place around the globe. The event provides an opportunity for people in homes, schools, businesses and other organizations to practice what to do during earthquakes.
Simpson Strong-Tie is helping increase awareness about earthquake safety and encouraging our customers to participate in the Great ShakeOut, which takes place next Thursday on October 15. It’s the largest earthquake drill in the world. More than 39 million people around the world have already registered on the site.
Earthquake risk is not just a California issue. According to the USGS, structures in 42 of 50 states are at risk for seismic damage. As many of you know, we have done a considerable amount of earthquake research, and are committed to helping our customers build safer, stronger homes and buildings. We continue to conduct extensive testing at our state-of-the-art Tye Gilb lab in Stockton, California, and next Wednesday, we’ll be performing a multi-story wall shake table test for a group of building officials at our lab. We are also working with the City of San Francisco to offer education and retrofit solutions to address their mandatory soft-story building retrofit ordinance and have created a section on our website to give building owners and engineers information to help them meet the requirements of the ordinance.
Our research is often in conjunction with academia. In 2009, we partnered with Colorado State University to help lead the world’s largest earthquake shake table test in Japan, demonstrating that mid-rise wood-frame buildings can be designed and built to withstand major earthquakes.
Earthquake articles like the one from The New Yorker also remind us how important it is to retrofit homes and buildings and to make sure homes, businesses, families and coworkers are prepared.
Like others in our industry, structural engineers play a role in increasing awareness about earthquake safety. We’d like to hear your thoughts about designing and retrofitting buildings to be earthquake resilient. Let us know in the comments below. And if your office hasn’t signed up for the Great ShakeOut Earthquake Drill, we encourage you to do so by visiting shakeout.org.
Vertical deflection resulting from live and dead loads – of both roof and floor framing components – is an important serviceability consideration in the overall design of the building. And while this could be a blog topic in and of itself, this post is instead going to focus on two other types of truss movements that often prompt questions: seasonal up-and-down movement (of the trusses relative to the walls) and horizontal movement (of scissor trusses).
On the one hand, these are completely different topics. But on the other hand, they both deal with movement; which needs to be properly addressed when incorporating trusses into the overall building. So it’s sensible to discuss them together in one blog post.
Seasonal Up-and-Down Movement
This type of movement goes by many different names that might sound familiar – truss arching, truss uplift, partition separation, or – to use the most formal name – ceiling-floor partition separation. All of these names describe the separation that develops between interior partition walls and ceiling finishes, which can cause gaps in the drywall to open in the winter and close in the summer. This movement is often considered to be a truss issue; however, it is not always the trusses that do the moving, but rather the walls or floors, or both, beneath the trusses.
This issue is also not limited to truss construction, but can also occur with other types of wood construction. The truss industry has information on this topic to help educate the market about the causes of ceiling-floor partition separation, best practices and construction techniques for minimizing the movement, and how to accommodate this movement in the structure to prevent drywall cracking.
For those who are interested in a very thorough and technical discussion of this issue and all of the factors that can contribute to it, there is a Technical Note available from the Truss Plate Institute (TPI) called Ceiling-Floor Partition Separation: What Is It and Why Is It Occurring? Although it was written several years ago (by the Small Homes Council-Building Research Council), the information remains relevant because the problem and its causes are the same now as they were then. The Technical Note discusses the potential causes of ceiling-floor partition separation, which may include one or more of the following: attic moisture (and the differential shrinkage and swelling of truss chords due to seasonal changes in moisture content), foundation settlement, expansive soils, excessive cumulative shrinkage of wood framing members and errors made during the construction process such as pulling the camber out of a truss to attach it to a partition. There is even an Appendix with a brief discussion of longitudinal shrinkage and an example calculation showing how much upward deflection results when a truss arches because of differential shrinkage.
For a condensed version, there is also a document available from the Structural Building Components Association (SBCA) called “Partition Separation Prevention and Solutions (How to Minimize Callbacks Due to Gypsum Cracking at the Wall/Ceiling Interface)”. This single-page document is particularly useful for educating the industry to take the appropriate preventive measures during construction, which help minimize problems later.
For example, the use of slotted roof truss clips – such as our STC (see below) – is one preventive measure, since these clips allow for vertical movement, but still provide lateral support at the top of the wall. DS drywall clips can be used in conjunction with the STC clips to secure the drywall to the wall. Then, to allow the drywall ceiling to “float,” the drywall is not fastened to the bottom chord within 16” from the wall. Taking these steps allows movement between the truss and the wall, without causing cracking in the drywall at the wall/ceiling interface.
It is important to note that, while foundation settlement may indicate a structural problem and can be prevented by proper design, truss arching resulting from the natural shrinking/swelling of wood does not indicate any structural problem and cannot be avoided in the design process.
Horizontal Movement of Scissor Trusses
In the typical design of a scissor truss, a pin-type bearing is used at one end, and a roller-type bearing is used at the other end, which results in some amount of horizontal deflection at the roller bearing.
