Experimental Study on the Seismic Behavior of Shell-Base Connections in Large Storage Tanks

When an unanchored tank is subjected to strong motion, the impulsive mass of the contained liquid generates a moment which if strong enough could cause partial uplifting of the base plate. As the tank undergoes uplifting, the shell-base welded connection is subjected to cycles of rotation which could lead to its failure. To address this issue, the Eurocode and the New Zealand’s recommendations limit the rotation amplitude that the shell-base connection of a tank may undergo to 0.2 radians. This limit is obtained from the following two assumptions: (1) a plastic hinge with a length equal to twice the thickness of the base plate forms at the base plate, next to the connecting weld, and (2) the maximum strain that can be sustained by the base plate is 5%. While these two assumptions are reasonable and have been the state of practice for many years, no research work is believed to exist backing these two assumptions and therefore the 0.2 radians limit. For this reason, an experimental research was conducted to determine the real rotational capacity of shell-base welded connections found in tanks. A total of 24 shell-base connections were tested considering the bending and membrane stresses that develop at the base plate when uplift occurs. Constant amplitude tests were carried at different amplitudes of rotation in order to create curves of rotation versus number of cycles to failure. The main finding from this investigation is that the current limit of 0.2 radians is very conservative and that a limit of 0.4 radians would be more realistic.

Experimental Determination of the Capacity of Wall-to-Base Connections in Storage Tanks

Gustavo Cortes, Eric Lattion, Alain Nussbaumer

Earthquakes around the world have affected many unanchored tanks used for storage of gasoline. When these unanchored tanks are subjected to strong motion, the moment generated at the base, by the mass of liquid contained in the tank, will cause partial uplift of the base (i.e. rocking of the tank), inducing cycles of rotation at the wall-to-base connection. Actual codes of standard practice used in Europe and New Zealand limit the rotation that may be allowed at this connection to 0.2 radians. This limit was based on a series of assumptions since no realistic tests had been conducted. This paper presents the experimental research conducted to determine the real rotational capacity of these connections. The tests were carried out considering the bending stresses and the membrane stresses that develop at the base plate when uplift occurs. Specimens from an old existing tank (dismantled) were used. New specimens were also fabricated and used to allow for a more comprehensive study. Results from this research are presented in the form of plots of maximum rotation vs. number of cycles to failure, called RN curves. These curves revealed that the current Eurocode limit of 0.2 radians is overly conservative. Based on the limited data, a limit of 0.4 radians would be more realistic.

Elsevier2011

Seismic Behavior of Shell-to-Base Connections in Large Storage Tanks

Gustavo Cortes, Alain Nussbaumer

A study of non-anchored tanks found in Switzerland (Koller, 2003) revealed that the plastic rotation of the shell-to-bottom connection in four of the six tanks studied did not satisfy the 0.2 radians limit established by the Eurocode (Eurocode-8, 2005). The fact that tanks could undergo fracture in their shell-to-bottom connection prompted further investigation with funding from Carbura and OFEV. The investigation focused on determining the real capacity of the shell-to-bottom connection since the current limit given by the Eurocode was based on a series of assumptions which had not been verified experimentally and omit essential parameters such as the number of cycles undergone by the connection and the ductility of the material used. 24 experiments were carried out to investigate connections with different thicknesses, condition (new vs. rusted) and with different levels of membrane loading. Experiments were performed under constant amplitude cycles ranged from 0 to maximum angle rotations of 0.2, 0.3 and 0.4 radians. The results from the experiments allowed the creation of plots of rotation vs. number of cycles to failure (called RN curves). These RN curves provide the number of cycles that may be resisted at a given rotation angle. Or, if the number of cycles is known (estimated), the maximum rotation the connection can resist may be obtained from the RN curve. The number of equivalent cycles (i.e., cycles with a rotation angle equal to the maximum rotation angle obtained during a given earthquake) expected in the connection was obtained from a dynamic analysis of two tanks representative of large capacity tanks found in Switzerland. The analysis was carried out in the finite element software ABAQUS. For the dynamic analysis, a suit of 8 ground motions form earthquakes having magnitude and distance according to the de-aggregation (for a 2500 return period) for the site of Sion, Switzerland and with soil conditions representative of type C and D soil (Eurocode-8, 2005) was used. From this analysis the maximum number of cycles was found to be 4. The estimated maximum number of cycles is conservative since the de-aggregation for the city of Sion, which has the highest earthquake hazard in Switzerland, was used for the analysis. From the RN curve (95% confidence interval) and the estimated number of cycles, the maximum permissible rotation in the connection would be 0.59 radians. However, since the maximum rotation angle applied during the tests was 0.4 radians, this rotation limit should not be exceeded. By using the limit of 4 cycles at 0.4 radians, rather than 0.59, the confidence interval around the regression line was increased from 95% to 99.999% (µ - 4.7σ). Results from this research suggest that the rotational limit of the shell-base connection of tanks may be increased to 0.4 radians based on the limited number of tests carried out. This limit is twice the original limit given by the Eurocode. It should be emphasized, however, that a number of aspects should be addressed before the suggested new limit can be recommended for design. A number of aspects shall be addressed to confirm the results obtained: - Additional tests at rotation angles of 0.2 and 0.4 radians should be carried out since only two tests were performed at these rotation angles. A minimum of two additional tests should be carried out at each level (0.2 and 0.4 radians). - New connections made of S235 steel grade shall be tested - The number of tests was limited to rotation angles of 0.2, 0.3 and 0.4 radians. Additional tests should be performed at greater rotations (e.g. 0.5, 0.6 radians). - Specimens from old tanks were limited to one tank (Konolfingen). Samples from two or three additional old tanks are recommended. - The dynamic analysis was limited to two tanks and eight ground motions. One or two more tanks should be studied. Additional ground motions should be used (12 to15 ground motions in total).

2010

Cyclic demand at the shell-bottom connection of unanchored steel tanks

Gustavo Cortes, Alain Nussbaumer, Gary Scott Prinz

Gasoline and other oil byproducts are commonly stored in steel tanks that rest on ground without any anchorage. Under strong motion, the impulsive mass of the contained liquid generates a moment about the base of the tank capable of causing partial uplift of the tank. The uplifting of the tank is accompanied by a distortion of the shell-base welded connection which may induce low-cycle fatigue failure. Failure of this connection is critical since the liquid contained is usually hazardous to the environment. Current codes of standard practice address the capacity of the shell-base connection. In the case of the Eurocode [1], it establishes that the maximum rotation that this connection may undertake is 0.2 radians. This limit came from a series of reasonable assumptions; however, no experimental studies existed to back it up. This paper shows a mathematical model, performed in the analysis software OpenSees [2], of a tank from which the expected number of cycles at the shell-base connection was estimated for the expected maximum seismic hazard in Switzerland. The mathematical model consists of a multiple-degree of freedom (MDOF) system which accounts for the initial (at rest) tank stiffness and mass, attached to a rigid beam (simulating the tank circular base) with a system of springs at the ends of the rigid beam. The springs account for the stiffness of the soil and, if there is uplift, for the weight of the liquid being lifted up by the base. This model was calibrated by using the method of the New Zealand’s recommendations [3] which is a modified version of the work by Clough [4]. A dynamic time history analysis was then performed and the number of cycles of uplift determined from this model. The number of expected cycles (cyclic demand) was then combined with capacity curves obtained from experiments of typical shell-base connections [5] found in steel tanks. The combination of the cyclic demand and the capacity curves allowed for the determination of the rotational capacity of the tank. The results of this research have revealed that the Eurocode limit of 0.2 radians is overly conservative and that a limit of at least 0.4 radians would be more realistic.