How Heat Straightening Works

A technical guide for bridge owners, DOT engineers and facility managers deciding whether a damaged steel member can be repaired in place instead of replaced.

Heat straightening is often described as “heating the steel and bending it back.” That description is wrong in a way that matters. The member is not bent while hot. Heat is applied in a defined pattern to a small area, that area tries to expand, the surrounding cool steel and an applied restraint prevent it from expanding freely, and the confined region upsets plastically. When the steel cools it contracts more than it grew — and the member moves toward its original shape. The movement happens during cooling, not during heating.

Because the correction comes from controlled plastic upset rather than force, the process is governed by three variables an engineer can specify and a crew can hold: temperature, heating pattern, and restraint. Get those right and the member's properties are largely preserved. Get them wrong — particularly temperature — and the damage is permanent and metallurgical.

The Governing Documents

Most specifications, and most people in the industry, still cite the 1998 FHWA manual. It has been superseded.

FHWA-HIF-23-003 (2023) — Manual for Heat Straightening, Heat Curving and Cold Bending of Bridge Components. The current FHWA reference; it states explicitly that it updates FHWA-IF-99-004.
FHWA-IF-99-004 (1998) — Avent & Mukai, Heat-Straightening Repairs of Damaged Steel Bridges: A Technical Guide and Manual of Practice. The document most often referenced; FHWA now flags it as superseded.
AASHTO Guide for Heat-Straightening of Damaged Steel Bridge Members (Avent, 2008) — the source of the temperature, restraint and repair-cycle rules below.
NCHRP Report 604 (2008) — Heat-Straightening Repair of Damaged Steel Bridge Girders: Fatigue and Fracture Performance. Establishes limits based on fatigue and fracture behavior.

If a repair procedure on your project cites only the 1998 manual, that is worth a question in the submittal review.

Temperature Is the Whole Ballgame

Above roughly 1,300 °F, steel begins to cross its lower phase-transition temperature and its molecular structure changes permanently. Every limit in the guidance sits below that line, with margin.

Maximum heating temperatures — AASHTO Guide / FHWA
Steel	Maximum temperature
Non-quenched-and-tempered carbon and HSLA (A36, A588, A709 Gr 36/50/50W)	1,200 °F
A514 and non-HPS A709 Gr 100 / 100W (quenched and tempered)	1,100 °F
A709 Gr 70W (quenched and tempered)	1,050 °F

In Pennsylvania, PennDOT is stricter — and requires approval every time

PennDOT Publication 408, Section 1050.3(c)5 sets a general limit of 1,150 °F — 50 °F below the FHWA figure — and imposes a separate, tighter limit near welds:

Steel	More than 6″ from a weld	Within 6″ of a weld
HPS 70W	1,050 °F	900 °F
HPS 100 / 100W	1,100 °F	950 °F
All other steels	1,150 °F	—

The specification also states that HPS 70W, 100 and HPS 100W may be heat-straightened only under rigidly controlled procedures, with each application subject to approval. On a PennDOT structure, heat straightening is never a field decision.

Other agencies add their own conditions. Caltrans, for example, prohibits live traffic under any girder being heat-straightened and requires work to stop if the member does not behave as predicted. Michigan DOT requires cooling to 250 °F between cycles, 48 hours' notice, and on-site supervision by a pre-approved specialist. Always read the owner's specification before assuming the FHWA limits apply.

Heating Patterns and Restraint

The four working patterns

Vee heat — corrects bending about the strong axis. Applied full-depth, three-quarter-depth or half-depth, typically at a 20°–60° vee angle.
Line heat — corrects bending about the weak axis; the torch is run in a line along the convex side of the damaged plate.
Strip heat — used with vee heats and to remove bulges; width matches the vee at the intersection.
Spot heat — localized dents, bulges, bellies and dishes.

Complex damage is repaired by superimposing patterns, not by inventing new ones.

Restraint: applied first, never increased

Jacking force is applied before heating and is left to relieve itself as the steel contracts. It is never increased while the member is hot — that is hot mechanical straightening, a different and riskier operation.

The AASHTO Guide limits the combined effect of self-weight and applied restraint to a maximum moment of 50% of the member's capacity at ambient temperature, and where residual moments are minimal, to half the plastic moment capacity (½M_p). Research from Purdue recommends a more conservative ceiling near 0.33M_y.

