L'art des structures propose une découverte du fonctionnement des structures porteuses, telles que les bâtiments, les toitures ou les ponts. Ce cours présente les principes du dimensionnement et les structures en câbles et en arcs. Un deuxième cours présente les structures en treillis, en poutres et en cadres. Après avoir suivi ce cours, vous serez capable d'identifier les structures en câble, en arc et en arc et câble. Vous pourrez déterminer les efforts dans les éléments de structure principaux et procéder au choix de leurs dimensions en fonction du matériau de construction choisi. Votre maîtrise des constructions de statique graphique ainsi que de l'applet de calcul i-Cremona vous permettra de comparer diverses solutions et de choisir la forme la plus appropriée pour un type de structure donné. De manière générale, vous pourrez, en observant les structures autour de vous, reconnaître leur mode de fonctionnement, expliquer le choix de leurs dimensions et de leurs proportions.
180 Applet i-Cremona
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From the course by École Polytechnique Fédérale de Lausanne
L'art des structures 1 : Câbles et arcs
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From the lesson
Introduction
Cette première semaine vous permettra de faire connaissance avec l'environnement du cours sur la plateforme i-structures ainsi de vous rappeler les principes de ce que sont les forces.
Le sous-système est un élément décisif que nous utiliserons dans toute la suite du cours. Apprenez donc bien à l'utiliser pour résoudre les problèmes qui vous sont proposés.
Prenez le temps de bien comprendre la terminologie que nous utilisons, faites les constructions graphiques en vous assurant que vous pouvez bien imprimer les feuilles et apprenez à utiliser l'applet i-cremona.
Meet the Instructors
Olivier L. BurdetDr
Laboratoire de Construction en Béton
Aurelio Muttoni
Hello. In this video, I am going to show you
how to activate and to use the applet i-Cremona which is a program that
we are going to use throughout this course to
have interactions dealing with graphical statics.
i-Cremona is a Java applet, that is to say
a small program which executes inside
an Internet browser.
Most the usual browsers like Firefox, Safari, Chrome, Internet
Explorer, Opera and so forth, support the use of the Java applets.
Although we are going to see later that it is necessary to activate them.
However, and it is a bad news, most of the mobile platforms, whether
it be cellphones or tablets working under IOS as well as
under Androïd, do not support the use of Java applets.
To use these applets, and it is a significant
part of the course's exercises, you will have to have access
to a classic computer like a PC, a Mac, or a Linux workstation to
be able to use a complete version of these browsers
which can host
a compatible Java virtual machine.
I am going to show you soon how to proceed.
Once we will have activated the applet, I will
show you how we can use the main commands to
save the documents, to add forces and divers
other elements inside, in relation to a structure.
Generally, in the Art of Structures
course, you will access an applet through a link
which will be given to you for example in an exercise or in a lecture.
When you follow this link, you arrive on a page where normally the applet
should appear. If you arrive on a such page with this
typical pink rectangle, it means that on the computer that you use,
either Java is not installed, either it is not activated.
In this case, you can follow the link which is proposed to you. Normally, here, on the right, in the
window, you should have a yellow window.
If it is pink again, it means that
there is no installed Java virtual machine at all.
You then have to go on the website of Oracle to
install a virtual machine which corresponds to
your platform whether it be Windows, Mac OS or the diverse versions of Linux.
Once again, it will not be possible
for a tablet or a cellphone, or at the least
for the models which are common in 2013 when I record this video.
When I have activated Java in my browser, I can
now, directly clicking on the link, start the applet.
Note that I receive here a security
warning telling me that I am going to execute an applet.
Why this ?
Because there have been quite important security problems linked to Java
the last few years and since then, the browsers give
different kinds of warnings, here we can see the one of Internet
Explorer on a PC, but there are different kinds
of warnings which encourage us not to use the Java applets.
In the present case, we want to do it, we can trust
the Java applet i-Cremona, but it's a question of accepting the risk.
We can also,
without any troubles, accept not to see anymore this warning
thereafter, since we will quite often start this applet.
Once we have done this, the Java applet appears
with its work zone which is here framed
in gray with a series of buttons, here on the top, which we are going to look at later.
A work zone on the middle and
an input zone which we are going to use very soon.
On the top-left corner, you can find a button to open
a file.
Actually, we are not going to use this very often since generally we
will open a file following a link from an exercise.
The save button is however very important because it
will enable us to keep the work we have done.
I am going to give you a very quick example.
Here, using the button "force", we can introduce
two forces here and we are going to push the "save" button.
Let's say that we have thereafter to stop our work on the exercise.
No problem, we can close the applet and when we will
open it again, we will find again in the environment
both arrows that we had introduced.
Well, I want to quickly delete them so I click on the arrow and I click on the
x to take it off and I am going to explain you
a little bit more how to introduce a force.
OK, I click on this button which stays down
and black, and then, on the bottom, a text area appears with
on the left, one hundred, on the middle, less than and on the right, minus ninety.
What does it mean ? Well, the "one hundred" is the amplitude of the force which
we are going to introduce, the "less than", symbolizes an angle and
the "minus ninety", indicates that we are going to get a force
which will point in the orientation minus
ninety degrees when counterclockwise.
Then, when we use the counterclockwise orientation, horizontally
to the right it is zero, vertically
upwards it is ninety degrees, vertically downwards it is minus ninety.
If I want now to introduce a force of only 10 Newtons then
I edit this field and I can directly introduce this force.
If I put the cursor on the force,
it shows indeed with its amplitude
of ten and its orientation ninety.
