Introduction to Evolutionary Structural Optimisation (ESO)

Dear Student, these pages will give you a short introduction to the use of ESO as a reference for your project work for our ‘Environmental Performance’ Studio this semester.

ESO is a fairly new method of deriving minimal structures that ‘evolve’ on the basis of some simple geometry-input, basic load-cases and an optimisation algorithm. I will not bore you with the exact inner workings of the optimisation process.

Within this studio it is not the goal for you to become structural engineers and do any sorts of calculations. The software we are using for the ESO process is called Evolve97 and its beauty is its absolute rawness. It allows you to set up sections of your design within seconds and to add values and constraints using rough estimates. Evolve 97 then develops a minimal structure in front of your eyes. In that sense the type of information it provides is very useful for decision support for you to develop your design. Evolve97 is not a commercial product so it certainly has a few flaws that we have to work around. Once you follow the guidelines given in this text, you should be on the safe side and you’ll soon recognize the great stuff that can be done with it!

First of all a bit of background information:

The basic concept of the ESO method is simple: by testing for redundancy/inefficiency, material is removed through digital sculpting from an oversized structure under a particular load-condition in an iterative process. The evolutionary algorithm can be applied on a 2D or 3D CAD-model to classify entities which get derived from a Finite Element Analysis (FEA) mesh. Those elements in the model bearing the lowest stresses get removed by the algorithm and loads get redistributed amongst their neighbouring elements. Through this process, the residual shape gradually evolves towards an optimum. At the end of the optimisation process, each component is indispensable within the structure.

Figure 1: Catenary ESO iterations: 0, 19, 38, 57 and 76

You can define ‘design’ and ‘non-design elements in your section. Whereas the first can be removed by the algorithm, the latter will always remain as part of the structure. ’Following a finite element analysis the stresses of all designable elements are compared, and a small number of the most low-stressed elements are removed from the model. The modified model is then sent back to FEA and the second iteration begins. Most often, after 10-100 iterations we can obtain a very efficient structure, in which the material distribution has been optimized.

When using ESO for structural detail design, we can modify the shape of some part of a structure to reduce the so-called stress concentration. In this way, the optimized structure could be more durable.

The ESO process has just recently been applied to a real life building project as can be seen in Fig ???. This project for an office building in Nagoya (Japan) displays a structure that has been developed to find the minimal structural components for a four storey high building in an urban context. The structural components of the building are materialised as concrete wall/columns integrated in the façade-plane. The component developed through ESO makes the flow of forces readable in a highly dynamic fashion.

Figures 2-4: Office building in Nagoya, elevation and perspective views of the final project

Another project which has been developed in 2002 with the help of ESO is a competition entry for the Florence New Station in Italy. Here the Japanese architect Arata Isozaki collaborated with the Japanese engineer Mutsuro Sasaki to develop a 150m long, 26m wide and 15m high structure.

Figure 5-7: Florence New Station, redefinition of geometry derived from ESO by using a 3D steel framework

Evolve 97 Guidelines:

As mentioned in the introduction, this software is not a commercial product and there are some rules you have to follow to get the best out of it!

Before you even start working with it, you have to create a folder on your hard-drive that you can easily find when you set up a ‘new’ file in Evolve97. It is not advisable to just put it in C: or D: as the software will write up to 20 files for each file that will contain the project information. The best way is to set up a folder that contains your own name to avoid others overwriting it! (e.g. “D:\Stuart_Smith\”)

The one thing that’s a bit annoying about Evolve97 is that it does not have an ‘undo’ function. In order to work around this, I advise you to save each major step of the configuration of you file under a different name so you can always go back to previous settings without losing information. The main steps to optimize a structure in Evolve97 are as follows:

1) Draw an oversized ‘Design’ shape (reflecting a section through your project)2) Add ‘non-Design’ areas3) Subdivide the shape in equal (finite) elements (meshing)4) Refine ‘design’ , ‘non-design’ and void elements5) Define loads and restraints and place them in your section6) Run Finite Element Analysis (FEA)7) Run ESO8) Observe the process

Let’s go through those steps one by one:

1) After you have set up a folder at a location accessible to you, go into Evolve97 and set up a new file. Next go to the ‘Drawing Tools Menu’ and chose between creating a rectangular, a round and a free-form shape (make sure the according button is highlighted)

When you chose for a freeform shape, you can closeit by clicking close to the starting point (it will turn green, try a few times if it doesn’t work right away.

2) You can add non-Design (orange) elements by clickingon the Stop-sign button and creating shapes like thoseof ‘Design’ elements

3) Now let’s generate a mesh for Finite Element Analysis: First select the mesh density (yellow button). Choose ‘Very fine’ 88x55, press ok then press the ‘Create FE mesh (orange& green) button for meshing. A dialogue will pop up – press start and at the end – press ‘close’

4) Go to the ‘Mesh Structure and ESO Parameter set up’ menu. Here you can refine the mesh by adding (Draw new plates to mesh)and deleting (Delete individual elements from FE mesh) elements.At this point it is advisable to save your file: give it a ‘just_mesh’ name and then save it again as ‘mesh_loads’ (that way you can always go back to the original mesh file)

5) Now let’s add loads and supports (nodes). As part of the ESO parameter setup you first define the type of loads (Specify load to apply) and then the type of supports (Specify Node freedom conditions) Use the following settings for the loads:

This reflects a vertical load (gravity) with no lateral (wind loads). You can play with these settings and see how results vary if you change them! The selection for the Freedom Conditions should be as seen on the left

Again you can try different variations (e.g. set a node as Y-Displacement only if you want to fix a support on the wall). Once you have made your selections you can start applying the loads and nodes by clicking once on the relevant button and then clicking on the areas on your mesh where you wish to place them. Nodes only require one click per support, loads can be repeated to approximate distributed loads over a distance.

6) Once all loads and nodes are placed we can conduct Finite Element Analysis (FEA) by simply pressing

the FEA button. An extra menu will appear press the ‘Start’ button. At the end of the process we can visualize the result by pressing the ‘Select what to draw on screen’ button (left upper corner). In the pop-up menu select the bottom tag (Plate Stress). As you can see- nothing has happened – you now have to press the ‘D’ button to refresh the mesh- display. Your display is now set to represent plate (element) stress and from now on you can switch between different representations.

7) Now we are ready for the ESO process. It is essential to reduce the ‘Zoom Factor’ by at least 0,5 as the computer is likely to crash otherwise (the smaller the zoom size – the less likely it will crash -I never quite figured out why that happens)! Once the size is reduced, simply click the green ‘E’ button and press ‘Start’ in the pop-up menu. Now the optimisation process unfolds in front of your eyes and you can see how the algorithm takes away elements step by step. You can pause the process at any time and take snapshots of the current iteration by clicking the walking man (Save Display Area to Bitmap Image file) button on the top right. I suggest taking a few of these snapshots as a record for later evaluation.

If, for some reason the process crashes you can simply restart Evolve97 and reopen the file. The ESO process will have saved the latest iteration and you can press start anew to continue.

Now interpret the process as well as the result to fit your design!