10.1 Clause 11 Evaluation, Base Metal Identification & Welding Under Load

Clause 11 Evaluation, Base Metal Identification & Welding Under Load

Clause 11 (Strengthening and Repair of Existing Structures) of AWS D1.1/D1.1M:2025 governs the engineering evaluation, base metal identification, procedure qualification, design detailing, workmanship, and quality assurance required when modifying, repairing, or retrofitting existing structural steel buildings and structures. Unlike new fabrication governed by Clauses 4 through 10—where material specifications, joint geometries, and boundary conditions are rigorously controlled and documented from mill to erection—Clause 11 addresses structural systems with unknown chemical compositions, decades of fatigue or cyclic loading, existing structural deformations, environmental corrosion losses, and active in-service stresses. Modifying an existing structure requires a fundamentally different engineering and inspection methodology to ensure structural stability throughout the thermal and mechanical repair cycle.

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1. Scope and Engineer Responsibilities (Clause 11.1 & 11.2)

Clause 11 applies exclusively to structural steel framing erected under prior building codes that must be strengthened to accommodate increased live loads, repaired after physical damage (such as vehicular impact, seismic deformation, or fire exposure), or retrofitted to comply with updated architectural and structural standards.

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|                 AWS D1.1 CLAUSE 11 CORE JURISDICTION                    |
+-------------------------------------------------------------------------+
| Clause 11.1: Scope covers repair, modification, and strengthening of    |
|              existing structural steel components.                      |
| Clause 11.2: General Engineer Responsibilities and structural integrity.|
| Clause 11.3: Base metal identification, chemistry & weldability.        |
| Clause 11.4: Design for strengthening and repair (load paths, stress).  |
| Clause 11.5: Fatigue life enhancement of existing details.              |
| Clause 11.6: Workmanship and technique (surface prep, consumables,      |
|              heat straightening, welding sequence).                     |
| Clause 11.7: Quality (inspection and acceptance of the repair).         |
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The Mandatory Role of the Engineer (Clause 11.2)

Under AWS D1.1 Clause 11.2, the Engineer bears primary statutory and technical responsibility for the execution of any strengthening or repair project. Fabricators, contractors, and welding inspectors cannot independently develop or modify repair procedures without the Engineer's formal written authorization. The Engineer's mandatory responsibilities include:

1. Structural Integrity Evaluation (Clause 11.2): Conducting a comprehensive structural engineering analysis of the entire existing framing system. This includes calculating existing load distributions, identifying secondary stress concentrations, verifying load paths, assessing diaphragm stiffness, and evaluating the stability of framing members.
2. Review of Historical Records (Clause 11.2): Reviewing original design drawings, mill test reports (MTRs), shop fabrication records, erection diagrams, and historical specifications to ascertain the original design criteria, steel grade designations, and fastener types (e.g., hot-driven rivets, ASTM A307 bolts, or early high-strength bolts).
3. Condition Assessment & Damage Survey: Performing on-site visual and non-destructive examinations to quantify corrosion section loss, measure permanent plastic sweep or camber distortions, locate fatigue micro-cracking, and identify previous undocumented field modifications or torch cuts.
4. Determination of Weldability (Clause 11.2): Establishing the chemical weldability of the existing steel through coupon sampling or non-destructive field spectroscopy, and prescribing mandatory preheat, interpass temperature, and consumable requirements.
5. Establishment of Load Restrictions & Shoring (Clause 11.4.1): Specifying whether the structure must be shored, jacked, or temporarily evacuated of live loads during thermal welding operations to prevent plastic collapse.
6. Approval of Strengthening Sequences: Authorizing the exact physical sequence of adding cover plates, stiffeners, or reinforcing trusses to minimize thermal distortion and prevent eccentric loading.

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2. Base Metal Identification & Chemical Evaluation (Clause 11.3)

One of the greatest challenges in structural rehabilitation is identifying the chemical composition, mechanical properties, and metallurgical weldability of legacy base metals. Buildings constructed prior to modern ASTM standards contain steels with widely varying carbon, manganese, phosphorus, sulfur, and nitrogen contents that dictate preheat and consumable selection.

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|                    HISTORICAL STRUCTURAL BASE METALS                    |
+-------------------------------------------------------------------------+
| Era / Designation       | Key Metallurgical Characteristics             |
| :---------------------- | :-------------------------------------------- |
| Pre-1900 Wrought Iron   | Fibrous silica slag stringers; extremely low   |
|                         | carbon; prone to hot tearing and delamination.|
| 1870-1920 Bessemer Steel| High phosphorus (>0.10%) & nitrogen (>0.015%);|
|                         | severe strain-aging & notch embrittlement.    |
| 1900-1960 ASTM A7 / A9  | Open-hearth carbon steel; variable carbon     |
|                         | (up to 0.40% C); requires moderate preheat.   |
| 1960-Present ASTM A36   | Controlled carbon-manganese structural steel; |
| / A572 / A992           | highly weldable; prequalified per Table 5.6.  |
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Field and Laboratory Identification Protocols (Clause 11.3)

