Dynamic Load Effect on Lift Slab Structures

The lift slab construction method is a very efficient and economical method; however, the structure is considered to be potentially unstable statically during the construction stages. Such vulnerability is an inherent characteristic of this method of construction; the columns are required to be unbraced laterally in order to facilitate the lifting of the slabs by the jacks, which are placed at the top of the columns. Furthermore, the method requires stacking several slabs at the top or near the top of the columns. Such stacking is needed before new sections of the columns are spliced to the ones already constructed. Stacking the huge masses of the slabs at the top of the columns reduces the natural frequency of the structure at the construction stage, which makes the structure more vulnerable to dynamic load, particularly horizontal dynamic loads (such as gusting wind) which may have a frequency equal or very close to the frequency of the structure at that stage of construction. Although such situations are very rare, it is possible for engineers to check each stage of construction against any possible dynamic load that could trigger high vibration—drastically increasing the stresses at various locations—when the structure becomes dynamically unstable. Dynamic instability is a rare occurrence, because it requires the frequency of the dynamic load to be very close to the natural frequency of the structure; however, this paper applies the principles of structural dynamics to derive a suitable formula, relating the shear at any level to the frequency of the dynamic load and the frequency of the structure at any construction stage—which is a function of the position of the stacked slabs during a specific stage as well as of other properties of the structure.