5.4 Direct Tension Indicator (DTI) Installation

5.4 Direct Tension Indicator (DTI) Installation

Introduction to Compressible-Washer-Type DTIs

Direct Tension Indicators (DTIs), governed by ASTM F959 specifications, offer one of the most reliable and direct methods for confirming that a high-strength bolt has achieved its required minimum pretension. Unlike the calibrated wrench or turn-of-nut methods, which rely on secondary measurements like torque or rotation to imply tension, a DTI provides a physical, load-indicating response. The DTI is a specialized, hardened washer manufactured with small arch-like protrusions pressed into one face. As the bolt assembly is tightened, the tension load compresses these protrusions, flattening them against the adjacent surface. By measuring the remaining gap between the protrusions after installation, an inspector can definitively verify that the minimum required compressive force—and thus the required axial bolt tension—has been applied. This section details the precise installation protocols and the exacting inspection criteria required for DTIs.

Orientation and Washer Placement Requirements

The correct orientation of a DTI is paramount to its function. The fundamental rule is that the protrusions on the DTI must bear against the bolt head or the nut, and they must always face away from the structural steel surface. If the DTI is installed backward, with the protrusions facing the softer structural steel, the steel will yield and deform before the protrusions compress, rendering the indicator entirely useless.

Furthermore, the placement of the DTI relative to the turning element dictates additional requirements for hardened washers. The installation process generates significant rotational friction. If the element being turned by the wrench (usually the nut) bears directly against the DTI protrusions, the abrasive rotation will carve into the protrusions rather than compressing them cleanly. Therefore, the RCSC specifies strict rules regarding the placement of an ASTM F436 hardened washer:

RCSC Section 6.2.4 covers four cases:

The common arrangement is the first: the DTI sits under the stationary bolt head with its protrusions bearing against the underside of the head, and the turned nut rides on its own F436 washer. When the DTI must go under the turned element, the washer between them keeps the rotating part from grinding the protrusions.

Special inspectors must visually verify the assembly stack-up—ensuring correct orientation and washer placement—before authorizing the tensioning sequence.

Snugging Pattern and Preparation

As with all installation methods, the connection must be brought to a snug-tight condition before final tensioning. The sequence remains the standard procedure: starting from the most rigid part of the joint and working systematically outward to the free edges.

During this snugging phase, the DTI protrusions should experience minimal compression. They must remain largely uncompressed to allow the joint plies to be drawn firmly together without exhausting the DTI's capacity. RCSC Section 9.2.4 makes this a firm check: before pretensioning, the appropriate feeler gage must be accepted in at least half of the spaces between the protrusions. If it is accepted in fewer than half, the DTI was crushed during snugging and must be removed and replaced. The commentary also notes that, with the plies in firm contact, partial compression of the protrusions is commonly taken as a sign that the snug-tight condition has been reached.

Pre-Installation Verification for DTIs

RCSC Section 8.2.4 requires pre-installation verification to show that, when the bolt pretension reaches the Table 7.1 value in the tension calibrator, the DTI gap is not less than the job inspection gap in ASTM F959. In other words, the DTI must not indicate full tension before the bolt actually has it. During installation, the installer confirms that the protrusions were not compressed below the job inspection gap during snugging (otherwise the DTI is replaced), then pretensions systematically from the most rigid part of the joint until the gap is less than the job inspection gap. If the gaps fail to close, or when the DTI lot changes, the commentary calls for another verification in the tension calibrator.

Feeler Gauge Inspection Protocol

The inspection of a DTI involves the use of a precise feeler gauge to measure the remaining gap size between the compressed protrusions. The standard involves attempting to insert the feeler gauge into the spaces (or "gaps") between the raised arches around the circumference of the DTI.

The thickness of the feeler gauge required depends on the coating of the DTI being inspected:

• For plain, uncoated DTIs: A 0.015-inch feeler gauge is utilized.
• For coated DTIs (e.g., galvanized or epoxy-coated): A thinner 0.005-inch feeler gauge is used (RCSC 2014 commentary to Section 9.2.4). The coating itself occupies space within the gap, so a thinner gauge is required to accurately measure the underlying metal-to-metal compression.

The inspector attempts to slide the appropriate feeler gauge into each space between the protrusions, pointing the gauge toward the center of the bolt. If the gauge cannot enter the space, that gap is considered a "refusal."

Refusal Criteria and Final Acceptance

After pretensioning, RCSC 2014 Section 9.2.4 requires the inspector to ensure by routine observation that the appropriate feeler gage is refused entry into at least half of the spaces between the protrusions. For a DTI with six spaces, that means at least three refusals. No further evidence of conformity is required.

The later RCSC 2020 edition tightens the final check to refusal in more than half of the spaces (four of six). Answer exam questions from the edition in your Manual.

Full Compression Is Not a Rejection

A DTI whose spaces all refuse the gage simply shows a high pretension. RCSC 2014 states that a pretension greater than the Table 8.1 minimum shall not be cause for rejection, and RCSC 2020 says expressly that feeler gage refusal in all locations is not cause for rejection. The real risk is the opposite situation: protrusions crushed during snug-tightening, before the joint was compacted, which is why the snug-stage check requires replacement. Over-tightening still deserves attention if it strips or cracks a nut, because nut stripping or cracking is cause for rejection.