A Practical Look at Creep and Shrinkage in Bridge Design

R f search has produced two major procedures for assessing creep and shrinkage; the ACI 209 method' and the CE$-FJP method.' Laboratory research into concrete creep and shrinkage has been conducted for decades, and excellent analytical procedures for evaluating structures for creep and shrinkage have been presented in the literature.'-5 Creep and shrinkage are factors in the design of a variety of bridge details. (The focus here will he on creep, for its treatment is somewhat more involved than that of shrinkage. However, the conclusions apply to both as elements of time dependent deformations.) Creep affects the setting of bearings and the size of sliding plates or laminated bearing pads, it affects the size and setting of expansion joints, and it affects the amount of girder shortening due to prestress and the corresponding loss of prestress, thereby also affecting the secondary moments in a prestressed girder bridge. The amount and character of creep influences the redistribution of forces in certain , structures where the statical system changes during construction, and can, therefore, play a major role in stress distribution for cornposite construction. Creep must be considered in the context of the entire design. The major effects of creep in concrete bridge structures can be summarized in three categories: — Camber and deflection Prestress loss — Stress redistribution Where stress and strength is concerned, it is general practice to base engineering design on a conservative upper bound of demand, not on average demand. However, with creep we have an interest in both. Camber and deflection control i ,or a free cantilever bridge