How do the balconies work and how do they hold the loads applied without collapsing?

A balcony is a architectural element of the building systemdelimited by a parapet or a railing, which guarantees usability of external spaces also on the high floors of a building. Its peculiarity lies in the fact that it is one structure without a base of support for one extremetherefore resulting in a part that substantially develops in the void. It includes in the more general family of cantilever structuresor simply “changes”. Of these there are various types in the world, all different from each other but united by a key concept that represents the actual “secret” that outlines its static operation.

How a balcony works

We try to group i physical concepts which distinguish the static functioning of a balcony (or of an overhang in general). In practice, in fact, despite each balcony can have a design solution different from the others, therefore different materials and shapes structural elements, the same retains an operating mechanism that remains substantially unchanged. Let’s try to do it simple:

• To ensure that there is a balance towards vertical actions, such as weight force, a structural system equipped with multiple support points counts on the fact that Each of these developments a reaction of bond that bits part of the load applied. Since there are many support points, these bond actions can – in general – to be vertical forces onlyjust like the loads applied;
• In the case of cantilever structures, like a balcony, what has been said above cannot happen. To guarantee balance, the only bond present must be able to develop not only reactive vertical forces, but also of the so -called moments of interlockingthat is, of the entities type forces who will have to oppose the rotations that the system would like to perform in their absence.

This reactive difference is the basis of the operating mechanism of a balcony: from the static point of view, in the first case we can generally speak of beamswhile in the second case we speak more properly than shelves.

How to make a balcony

Here too we try to give an example, referring to one practice technological application, like that of the balconies made with Armed concrete structures and bricks. How is this structure made? There is a higher part called insolewhich is a huge reinforced concrete slab that covers the top. At the bottom, from this insole they emerge, at regular intervals, of rib. Among the various ribs, the void is filled with a light material, which could be a classic Frato brick. This same system thus described, reported schematically in the following figure, is also re -proposed in the floors of our homes on the inside, therefore far from the balconies.

So what changes between the two systems? Simply, the position of the armor irons present inside the concrete:

• In the case of the attic on support for both ends, the irons are mainly found in the lower part of the ribs;
• In the case of the balconies, these same irons are found in the upper part of the ribs, inside the so -called slab.

How come? Simplifying, the flow of forces that must be transferred from the balcony to the joint must pass into this resistant element. Since the interlocking must develop, by balance, an action that opposes the rotations of the balcony under the effect of one’s weightwe can imagine that this resistant action is formed by one couple of forces arranged at a roughly distance equal to the height of the structural section of the balcony itself. Well, between these two forces, the one who would like to pull the balcony to keep it on is always the one in the upper area. For this reason, In this area a material that is able to absorb it must necessarily be found, As the steel armor inside the concrete jet do.

The changes in the world

What is described so far happens on the balconies, but also on many covers that develop with a cantilever scheme (as for example it happens for the coverage of the Maradona stadium). The cantilever structures are also used on many bus waiting canopies or in the creation of the steps of some particular types of stairs.

The more you move away from the interlocking point, the more the vertical movements of the system grow, until they reach the maximum in correspondence with the most distant point. It follows that the bigger the length of the overhang, the more complicated the system and the more The one that we first have identified as a traction force to the upper strip of the structural element is growing. For these reasons, we can deal with changes made with:

• reinforced concrete elements ribbed and lightened, or full;
• More complex structures formed by different systems of beams and orthogonal shelves to which the beams connect.
• Flooring systems, possibly with variable section along the overhang. This system optimizes the structural geometric conformation to provide maximum yield by minimizing the weight of the materials used.