Tech Tips - Understanding Seismic Requirements
ESAB offers a new line of Seismic Certified™ filler metals to meet the needs of seismic applications. But are you familiar with the testing requirements for these products and how they came about?
In 1994, an earthquake in Northridge, California, 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 a variety of investigations into problems with welding steel moment-frame connections. In 2000, FEMA issued FEMA 353, Recommended Specifications and Quality Assurance Guidelines for Steel Moment-Frame Construction for Seismic Applications. This document deals with overall structural design, connection design and details, materials, workmanship and inspection.
While the FEMA document was being drafted, the American Welding Society (AWS) also began to evaluate their respective specifications and codes to incorporate the results of the FEMA studies. The document they produced, AWS D1.8 Structural Welding Code-Seismic Supplement, focuses on welding procedures, materials testing and inspection.
The existence of two documents produced some confusion among architects, contracting engineers, welding engineers and welding manufacturers. Which document should you follow? How can you be sure you are correctly meeting the testing requirements and standards to assure that your structures will be safe and secure for many years?
The FEMA document focuses exclusively on moment-resisting connections. The AWS document also addresses other Seismic Load Resisting Systems (SLRS) thus justifying some additional provisions not found in FEMA 353. It is expected that AWS D1.8 will eventually replace FEMA 353 but this transition may take several years. In addition AWS inserted a test into the A5.20:2005 specification to cover the seismic requirements and used an optional “D” designation.
AWS D1.8 is used in conjunction with other AWS documents, such as AWS D1.1 Structural Welding Code. 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 is also critical when working in seismic applications. Additionally, an engineer may use the Contract Documents to customize requirements for a particular project, so two projects governed under D1.8 may have different requirements.
The AWS documents divide the welds in a seismic application into three categories. If the weld is done on a part of the structure that is not considered part of the Seismic Load Resisting System (SLRS), the weld must meet only the requirements of D1.1. If the weld is part of the SLRS, it must meet the requirements of D1.8. In addition, certain welds in the SLRS can be designated by the engineer as demand critical welds that 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 and 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.
One key difference between AWS D1.8 and FEMA 353 is that the AWS document requires that contractors specify the filler metal manufacturer and trade name of the filler metal in the welding procedures, not just the AWS classification of the filler metal to be used.
The requirements for welds used in the SLRS include:
• A minimum Charpy V-notch requirement of 20 ft-lbs (27 J) at 0º F (-18º C)
• All FCAW wires to be used for demand critical welds are required to be capable of depositing weld metal with a maximum diffusible hydrogen content of 16 ml/100 grams of deposited weld metal (H16). Provision is made in the AWS D1.8 spec. for exposure periods over 72 hrs @ 80°F/80%RH. Longer exposure times are permitted providing the supplier can ensure the diffusible hydrogen level will not exceed 16 ml/100g. The code states that electrodes shall be provided in packaging that limits the ability of the electrode to absorb moisture. Once removed from the packaging, the electrode shall be capable of depositing weld metal with diffusible hydrogen content of 16 ml/ 100 grams of deposited weld metal.
• When self-shielded flux cored filler metals are combined with filler metals deposited by other processes, the combination of the two must be checked to ensure that the minimum required Charpy V-notch is obtained. This is known as intermix testing.
• In addition, for demand critical welds, Heat Input Envelope Testing (HIE) is used to evaluate the weld metal mechanical properties at high and low heat input levels at specified preheat (PH) and interpass temperatures (IPT)
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. Charpy V-notch toughness values are typically optimal at an intermediate cooling rate, with lower values found with 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 must be evaluated in tests run at high and low levels of heat input and specified PH and IPT. HIE testing involves welding plates at low heat input (30 kJ/in is suggested) and high heat input (80 kJ/in is suggested) and testing to show that both welds meet minimum strength and toughness properties. In the welding procedures, the contractor may then use any heat input within the qualified heat input envelope.
Filler metal manufacturers must supply certification documents 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. AWS D1.8 does not specify a particular position for welding on these tests. 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 where the SLRS is subjected to service temperatures below +50° F (+10° C) following completion of the structure, AWS D1.8 states that a minimum CVN of 40 ft-lbs (54J) shall be provided at a test temperature not more than 20° F (10°C) above the Lowest Anticipated Service Temperature (LAST). For example, a LAST of -20° F (-30° C) is tested at 0° F (-18° C).
Materials designated as 7018, 7018-X, 7018-C3L, 8018 C3, solid GMAW and FCAW electrodes covered under AWS A5.20 and AWS 5.29 with “D” designators are currently exempt from heat input envelope testing.
Filler metals for demand critical welds must also be tested to ensure lot-to-lot consistency. FEMA 353 requires that Heat Input Envelope 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, Lloyds Register of Shipping, ASME or the Department of Defense provided they have 3 different lot tests performed and tested 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” or the AWS D1.8 seismic supplement or some custom requirements as dictated by the engineer in charge of the project. If you have questions about your filler metal, contact the manufacturer to see what testing has been done and if the product meets the requirements for your project.