How to Choose Anchor Bolts: The Complete Buyer's Guide
When you connect something to concrete, it should hold a lifetime. That's why it is essential to select the right bolts. When anchoring structural and non-structural elements to concrete on a construction job, you need anchor bolts. However, there are many things to consider when choosing the right anchor for the job.
Before starting your anchoring job, you'll need to know more about the base material that you will be drilling into. Some anchors are made for newer concrete, some for cement, while others are made to hold heavy non-structural loads to the concrete.
Choosing the right anchor bolt should start with understanding the base material and the structural or non-structural material that you will be anchoring to the base material. This guide will help you to ask the right questions about the base material and the non-structural material you are anchoring into the base material so that you can do a thorough job.
Types of Anchor Bolts
Anchor bolts can be divided into two main categories: bent-bar anchors and headed anchor bolts. How the two provide stability for the structure is important for understanding which anchor bolt you should use on your construction project.
Headed Anchor Bolts
There are a few different head types for the headed anchor bolt, including the square-head and hex-head threaded bolts. Plate anchors are when a metal plate is welded to each side of the bolt, and these are also considered headed anchor bolts.
Headed anchor bolts are also appropriate for giving a uniformed look to a project. These types of anchor bolts are used when the aesthetic quality of the fixture is needed and when you don't want anchor bolts sticking out at different lengths from the installation, which can be unsightly.
Bent-Bar Anchor Bolts
Bent-bar anchor bolts are threaded steel-rods with hooks on the end that is embedded in the masonry. They come in two main categories, which are either in the shape of a J or an L.
All allowable stress ratings for these bolts are only 20% of the average of 5 tests done on the specimens. This means that many of them are much more reliable than the rating indicated by the producer of the anchor bolt.
Which Anchor Bolts Should be Used?
There are many considerations to make when deciding on the size and type of anchor bolt that you will use to fasten something to the concrete of a structure. However, the two most important aspects of the job to consider are the size of the hole for the fixture fasteners and the length of the bolt.
Length of the Fastener
The simple formula used to determine the length of the male fastener is done by adding the thickness of the material being fastened plus the minimum depth of embedment for the diameter of the anchor bolt, plus the thickness fo the washer and nut.
If it is a female fastened being used, the formula used to determine the length of the fastener is done by adding the number of threads of the anchor, plus the thickness of the material being fastened, plus the thickness of any washers that may be used.
Hole Size in the Fixture Being Fastened
The hole in the fixture may determine the gap that needs to be drilled in the concrete. If this is the case, it is essential to know that the diameter of fasteners will not match the holes of the fixture. This is because the working end of the anchor bolt will not fit through the hole, so you will need to size down to ensure that the entire bolt is able to fit through the opening of the fixture hole. Below is a table of anchor bolt diameters and the fixtures holes that they can fit into.
Anchor Dia. Fixture Hole Dia.
1/4″ 5/16″
5/16″ 3/8″
5/8″ 11/16″
3/4″ 7/8″
7/8″ 1″
1″ 1-1/8″
1-1/4″ 1-3/8″
Choosing the Right Anchor Bolt for Cement or Concrete
Often, the terms cement and concrete are used interchangeably. However, they are two different materials with different compression strengths and reasons for being used. Concrete is the stronger of the two and is used in building skyscrapers, superhighways, and dams. So the next time you see a "cement-truck" remember, its a concrete truck.
In general, cement is used as a mixture in concrete. Concrete is a mixture of paste, which is Portland cement and water, and aggregates or fine and coarse rocks.When it hardens around the aggregates, the paste gets much more reliable. It becomes a rock structure known the world over as concrete. Concrete is made up of up to 8% of air, 7% to 15% of cement, 14% to 21% of water, and 60% to 75% of fine and coarse aggregates.
What are the Pounds per Square Inch of the Concrete
PSI refers to the compression strength of concrete. It's an important marker for understanding how secure your concrete is. The PSI can be calculated for concrete by having the wet mixture poured into a cylinder. The cylinder of concrete is then allowed to dry. The force needed to break the dried cylinder of concrete gives the PSI or compression strength of the concrete.
The compression strength of concrete will tell you how durable the concrete is when it is thoroughly dried. Concrete usually takes up to a month to dry thoroughly. So, if you have concrete that is rated at 3,500 psi compression strength, that is the strength of the concrete after it has dried for a month.
Concrete has various strength types for the different building projects that it could be used for. For example, smaller residential projects can be built with 3,000 psi compression strength concrete. On the other hand, higher strength concrete for large building projects can be 10,000 psi compression strength or more significant.
