Anchor bolts is used to secure steel baseplates to concrete pads. The anchor bolts have to be installed correctly in order to prevent the anchor bolts from pull out of the concrete. If anchor bolts are not installed with the correct torque, the anchor bolts may fail when the steel columns is under tension.
Anchor bolts work by either expanding, adhering to the concrete or utilizing friction between the anchor bolt and the concrete pads. Each type of anchor bolt require certain procedures to be performed during installation. Mechanical anchor bolts expand outward against the walls of the drilled hole when the nut is tightened.
How to Install Anchor Bolts Correctly
The friction between the anchor bolt and the walls of the hole resist any attempt to pull the anchor bolt out of the concrete. There are many varieties of mechanical anchor bolts. Some of the most common types is wedge anchors, quick-set strike anchors, and self-tapping concrete screws.
Wedge anchors are typically used in solid concrete slabs with high tension load. Self tapping concrete screws do not expand and are used in situations where edge spalling is a concern. The first step is to apply the correct amount of torque to each anchor bolt.
The amount of torque applied will create the proper amount of tension in the steel rod. Applying too little torque will cause the anchor bolt to spin within the hole when the steel column is under load. Applying too much torque may strip the threads of the bolt or fracture the concrete.
Torque value are typically found on a torque chart that the manufacturer provides. The values are based on the diameter of the anchor bolt. However, each type of concrete has a different strength; thus, it is important to check the manufacturer engineering report to determine if the torque value is appropriate for the strength of the concrete pad.
Proper preparation of the hole where the anchor bolt will be installed is essential for the success of the installation. Any dust or debris within the hole will act as a lubricant that will significantly reduce the grip of the anchor bolt. It is necessary to perform a specific ritual to clean the hole; a stiff nylon brush and compressed air can be used to accomplish this task.
The hole should first be drilled to a depth that is slightly deeper than the embedment depth of the anchor bolt. Following drilling the hole, the nylon brush will brush the hole and the compressed air will blow it. This process will be repeated two more times.
If the hole is not properly cleaned, especially for epoxy anchors, the pull out strength of the anchor bolt will drop by 60 percent. There are specific rules that must be followed regarding the geometry of the installation. The embedment depth is the distance that the anchor bolt extend into the concrete.
The embedment depth should be between four and nine times the diameter of the anchor bolt. The distance from the edge of the concrete pad should be maintained; the anchor bolt should be installed at least six times its diameter from any free edge of the pad. Additionally, the anchor bolts should be spaced at least ten times the diameter of the bolt from any other installed bolt.
Ignoring these rules may lead to the failure of the concrete in a conical shape; the anchor bolt will simply pull out of the concretesupport pad. Cast-in anchor bolts, such as J-bolts or L-bolts are inserted into the wet concrete prior to the pour being allowed to harden. These anchor bolts dont require the application of torque.
For steel rods, it is important to use the correct ASTM F1554 grade for the strength requirement of the installation site. Grade 36 steel rods are used for general welding applications. Grade 55 is used for applications with higher strength requirements.
Grade 105 is used for the heaviest of installations. The other important factor for cast in anchor bolts is using the correct coating for the steel rod. Galvanized or stainless steel coated rods should be used for outdoor installations to prevent rusting.
Plain carbon steel rods should only be used for indoor installations. Anchors that use epoxy have high pull out strength because the epoxy creates a chemical bond between the threads of the rod and the concretesupport pad. Following installation of the rod into the drilled hole the epoxy should be allowed to cure according to the instructions on the epoxy container.
The holes should be warm and dry to allow for proper curing of the epoxy. If the temperature of the hole is too low or the hole is damp the epoxy may take longer to cure or may not be able to reach its capacity for the strength of the anchor bolt. Additionally, the technician should inject the epoxy into the hole from the bottom of the hole with the anchor bolt being twisted into the epoxy.
Finally, the anchor bolt should be allowed to remain in place until the epoxy completes its curing cycle. In order to properly install the anchor bolts and ensure that they achieve the proper amount of torque it is important to use proper torque tools. Click-type torque wrenches are the most common type of tool to use.
These torque wrenches will provide an audible signal when the proper amount of torque is applied to the bolt. The wrench should be pulled smoothly from the handle of the tool and should be returned to zero after use. Digital torque wrenches are used to log the amount of torque that was applied to each anchor bolt for inspection logs.
Additionally, care should be taken to avoid common mistakes during installation; such mistakes would be drilling oversized holes for the anchor bolt, applying too much torque to the bolts, using shallow embedment depths for the anchor bolt and leaving the holes dirty after installation. These mistakes will lead to the failure of the anchor bolt installation.
