May 18, 2009
FEMA, AWS, and AISC have all released documentation andspecification guidance for structural steel construction projects.Determining which to follow, however, can be a source of confusionfor architects, contract engineers, welding engineers, and weldingmanufacturers.
In 1994 an earthquake in Northridge, Calif., caused devastating damage to structures in the Los Angeles area. Determined to avoid this type of devastation in the future, the Federal Emergency Management Agency (FEMA) funded numerous investigations into problems with welding steel moment-frame connections.
In 2000 FEMA issued a document, "Recommended Specifications and Quality Assurance Guidelines for Steel Moment-Frame Construction for Seismic Applications" (FEMA 353), which addresses overall structural design, connection design and details, materials, workmanship, and inspection.
While the FEMA document was being drafted, the American Institute of Steel Construction (AISC) and the American Welding Society (AWS) began to evaluate their own respective specifications and codes to incorporate the results of the FEMA studies. The resulting AISC publication focuses largely on the design of structures intended to resist seismically induced loads. The AWS document, D1.8, "Structural Welding Code-Seismic Supplement," focuses on welding procedures, materials testing, and inspection.
The AWS document was printed in 2005. With lead-times of three to four years not uncommon in the building construction field, much of this specification is just coming into practice today, causing some confusion among architects, contracting engineers, welding engineers, and welding manufacturers.
Which document should be followed? What's the best way to navigate the testing requirements and standards to be certain that the structures built will be safe and secure for many years?
The AWS and FEMA documents provide the key for filler metal selection and testing. While many of the requirements in the AWS standard are very similar to FEMA 353, some of the FEMA provisions have been modified or eliminated in the AWS document.
Also, while the FEMA document focuses exclusively on moment-resisting connections, the AWS document addresses other seismic load-resisting systems (SLRS), thus justifying some additional provisions not found in FEMA 353. In addition, AWS inserted a test into the A5.20:2005 specification to cover the seismic requirements and used an optional D designation.
One key difference between AWS D1.8 and FEMA 353 is that the AWS document requires contractors to specify the filler metal trade name and manufacturer in the welding procedures, not just the AWS classification of the filler metal to be used.
AWS D1.8 is expected to replace FEMA 353 eventually, but this transition could take several years. And while AWS D1.8 is used in conjunction with a number of other documents, such as AWS D1.1, "Structural Welding Code," and might modify parts of these other documents, it does not replace them. All the requirements of D1.1, for example, still apply when D1.8 is specified unless modified by D1.8.
AWS D1.8 also references the AWS filler metal specifications, particularly AWS A5.20:2005, and knowledge of these other documents also is critical when working in seismic applications. Additionally, an engineer may use the contract documents to customize requirements for a particular project, so two projects both governed under D1.8 may have different requirements.
The AWS document divides the welds in a seismic application into three categories:
1. If the weld is performed on a part of the structure that is not considered part of the SLRS, the weld must meet only the requirements of D1.1.
2. If the weld is part of the SLRS, it must meet the requirements of D1.8.
3. If the weld is part of the SLRS and can be designated by the engineer as demand-critical, it must meet even higher standards as defined in AWS D1.8.
Demand-critical welds generally represent a small percentage of the welds made on any structure, but most fabricators and erectors are likely to specify one filler metal for the entire job. They usually use a filler metal that meets the requirements for demand-critical welds rather than take the chance that the wrong filler metal might be used in a demand-critical application.
Welds used in the SLRS have a minimum Charpy V-notch requirement (CVN) of 20 foot-pounds at 0 degrees F. In addition, all flux-cored arc welding (FCAW) wires to be used for demand-critical welds must be able to deposit weld metal with a maximum diffusible hydrogen content of 16 milliliters per 100 grams of deposited weld metal (H16). AWS D1.8 makes provisions for exposure periods over 72 hours at 80 degrees F or 80 percent relative humidity (RH). Longer exposure times are permitted providing the supplier can ensure the diffusible hydrogen level will not exceed 16 ml/100 g.
