CEMP-E
TI 809-26
1 March 2000
(2) Various design codes may be used for fatigue design, and all are based upon the same
principles and research data. Occasional revisions to these provisions and details are made by the
various code organizations, so there may be minor differences between codes. Generally, AISC and
AASHTO specifications are the most current and comprehensive, including bolted details. AWS D1.1
provisions are limited to welded details. AASHTO and AWS currently use S-N curves, and AISC uses
tabular values based upon the S-N curves. All three organizations are currently changing to equation-
based design.
(3) AASHTO and AWS provide fatigue design curves for both redundant and nonredundant
structures. The AWS nonredundant structure fatigue provisions are based upon bridge principles, where
failure of the welded component would result in collapse of the structure, but special provisions for
nonredundant structures are not required. The AASHTO code, however, requires the use of the AWS
D1.5, Section 12, Fracture Control Plan for Nonredundant Structures. As a specification for building
construction, AISC does not address nonredundant applications.
c. Allowable Stress Ranges. Stress ranges at the lower number of cycles, for the better fatigue
categories, are often limited by the static stress applied. Because the number of cycles is usually
established for the application, and often the type of detail needed to make the component or connection
is established, the design must be established to keep the stress range below that permitted. Fatigue
design begins with the sizing of the member and the connection for the maximum applied static load,
then checked for the applied stress range. Adjustments are then made to increase the component or
connection size as needed. Should the size become excessive, other improved details may be
considered. This includes, for some groove details, grinding of the surface and NDT to improve the
fatigue design category. Some joints may be changed from PJP groove or fillet welds to CJP groove
welds. Another alternative is the use of fatigue life enhancement details to improve fatigue life. Fatigue
life enhancement details are not to be used to increase allowable stress ranges.
d. Fatigue Life Enhancement. At the toe of every weld, with the exception of welds made using
Tungsten Inert Gas (TIG) welding with no filler metal, a microscopic slag intrusion line is present. This
line, for fatigue purposes, acts as a small crack. Fatigue life of welded joints, therefore, begins with an
initial crack, and fatigue life is limited to crack propagation. With plain material, there is no pre-existing
crack, so fatigue life is spent in both crack initiation and crack propagation. By applying fatigue life
enhancement techniques, as described in AWS D1.1 Section 8, fatigue life may be extended. The
process of TIG dressing can be used to remelt the weld toe area to a limited depth, melting out and
removing the microscopic slag intrusion line. Burr grinding of the weld toe, to a depth of approximately 1
mm (1/32 in.), may also be used to remove the slag line. Toe peening, in which localized mechanical
compressive stresses are induced into the weld toe area, does not remove the slag line, but induces
residual compressive stresses around the slag line to prevent the introduction of the tensile stresses
necessary for crack propagation. Any of these enhancement processes typically double the fatigue life of
the treated joint. Performing both toe grinding and hammer peening will provide additional benefits,
achieving typically triple the fatigue life of the untreated weld toe. Caution should be used when
extending fatigue life expectations, as other areas of the welded joint may now fail before the weld toe.
Inspection of the weld should be performed prior to implementing fatigue life enhancement techniques,
with any required inspection for surface discontinuities repeated following the work.
5. HIGH SEISMIC APPLICATIONS.
a. Latest Guidance. Recommendations for the design of welded connections in high seismic regions
are undergoing substantial revision as of the date of this document. Users are advised to seek the latest
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