The Tension Between Us!

09.14.18   Tiffany D. Vorhies, NACE CIP-2 | More by this Author

The Tension Between Us!

High strength bolt tension verification is an important part of making sure your structure is safe. Unfortunately, when it comes to the inspection process, this critical step is often overlooked.

Many times, the realization that bolted connections need verification comes well after installation is complete. By then, they are buried in insulation, fireproofing and drywall. Tension verification to ensure your bolts are meeting required specification now goes from a relatively simple review to an expensive, painful process.

It is critical to know what level of tensioning is required for your structure, both for installation and verification. Can it be just snug-tight? Or, is pretensioned (i.e. fully-tensioned) required?

Here are a few things you should consider when starting steel erection with high-strength bolting, and a few ways SME can help keep this train on the tracks.

First, let’s look at what tension in a bolt does. When a nut is tightened, the bolt stretches and applies tension within the bolt. This creates a clamping force on the steel members being connected. The tighter the bolt and nut assembly, the greater the clamping force imparted on the joint. Having the proper amount of force keeping the members together is critical, as connections are designed to transfer forces differently. Some joints are under more stress than others, and some may need to restrict movement.

Example of the relationship between torque, tension and clamping force.

Now, let’s look at snug-tight and pretensioned connections, where we see these connections, and what tension verification is usually required.

Snug-tight connection is defined as tightness in a connection attained by a few impacts of an impact wrench OR the full effort of an ironworker using an ordinary spud wrench. Either method must bring the plies into firm contact.

Snug tight connections are commonly found at:

  • Column bases
  • Non slip-critical connections
  • Connections without significant reversal of load
  • Connections where movement in multiple directions is not being restricted

Pretensioned connections create a minimum level of tension (past the friction load) on bolts through installation.
We commonly find pretensioned connections at:

  • Slip critical connections
  • Moment connections
  • Column and beam splices
  • Connections subject to cyclical loading
  • Other special circumstances/specifications

Multiple bolted connection and conditions on a structure.

There are several ASTM approved methods to achieve pretensioning:

  • Turn-of-Nut
  • Twist-off Tension Control bolts
  • Direct Tension Indicators
  • Calibrated Wrench

These are the methods of achieving pretensioning prescribed by building, bridge, or other applicable codes. Erectors and inspectors cannot deviate from the approved methods or use methods other than those approved.

So how do you verify the appropriate tension is being achieved in the field? SME can test the tension using a tension calibration device such as Skidmore-Wilhelm’s Testing unit. Three bolts from each lot are “installed” in the device using the appropriate pretensioning method being used in the field to verify the tension. This can be performed by SME, or the contractor can perform this step if they have the device, and we can witness/document the testing.

Note: The bolts used in the test absolutely cannot be reused if they are galvanized or ASTM Grade A490. If the bolts are ASTM Grade A325 and not galvanized, approval can be obtained by the engineer of record, but in most cases they should not be reused.

For assistance with your bolt tension verification, contact Christopher M. Buyle, NACE CIP-3, SFSI.

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