When original mill test reports are unavailable, Clause 11.3 mandates metallurgical identification through field non-destructive screening or destructive laboratory testing:

• Spark Testing: An experienced technician presses an abrasive grinding wheel against an inconspicuous section of the existing member. The color, length, carrier lines, and explosion density of the spark stream provide rapid qualitative estimation of carbon content (e.g., carbon steel exhibits bushy, multi-bursting white sparks, whereas low-carbon wrought iron produces long, straight, straw-colored carrier lines with minimal bursting).
• Chemical Spot & Chip Testing: Nitric acid or copper sulfate spot etching identifies gray cast iron (brittle, non-weldable without specialized nickel-alloy procedures) versus malleable iron or structural carbon steel.
• Portable Optical Emission Spectroscopy (OES) & XRF: Modern field-portable OES analyzers vaporize a microscopic spark spot on the cleaned steel surface to measure full chemical composition, including light elements like carbon, phosphorus, and sulfur. Portable X-Ray Fluorescence (pXRF) measures heavier alloying elements (Mn, Cr, Mo, Ni, Cu, V) but cannot quantify carbon directly.
• Destructive Coupon Extraction (Clause 11.3): When field testing indicates high carbon or unlisted chemistries, core samples or rectangular coupons must be removed from low-stress regions of the existing structure (approved by the Engineer). Coupons are subjected to wet chemical analysis, tensile testing, and Charpy V-notch (CVN) impact energy testing.

Carbon Equivalent ($CE$) & Weldability Evaluation

To prevent Hydrogen-Induced Cracking (HIC) or underbead cracking in the Heat-Affected Zone (HAZ) of legacy steels, the Engineer calculates the Carbon Equivalent ($CE$) using standardized empirical equations:

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|                       CARBON EQUIVALENT FORMULAS                        |
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| 1. IIW (International Institute of Welding) Formula:                    |
|    CE_IIW = C + (Mn / 6) + ((Cr + Mo + V) / 5) + ((Ni + Cu) / 15)       |
|                                                                         |
| 2. Ito-Bessyo Cracking Parameter (Low Carbon Steels, C <= 0.18%):       |
|    Pcm = C + (Si / 30) + ((Mn + Cu + Cr) / 20) + (Ni / 60) +            |
|          (Mo / 15) + (V / 10) + 5B                                      |
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Carbon Equivalent Interpretation & Preheat Regimes

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3. Welding on Members Under Stress / Cyclic Load (Clause 11.4)

A defining characteristic of structural retrofitting is performing welding operations on members that are actively supporting live and dead loads. Applying localized welding heat to a loaded structural shape causes complex thermo-mechanical interactions that can degrade load-carrying capacity if not properly engineered.

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|               THERMO-MECHANICAL EFFECTS OF IN-SERVICE WELDING           |
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| 1. Thermal Softening: Steel yield strength (Fy) drops by >50% at        |
|                       temperatures exceeding 800°F [425°C], and drops   |
|                       to near zero at 1200°F [650°C].                   |
| 2. Plastic Upsetting: Constrained hot steel expands against cold fibers,|
|                       undergoing compressive yielding.                  |
| 3. Load Shedding:     Acting stress is forced into adjacent cold cross- |
|                       sectional fibers, increasing their stress state.  |
| 4. Thermal Contraction:Upon cooling, the welded zone contracts,         |
|                       inducing high tensile residual stresses.          |
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Maximum Allowable Stress During In-Service Welding (Clause 11.4.1)

Clause 11.4 mandates that members shall not be stressed beyond their allowable yield limit during the thermal welding cycle. Because localized heating temporarily removes the heated zone from the structural cross section, the remaining unheated portion of the member must safely carry the entire acting load.

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|                 IN-SERVICE STRESS THRESHOLDS & SHORING                  |
+-------------------------------------------------------------------------+
| Acting Operating Stress (f_existing) | Mandatory Engineering Action     |
| :----------------------------------- | :------------------------------- |
| f_existing <= 0.55 Fy                | Welding permitted under live load|
| (Standard Design Stress Level)       | with heat input controls.        |
| 0.55 Fy < f_existing <= 0.75 Fy      | Live load reduction, restricted  |
|                                      | bead width, or staggered passes. |
| f_existing > 0.75 Fy                 | MANDATORY TEMPORARY SHORING /    |
| (High Stress / Overload Condition)   | JACKING to relieve dead/live load|
+-------------------------------------------------------------------------+

Under standard AISC and AWS D1.1 engineering guidelines, if the existing dead plus live load stress ($f_{existing}$) exceeds $0.55 F_y$ (55% of specified minimum yield strength), temporary shoring, hydraulic jacking, or live load removal is mandatory. If welding is performed on a member stressed near yield, the temporary loss of cross-sectional area in the fusion zone can trigger catastrophic local flange buckling, web crippling, or lateral-torsional instability.

