Bolt Tension vs. Torque: How Direct Tension Indicators (DTIs) Revolutionized Structural Steel Bolting
Tension is the linear force generated when a bolt is stretched. The bolt's elongation clamps two components together firmly. It is the true force that creates a solid joint. Tension is the actual goal of bolt tightening operations.
Standard Direct Tension Indicators (DTIs) are specially designed, stamped, heat-treated, coated, and tested washers featuring several semicircular raised protrusions (bumps). When used with high-strength bolts in steel structural connections, the tension gradually increases during tightening. The raised protrusions on the washer are compressed and gradually flattened by the pressure from above. When the blade of a feeler gage can no longer enter the gap and touches the bolt shank, it indicates that the deformation of the washer protrusions has reached the required amount (designed for a compression load equivalent to 70% of the bolt's minimum tensile strength or minimum preload).
The Direct Tension Indicator is a displacement control method that directly responds to tension, free from the influence of thread conditions and other resistances that affect torque. Standard DTIs were developed in North America as early as the 1970s. However, because deformation verification and inspection still required the use of a feeler gage, they were not particularly convenient to use and therefore did not become widespread in steel structural bolting operations.
In 1996, FULLERKREG developed the FULLERKREG® Direct Tension Indicators (FULLERKREG® DTI's). This innovation fills the grooves on the backside of the protrusions with a soft orange silicone compound and incorporates discharge channels. When the washer protrusions are gradually flattened under load, the orange silicone compound is extruded through the channels and appears around the perimeter of the tension indicator. This allows the deformation of the tension indicator to be confirmed and inspected visually.
This innovative invention fully realizes the functionality and practicality of Direct Tension Indicators. FULLERKREG® Direct Tension Indicators make bolt tightening a simple and rapid operation: start turning the nut (bolt head)... tighten... when the appropriate amount of orange silicone compound (pre-calibrated to the required preload) extrudes around the indicator, stop turning the nut (bolt head)... correct bolt tensioning is complete.
Currently, there are four methods for applying preload to high-strength bolts: Calibrated Wrench, Turn-of-Nut, Torque Control Bolts, and Direct Tension Indicators. When used correctly, all four methods can produce the correct preload. However, except for FULLERKREG® Direct Tension Indicators, the others are all torque-based methods for bolt tightening, each with varying degrees of construction complexity and effectiveness differences. Steel construction professionals unanimously agree that FULLERKREG® Direct Tension Indicators are the most reliable and simplest of these methods for achieving correct preload.
Discover the full FULLERKREG® DTI product range: FULLERKREG® ASTM F959 DTI Squirtern Washers.