Is a steel frame cheaper than wood
Material properties in comparison
steel is a very precisely calculable material. The material behavior is the same in all directions of loading and for different types of loading. The industry supplies steels with very low material fluctuations. This allows a greatly reduced safety factor on the material side. Steel components can be connected to one another relatively easily with the help of weld seams. One hundred percent load-bearing connections are created, i.e. the strength of the connection cross-section can be completely transferred to another component.
Wood is anisotropic and very inhomogeneous. The behavior under load depends on the grain direction, and there are also differences between pressure and tension. Even the duration of the load plays a role, as it creates additional permanent loads. The moisture content in the wood also has an influence on the behavior of the material. Due to the natural growth conditions, the material characteristics fluctuate in a relatively large range. Therefore, the safety factors on the material side are significantly higher here. The spread of the characteristic data can be reduced by appropriate remuneration. Components made of glued laminated timber, cross laminated timber and others are more resilient than solid wood. Only round wood can withstand somewhat higher loads if the fibers are not cut. From a static point of view, however, a round cross section is not optimal.
Dimensioning Compared to wood, steel offers the possibility of very low beams. Only a possibly necessary fire protection cladding can increase the structural height of the component a little.
A glued laminated timber beam with a cross section of 20 by 44 cm (GL 32) corresponds to a HEB beam with a height of 22 cm. Although this costs twice as much as the wooden beam, it is significantly lower.
Steel has a significantly higher shear resistance. This is particularly important for shorter girders on which wide-span ceilings rest.
The already mentioned girder cross-section of 20 by 44 cm can transfer a working load of approx. 80 kN as shear force, an IPE 300 almost three times as much.
Steel supports made from shaped profiles can also be slimmer than wooden supports.
A 3 meter high wooden support for a load of approx. 180 kN is feasible with a cross section of 16 by 16 cm (GL 24), a steel support can be implemented as a square shaped tube with 9 by 9 cm.
Load forwarding When placing girders on supports, steel has an enormous advantage, as it can transfer very high loads in all directions. Wooden components, on the other hand, can only absorb a maximum of 25 percent of the load that they can transfer in the direction of the grain when pressure is normal to the grain direction - a fact that is particularly problematic in the support area. There are possibilities to strengthen the pressing surfaces, but this is associated with higher costs and only possible up to approx. 45 percent of the load in the direction of the grain. With special measures and more massive steel parts, a little more is possible.
Building physics Wood has a much lower thermal conductivity than steel. With solid wood, cold bridges are only an issue for passive houses, but always for steel structures.
Machinability Wood can be processed more easily, for example by milling, cutting and drilling. This applies in the manufacturing plant, but also on site if you B. has to make changes to existing components. Steel parts, on the other hand, have to be prepared relatively precisely, reworking on site is more complex or, if the parts are galvanized, not advisable, as the galvanizing cannot be restored on site.
Fire behavior Wood has a calculable fire behavior. Its disadvantage is that it provides an additional fire load. Steel, on the other hand, does not burn, but loses its load-bearing capacity relatively quickly.
Support systems Due to the possibility of welding, almost any shape can be produced with steel. Vertical and horizontal loads can be derived using the same load-bearing system. The steel frames can be joined together without any loss of load-bearing capacity. Bumps can be formed in places that are less statically stressed. In the case of wooden structures, the boundary conditions for rigid finger-jointed frame corners must be optimal so that such structures are worthwhile.
However, very simple, disk-based load-bearing systems (folded structures = load-bearing structures that transfer loads via flat components) can be produced with wood. With modern wood-based materials such as cross-laminated timber, for example, wall panels can take over the function of the framework.
A wall panel with a 6 meter span can consist of a 9.5 cm thick cross laminated timber panel including a door and transfer line loads of approx. 20 kN / m. A steel frame must consist of at least HEB-180 profiles and show considerable horizontal deformation at 20 kN / m, whereas the wooden wall panel has almost none.
Resistance to environmental influences Water is not good for either material. In the event of moisture ingress, wood can rot and steel rust. The decay of the wood is a little faster than the rusting of the steel. B. are given in indoor swimming pools, fewer problems.
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