The bearing assumptions used in the design of a scissor truss are important not only to the truss, but they also have design implications for the building as well. Using a pin-type bearing at both ends of the truss has undoubtedly been a temptation to every truss technician at one time or another, when the same scissor truss that is failing the analysis suddenly works as soon as the bearings are switched from pin-roller to pin-pin. Unfortunately, that isn’t a valid option unless the walls are infinitely stiff (which they typically aren’t), or unless special measures are taken to resist the horizontal thrust that develops at the pinned reactions. In most cases, such measures won’t be taken which means with the exception of some rare cases, scissor trusses must be designed with pin-roller bearings.
The horizontal deflection that results when a scissor truss is designed with a roller bearing on one end prompts further questions and discussion. What happens when a scissor truss is rigidly secured to the walls of the building – how does that horizontal movement happen? How much horizontal movement is too much? Should the scissor truss be attached to the wall with a sliding (roller-like) connection?
First, a scissor truss that is rigidly secured to both walls will still experience horizontal movement due to the flexibility of the building’s construction in most residential and light commercial construction. How much horizontal movement is too much for the building? This is definitely a question that the Building Designer needs to answer based on his/her evaluation of the overall structure. However, there are a couple of resources that can provide some insight.
ANSI/TPI 1 has the following provision:
Per ANSI/TPI 1, a scissor truss can have up to 1.25″ of total horizontal deflection in the absence of stricter limits from the Building Designer. Scissor trusses may even be designed with more than this amount of horizontal deflection, along with a warning that special provisions for lateral movement may be required. It is important for the Building Designer to be aware of the calculated horizontal movement of the scissor truss, as reported on the truss design drawing, to ensure that it is an acceptable amount of horizontal movement for the supporting structure and/or to determine whether special provisions for the lateral movement need to be made.
While 1.25″ of total horizontal deflection may seem like a lot of horizontal movement, these calculated horizontal deflections are considered to be conservative; many Designers agree that the predicted movement from the pin-roller bearing combination is greater than will actually occur in the constructed building. This is based on the fact that the design loads may be overstated and the contribution of the sheathing (and drywall if applicable) to resist the horizontal movement is not taken into account during the analysis of the truss.
The National Building Code of Canada (NBC) references Section 5.4.4 of the 2009 Engineering Guide for Wood Frame Construction, which limits lateral movement at the top of each wall to h/500. This correlates to a total allowable horizontal movement of 3/8″ for 8ˈ walls. However, the Canadian truss design standard (TPIC-2014) permits trusses to have a horizontal deflection (at the roller support) of up to 1″. In this case, since the horizontal deflection of the truss exceeds the allowable horizontal deflection of the wall, a sliding connection needs to be used between the truss and the wall.
There are different opinions on the use of sliding connections, such as the slotted TC24 or TC26 connectors (see below), which allow for horizontal movement of the trusses without pushing out the wall, and also provide uplift resistance. The use of these clips also varies greatly by region. There are many places where these clips are used regularly and successfully. However, some Designers prefer to restrict the truss horizontal deflection and require the use of a positive connection between the scissor truss and the wall plate due to concerns regarding the transfer of lateral loads from the top of wall to the roof diaphragm. When TC connectors are used, they are often used on alternating ends of the trusses so that there is a positive connection along each wall at every other truss. Some Designers feel this approach minimizes the horizontal movement between the truss and the wall after the building is constructed and fully sheathed and braced.
There is not a single correct answer to address horizontal truss movement for every building. The amount of horizontal movement that is acceptable for the structure and whether or not a sliding connection should be used will depend on the building, the loading conditions, the designer’s experience and/or judgment, and, in some cases, the local building jurisdiction. What is more important than the decision to either restrict horizontal deflection or utilize sliding connectors like the TC24/TC26 (both have been successful) is that the bearing assumptions used in the design of the scissor truss are accounted for in the design of the building. The worst-case scenario is when a scissor truss is designed with a pin-pin bearing and installed in a building where absolutely no measures have been taken to supply the needed resistance to the calculated horizontal thrust.
What are your thoughts or experiences with either seasonal up-and-down movement or horizontal movement? Let us know in the comments below!
This week’s post comes from Brad Erickson, who is the Engineering Manager for the Composite Strengthening Systems™ product line at our home office. Brad is a licensed civil and structural engineer in the State of California and has worked in the engineering field for more than 17 years. After graduating from Cal Poly, San Luis Obispo with a B.S. in Architectural Engineering, he worked for Watry Design, Inc. as an Associate Principal before coming to Simpson Strong-Tie. Brad is the Engineering Manager for Composite Strengthening Systems and his experience includes FRP design, masonry and both post-tensioned and conventional concrete design. While not at work, Brad enjoys spending time carting his three kids around to their competitive soccer games and practices.
Have you ever had a concrete or masonry design project where rebar was left out of a pour? Chances are, the answer is yes. Did you wish you could solve this problem by putting rebar on the outside of that element? That’s exactly what Simpson Strong-Tie Composite Strengthening Systems™ (CSS) can do for you and your project. In effect, composites act like external rebar for your concrete or masonry element. Composites can be used in similar configurations to rebar but are applied on the exterior surface of the element being strengthened.
