COMPARATIVE MODELS OF PRECAST WAFFLE SLABS DUE TO POST-TENSIONING

Inspecting the IMS structural system tensioned together from precast reinforced concrete elements, considerable corrosion, in some cases rupture of the reinforcing tendons was found in the critical floor cross-sections. On the basis of the results of investigations carried out on a typical floor panel, assuming a corrosion predictable from disclosures, possibility of failure of the critical cross-section is 15 years after its construction by about two orders of magnitude higher than prescribed in the Hungarian Standard. Complete corrosion or rupture of the reinforcing tendons in either of the directions can lead to progressive collapse of the floor due to the rearrangement of stresses. Its prevention calls for structure strengthening. Insufficient load bearing capacity of the floor structures - especially in the case of multielement systems - can be most favourably guaranteed by supplementary post- tensioning. Efficiency of the strengthening by post-tensioning depends essentially on the distribution of the normal force originating from the stressing force applied -concentrically in the block floor ribs - that can be regarded as monolithic in respect of the supplementary stressing -, as well as on moments and reactions due to concentrated forces arising in consequence of the polygonal cable arrangement and acting perpendicularly to the floor plane. For design the strengthening effect of subsequent steps of the post-tensioning and of the location of force application, the role of columns in stiffening, influence of the possible lattice models as well as the role of the 3.5 cm floor plate in stiffening and effect of joint eccentricity joint of structural elements have been investigated