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DTI Washers and TC Bolts: Helping Ensure Torque and Tension

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We’ve talked about torque and tension before, and why it’s important to understand that you must achieve the proper tension when installing structural bolts. Fortunately, there are special structural bolting components on the market that can make it easier to ensure proper torque and tension. Tension control (TC) bolts come built with a spline that pops off when the nut is properly torqued. For hex bolts, DTI washers can be just as helpful in displaying the proper tension. Two of the four methods of installing structural bolts cover tension control (twist-off type bolts) and DTI washers, respectively. Let’s look at these two components and how they’re helping workers on the jobsite every day.

Tension Control Bolts

Tension control bolts are pre-assembled fasteners with a rounded head and a splined end that shears off when the nut achieves proper torque. This is a great advantage to using TC bolts, as it provides a quick installation procedure that maintains accuracy. TC bolts are also faster to install for this reason. It is still important to remember that though TC bolts help confirm that you have achieved proper torque, you still need to ensure your bolts are properly tensioned.

Direct Tension Indicators 

Direct tension indicators (DTI) are washers manufactured by Applied Bolting Technology and Turnasure. These washers are used on a hex assembly and are designed to ensure proper tension on bolts. There are two types of DTIs, both resulting in the same outcome. Traditional DTIs have bumps around the washer that flatten once proper tension is achieved. This allows you to have a visual confirmation that tension is achieved.

The other type of DTI, also made by ABT, are DTI Squirters®. When properly tensioned, these washers will squirt out a colored silicone that gives you a visual confirmation that tension has been achieved. An extra advantage to this is that inspectors can easily see the bolts were installed properly, saving time on the jobsite.

Bottom Line

Both tension control bolts and DTI washers can help save you time on the jobsite by easily confirming tension or torque is achieved. Regardless of what you use, remember that bolts are tightened in order to achieve tension – torque is only a by-product of this relationship to reach proper tension. 

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Structural Nut Color: Why Color Doesn’t Equal Coating

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Picture it: you receive your order of hot-dip galvanized structural nuts, open the container, and see that the nuts are blue. There must be a mistake. Aren’t hot-dipped nuts supposed to be green? That’s not always the case. Let’s go over dyes and why the color of the structural nut may not be what you were expecting.

Why are Structural Nuts Dyed?

While there’s no requirement that the lubricant should be dyed, many manufacturers put colored dye into the lubricant to easily distinguish the structural nuts. Because there is no set standard, different manufacturers use different colors for each kind of structural nut. Though you may be used to hot-dip galvanized nuts being green and mechanical galvanized nuts being blue, this can be the opposite for different companies. Some companies will use an entirely different color at their discretion. The colors are not present for the purpose of the customer, but for the manufacturer’s organization. 

Do I Have the Correct Structural Nuts?

Nine times out of ten, yes, you received the correct structural nuts. The best way to tell the difference in a hot-dip galvanized and a mechanical galvanized nut is by feel. Hot-dip galvanized structural nuts will feel grittier, while mechanical galvanized nuts will feel smoother.  That said, if you are still unable to tell the difference, our sales representatives are more than happy to assist you in ensuring you received the correct order.

Bottom Line

The colored dye in the lubricant on the structural nut serves two purposes: it shows that the nut is covered in the lubricant, and it helps the manufacturer keep each type of nut separate and organized. Different manufacturers use different colors to signify types of coatings, so it can get confusing if you are used to one color, and you receive another. You can usually confirm you received the right nuts by feeling the coating.

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Structural Bolt Coatings

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Coating structural bolting assemblies can protect the steel and ensure durability of the bolt, nut, and/or washer. There are a multiple types of structural bolt coatings, and we will touch on a few here, focusing on their characteristics, advantages, and things to keep in mind when choosing a coating. The two most common types of structural bolt coatings for F3125 Grade A325 bolts are hot-dip galvanized and mechanical galvanized. F3125 Grade A490 bolts can be coated in a zinc/aluminum corrosion protective coating or a zinc-rich base coat and aluminum coating. Zinc is used to coat structural bolts, because this metal is more reactive than iron. This ensures that the zinc coating will corrode first, thus protecting the steel fastener underneath it.

Hot-Dip Galvanizing F3125 Grade A325 Fasteners

Hot-dip galvanizing involves submerging a fastener into molten zinc to coat it. This coating is very thick, and there is a strong bond between the zinc and steel. Sometimes, the finished product will look slightly uneven compared to mechanically galvanized parts, because of the thickness of the coating. That said, hot-dip galvanized coating has more zinc per unit volume than mechanical galvanized parts, making it marginally more corrosion resistant. Hot-dip galvanizing is also generally less expensive than other types of coating.

It’s important to note that because of the thicker coating, more friction is caused when installing a hot-dip galvanized bolt than other bolts. Therefore, a rotational capacity (ROCAP) test is required for hot-dip galvanized bolts.

Mechanical Galvanizing F3125 Grade A325 Fasteners

The alternative to hot-dip galvanizing is mechanical galvanizing. This method provides a much cleaner, neater finish. The threads are smoother, and the bolt is easier to install. Mechanical galvanizing is a cold process, where the fasteners are placed in a rotary barrel that allows for a uniform coating. Though mechanical galvanizing produces a cleaner fastener, the protective coating is slightly less corrosion resistant than hot-dip, due to a weaker bond between the steel and the zinc. That said, mechanical galvanizing is a great way to ensure your bolts have a clean finish and are easy to install.