The code states that electrodes shall be provided in packaging that limits the electrodes' ability to absorb moisture. Once removed from the packaging, the electrode must be able to deposit weld metal with diffusible hydrogen content of 16 ml/100 g of deposited weld metal.
When self-shielded FCAW filler metals are combined with filler metals deposited by other processes, the combination must be checked to ensure that the minimum required CVN is obtained. This is known as intermix testing.Heat Input Envelope Testing
The mechanical properties of deposited weld metal, such as tensile strength, elongation, and CVN toughness, result from a variety of factors, including the cooling rate experienced during the welding cycle. Faster cooling rates generally increase the yield and tensile strength of the weld deposit but decrease the elongation. Slower cooling rates produce lower-strength deposits with greater elongation. CVN toughness values typically are optimal at an intermediate cooling rate, with lower values resulting from significant changes in either direction (increase or decrease).
Heat input, as well as preheat (PH) and interpass temperature (IPT), is a significant determinant of the cooling rate. High heat input levels decrease the cooling rates, and low heat input levels increase the cooling rate. D1.8 requires that the filler metals used for demand-critical welds be evaluated in tests run at high and low levels of heat input and specified PH and IPT.
Heat input envelope (HIE) testing is used to evaluate the weld metal mechanical properties at high and low heat input levels at specified PH and IPT. This test involves welding plates at low heat input (30 kilojoules per inch) and high heat input (80 kJ/in.) and testing to show that both welds meet minimum strength and toughness properties. In the welding procedures, the contractor then can use any heat input within the qualified HIE.
Filler metal manufacturers must supply documents certifying that their filler metals meet the HIE test requirements. As an alternative, the contractor may provide the testing himself or have it done by a third party.
One difference between FEMA 353 and AWS D1.8 is that FEMA requires the HIE testing to be done in the flat position, while AWS D1.8 does not specify a particular position. In addition, the root pass can be made in a single pass rather than the split pass required in FEMA 353.
The AWS A5.20:2005 D classification requires the same HIE testing, but if the electrode is classified as an all-position electrode, the high-heat-input weld must be welded in the vertical-up position. All low-heat-input welds covered under this classification are welded in the flat or 1G position.
For demand-critical welds in applications in which the SLRS is subjected to service temperatures below 50 degrees F following completion of the structure, AWS D1.8 states that a minimum CVN of 40 ft.-lbs. shall be provided at a test temperature not more than 20 degrees F more than the lowest anticipated service temperature (LAST). For example, a LAST of -20 degrees F is tested at 0 degrees F.
Materials designated as 7018, 7018-X, 7018-C3L, and 8018 C3 solid GMAW and FCAW electrodes covered under AWS A5.20 and AWS 5.29 with D designators are currently exempt from HIE testing.
Filler metals for demand-critical welds also must be tested to ensure lot-to-lot consistency. FEMA 353 requires that HIE testing be run on each lot of filler metal unless the engineer grants a waiver.
AWS D1.8 has a similar requirement but exempts from lot testing any filler metal that is produced by a manufacturer audited by ABS, Lloyd's Register of Shipping, ASME, or the Department of Defense, provided the manufacturer has three different lot tests performed and tests at the high and low input as dictated in Annex A of AWS D1.8.
In these cases, after three lots have been successfully tested, only one subsequent lot must be tested within three years. This is trade name- and product size-specific, so filler metal manufacturers must test every diameter of every filler metal brand individually.
Filler metal manufacturers are diligently testing their products to ensure that they meet the requirements of the structural welding code—seismic supplements. In some cases, the welding procedures may require that the materials meet the specifications of FEMA 353, AWS A5.20 D, AWS D1.8 seismic supplement, or some custom requirements as dictated by the engineer in charge of the project.
Filler metal users should contact the manufacturer to see what testing has been done and whether the product meets the requirements for their projects.
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