Structural Thermal Break Detailing for Steel Connections

A structural thermal break is a load-bearing insulating layer, usually a pad, installed between two steel parts at a connection where steel crosses or ties through the building envelope. It interrupts the continuous steel path that would otherwise carry heat past the insulation, while the connection still transfers the forces the EOR designed it for.

Steel conducts heat far more readily than insulation, so an uninterrupted canopy beam or roof post becomes a thermal bridge: heat bypasses the insulation, nearby interior surfaces run colder, and condensation risk rises. Recent editions of ASHRAE Standard 90.1 and the International Energy Conservation Code (IECC) address thermal bridging, and project specifications increasingly call for structural thermal breaks at canopies, balconies, parapets and envelope-penetrating steel.

Whether a connection gets a break, and which product and thickness, is the design team's decision, not the detailer's. The reference project below used Armatherm 500 series material. Other manufacturers make thermal break products in different materials; whichever is specified, the manufacturer's published data and the EOR's details govern.

Thermal breaks appear wherever exterior steel connects to structure inside the insulated envelope: canopies, balconies, parapet posts and braces, roof posts and equipment dunnage, slab-edge or wall-penetrating steel, and secondary frames bolted to primary framing. They can be scattered across a package, so the first detailing task is finding every one.

Read the architectural wall sections and structural details together. A general note such as "provide thermal breaks at exterior steel connections" with no detail for a given condition calls for an RFI, not a license to invent a detail. Canopies and platforms fall within miscellaneous steel detailing; the thermal break work concentrates at their connections.

A thermal break adds a non-steel layer to the grip. Bolt lengths are recalculated, thread position is rechecked, pad holes are detailed separately from steel holes, and bushings and washers are called out. The EOR's joint type still governs, but a standard slip-critical detail cannot be assumed to work through a pad.

Bolt length and grip stack-up

The pad, and any bushing flange in the stack, adds to the grip; each specified washer adds on top. A bolt length copied from the same connection without a pad will be short. Hypothetical example: two 1/2" plies make a 1" grip, a 1" pad between them makes it 2" before washers, and the bolt length follows from that stack plus the nut and stick-through allowance for the diameter, rounded up to an available length.

For threads-excluded (X) bolts, recheck thread position at the new length, since the pad creates two interfaces where there was one. For threads-included bolts such as the A325N bolts on the reference project, confirm the bolt end still reaches at least flush with the outer face of the nut. Bolt shear and bending across the pad thickness are for the EOR to address, not the detailer.

Bushings and washers

Show every bushing and washer the EOR or manufacturer specifies, call it out, count it and add its thickness to the stack-up. Where the manufacturer's washer arrangement differs from what the RCSC Specification requires for the bolt grade, hole type and installation method, raise an RFI.

Holes, tolerance and alignment

Steel holes follow AISC 360 Table J3.3, which gives 13/16" as the standard hole for a 3/4" dia. bolt. Pad holes may differ, for example to accept a bushing, so detail them on the pad, dimensioned from the same edges and gage lines as the mating steel; misalignment accumulates across three plies. Do not switch to oversized or slotted holes for fit-up without EOR approval, because AISC 360 and the RCSC Specification limit those holes by joint type and load direction.

Bearing, slip and joint type

The EOR sets the RCSC joint type: snug-tightened, pretensioned or slip-critical. A slip-critical joint relies on friction between prepared steel faying surfaces, and a pad means those surfaces no longer touch, so a standard slip-critical note is never carried over by default. How the pad affects pretension and slip is covered by the manufacturer's published data and the EOR's design.

Dimensions and work points

A 1" pad moves the supported steel 1" off its support. If the design dimensions do not include it, member lengths and work points shift, and that goes to an RFI rather than being absorbed quietly in the model.

Shop welds at the break are completed before the pad is fitted, keeping the pad out of the weld zone. The shop also needs to know who supplies and installs the pad, which parts ship loose, how the longer bolts are listed and how the steel surfaces in contact with the pad are finished.

