Sp-287—7 7.1 Special Considerations in the Reinforced Concrete Slab Design for the St. Cloud Hospital East Addition

Synopsis: The structural design for the East Addition to the St. Cloud Hospital facility in central Minnesota incorporated punching shear and cracked section design criteria that are not currently specified in ACI 318-05. The complexity of the column layout and shallow floor-to-floor spacing were the primary reasons for choosing a twoway 12 in. (300 mm) reinforced concrete flat slab for the 450,000 square foot (42,000 m) addition. The use of continuous top and bottom reinforcing mats and the use of column capitals were early design decisions. Due to the complex column layout neither the Direct Design Method, nor Effective Frame Analysis would have been ideally suited to this project. A finite element analysis based program was employed to determine required flexural reinforcing, column joint forces and slab deflections. Design methodologies were investigated, and a method from the literature was chosen that incorporates slab depth, aggregate size, and reinforcement ratio when determining punching shear resistance, resulting in a reduced punching shear capacity. Another design consideration was the use of a reduced modulus of rupture to better predict deflection performance. Construction of the primary concrete structure has been completed and no performance issues have been observed.