Restraint is what makes the process controllable. It is also where an unqualified crew does the damage.

Classifying the damage

The FHWA method sorts damage into four categories, and real bridge-strike damage is almost always a combination of them:

Category S — bending about the strong (major) axis
Category W — bending about the weak (minor) axis
Category T — torsion or twist about the longitudinal axis
Category L — localized damage: buckles, crimps, web crippling, plate bends

Research supports repair of steel with plastic strains up to roughly 100 times the yield strain. Past that, the data thins out and it becomes an engineering judgment rather than a procedure.

What It Actually Does to the Steel

This is the question an owner's engineer asks, and it deserves a real answer rather than reassurance.

Strength. FHWA reports yield stress, tensile strength and modulus of elasticity as relatively unaffected. The Purdue synthesis measured yield stress generally increasing — about 10% in carbon steel, with a coupon at the apex of a vee heat reading 88% above unheated material — while quenched-and-tempered steels dropped about 6%.
Ductility. This is where the cost shows up. Damaged-and-repaired A36 lost roughly 10–30% of elongation; heated regions in wide-flange beams showed reductions ranging from 15% to 66%.
Toughness. Charpy V-notch testing showed insignificant change in upper-shelf energy absorption before versus after.
Repeat repairs. The guidance limits the same location to two damage-and-repair cycles. Each cycle expands the yield zone and raises cracking potential.

When heat straightening is the wrong answer

A fracture is present — not to be used unless combined with a mechanical or welded repair
The steel has previously seen temperatures above the lower phase-transition point without metallurgical verification
Nicks, gouges or discontinuities in the damage zone have not been ground smooth first — they are crack initiation sites
The location has already been repaired twice
Hot working is proposed — the guidance prohibits it outright

Fracture-critical members warrant additional scrutiny and, on most structures, a more demanding submittal and inspection regime. Confirm the owner's position before proposing a repair on an FCM.

Why Owners Choose Repair Over Replacement

When heat straightening is approved, it removes the two things that usually drive the schedule on a struck bridge: fabricating and shipping a replacement member, and the demolition required to get the old one out. FHWA notes that these repairs typically do not require temporary shoring and can eliminate or greatly curtail the need for traffic detours.

600+Pennsylvania bridges hit by over-height vehicles in the last ten years — PennDOT, July 2025

15,000+Bridge hits per year nationally, per NHTSA data cited in NCHRP Project 08-139 (2025)

$6.7MAverage annual bridge-strike repair cost in Texas over three years — NCHRP 08-139

The same NCHRP study reports Virginia bridge strikes costing approximately $4 million since January 2018.

How HSE Executes the Repair

Assessment first. Damage categorized, strain estimated, and the decision of repair versus replacement made against the governing specification — not against what is convenient to mobilize.
An engineered procedure. Heating patterns, sequence, temperature limits, restraint and jacking forces developed in house by licensed Professional Engineers and submitted for the owner's approval.
Temperature held and documented. Temperature-indicating methods on every heat, with records that support acceptance of the member back into service.
Inspection support. Documentation packaged for the engineer of record and the owner's inspector.
Replacement when straightening is not approved. HSE removes and replaces the member, with replacement steel available from High Steel Structures inside the same corporate family.

HSE is an AISC Certified Erector with Bridge Endorsement, holds PE licensure in ten states, and responds across Pennsylvania, New York, Delaware, Maryland, Virginia and Washington, D.C.

References: FHWA-HIF-23-003, Manual for Heat Straightening, Heat Curving and Cold Bending of Bridge Components (2023) · FHWA-IF-99-004, Avent & Mukai (1998) · AASHTO Guide for Heat-Straightening of Damaged Steel Bridge Members (Avent, 2008) · NCHRP Report 604 (2008) · PennDOT Publication 408, Section 1050.3(c)5 · FHWA-IN/JTRP-2007/3, Lackowski & Varma, Purdue University (2007) · FHWA-RD-01-068 (2001) · Caltrans BCM 60-3.05D (2022) · MDOT Standard Specifications Section 713 · NCHRP Project 08-139, Prevention and Mitigation of Bridge and Tunnel Strikes (2025) · PennDOT news release, July 2025.