Additionally, knowing the age of the concrete can provide information on which anchor bolts to use. The longer concrete is left to dry, the harder and more robust it will get. Remember, after a month, concrete is fully cured and very hard. Harder concrete will be more challenging to drill a hole for your anchor and may be harder to fasten an anchor to.
The general rule is that lead fasteners are not suitable for old cement. For example, a lead material anchor bolt like the Hard-To-Find Fastener would not be an ideal choice for more aged concrete because its lead body would not fasten well into the concrete. Lead fasteners should only be used in what is called "green" concrete, or concrete that is less than a month old and thus, not done curing.
How is the Concrete Composed? (Thickness, Rebar, etc.)
Before buying the anchor bolts, it's important that you know at least a few things about the cement that you will be drilling into and anchoring from. Understanding the composition of the concrete can help you to plan out which anchor bolts to use and also how you will drill the holes for the anchors to be set in.
The most important part of the concrete's composition to consider is the thickness and whether it is steel rebar reinforced. Odds are that if you are working on a job for a large structure, you will be dealing with concrete that is reinforced with steel rebar.
You will chew up standard bits trying to drill into the steel rebar. There are individual drill bits that can handle the job of digging through the steel rebar. However, they are expensive and will take a longer time to drill than standard concrete.
Drilling Products for Steel Rebar Reinforced Concrete
Hammering Or Diamond Drills
Many of the drills that you will find on the market tend to be pricey. A steel rebar drill needs to be durable, but it shouldn't break the bank. A good option is the Bosch RC2124 3/4-Inch by 12-Inch SDS Plus Rebar Cutter. It comes in various sizes from 1 inch in diameter to 9/16ths of an inch in diameter.
Scanning Before Drilling
Another option is to use a device that actually scans the concrete for where the rebar is located. This will help you drill the holes around the rebar instead of through it. Scanning before drilling is the best option for anchoring into concrete with rebar.
Is the Concrete Thick Enough for your Anchor Bolts?
The depth at which the anchor bolt needs to safely sit to bond the outer layer to the concrete is called the "minimum embedment". A simple rule to remember is that the smaller the diameter of the bolt, the less the depth of the drilled hole needs to be.
The end that is transferring shear and tension through the system should not be too close to the bottom end of the concrete as this can cause failure and crack. The holding values will be significantly reduced the closer to the bottom of the concrete the anchor is placed because of the unsupported edge that you have created.
Proper Embedded Depths
When anchoring to masonry, the general rule is that the embedded depth should be at least four bolt diameters or 2 inches (51 mm) deep, whichever is greater.
The embedded depth for headed anchor bolts is determined by measuring the surface of the masonry to the bolt head bearing surface.
The embedded depth for bent-bar anchor bolts is determined by measuring the from the masonry surface to the surface bearing weight on the bent end minus the diameter of one bolt.
Forces Acting on the Whole System of Masonry Buildings
The two main types of force that are acting on the masonry structure of a building are shear and tension. Anchor bolts are designed to transfer the shear and tension from the structural parts.
Shear is the force from a load that is projected laterally against something that cannot be moved (like a concrete structure). These forces will do one of two things; either force through the entire system of masonry causing the failure of the system, or the masonry will remain standing.
Tension force is the movement of two things caused by sliding across one another. In other words, it is the force that is attempting to make the masonry and other materials, longer.
Other Factors to Consider When Choosing Anchor Bolts
When choosing the right anchor bolt for the job, you will want to consider some aspects of the things you will be securing to the concrete structure. You want the anchor bolts to be able to hold the strain of the load without sacrificing the stability of the entire structure.
Also, you want to keep in mind how heavy the weight that is being fastened is. Finally, there are different anchor bolts used for loads propped up off of the ground versus those which will be sitting on the ground.
When considering the weight of the load that the anchor bolt must hold, it's important to understand that the bolt is the weakest point in the concrete structural system. With this in mind, you need to determine which diameter of the anchor will be required depending on the weight of the load it is supposed to hold. The strongest and deepest set anchor bolts with the largest diameters should be used for heavy loads fastened to the concrete.
In general, the larger the diameter of the anchor bolt, the more carrying power that it has and more massive a load it can bear. Also, the best holding value occurs in deeply embedded anchors within very hard(compression strength-PSI) concrete. So, a 1/2″ anchor set with a minimum embedment of 3.5 inches will be a stronger hold than a 1/4″ anchor with a minimum embedment of 1″.
Another thing to consider is where the load that the anchor bolts will be holding will be fastened to the concrete. Stronger anchor bolts with greater diameter need to be used if the load is attached overhead on a wall. Smaller diameter less deeply embedded anchor bolts can be used for loads that are fastened with on edge resting on the ground.
