Direct Tension Indicators (DTIs) have long served as an established mechanical method for verifying bolt tension in structural steel connections. In recent years, Squirtern DTIs featuring flexible silicone indication channels have undergone rigorous evaluation through independent academic research and comprehensive validation reviewed under Research Council on Structural Connections (RCSC) specifications. For structural engineers, specification writers, and quality assurance managers, empirical load-displacement behavior and controlled field testing offer conclusive proof that flexible silicone indicators deliver dependable tension feedback without compromising mechanical integrity. Structural steel teams can explore fully certified options via the FULLERKREG DTI washer collection or reach out directly to technical specialists at sales@fullerkr.com for full product submittals, testing documentation, and engineering support.
ASTM F959 Compliance of Silicone-Indicated DTIs
A critical consideration for structural engineers is whether the addition of flexible silicone to indication cavities alters the washer’s compliance with standard manufacturing and performance specifications.
Silicone Does Not Violate ASTM F959 Geometrical Requirements
- Full Specification Conformity: FULLERKREG Squirtern DTIs fully qualify as standard ASTM F959 compressible-washer-type direct tension indicators, adhering strictly to required steel chemistry, hardness, protrusion dimensions, and gap limits.
- Preserved Feeler Gauge Inspection: Field inspection remains 100% possible using standard 0.005" (or 0.015") feeler gauges; the inclusion of silicone does not obstruct mechanical gap measurement.
- Supplementary Visual Indication: The elastomeric silicone serves strictly as an auxiliary visual aid to streamline bolt tensioning operations, operating in tandem with rather than replacing the governing mechanical gap criteria established by ASTM and RCSC.
Dual Verification Capability
- Visual Silicone Extrusion: Ironworkers and inspectors can instantly verify tension development from a safe viewing distance as bright silicone squirts from arch openings once target compression is achieved.
- Mechanical Feeler Gauge Inspection: Third-party inspectors retain the ability to perform physical verification with standard feeler gauges in accordance with RCSC Section 8 bolting procedures.
Washer Performance and Load-Displacement Behavior
Extensive laboratory compression testing demonstrates that compressible-washer-type DTIs exhibit a three-phase load-displacement curve when subjected to axial clamping loads.
Zone 1: Overall Washer Flattening
At initial low load levels, initial displacement is dominated by the elimination of slight manufacturing dishing and macro-surface seating against the bearing face. This phase establishes stable, uniform contact across the washer body and effectively zeroes the measuring device before significant protrusion deformation begins.
Zone 2: DTI Bump Compression Range
Throughout Zone 2, protrusion deformation exhibits a highly linear and predictable relationship between applied bolt tension and residual protrusion gap. This uniform plastic yielding of the cold-formed bumps provides a stable window where bolt tension correlates directly with gap reduction, forming the primary engineering basis for reliable installation acceptance criteria.
Zone 3: Complete Bump Compression
Once protrusions approach complete flattening, washer stiffness increases exponentially. This dramatic surge in compressive resistance serves as a natural mechanical safeguard against over-tightening, preventing bolt yielding or thread stripping during field erection.
Rigorous compression data obtained from tests on 7/8" ASTM A325 high-strength structural bolts confirms this characteristic response. As axial clamping force advances through the minimum required tension of 39 kips up to the 45 kips target installation threshold, protrusion gap displacement proceeds in an exceptionally linear, repeatable manner across all evaluated lots.
Pre-Installation Verification Protocol
Field implementation of direct tension indicators requires a rigorous verification protocol prior to production bolting, confirming that bolting crews and tool setups achieve proper clamping loads.
Skidmore Calibrator Training
- Target Load Calibration: Crews calibrate installation tools to a target load of 45 kips, ensuring the assembly meets and exceeds the minimum specified tension of 39 kips for 7/8" A325 bolts.
- Pre-Installation Safety Margin: Calibrating to 45 kips builds in an intentional ~15% safety margin above RCSC minimums to accommodate field friction and tool variation.
- Blind Indication Evaluation: Bolting participants were initially trained on a hydraulic Skidmore-Wilhelm calibrator with the analog dial visible, after which the dial was completely covered; operators then tightened assemblies solely relying on visual silicone squirt, demonstrating exceptional visual accuracy.
Plate Specimen Testing
To evaluate performance in real-world structural geometries, verification testing progressed to multi-bolt plate specimens featuring four 7/8" high-strength bolts per connection. This setup simulated realistic structural joint rigidity, bolt interaction, and progressive clamping effects typical of steel framing connections.
