This paper is compilated as a masters degree assignment at the Faculty of Mathematics and Natural Science, University of Oslo.
Today, Bubble Deck is used to reduce the net weight of a concrete slab. This is done by casting plastic pellets for an optimalized cross-section of the slab. It is desirable to reduce the net weight, to increase the load capasity for maximum loads, or to increase span between the loadbearing structures for the slab. BubbleDeck is mostly used in industrial buildings where the need for large, open spaces are a part of the design criterias.
This paper looks at how BubbleDeck behaves at consentratet maximum loads. The thesis is solved by looking at the shell theory, to work out an equation that describes the membrane-forces in the cross section over the plastic pellet in BubbleDeck, under a variable concentrated load. The results are compared with an element analasys in Robot. The shell is simplified to describe the concrete above the slab with a constant thickness. The analasys shows increased bending stress at reduced load surface. To take a closer look at the tensions at the top of the slab, two new models are established, one shell with varied thickness in Robot and one model with volume-elements in Patran (SESAM program package). The results of these models show that longitudinal tension occurs in the concrete towards the plastic pellets. Cracks appear to be evident by obtained dimensional tensile strength for the concrete. The tensile strengths are also estimated for different placement of load above the plastic pellet, and for varying load surface.
During testing, this assumption proved to be very conservative. In addition, the crack came as a clean shear fracture around the load surface. This states, that even for small loads, it would be relevant to consider shear fracture of the slab, both locally over the plastic pellet and globally for the slab.