Loads Fastened to Concrete Overhead
Loads fastened to concrete overhead are essential to consider when choosing the anchor bolts for the job. These loads are known as heavy load values and have a few more considerations to take place than if the load was simply resting on the ground. The strongest and deepest set anchor bolts with the largest diameters should be used for loads fastened to concrete overhead.
Overhead Loads Consideration #1: Tension
Since the load is being held in place to the concrete, the tension weight is directly downward, putting a strain on the anchor bolts in this way. L-shaped anchors work well in these situations for transferring the strain of the tension from the fixture to the bolt. For projects where the load is being anchored to concrete overhead and especially for heavy loads, tension is of immediate concern.
Overhead Loads Consideration #2: Catastrophic Failure
Masonry's catastrophic failure goes beyond cracking. Even a crack would be a failure that needs to be re-engineered. But catastrophic failure means that the structure is coming apart and could fall onto someone, injuring or even killing them!
Overhead Loads Consideration #3: Shear
When the load is fastened to the wall, the load is at a ninety-degree angle to the anchor. This situation is known as a shear loading situation for the concrete fastener. The values of the shear for any particular job depending on the diameter of the anchor. Shear loading situations are not affected by embedment depth as long as the anchors have placed at their minimum required depth.
Other Considerations for the Load Being Fastened to the Concrete
When deciding on the anchor bolt for the job that you are completing, it's important to take into consideration the environmental effects that the load being fastened to concrete will have to withstand on a consistent basis.
This means considering if the fixture of the load will be in an environment where constant vibrating will happen. Also, is this an environment that needs to change over time, known as a shock load. Finally, is this a concrete fixture that needs to be permanent?
Will the Load Being Fastened to Concrete be in an Environment that Vibrates?
A fixture bonded to concrete by anchor bolts that vibrate can cause added problems that need to be considered before installing those anchors. Fans, conveyor belts, or even objects like signs that may blow in the wind all need to be considered fixtures that will vibrate on a continual basis.
Since a mechanical anchor bolt transfers shear and tension through friction if the friction against the wall of its hole is lost due to continual vibration and deterioration, the holding power may be greatly diminished. Great diameter and deeper embedment can be used to alleviate this issue.
What is Shock Loading?
Any load that is fastened to concrete where there is a shock load will have to be able to withstand changes over a period of time. A common example of a shock load would be a bumper on a dock.
Since mechanical anchors can provide holding values based on the friction of the bolt against the walls of the hole, if the friction is broken, the holding value will diminish. Shock load can slowly loosen the friction due to the shock of use over time.
Finishing Considerations for Anchor Bolts
As you finalize your plan to purchase the correct anchor bolts for your job, there are a few more considerations that you want to consider after the job is done. These considerations include whether or not this is a permanent fixture to the concrete structure if the fixture needs to be adjusted with leveling or shimmying, and whether the job needs to have a "finished" look for aesthetics.
Is the Fixture Permanent?
A vast majority of anchor bolt jobs are made to be permanent. However, sometimes you will need to anchor bolt something to concrete that will be taken down in the future. Examples of this may be things attached to fences.
Leveling or Shimmying of the Fixture
If the fixture needs to be leveled or shimmied, then you must make sure that the anchor bolt goes through with enough treads to allow for up and down movement of the anchor. For a job like this, you will need to use a concrete wedge anchor. One of the best designs for concrete wedge anchors is the CONFAST 1/2″ x 7″ Wedge Anchor Zinc Plated, which comes in a box of 25.
Finished Aesthetic Look
Anchor bolts are designed with the bolt coming through the fixture. The length of the bolt extending through is determined by how many times it is tightened and how the nut was placed in the concrete before the bolt was hammered into place. For a finished look, you will need to use a uniform headed anchor bolt. Flat or round-headed sleeve anchors have smooth heads that protrude from the fixture and look uniform, giving a finished look to the project. You could also use female anchors such as the single and double expansion anchors, machine screw anchor, or the drop-in anchor to achieve the same uniform look.
There are many different characteristics when choosing the right anchor bolt for your project with concrete. You have to think about the type and strength of the concrete, the fixture size and location, the type of environment, and the desired look of the finished project to consider when choosing the right anchor bolt for the job.
The correct sizing and spacing of anchor bolts is critical in any structural application. For this reason, it's imperative to reference the structural drawings and specifications of the building or other structure – if you have them. If not, your best option is to consult with a structural engineer who can produce this for you.
https://www.cement.org/cement-concrete-applications/how-concrete-is-made
https://www.nrmca.org/aboutconcrete/cips/35pr.pdf