Note that it has exactly the same size than
the force of one hundred I had introduced before.
Why?
Because the applet will automatically scale the first
force that you introduce to have this size on the screen.
If I introduce a second force which has
an amplitude of twenty Newtons, it
is clearly going to be longer and if I introduce a third force with only
five Newtons, it will be smaller.
If I want to move one of these forces, I can grasp it anywhere or
particularly by the little ball which is at the lower end
of the arrow and I can move it.
Let's say that I am going to place
these three arrows on top of each other.
The three forces.
Notice that when I move it, I can see the line of action
of the force, which enable me to work with quite great precision.
If I want to change the amplitude of a force, I can go at
the other end of the force and choose, to change its amplitude, there we go.
That is what I have done here.
If I click again on this force, I can see that
now it has an amplitude of 35.5 and that it points at minus 119 degrees.
If I want to delete a force as you have seen before, I simply click,
once it is selected, and we see it
in pink, I click on the "delete" button.
If I want to select several forces, I can do it.
For that, I click on the button "control", do it rather with the keyboard but
here I do it with this button on the mouse, and afterwards I can move them
simultaneously or for example modify them making them turn like here.
If I stop pressing "control", I
can also make change the amplitude of the forces,
delete them at the same time. Here we have a button "support"
which we will use to represent the supports
when we will arrive there in the structures course.
We can introduce at most two supports and like for
forces, if we select them, we can delete them.
There is also a tool which enable us to draw a free-body.
Then, I draw my free-body here in a quite simple way.
Once the free-body is created,
we can move it for it to
correspond exactly to what we want.
And then if we are patient, we can move and activate when we are
individual points of the free-body, which we make appear
as intermediary points. They are
used for the drawing of the free-body and you
can see that with this, we can improve the shape,
that we have given to our free-body, if
we want to get something more accurate.
Please do not hesitate to delete your free-body and draw it again
more accurately because it is maybe easier
to draw it again in the good way the second time.
A button which is also very important withing the framework of the applet is
the button which is always above the applet, with the label "reset".
When we click on "reset", we
exactly get back where we started from at the beginning of the exercise.
It is something which is very recommended, why ?
Because the activities that you will do
within the applet are generally very short.
A couple of minutes at most and once you have understood what
you have to do, maybe there will be something that does not work
well but you have understood thus it is often easier to reset
the applet and to do over everything we asked you to do.
So, here you have on the right a
button to replace the control key, to replace the
shift key and to replace the alt key of the keyboard.
When you hold them down, do not forget to inactivate them afterwards because
otherwise, there will be secondary effects a little bit everywhere.
These two buttons here allow you to introduce
either red lines, or blue lines.
These lines are really, completely
free.
They look like a free-body, and work a little bit in the same
manner, that is to say that if you want to edit it
you go to the end of this line,
there will be a little ball which is going to form and then we will be able to edit
and create drawings which combine blue elements and red elements.
For some operations, we will ask you
to create this kind of drawing to indicate where there is
tension and where there is compression in a given structure.
Finally, here this button of help activates the online help.
It is essential to allow the website i-structure
to display pop-ups for this to work.
The first time, it is not going to work.
We are going to make
reappear the applet and then, it is necessary to have
saved your work,
otherwise we would have problems, but the second
time, the online help is going to appear and for
example, here we can go to the online
help which explains how to create a free-body,
how to modify it, how to delete it, and the all of the functions
of the applet are described in the online help.
The last button that I want to show you
here is the background image.
The background image is what you see in the applet.
You will sometimes have a photo, you will have a little drawing, a sketch, etc.
We can make it disappear by clicking on this button.
If you have been worried, "But, I do not see anything
anymore !", click on this button and generally, the background image will reappear.
Another reason for which the drawing
can disappear is because you will have zoomed too much.
Here, I click on "zoom" and I zoom around the point where I am.
If I want to unzoom, I have to click in the same time
on "shift" and then now, I am dezooming.
If I click on "control", I can pan the zoom.
Most of you, at the least
if you have a computer will prefer to use
the scroll function of the middle wheel and to point, by clicking on the middle button. This enables to
do that much more easily than what I do with the applet.
However, sometimes there are secondary effects regarding
the environment which can be a little bit complicated.
Do not forget to click again on the zoom button to deactivate this mode at the end.
To conclude,
I want to summarize the buttons that we have see in the applet and their main
functions. This first button here, it is open a
file, it is relatively classic. But actually, it is rarely used.
As I said it, generally you will open a file through a link.
This button, however, it is used to save
and it is very important. This button enables
to activate or to deactivate
the background image. It is
also an important function, especially if
you lose the background image. This button enables you to zoom
or unzoom using the shift key or to
move using the control key. This button enables to introduce a support.
We will see later what a support is.
This button enables you to introduce a force.
We have seen that we can specify the amplitude and the orientation of the force
or that we can change it interactively at a later time.
This button enables
you to draw a free-body.
These two buttons, here, enable you to introduce red
lines, respectively
blue lines which represent lines in tension and in
compression. This button, here, enables to delete a force
or a support or a free-body. And then finally,
here, these buttons here are buttons equivalent to keyboards button which
reproduce the functions of the control,
shift an alt keys. Finally, here, very important,
this button enables you to have access to the online help, which I recommend you
to use to get more information about the applet.
During this lecture, we have seen the principle of activation
of the Java applet i-Cremona which is used within the framework of this course of structure.
We have seen how to open and save
an applet's file with data inside.
We have seen how to introduce
forces and supports
We have also seen how to introduce a
free-body and how to use the online help.
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