Jämförelse mellan olika träbjälklag
Wooden joist slabs is a common type of floor in residential buildings. Higher demand and production of residential buildings increases the possibilities of profiting by optimizing the system of joists in a variety of areas. Spring and deflection is something most people can relate to since it?s not that uncommon to hear furniture rattle when you walk over a wooden floor. To remove the presence of those kinds of vibrations the system of joists have to be made stiffer than the standard joist construction that is used in today?s constructions. A system of joists has to be dimensioned, besides the tensile strength dimensioning, be dimensioned against excessive deflection and spring when someone is walking across the floor. To achieve a stiffer floor it can be constructed with higher rafter profiles since the deflection and spring decreases as the rafter is raised. Besides the rafter dimensions it is possible to change the materials in the slab to stiffer substitutes, this leads to a stiffer joist with a lower material consumption for the rafters. Solid wooden slabs are a type of self-supported slab that makes it possible to have a longer distance between the rafters. With these kinds of slabs it?s possible to build a floor with one big rafter in the middle of the room that carries one end of the slabs. The objects studied are two fictitious floors in a residential building with a span of four meters with two supports and the second floor eight meters with three supports. The floor is dimensioned according to SS EN-1990 and SS EN-1995-1-1:2004. The result from the calculations showed that it isn?t possible to increase the space between the rafters if the slab is a chipboard slab since the chipboard doesn?t have enough tensile strength. If the joist is constructed with solid wooden boards it is possible to increase the rafter spacing up to at least 2400mm, but the joist system gets a higher utilization if the rafter spacing is 1800mm because if the spacing is 2400mm the deflection is so high that the rafters get so little space to move on since the slabs deflection takes up more than half of the allowed deflection, this isn?t a good solution in practice, even if the rafter and the slab have enough strength to manage it the construction will be too high for a relatively small residential building. If the construction would be raised in an arch the rafters could be designed lower since in all the studied cases it was the deflection that was the weakest link