When designing concrete slabs, it is rather difficult to make a performant design that already takes the effect into account of unexpected openings. This is needed because it becomes unfortunately a common practice to add some openings at a final stage, which then requires rough time-saving rules. Even when the concrete is already casted some openings will be cut afterwards, certainly when a building gets refurbished in his life-time. Some standards require that if the reinforcement is designed for a homogeneous slab, the interrupted amount of reinforcement by the opening must be distributed around the opening. However, it is logical that for bigger openings, the flow of internal forces will be influenced by the opening itself. So it is clear that the previouslymentioned method can overestimates and, more dangerously, underestimates some reinforcements. To develop a more accurate design method, FEM analyses were conducted for different spans and opening sizes in slab areas with one- (central part) and two-way bearing (towards the edges) behaviour. Because flat slabs are much less sensible for openings, they are not included in this study. As result of this study, reduction and magnification factors were found which may be applied with an easy rule to obtain a better-performing method. These factors depend on the opening width/span ratio and must be normalized for the span in meters. In addition to the ultimate limit state, the service limit state was also verified. A magnification factor on the deformations must be applied, of the same order as that one on the lower reinforcement. This work was done in the scope of a new technical document about reinforcement for the Belgian Centre of Science and Technology for Construction Works.
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