Concrete is one of the most commonly used construction materials. But it can’t be an appropriate substitute for steel and wood in structural systems it reacts improperly against tension. To overcome this malfunction, a combined structure of concrete and steel is used.
At first, engineers used a mixture of steel and concrete which resisted against tensile and compressive forces (reinforced concrete 1800 AD). Afterwards using composite roof covering, causes the concrete to be one of the most practical materials in covering the roofs. In Iran one of the most common methods to perform the roof is using block and joist ceiling. Currently in other parts of the world using the steel composite roof technology is known as one of the most common methods in roof covering. The idea of applying this technology was firstly propounded in early of 20th century in European countries and then spreads through other countries.
Advantages of this method comparing to other methods are:
Versatility over the scheme
Reduction of structure weight
Integration and continuity of the ceiling in seismic region
The steel composite roofs are performing in a way that galvanized steel sheets, which are designed in advance, would be installed in the considered place and then these sheets would be used as working platforms and shuttering during the concreting. These sheets were designed in a way that could bear performing equipments and recently made concrete weights and when the concrete is set can be used as rebar in the slab. This method does not require shuttering, piling and reinforcing.
Performance in seismic region
Today the main element in structures design is considering the vulnerability against hazards caused by seism. Therefore all structural elements are designed according to specific criteria and standards to resist against seismic forces.
Basically, slabs and structural roofs are not considered as resistant systems against lateral forces and they only transfer them to the bearing parts. Regarding this fact, integration and rigidity of the roof ensure a proper and complete transfer of the seismic forces.
The most important defects of block and joist ceilings during seism are falling of earthenware materials, detachment of the yonolit (poly styrene) phase, falling of the roof downer covering materials, and improper function in transferring lateral forces, which has been overcome in steel composite roofs. In other words, in the new method of steel composite roofs the risk of covering material falling would be minimized due to the roof integration and thorough consolidation.
Totally, block and joist ceilings have considerable rigidity in comparison to the former Jack arc ceilings, but rigidity of steel composite roofs is definitely more than block and joist ceilings. In this method, steel composite roof creates a totally integrated surface that has an appropriate rigidity in turn to transfer seismic force to the bearing elements.