How does a slab work?

imageIn the book Why Buildings Stand Up Mario Salvadori once wrote that the purpose of a structure is to convey to the ground the loads of a building in a way similar to that of the water flowing along a network of pipes: columns, beams and other structural elements act as pipes for the flow of loads.

Most of the buildings made of reinforced concrete floors built in the last century have slabs made with hollow brick for lightening and reinforced concrete beams and joists. This kind of slabs are widespread particularly in areas of low seismicity. Their flat surfaces, both at the extrados and intrados leave complete freedom to the architectural designer to place partitions and dividers in the way that is most comfortable and ergonomic for the end user.

This technological version of concrete slabsï is the final stage of the concept of free plan , one of the five points at the base of the architecture of Le Corbusier: no beam annoies the use of built space .

The structures that make up the floors are therefore completely hidden from view, making it difficult to read the backbones of these buildings and to understanding how the skeleton of the building withstands loads and stresses placed upon them.

Intuitively, we can assume that each structural element discharges the loads and stresses it bears upon gradually larger elements until all loads (live loads, dead loads, winds, snow) are discharged to the ground:

  • the hollow bricks and the thin concrete slab coverting them will build upon the joists,
  • the joists are supported by beams,
  • the beams are supported by pillars
  • the pillars are inserted in the works of foundation
  • foundations – footings, reverse beams , slabs , pilings – spread the load on the ground.

All this is well illustrated in the church of SS. Peter and Paul in Desio that clearly shows its structure to its visitors; it is a valuable witness for anyone studying reinforced concrete. Rarely we are able to read with such clarity the structure of a floor as current designs most often tries to hide it. A cultured esthete reading these humble notes or visiting the actual church probably would consider very unfortunate the juxtaposition between the brutalist concrete structures and exposed bricks as that building could be judged almost a religious refitting of an industrial shed.

Althought we are not interested in actual aesthetic judgments we may just note that this church follows the architectural fashion of the period in which it was built; actually I think there was even worse design, but this is a just a matter of taste.

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The structure of this church has at least two other interesting features for the lovers of structural engineering: flares dovetail coverage of the joists and windows ribbon that will be dealt with in future articles.

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