DIRECT TENSION INDICATORS (DTI'S)

What Are Direct Tension Indicators (DTI's)?

Direct Tension Indicators (DTIs) are specialized washers used in structural bolting assemblies to visually confirm that proper bolt tension (preload) has been achieved during installation. Commonly used in steel construction and infrastructure projects, DTIs provide an accurate, low-cost method to verify compliance with bolting specifications, such as those defined by the Research Council on Structural Connections (RCSC) and ASTM F959.

How DTI's Work

Each DTI washer contains a series of raised protrusions (bumps) on one face. When the assembly is tightened, these protrusions are compressed under bolt tension. As the bolt is torqued, the protrusions flatten in response to the clamping force. A feeler gauge is then used to check the gap between the protrusions to confirm that the minimum required tension has been achieved.

Some DTIs feature indicator media, such as Squirter® DTIs, which release a colored material when properly compressed, providing immediate visual confirmation without tools.

DTI Standards & Compliance

DTIs are manufactured and tested in accordance with the following standards:

ASTM F959: Standard Specification for Compressible Washer-Type Direct Tension Indicators for Use with Structural Fasteners.

RCSC Specification: Accepts DTIs as a reliable method for bolt tension verification.

AISC 360: Recognizes DTIs as an approved method for installation verification in structural steel connections.

Types of DTI Washers

Direct Tension Indicators come in several configurations to accommodate different installation conditions, inspection preferences, and coating requirements. Below is a breakdown of the most commonly used DTI types:

Standard DTI's

Standard Direct Tension Indicator (DTI) washers are hardened, washers with protrusions on one face that compress in a controlled manner when subjected to a compressive load. These protrusions are designed to flatten and reduce a gap between the washer and the bolted connection, visually indicating the proper tightening of the bolt. DTIs are used to ensure that structural bolts are tightened to the correct tension, providing a visual confirmation of proper clamping force.

Squirter® DTI's

Squirter® DTI washers are specialized washers used in structural steel connections to ensure proper bolt tension. Unlike standard DTIs that require a feeler gauge for inspection, Squirter® DTIs have silicone embedded under protrusions. When the bolt is tightened to the correct tension, the silicone is expelled, providing a visual indication that the bolt is properly tensioned.

DTI Installation Process

1. Positioning - Place the DTI under the bolt head or nut, with the bumps facing the hardened surface.

2. Tightening - Tighten the bolt using standard methods (Turn-of-Nut, calibrated wrench, or TC bolts).

3. Verification - Use a feeler gauge to check the gap on a specified number of protrusions (typically 3–5). Or, for Squirter® DTIs, observe the expulsion of colored silicone.

4. Acceptance Criteria - The joint is accepted when the number of protrusions not accepting the gauge is within specification (e.g., no more than 1 out of 5).

Minimum Bolt Pretension Per AISC/AISC Table 5.2

Bolt Diameter, in.	A325 Bolts	A490 Bolts
1/2	12	15
5/8	19	24
3/4	28	35
7/8	39	49
1	51	64
1 1/8	64	80
1 1/4	81	102
1 3/8	97	121
1 1/2	118	148

Equal to 0.70 times the minimum tensile strength of bolts as specified in ASTM F3125/F3125M for grades A325 and A490 bolts, with UNC threads, rounded to the nearest kip.

Minimum Bolt Pretension Per AISC 360-16, Table J3.1M

Bolt Diameter, in.	A325 Bolts	A490 Bolts
M16	91	114
M20	142	179
M22	176	221
M24	205	257
M27	267	334
M30	326	408
M36	475	595

Equal to 0.70 times the minimum tensile strength of bolts, as specified in ASTM F3125/F3125M for grades A325M and A490M bolts, with UNC threads, rounded to the nearest kN.