Multilevel formwork load distribution with posttensioned slabs

Formwork and the associated shoring represent a significant proportion of the costs associated with the construction of multilevel concrete structures. To minimize these costs, a limited number of formwork and shoring sets are recycled up the structure as construction progresses, eliminating the need for a new set of formwork and shoring with each new slab. When a slab is posttensioned using draped tendons, slab lift occurs as a portion of the slab self-weight is balanced. The formwork and shores supporting that slab are unloaded by an amount equivalent to the load balanced by the posttensioning. This produces a load distribution through the structure that is inherently different from that of a conventionally reinforced slab. This paper presents two design methods suitable for modeling the multilevel formwork process for posttensioned slabs: A modification to the simplified analysis method and a finite element model - both techniques will be of immediate use by industry practitioners and of interest to researchers examining the load distribution phenomenon. The paper also summarizes the findings of one of only a few research projects in which actual shore loads were monitored during the construction of a multilevel posttensioned building, which is used to validate the proposed design models.