The trade of the structures consultant (I) | Juan Carlos Arroyo

Can you do the pine with the arms opened 45º?

For years I formulated (for me itself) the secret of “having trade”, something that a good shopkeeper must have, a good doctor, a good taxi driver, a good engineer and a good architect. The trade consists of three ingredients, the deep knowledge of the concept, the order of magnitude and the technology.

In the university they are not taught, as other many things. These things about which we are going to speak here are learned later, when you do not have any more remedy because the life knocks down you. And in the University they are not taught because the University is in the habit of being to the fourth derivative and the concept is the first one.

“To the facts I repeat myself”.

In structures these three legs are very identifiable. The intimate knowledge of the concept, of the physical or mathematical phenomenon, is the first one of them.

Famosa foto de Eduardo Torroja pensando1: “Antes y por encima de todo cálculo está la idea, …”
Eduardo Torroja’s famous photo thinking: “Before and over any calculation the idea is, …”

Is it possible to be a good structural consultant without knowing how one endures a shoe? Even, without devoting yourself to the structures, do not you think that you should know this type of concepts?

Do you know why armor needs a rigid shoe?

A rigid shoe behaves like it indicates the figure. The load goes down for the sloping connecting-rods and finishes in the area in two vertical forces. On having spent the connecting-rods inclined to the area there is generated a force of traction that has to be taken by armor.

If you do not understand it well I put an example. It tries to do the pine with the arms lightly opened in a slippery surface do you guess what is going to happen?

“Puef que te puedef quedar fin dientef…”

Equilibrio | Ed Marks
Balance | Edward Marks’s cartoons

How can you solve it?

El oficio del consultor de estructuras (I) Equilibrio Ed Marks 2
Solution | Edward Marks’s cartoons

Sticking a rope to the wrists so that you guarantee the balance of your hands by means of one horizontal element traccionado. We it can enunciate of another form: The sloping connecting-rod needs a horizontal stress that turns it aside to turn into the vertical reaction of the area.

Esa cuerda que necesita el gimnasta es la armadura que necesita la zapata rígida para equilibrar los dos nudos inferiores.
This rope that the gymnast needs is the armor that needs the rigid shoe to balance both low knots

This type of structure is very rigid, little is deformed, the load does a very short tour, going down for the arms. The only deformation is that of the shortening of the arms (the connecting-rods).

El ingeniero Peter Rice explicando su estructura. Se nota que la entendía.
The engineer Peter Rice explaining his structure. One notices that he understood it.

Understood the concept of the connecting-rods and the brace arises another question ¿ and if the shoe was bigger and had we to separate very much the arms? There turns out to be at the time the concept of flexible shoe.

Do you know what difference is between a rigid shoe and a flexible shoe?

How can it come to load so far away with the simple mechanism of the connecting-rods? Inclining very much the connecting-rods; but it is not possible.

Los brazos del gimnasta, salvo si eres olímpico, no pueden generar un ángulo tan agudo, tenga o no tenga cuerda en las muñecas.
The arms of the gymnast, except if you are Olympian, cannot generate such a sharp angle, have or do not have rope in the wrists | Edward Marks’s cartoons

You notice as the arms of the gymnast they are not working only to compression? The arms on having been so opened already are not capable of lowering the load for compression And begin to flectar.

What happens is that the connecting-rods cannot be opened so much and to there be opened only what they can, approximately 45º. Then to come up to the end of the shoe there is generated a curious mechanism of transport of the loads that consists of repeating the module connecting-rod – brace up to the end of the shoe.

Para pasar la carga de un módulo al siguiente se necesita una fuerza de tracción vertical que la traspase. Esa fuerza se llama ¡cortante!
To spend the load of a module to the following one he needs a force of vertical traction that penetrates it. This force it calls butcher!

This combined mechanism is much more flexible, there has just appeared the flexion, the flexibility, the arrow.

And the flexion and the butcher appear simultaneously, because they are two indivisible phenomena. The butcher is a consequence of the flexion, without flexion, there is no butcher (pardon for remembering old old monsters, but the butcher is the derivative of the moment).

En un zapata flexible, la necesidad de armadura aparece para resistir el momento flector que provocan las tensiones del terreno. Calcular el momento flector es muy fácil y calcular la armadura necesaria, tanto más fácil. Otro día lo vemos.
In a flexible zapata, the need of armor appears to resist the moment flector that provoke the tensions of the area. To calculate the moment flector is very easy and to calculate the necessary armor, so much easier. Another day we see it.

I assure you that (at least the most habitual) they can understand almost all the structures of simple form, since they are not any more than physical easy phenomena. When you understand the phenomenon, there is no thing easier that to put numbers, a couple of sums and multiplications and …

Pull, already you can calculate!

But also I say another thing to you, if you you it do not know well, well, equally of good that the name of your son …

It does not happen to you to calculate, people can die!

Juan Carlos Arroyo ( structural engineer
Madrid, February 2017

In this hollow that veredes gives me I feel very comfortable because I can write things that I have thought walking along my people. Thank you Alberto.


Eduardo Torroja is assumed by some phrases that if they are not his would deserve it. For example:

“The structures behave since they arm themselves not since they are calculated”.

“The less steel takes the best concrete behaves”.

In a next post we will explain the intelligent incongruity of these two phrases.

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