Iron Worker Field Test Results Table
The table below summarizes empirical data collected across 240 structural bolt installations performed by field crews under blind testing conditions:
| Metric | Value | Significance |
|---|---|---|
| Mean Bolt Tension | 45.0 kips | Exactly matched the 45.0 kips pre-installation target calibration load. |
| Standard Deviation (Entire Group) | 5.0 kips | Demonstrates fully acceptable field variability within RCSC guidelines. |
| Average Standard Deviation (Individual Workers) | 3.3 kips | Highlights strong repeatability and consistent operational technique per worker. |
| Extreme Individual Standard Deviations | 2.1 to 5.7 kips | All individual operators remained within acceptable engineering limits even at extremes. |
What the Data Means
- Exceptional Mean Accuracy: Achieving a mean tension of 45.0 kips verifies that visual silicone extrusion reliably indicates target preload without systemic over- or under-tensioning.
- High Repeatability Across 240 Bolts: Tight individual standard deviations confirm that field crews achieve consistent tension across extensive production runs.
- Benchmarked Against Industry Standards: Overall scatter is fully comparable to, and often narrower than, calibrated wrench or turn-of-nut methods tested in Skidmore verification units.
ASTM F606 Certification Testing
Product certification of direct tension indicators relies on standardized test methods defined in ASTM F606. Every production lot undergoes the following sequence:
- Initial Flattening and Zeroing: The washer is placed in a certified test fixture and subjected to a conditioning load to eliminate dishing and establish a baseline zero reference.
- Protrusion Compression: The washer is inverted and compressed until the mean residual protrusion gap reaches exactly 0.015" (or 0.005" for coated assemblies).
- Load Measurement: The compressive force required to achieve this target gap is measured; it must fall within the acceptance range specified by ASTM F959.
- Lot Representative Sampling: A statistically representative sample size from each manufacturing heat and lot is tested to guarantee mechanical compliance.
Note: FULLERKREG Squirtern DTIs follow this identical certification protocol, confirming that silicone visual indication occurs precisely within the mandated ASTM F606 load window.
Preliminary Recommendations from Field Research
- Mandatory Pre-Installation Verification: Bolting crews must conduct routine pre-installation testing with a hydraulic tension calibrator to familiarize workers with tool dynamics and specific silicone extrusion patterns.
- Experience Enhances Consistency: While even novice operators achieve required tensions, brief crew familiarization measurably narrows individual standard deviation and cycle times.
- Release-Stretch Measurement Validation: Ultrasonic bolt measurement and release-stretch verification provide the ultimate benchmark for confirming clamping retention in mission-critical joint designs.
Frequently Asked Questions
Does adding silicone to a DTI washer violate ASTM F959?
No. The elastomeric silicone fills the cavity beneath the protrusion without modifying steel dimensions, metallurgical composition, or gap requirements defined in ASTM F959. The indicator remains fully inspectable via feeler gauges.
What is the load-displacement behavior of a DTI washer?
The load-displacement curve consists of three distinct phases: initial macro-flattening (Zone 1), highly linear protrusion compression (Zone 2), and a steep stiffness increase upon complete bump compression (Zone 3), providing both linear tension verification and over-tightening protection.
How accurate is Squirtern visual indication in the field?
Field research demonstrates outstanding accuracy: blind-tested ironworkers achieved a mean tension of 45.0 kips against a 45.0 kips target, with individual standard deviations averaging just 3.3 kips across 240 production-style bolt installations.
What is the recommended pre-installation verification protocol?
The recommended protocol entails tightening sample assemblies in a Skidmore-Wilhelm calibrator to establish tool shut-off or manual timing, followed by visual-only verification tests to ensure the required tension is reached prior to production erection.
Can individual worker technique affect DTI installation consistency?
While minor differences exist between operators, standard deviations range from 2.1 to 5.7 kips, proving that the visual squirt mechanism reliably guides operators to safe clamping levels regardless of slight variations in technique.
Where can I get Squirtern DTIs with documented research and test data?
You can procure fully certified ASTM F959 direct tension indicators directly from the FULLERKREG DTI washer collection or contact our technical engineering team at sales@fullerkr.com for test reports, MTRs, and submittal packages.
Conclusion & Technical Support
Independent research and empirical validation confirm that silicone-indicated DTIs combine the structural rigor of ASTM F959 with the speed and reliability of real-time visual inspection. By eliminating guesswork and streamlining third-party QA checks, FULLERKREG Squirtern DTIs deliver unmatched confidence to structural steel projects worldwide.
For project quotations, engineering submittals, or factory-direct inquiries, contact our engineering department at sales@fullerkr.com.