Scope
ASTM F959 / F959M covers compressible washer-type direct tension indicators for use with fasteners meeting F3125 and related structural bolting standards. Commonly called DTIs (direct tension indicators), and colloquially "squirters" for the visible orange sealant some versions emit when compressed.
DTIs are one of four RCSC-approved methods for pretensioning structural bolts, alongside turn-of-nut, calibrated wrench, and twist-off (F1852 / F2280) methods. DTIs offer the advantage of per-bolt visual verification — every installed bolt carries a measurable gap indicating its actual installed tension.
How DTIs work
A DTI is a flat washer with small raised bumps (protrusions) on one face, arranged around the ID. At installation:
- The DTI is placed under the bolt head (or under the turned element, depending on assembly arrangement)
- As the bolt is tightened, clamping force flattens the bumps
- A feeler gauge of specified thickness is inserted between the washer and the mating surface
- When the gap is smaller than the feeler gauge thickness, the bolt has reached the specified minimum pretension
Typical feeler gauge requirements are 0.015" for uncoated DTIs and 0.005" for coated or weathering variants. The number of refusals (readings where the feeler won't enter) determines whether the bolt is properly pretensioned per project specification.
Types
F959 covers DTIs for the full range of F3125 bolt grades:
| Type | Intended use |
|---|---|
| 325 | For use with F3125 A325 heavy-hex structural bolts |
| 490 | For use with F3125 A490 heavy-hex structural bolts |
DTIs must match the bolt grade because the compression force required to flatten the bumps is calibrated to the minimum pretension of that grade. A "325" DTI under an A490 bolt will flatten too easily and provide no useful indication; an "490" DTI under an A325 bolt may never fully compress.
Materials and coatings
- Plain (uncoated) — The default DTI for Type 1 bolted assemblies
- Mechanically galvanized per B695 — The typical coating when used with galvanized A325
- Weathering — Atmospheric-corrosion-resistant for use with A325 Type 3 (weathering-steel) bolts
- Epoxy-coated — Some manufacturers offer epoxy coatings for specific project requirements
Hot-dip galvanized DTIs are not standard because HDG coating thickness and adhesion can interfere with the calibrated flattening behavior. Mechanical galvanizing is the preferred coating for DTIs used with HDG A325 bolts.
Squirter variant
A variation on the standard DTI embeds a bright-orange silicone sealant in cavities on the washer face. As the bumps flatten under bolt tension, the sealant is squeezed out through small channels in the washer, giving a visible, unambiguous indication of compression. Popular on bridge and public infrastructure work where visual third-party inspection is preferred over hand-held feeler-gauge inspection on every bolt.
Placement in the assembly
RCSC / AISC rules for DTI placement:
- Place the DTI under the unturned element — typically under the bolt head, if the nut is being turned
- Place a standard F436 hardened washer between the DTI and the turned element (the nut), so the turned element rotates against a smooth hardened surface
- The feeler gauge is inserted into the gap between the DTI bumps and the mating structural steel, not into a gap between the DTI and the washer
For special hole configurations (slotted or oversized holes), additional plate washers or different arrangements may be specified.
Common installation errors
- DTI installed with bumps facing the wrong way. Bumps must face the smooth structural steel — not the bolt head. Reversed installation defeats the indication.
- DTI used with the wrong bolt grade. A325 DTI under A490 (or vice versa) gives false readings.
- DTI replaced by a standard F436 washer. Obvious but does happen in the field; inspection should verify DTI presence on every bolt.
- Feeler gauge used inconsistently. The inspector must test all the bumps, not just one, and know the correct feeler thickness for the DTI type.
- HDG DTI used in lieu of mechanically galvanized. Can give false readings due to coating thickness variation.
Applications
- Steel bridge fabrication (AASHTO and state DOT structural connections)
- Slip-critical connections in high-rise and infrastructure construction
- Stadium and arena steel framing
- Wind turbine tower connections
- Any F3125 structural bolted connection where per-bolt pretension verification is required
Recommended pairing
| Bolt | DTI | Companion washer | Nut |
|---|---|---|---|
| F3125 A325 Type 1 | F959 Type 325 | F436 Type 1 | A563 DH or A194 2H |
| F3125 A325 Type 3 | F959 Type 325 weathering | F436 Type 3 | A563 DH3 |
| F3125 A490 Type 1 | F959 Type 490 | F436 Type 1 | A563 DH or A194 2H |
| F3125 A490 Type 3 | F959 Type 490 weathering | F436 Type 3 | A563 DH3 |
Related specifications
- F3125 — Structural bolts (DTIs exist to pretension these)
- F436 — Hardened washer (installed together with DTI)
- A563 / A194 — Matching nuts
- RCSC Specification — Installation and inspection methods for pretensioned bolts (DTI rules are here)
- B695 — Mechanical galvanizing (the coating method compatible with DTIs)
Documentation
California Fastener F959 orders ship with mill certificates showing type (325 or 490), coating, dimensions, and calibration/compression test results verifying the DTI flattens at the correct clamping load. Feeler-gauge inspection procedures and required gap values for the product supplied are provided on request.