Coatings for F3125 Grade A490 Fasteners

Due to the risk of hydrogen embrittlement, A490 bolts cannot be galvanized like A325 bolts. Instead, a zinc/aluminum corrosion protective coating or a zinc-rich base coat and aluminum coating must be used. Common brands of these coatings include Geomet® and Magni®. The coating is sprayed on or dip spun, and it has a strong corrosion resistance. This coating reduces the risk of hydrogen embrittlement on the bolt.

Bottom Line

No matter what type you use, coating fasteners with a zinc barrier can help protect them from corrosion. While mechanical galvanizing produces a cleaner bolt with smooth threads, hot-dip galvanizing yields a stronger coating. F3125 Grade A490 fasteners cannot be galvanized due to the possibility of hydrogen embrittlement, but there are other great options for zinc-based coating to protect them.

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Who Creates Structural Bolt Specifications and Standards?

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When we talk about structural bolt specification, we drop a lot of acronyms: RCSC, ASTM, ASME, and more. All of these are organizations which provide the standards over structural steel and bolts. It’s important to keep up with these organizations to ensure that all standards are being followed. Here’s an overview of each organization and their responsibilities.

RCSC

The Research Council on Structural Connections (RCSC) is a non-profit organization made up of leading experts across all fields related to structural steel. These fields include engineering, fabrication, bolting, and erection. Many fastener companies, Birmingham Fastener included, have representation in the RCSC. The purpose of the RCSC is to “promote the knowledge of economical and efficient practices relating to structural connections.” They create and publish structural bolt specifications and standards which are used for the installation of high-strength bolts. In our blogs, you will often see us refer to the RCSC’s Specification for Structural Joints Using High-Strength Bolts. They have provided an online version, available here.

ASTM International

ASTM International is an all-encompassing organization that produces over 12,000 standards for many different types of products globally. In relation to structural bolt specification, the ASTM provides the standards for how the bolts are made and what raw materials can be used to create them. Each ASTM standard is reviewed at a minimum of every three years, meaning standards can change. For example, prior to 2015, ASTM A325 and A490 were separate specifications. After a review of the standards, these were replaced by F3125, making A325 and A490 grades under the F3125 specification. ASTM International also provides the standards on testing methods and what tests have to be done on all structural bolts prior to installation. To learn more about ASTM International, visit their website.

AISC

The American Institute of Steel Construction (AISC) is a technical institute and trade association that “serves the structural steel design community and construction industry in the United States.” The AISC provides continuing education in the industry, steel design guides, and the Steel Construction Manual. This manual provides design considerations which can relate to the RCSC. To learn more about the AISC’s resources, visit their website.

ASME

The American Society of Mechanical Engineers (ASME) is a not-for-profit organization that works towards collaboration and skill development for engineering disciplines. They produce codes and standards for many different technical fields, including the fastener field. In regards to fasteners, the ASME provides manufacturers with dimensional standards for all types of bolts, including structural bolts. For more information on the ASME, visit their website.

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What is the Difference in Type 1 vs. Type 3 Structural Steel?

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When we’re talking about structural bolts, particularly ASTM F3125 Grades A325 and A490, there are two types of structural steel: Type 1 and Type 3. On the outside, they look the same, but there’s one key difference: Type 3 structural steel is designed to weather, or rust, as a means of protection. Type 1 bolts can be galvanized to protect the bolts from corrosion. There are advantages to each type of steel in regards to structural bolts. Here, we’ll discuss their make-up, what types of coatings they can have, and their differences.

Type 1 Medium Carbon and Alloy Steel

According to the RCSC, Type 1 covers medium carbon steel for F3125 Grade A325 bolts and alloy steel for F3125 Grade A490 bolts. Type 1 Grade A325 bolts can be mechanically coated or hot-dip galvanized to protect them from corrosion.  Type 1 Grade A490 bolts on the other hand are only approved for F1136 Grade 3 and F2833 Grade 1 coatings per ASTM F3125 and the IFI 144 standard. Unlike Type 3 weathering steel, any rust on Type 1 bolts will not act as a protective coating to the bolt. Though Type 1 A325 bolts can have a protective coating (mechanical or hot-dipped), it is extremely important to ensure proper jobsite storage throughout your project to prevent any corrosion.

Type 3 Weathering Steel

The alternative to Type 1 steel is Type 3 weathering steel. These high strength fasteners have atmospheric corrosion resistance and weathering properties. This means that Type 3 structural bolts are designed to weather and rust over time. Unlike Type 1, their rust acts as a protective barrier or “coating” that seals the bolt.

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Structural Bolt Head Markings

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The ASTM requires structural bolts to be distinctively marked. These bolt head markings include the manufacturer’s mark, the grade of bolt (ex. A325, A490), and markings to classify the type of bolt (Type 1 or Type 3).

In addition to the mandatory markings, certain manufacturers may include additional markings to distinguish them from others. Below are the mandatory bolt head markings using our manufacturer’s mark. To learn more about our structural bolting products, visit our main structural bolting page.

Bolt Head Markings Table

bolt-head-markings

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