On the low-cycle fatigue capacity of unanchored steel liquid storage tank shell-to-base connections

Abstract

Under strong earthquake motions, the liquid stored within large unanchored steel tanks can cause rocking and uplift of the tank base from the supporting foundation. Repeated rocking can lead to low-cycle fatigue fractures in the tank base-plate, and ultimately spillage of tank contents. Due to limited research on tank connection capacity, current European and New Zealand tank standards (EuroCode8 part 4 and NZSEE) impose unjustified rotation limits on tank shell-to-base connections (current limit being 0.2 rad). These rotation limits often govern the design of new tanks, and many existing tanks require retrofit to meet compliance. This paper experimentally investigates the fatigue capacity of tank shell-to-base connections, with a focus on the effects from material ductility and applied strain range (rotation range). Twenty-seven tank connections representing two steel material grades are fatigue tested under constant range rotation cycles to generate fatigue-life curves. Tensile loads applied to the base-plate simulate membrane action present in tanks during uplift. Results indicate that the strain capacity of the base-plate base material strongly influences connection fatigue capacity as fatigue fractures originate in the base material away from the weld toe and weld heat affected zone. Increasing base-plate ductility drastically increases connection fatigue life. All connection specimens sustained multiple uplift cycles at rotations greater than the existing EuroCode8 and NZSEE limits.

Effect of radial base-plate welds on ULCF capacity of unanchored tank connections

“…Experimental testing of tank shell-to-base connections having only circumferential shell-to-base welds indicate a significant increase in fatigue capacity with increased base-plate ductility [8], and significantly higher rotation capacity than the current EC8-4 limit. Because baseplate ductility has such a strong influence on fatigue performance, potential changes due to the presence of radial welds could have significant effect on fatigue capacity.…”

“…Note that the tank connection specimen is rotated 90°such that the base-plate is parallel with the vertical actuator. To help transfer the base-plate tensile load into the strong floor and prevent plastic hinge formation in the tank shell, a portion of base-plate extending beyond the tank shell is welded to the self-leveling frame (similar to the testing by [7,8]). …”

“…Due to this 150 mm stroke limitation and a chosen peak base-plate rotation range of 0.4 rad, the specimen base-plate length had to change from the previous geometries used in [7,8]. This test modification is described in detail in the next section.…”

“…1 shows a typical tank assembly, with the tank base having multiple pentagonal sections near the tank edge, forming a ring for connection of the tank shell. Connections having only circumferential welds have been fatigue tested in two recent studies [7,8]; however, connections with both radial and circumferential welds have not been tested.…”

“…The performance of tank connections containing radial welds relative to the existing EC8-4 limit is unknown. This paper expands upon the experimental work in [7,8] by: 1) examining the effects of radial base-plate welds on connection capacity, 2) expanding current experimental data taken from existing tank connections, and 3) creating additional fatigue-life curves for fatigue evaluation of tank connections having radial welds.…”