• Weld plates, clips and stiffeners at the break in the shop, follow the manufacturer's installation requirements on heat, and flag any field weld shown near a pad by RFI. Welds follow AWS D1.1 and are shown with AWS A2.4 symbols.
• Check that the longer bolts can still be inserted and tightened near a flange, web or HSS wall.
• Pads the fabricator supplies ship loose under their own mark; pads furnished by others are noted as such.
• Contact-surface finish follows the specification and manufacturer. The reference project called for SSPC-SP2 cleaning and shop primer at 1.5 mils DFT. Put the requirement on the assembly detail, not only in general notes.

Almost every call-out at the connection changes. A standard bolted connection clamps steel to steel under shop-standard bolt lengths, holes and faying notes; a thermally broken one adds a marked pad, longer bolts, specified hardware, an interface finish note and member dimensions adjusted for the pad.

The wider rules for showing bolts, welds and plates to the EOR's design are covered under connection detailing.

The pad gets its own piece mark and part detail with material, thickness, plan dimensions and a dimensioned hole pattern, plus a ship mark when it ships loose. Bolts are called out by quantity, diameter, grade and length, bushings and washers are noted at each bolt group, and each appears as its own report line.

• Assembly detail: the pad in section at full thickness with a leader to its mark; a bolt call-out such as (4) 3/4" dia. A325N bolts with the calculated length; hardware and finish notes; a shop or field bolt designation.
• Pad detail: material as the specification names it; thickness, width and length; hole diameter, gage, pitch and edge distances; quantity.
• Reports: bill of material lines for pads and any fabricator-supplied hardware; field bolt list lines for bolts through the break, kept apart from same-diameter standard bolts so the erector pulls the right length.

The team runs SDS/2 and no other detailing platform, so drawings, bolt lists and DSTV NC files come from one model revision. The rest of each sheet is covered under steel shop drawings and erection drawings.

The detailer needs the pad material, thickness, plan size and hole pattern; the bolt diameter, grade and joint type; the bushing and washer specification; who supplies the pad; interface finish; the manufacturer's installation requirements; the connection details or reactions; and a named route for resolving conflicts. Missing items become RFIs rather than assumptions.

When the information is not there yet

Vertex drafts the RFI and your team submits it through its contractual channel to the EOR or architect; connections that depend on it are held or clearly flagged. The team works on India Standard Time, so a question list sent at 5:00 p.m. EDT arrives at 2:30 a.m. IST, is worked during the US night and returns for next-morning review. Same-hour answers during the US afternoon are limited.

The reference project is an airport terminal rehabilitation and addition in New England, detailed for an AISC-certified fabricator, with structural thermal breaks at its frame-to-structure connections.

Six days separated the approval issue from the fabrication issue. The airport terminal thermal break framing case study covers the scope in full.

The SDS/2 model gets a clash review and a review against the design drawings before drawings are extracted. Every drawing is then checked by a second person who did not draw it, against the design drawings, specifications, RFI responses and shop standards. The detailer corrects, the checker back-checks the corrections, and only then is the package released.

At a break, the items worth checking line by line are the pad thickness in the model, bolt stack-ups, pad holes, hardware counts and field bolt list lines. Under AISC 303 the approval issue goes to the owner's designated representative for design; comments are then clouded and re-checked, and NC files and reports are regenerated from the same model revision for the fabrication issue. The full sequence is under how drawings are checked.

Vertex publishes an hourly rate of $15–20 USD, varying with project complexity. Thermal break connections move a package toward the upper end because they add connection complexity: separately detailed pads, recalculated bolt lengths, hardware call-outs, sequencing and finish notes, and RFIs wherever design information is incomplete.

Repetitive framing with one recurring break condition, complete documents that name the pad product, established shop standards and a reasonable schedule pull toward $15. Many unique break connections, skewed or moment framing at the break, design changes during detailing and compressed schedules pull toward $20. The work comes from a team that runs SDS/2 daily, and the lead detailer has 8 years of SDS/2 experience. See steel detailing rates and detailing support for steel fabricators.