The initial offering in our CSS line is our fiber-reinforced polymer (FRP) product group. An FRP composite is created by taking carbon or glass fabric and saturating it with a two-part epoxy which, when cured, creates the composite. Together, the weight of the fabric and the number of layers in the composite determine how much strength it will add to your concrete or masonry element.
Another form of FRP composite is a precured carbon laminate. The carbon fibers are saturated in the manufacturing facility and are attached to the structure using CSS-EP epoxy paste and filler, an epoxy with a peanut butter–like consistency. We also carry paste profilers (pictured below) that help contractors apply the proper amount of paste to a piece of precured laminate.
Of course, before any strengthening project can succeed, proper surface preparation is of the utmost importance. Without a good bond with the substrate, a composite will not be able to achieve the intended performance. Concrete voids must be repaired, cracks must be injected and sealed, and any deteriorated rebar must be cleaned and coated. Prior to composite placement, the surface of the substrate must be prepared to CSP-3 (concrete surface profile) in accordance with ICRI Guideline No. 310.2. Grinding and blasting are the most common surface-preparation techniques.
The following are just a few applications where composites can be used for concrete and/or masonry retrofits. The orange arrows show the direction of the fibers in the fabric – in other words, the direction in which the composite provides tension reinforcement.
This is a summary of the basics of composites and their installation on strengthening projects. As composites are not yet in the design codes in the United States, the American Concrete Institute has produced 440.2R-08: Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures. This guide has numerous recommendations for using fiber-reinforced polymer systems to strengthen your concrete or masonry construction.
If you have any questions about composites, post a comment below and I’ll be happy to respond.
Modern code-listed adhesive anchors offer high-strength connection solutions for a variety of applications. However, as in all construction projects, good product performance requires proper selection and installation. In this blog post, we will discuss the challenge of installation orientation and an accessory that can help installers more easily make proper adhesive anchor installations—the piston plug adhesive delivery system.
ACI 318-11 Appendix D (Anchoring to Concrete) calculations use a uniform bond stress model to calculate an adhesive anchor’s resistance to bond failure. According to this theory, an adhesive anchor is assumed to transfer applied loads into the concrete base material uniformly along its effective embedment depth, hef. The equation for an anchor’s basic bond strength (expressed in pounds of force) is simply the adhesive formulation’s bond strength per unit area (λ * τcr) multiplied by the idealized cylindrical surface area of the insert that is in contact with the adhesive (π * da * hef):
Nba = λ τcr π da hef (ACI 318-11, Eq. D-22)
Although the model is a simplification of reality, the mathematical expression represents the core assumption that the adhesive is able to transfer stress completely along the entire depth of the anchorage. This is a key requirement in installation: Anchoring adhesives must be installed such that air entrapment and significant voids are prevented.
Downward installations (Figure 1) have historically presented relatively few challenges for adhesive injection in this regard. In such applications, gravity is helpful; the adhesive naturally flows to the bottom of the drilled hole while being dispensed from the cartridge through a static mixing nozzle. The installer maintains the open end of the nozzle below the free surface of the adhesive until the drilled hole is filled to the desired level. For deep holes, extension tubing is affixed to the open end of the nozzle to increase reach. This procedure avoids entrapping air bubbles in the adhesive material.
Installations into horizontal, upwardly inclined or overhead drilled holes (Figure 2) require more care on the part of adhesive anchor installers. Although the installation principle to avoid entrapping air is similar for these orientations, a key difference is that gravity does not help to keep the adhesive towards the “bottom” (deepest point) of the drilled hole. At worst, it can work against the installer when ambient temperatures may cause the adhesive to run out of the hole during injection. These adhesive anchor installations can be more difficult for an untrained installer and can slow the rate of work. This is one of the reasons that ACI 318-11 Section D.9.2.4 requires continuous special inspection of adhesive anchor installations in these three orientations when the application is also intended to resist sustained loads.
To aid the installer, Simpson Strong-Tie offers a piston plug adhesive delivery system (Figure 3). Consisting of pre-packaged flexible tubing, piston plugs and an adhesive retaining cap, this system allows installers to more easily and consistently make high-quality installations while completing their work efficiently. The installation sequence is provided in Figure 4.
The system consists of three components:
- Piston plug – The key component of the system, it is slightly smaller in diameter than the drilled hole. As the adhesive is dispensed into the drilled hole, the piston plug is displaced out of the hole by the advancing volume of the injected adhesive. The displacement creates a more positive feel for the installer to know where the free surface of the adhesive is.
- Flexible tubing – For use with the piston plug to facilitate injection at the deepest point of the drilled hole.
- Adhesive retaining cap – Provided to prevent adhesive material from flowing out of the drilled hole after dispensing and to provide a centering mechanism for the insert. For heavy inserts in overhead conditions, other means must be provided to carry the weight of the insert and prevent it from falling or becoming dislodged from the hole before the adhesive has fully cured.
What do you think about the piston plug adhesive delivery system? Let us know by posting a comment below.