T-Bolts for Cast-in Channels: Sizes & Selection Guide

A cast-in anchor channel is only as strong as the bolt that locks into it. On curtain wall and structural steel connections, the T-bolt is the single removable component in an otherwise permanent embed system. Get the thread size wrong and the bolt head won’t seat under the channel lip; get the tooth profile wrong and lateral wind loads can slide the bracket along the channel after installation. This guide covers how T-bolt sizes map to channel profiles, when serrated bolts are mandatory, and which material grade to specify for exterior exposure.

How a T-Bolt Locks Into the Channel

A T-bolt has a flat rectangular head that matches the channel slot. After concrete cures, the bolt is inserted from either end of the channel (or dropped through a pre-poured keyhole opening), rotated 90 degrees, and the nut is tightened. The T-head bears against the underside of the channel lips, transferring tension loads straight into the embedded anchors and surrounding concrete. Per ETA-09/0339 and CEN/TS 1992-4, the connection must pass six failure checks: bolt steel fracture, channel lip bending, anchor weld failure, concrete cone pullout, splitting, and side-face blow-out. The bolt is the easiest component to specify incorrectly because it is ordered as a line item rather than as part of the channel assembly.

Thread Size and Channel Profile Compatibility

Channel profiles are designated by two numbers: height times slot width, in millimetres. Common hot-rolled profiles run from 28/15 through 53/34. Each profile has a maximum bolt thread size it can accept — the T-head must fit under the lip without jamming, and the thread must clear the slot opening.

Channel profile (H×W)	Typical slot width	Max bolt thread	Typical application
28/15	15 mm	M12	Light cladding, MEP hangers
38/17	17 mm	M12	Façade panels, medium brackets
40/22	22 mm	M12 or M16	Standard curtain wall anchors
40/25	25 mm	M16	Heavy curtain wall, structural
50/30	30 mm	M16 or M20	Crane rails, heavy steelwork
53/34	34 mm	M20	Bridges, seismic zones

The most common mismatch on site is ordering M16 bolts for a 38/17 channel. The T-head either will not insert through the 17 mm slot, or it jams halfway and cannot rotate. Conversely, using M12 in a 50/30 channel leaves the bolt severely undersized for the load — the channel itself may be rated for 60 kN tension while an M12 bolt yields at roughly 17 kN. Always cross-reference the channel manufacturer’s ETA datasheet for the approved bolt size and the corresponding design resistance.

Standard vs Serrated T-Bolts

Not all T-bolts are smooth. Two profiles exist in the market, and the choice between them is governed by the direction of load, not by cost.

Standard (plain) T-bolts have a smooth underside. They work for pure tension loads where the bracket bears perpendicular to the concrete surface and there is no lateral sliding force. Interior wall bracing, MEP supports, and sign brackets typically fall into this category.

Serrated (toothed) T-bolts have milled grooves on the underside that mesh with matching serrations on the channel inner lips. This creates a positive mechanical interlock that resists lateral shear movement along the channel length. They are required for any connection subject to wind suction, seismic loading, vibration, or overhead hanging loads. On a curtain wall bracket, wind pressure pushes the bracket sideways against the channel; without serrations, the bracket can creep along the slot over the service life of the building. Hilti designates its serrated line as HBC-T and its plain line as HBC-C; fischer uses the FBC range for plain channel bolts.

As a rule: if the load has any component parallel to the channel slot, specify serrated. This is non-negotiable for exterior façade fixings in seismic zones or high-wind regions.

Material and Corrosion Selection

T-bolts are manufactured in three material grades, and the default choice on domestic projects is often wrong for coastal or pool environments.

Hot-dip galvanized carbon steel (8.8 grade) is the standard for interior and sheltered exterior applications. Zinc coating averages 45 micrometres per EN ISO 1461. It is the lowest-cost option but must not be used where chloride exposure exceeds 0.4 percent by cement weight — that includes coastal locations within 5 km of seawater, bridge decks over salt water, and pool enclosures.

Stainless A2 (304) handles moderate atmospheric exposure and interior garage environments. It is the minimum grade for covered exterior parking structures.

Stainless A4 (316L) is mandatory for coastal, marine, tunnel, and de-icing salt environments. The molybdenum content in 316L prevents pitting corrosion that 304 cannot resist. For a curtain wall on a high-rise within sight of the ocean, specify A4 T-bolts regardless of the channel material — the channel may be hot-rolled carbon steel with HDG coating, but the removable bolt is the component most exposed to weathering and galvanic degradation.

Bolt Length and Installation Torque

T-bolts are stocked in standard lengths of 60, 80, and 100 millimetres. The required length depends on the bracket thickness plus one flat washer, one spring washer, and the nut height. A typical curtain wall bracket (8–10 mm thick) with a standard nut and washers needs an M16×80 bolt. Undersized length leaves no thread engagement past the nut; oversized length adds weight and protrudes beyond the bracket.

Installation torque follows the channel profile and bolt size. Small profiles (28/15, 38/17) with M12 bolts require approximately 40–60 Nm. Standard curtain wall profiles (40/22, 40/25) with M16 bolts require 80–100 Nm. Heavy profiles (50/30, 53/34) with M20 bolts can require up to 150 Nm. Always use a calibrated torque wrench — never impact-wrench a T-bolt. Under-torquing leaves the bracket loose; over-torquing can strip the channel lip or crack the surrounding concrete.

Three Common Specification Mistakes

The most frequent errors we see on shop drawings are: (1) ordering smooth bolts for exterior wind-loaded brackets, which leads to lateral creep; (2) specifying galvanized bolts for a coastal project where 316 is required, resulting in visible rust within two to three years; and (3) mixing bolt lengths on the same bracket when the engineer intended a single torque value. Cross-check the T-bolt spec against the channel ETA datasheet before releasing the purchase order — the channel is certified as a system, and using a non-approved bolt voids the design approval.

For more on how T-bolts compare to post-installed anchors, see our guide on expansion vs chemical anchors. If you are specifying the channel itself, read our overview of cast-in anchor channels for curtain wall, and check our step-by-step notes on how to install cast-in channels before the pour.