Influence of a Vertical Shear Force on the Hogging Bending Moment Resistance in Composite Slabs

Abstract

Composite steel-concrete floor systems consist of a trapezoidal shaped steel deck, reinforcement and cast-in-place concrete. Depending on the span, height restrictions and application shallow or deep decks can be chosen. The shallow deck used within this thesis is the ComFlor75. Due to the low self weight, a bundle of steel decks can be lifted to the desired floor and the individual decks are placed by hand to the correct location. Shallow decks normally are placed on top of the supporting beams and one ComFlor75 can cover multiple spans. The deep deck used within this thesis is the ComFlor210, this deck type is usually integrated with the supporting beam by placing it on top of the bottom flange or a steel plate that is welded below the supporting steel beam. This allows for a bigger internal lever arm, while reducing the construction height. After reinforcement is placed in the ribs of the ComFlor75, meshes or additional reinforcement bars are placed in the top layer. This layer does continue over the support beams and creates a continuous floor system. Advantages of this composite steel-concrete floor system are: fast construction, low weight and a small construction height. At the supporting beams of this continuous floor, a hogging bending moment and vertical shear force occur. Within the Eurocode 4, the hogging bending moment and vertical shear resistance are calculated independently. The deep decks are not covered by the scope of the Eurocode 4. At a certain project (Case study: “town hall - Almelo”) the authorities asked if the vertical shear could influence the hogging bending moment resistance as both were near the calculated resistance. This question is being answered by first looking into current researches [1; 2; 3; 4] and calculation methods. These have been used to find a suitable test setup. This test setup has been adjusted to practical values to cover the critical spans where M-V interaction could be a concern. A total of 5 experiments, 3 on deep deck (ComFlor210) and 2 on shallow decks (ComFlor75) have been conducted. In both cases first an experiment is done to determine the hogging bending moment resistance with a low vertical shear force followed by an experiment where the specimen was fully loaded by a vertical shear and a hogging bending moment. For the shallow as well as the deep decks no reduction in hogging bending moment was found. All specimens failed in bending, even though the specimens were loaded by a vertical shear force surpassing the vertical shear resistance based on the Eurocode 4 and calculations done in practice. For the deep decks a higher hogging bending moment was found compared to the calculated resistance. The steel plate underneath the integrated support beam was not included in the calculation, but did contribute. All 3 specimens failed close to the calculated resistance. It was concluded that with the maximum shear resistance V_Rd used in practice no M-V interaction was found. The actual vertical shear resistance V_U could be far greater than the conservative value of V_Rd used according to Eurocode 4 [5]. Under normal distributed load patterns other criteria will govern. Vertical shear can become critical at shorter spans, however this implies a significant high distributed force compared to common values. The question is if it therefore is of interest to know the exact V_U as it generally is not the critical criteria. There is therefore no influence of a vertical shear force on the hogging bending moment with a 〖V≤V〗_Rd. To get a wider statistical base more experiments are advised.