Effects of slab post-tensioning on supporting steel beams in steel framed parking deck structures

An experimental program of monitoring the during- and post-construction behavior of a steel-framed open parking deck having a cast-in-place post-tensioned concrete slab is presented. Monitoring was carried out following steel erection and ended following completion of the structure. Data were acquired at a variety of stages throughout the construction process. Additionally, two live load tests were conducted. The objective of this field study was to quantitatively assess the effect that slab post-tensioning forces have on their supporting steel members. The following conclusions were drawn: 1. The structure carried its self-weight dead load in a manner consistent with its simply supported design. 2. The relatively small magnitude live loads applied were carried in a fully composite manner with the effective width equal to the beam-to-beam spacing. 3. While the form shores are in place, the steel beams carry only concrete deck loads tributary to the beam relative to the shores. 4. If shores are in place during the post-tensioning operation, only PT-induced axial stresses are effectively transferred to the supporting steel beams. These stresses may be calculated based on an equivalent transformed gross section analysis. 5. The shores are sufficiently stiff to resist flexural stresses induced by the post-tensioning operation. Once the forms are released, PT-induced flexural stresses are transferred to the supporting steel beam. These stresses should be assessed using a transformed sections analysis and material properties at the time of the form shore release. 6. For the beam and slab geometry considered, the PT-induced flexural strains were approximately 10% of those associated with the transfer of full dead load to the beams (as the forms are released). Additionally, for the geometry considered, the bottom flange strain associated with PT-induced axial force and that associated with flexural forces virtually cancel each other. Resulting in negligible additional stress in the bottom flange of the beam as a result f the two-directional slab post-tensioning. The top flange axial strains are reduced marginally because the neutral axis (in this case) has migrated into the slab.