Safe Heat Input & Temperature Monitoring

To minimize the depth and width of the thermally degraded zone during in-service welding, Clause 11 requires strict heat input management:

$\text{Heat Input } (H) = \frac{60 \cdot E \cdot I}{1000 \cdot v} \quad \left[\text{kJ/in. or kJ/mm}\right]$

Where $E$ is arc voltage (volts), $I$ is welding current (amperes), and $v$ is travel speed (in./min or mm/min).

1. Electrode Size Limitations: Welds on loaded members shall be deposited using small-diameter electrodes (e.g., $3/32\text{ in.}$ [2.4 mm] or $1/8\text{ in.}$ [3.2 mm] SMAW electrodes) to maintain low heat input.
2. Stringer Beads vs. Weave Beads: Welders must deposit narrow stringer beads rather than wide weave passes. Weaving concentrates excessive thermal energy in a single location, widening the plastic softening zone.
3. Interpass Temperature Control: Continuous temperature monitoring with calibrated contact pyrometers or temperature-indicating crayons is mandatory. The interpass temperature must not exceed the maximum limit established by the Engineer (typically $\le 450^\circ\text{F}$ [$230^\circ\text{C}$]) to prevent global heating of the structural member.

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Summary Matrix: Clause 11 Evaluation & Load Protocols

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Worked Engineering Example

Problem Statement

An engineering team is retrofitting a 1928 historic department store with steel framing. A main floor girder (built-up riveted plate girder with $F_y = 33\text{ ksi}$ [230 MPa]) is being strengthened by welding $1/2\text{ in.} \times 10\text{ in.}$ ASTM A36 cover plates to the bottom tension flange to support a new library archive. Mill test reports do not exist. Optical Emission Spectroscopy (OES) field analysis of the girder flange reveals the following chemical composition:

• $\text{Carbon (C)} = 0.28%$
• $\text{Manganese (Mn)} = 0.72%$
• $\text{Chromium (Cr)} = 0.15%$
• $\text{Molybdenum (Mo)} = 0.04%$
• $\text{Vanadium (V)} = 0.01%$
• $\text{Nickel (Ni)} = 0.18%$
• $\text{Copper (Cu)} = 0.22%$
• $\text{Silicon (Si)} = 0.05%$

Structural analysis indicates that under existing dead load plus restricted live load during construction, the acting bending stress in the bottom flange is $f_{existing} = 21.5\text{ ksi}$ [148 MPa].

1. Calculate the IIW Carbon Equivalent ($CE_{IIW}$) of the existing girder steel.
2. Determine the weldability classification and mandatory preheat/consumable requirements.
3. Evaluate whether in-service welding is permissible without temporary shoring under Clause 11.4.

Step-by-Step Engineering Resolution

1. Calculate Carbon Equivalent ($CE_{IIW}$): $CE_{IIW} = \text{C} + \frac{\text{Mn}}{6} + \frac{\text{Cr} + \text{Mo} + \text{V}}{5} + \frac{\text{Ni} + \text{Cu}}{15}$ $CE_{IIW} = 0.28 + \frac{0.72}{6} + \frac{0.15 + 0.04 + 0.01}{5} + \frac{0.18 + 0.22}{15}$ $CE_{IIW} = 0.28 + 0.120 + \frac{0.20}{5} + \frac{0.40}{15}$ $CE_{IIW} = 0.28 + 0.120 + 0.040 + 0.0267 = 0.4667 \approx 0.47\%$

2. Weldability & Preheat Determination:

   • Because $CE_{IIW} = 0.47% > 0.45%$, the steel is classified as High Crack Sensitivity (Category C).
   • Mandatory Actions:
     • Consumables must be low-hydrogen SMAW electrodes meeting E7018-H4R (less than $4\text{ mL}/100\text{ g}$ diffusible hydrogen) or gas-shielded FCAW with H4 designation.
     • Minimum preheat and interpass temperature must be elevated to $300^\circ\text{F}$ [$150^\circ\text{C}$].
     • Slow cooling under thermal insulating blankets is required to allow hydrogen effusion from the HAZ.

3. In-Service Stress & Shoring Evaluation (Clause 11.4.1):

   • The specified yield strength is $F_y = 33\text{ ksi}$.
   • The maximum allowable stress for in-service welding without shoring is: $f_{allowable} = 0.55 \cdot F_y = 0.55 \cdot 33\text{ ksi} = 18.15\text{ ksi}$
   • The acting operating stress is $f_{existing} = 21.5\text{ ksi}$.
   • Since $f_{existing} (21.5\text{ ksi}) > f_{allowable} (18.15\text{ ksi})$, the girder CANNOT be welded in its current loaded state.
   • Mandatory Action: The contractor must install hydraulic shoring posts beneath the girder to relieve dead and live loads until the acting flange stress is reduced below $18.15\text{ ksi}$ (or preferably zero load) prior to striking an arc.