TECHNOLOGY OVERVIEW
Selecting filler metals
for seismic requirements
Navigating FEMA and AWS specifications
By Roger Bushey
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?
Comparing the Standards
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.
Weld Types in
Seismic Applications
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
W10 2009 WELDING UPDATE FOR INFRASTRUCTURE
