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Effects of fabrication procedures and weld melt-through on fatigue resistance of orthotropic steel deck welds.
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2008-12-01
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Abstract:A common practice for the fabrication of orthotropic bridge deck in the US involves using 80% partial-joint-penetration groove welds (PJP) to join
closed ribs to a deck plate. Avoiding weld melt-through with the thin rib plate may be difficult to achieve in practice because a tight fit may not
always be achievable. When weld melt-through occurs, which is difficult to inspect inside the ribs, it is not clear how the geometric discontinuities
would affect the fatigue resistance. Furthermore, a distortion control plan, which involves heat straightening or even pre-cambering, is also used for
the fabricated orthotropic deck in order to meet the flatness requirement. It is unclear how repeated heating along the PJP weld line would affect the
fatigue resistance.
Six 2-span, full-scale orthotropic steel deck specimens (10 m long by 3 m wide) were fabricated and tested in order to study the effects of both
weld melt-through and distortion control measures on the fatigue resistance of the deck-to-rib PJP welded joint. Three of the specimens were only
heat straightened, and the other three were pre-cambered to minimize the need for subsequent heat straightening. For the two distortion control
schemes one of the three weld conditions [80% PJP weld, 100% PJP weld with evident continuous weld melt-through, and alternating the above
two weld conditions every 1 m] was used for each specimen. Up to 8 million cycles of loading, which simulated the expected maximum stress range
corresponding to axle loads of 3×HS15 with 15% impact, were applied at the mid-length of each span and were out of phase to simulate the effect
of a moving truck. The load level and boundary conditions were modified slightly based on the observed cracks that occurred in the diaphragm
cutouts in the first specimen.
Based on the loading scheme applied and the test results of the remaining five specimens, it was observed that three specimens experienced
cracking at the rib-to-deck PJP welds at seven loaded locations. It was thought initially that weld melt-through which creates geometric
discontinuities at the weld root was the main concern. But only one of the seven cracks initiated from the weld root inside the closed rib, and all the
other six cracks initiated from the weld toe outside the closed rib. Based on the loading pattern applied, therefore, it appears that these welds are
more vulnerable to cracks initiating from the weld toe, not weld root. Of the only one crack that developed at the weld root, the crack initiated from a
location transitioning from 80% PJP weld to 100% PJP weld. This type of geometric discontinuity may be representative of the effect of weld melt-through in actual production of orthotropic steel decks. Two of the five specimens did not experience PJP weld cracks, and were the ones that were
effectively pre-cambered; a third panel was insufficiently pre-cambered and the resulting distortion and heat straightening were the same as
required for the un-cambered panels. Therefore, effective pre-cambering is beneficial to mitigate the crack potential in rib-to-deck PJP welds
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