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Live loads on one span tend to cause uplift of outer column of the split piers (twin leaf piers). When the two split piers are designed too close, the uplift may be greater than the dead load reaction of the outer pier so that tension is induced in the outer pier.
However, if the two split piers are designed to be adequately far apart, the uplift could hardly overcome the dead load leaving both piers in compression. As such, this allows for the use of pinned bearing. The optimum spacing of twin piers is about 1/25 to 1/13 of adjacent span.