· · · Institutional Repository

In dit rapport is ingegaan op de consequenties van de VBC 1990 voor de toepassing van VZA en VMA bij vloeren in de utiliteitsbouw. In hoofdstuk 2 is de ontwikkeling van VMA en VZA in vloeren behandeld. Om de wijzigingen in de VBC 1990 ten opzichte van de VB 1974/1984 goed te kunnen onderbouwen is het gedrag van vlakke naspanvloeren in hoofdstuk 3 beschreven. Er is ingegaan op de gebruiks- en bezwijkfase en de bijbehorende berekeningsverschillen volgens de VBC 1990 en de VB 1974/1984. Ontwerpaspecten met betrekking tot kabel lay-out en uitvoering zijn in hoofdstuk 4 behandeld. In hoofdstuk 5 is aan de hand van een rekenvoorbeeld een vergelijking gemaakt tussen de toepassing van de VB 1974/1984 en de VBC 1990. De belangrijkste verschillen tussen beide voorschriften met betrekking tot het ontwerp en de berekening van VMA en VZA-vloeren zijn aangegeven. In hoofdstuk 5.4 zijn de berekeningsresultaten met elkaar vergeleken. De belangrijkste conclusies en aanbevelingen zijn in hoofdstuk 6 gegeven. Een overzicht van de literatuur besluit dit rapport.

It is becoming increasingly necessary to investigate the strength of reinforced concrete structures subjected to dynamic loading. Experience and knowledge relating to the non-linear dynamic behaviour of such structures is still limited, however. Attempts to solve this type of problems with the aid ofa finite element approach soon encounter difficulties. An example of this consists in the correct representation of the appropriate collapse mechanism and more particularly in the problem of the numerical stability for the integration process required for solving the equations of motion with respect to time and made additionally awkward by the non-linear behaviour. These problems are associated with mathematical algorithms and are not relevant to the structural problem under investigation. The authors anticipate considerable improvement in this sphere in the future, but at present they prefer an approximation which provides direct insight into the response of structures without involving too many difficulties with numerical problems. For this reason a simple well-tried beam model is applied. This discrete beam model consists of a number of indeformable segments (the elements) with hinges (the nodes) at their ends andjoined to one another by means of flexural springs. The mass of each segment is conceived as concentrated in the hinges, as is also the dynamic load. The material properties are assumed to be elasto-plastic. The effect of loading rate on the material properties has also been taken into account. Two failure criteria are applied in the discrete mathematical model. Thus, in the elastic range (M < Mp) the concrete section is checked for strength, and in the plastic range the rotational capacity is not allowed to be exc~eded. In other words, the shear strength (loadbearing capacity in shear) is calculated as a function of the moment-shear combination that occurs. The treatment of the subject starts from formulae derived for static moment-shear combinations. It emerges that the (static) formula given by Rafla can be modified and suited to dynamically loaded structures (M < Mp). The effect of shear on the permissible rotational capacity can be expressed in a simple relation. Thus, the rotational capacity will have its maximum value if the shear force is zero; but the presence of shear force will reduce the rotational capacity. The discrete model described here has been applied to analysing the elasto-plastic response of a beam subjected to an impulsive load. Two different examples are presented. The first example is concerned with the response ofa simply-supported beam under a uniformly distributed impulsive load. It appears that the distributions of the bending moments and shear forces are very different from those obtained for a comparable static load. Presupposing that no shear failure will occur (adequate shear reinforcement), plastic moments will be formed at some distance from mid-span. From here the plastic hinges will then move towards the middle of the span. The second example considers a beam with fixed (fully restrained) ends. It approximately represents a strip of the roof of a road tunnel. The situation where a gas explosion occurs in the tunnel is investigated. The distribution of the bending moments which is then produced bears a closer similarity to that associated with a static load, but the shear forces are still different, though less so than in the case ofthe simply-supported beam. Ifno stirrups are provided, a shear failure criterion must be introduced. This will very greatly reduce the permissible explosion load, so that in most cases no plastic hinges will even be formed.

concept v. 29-07-2011

Poster. In the Netherlands, 60% of the existing bridges were built before 1975, while the traffic volumes and loads have increased over time. The results of a first assessment of the existing bridges showed that particularly the shear capacity of reinforced concrete solid slab bridges is often lower than the resulting shear stresses due to dead loads and traffic loads.

Several existing reinforced concrete solid slab bridges in the Netherlands do not meet the criteria for shear when calculated according to the recently implemented Eurocodes. The shear capacity is assessed by comparing the design beam shear resistance to the design value of the applied shear force due to the dead load, permanent load and live load. Transverse load redistribution which occurs in slabs is not taken into account. To evaluate a large number of slab bridges, a first round of assessments is necessary to determine which bridges need a more detailed shear analysis. A series of 26 slabs and 12 slab strips are tested until shear failure. The results of these experiments are compared to the state-of-the-art in beam shear research to compare the shear behavior of beams nd slabs. Recommendations for the shear assessment of slabs are formulated, and used to verify the shear capacity of 10 cases of slab bridges. This “Quick Scan” approach is compared to the AASHTO provisions, which are found to be less conservative. However, the underlying target reliability index is significantly smaller for the AASHTO provisions. For the existing bridges in the Netherlands, the proposed method can analyze a large number of cross-sections and thus help prioritize the efforts f the owners such that cases which need a more detailed shear analysis are identified.