Formworks for Concrete Shells | DigitalFUTURES 2019

Computational Design And Engineering Of Tensioned Formworks For Concrete Shells

Instructor: Prof. Philippe Block | Tom Van Mele

TA: Xiao ZHANG

DigitalFUTURES 2019 Summer Workshop “Architectural Intelligence”, has ended up with great success. In Group1, Prof. Philippe Block and Dr. Tom Van Mele, University of ETH, has led 22 students from universities across the world to explore new dimensions in architecture with the theme of ” Computational Design and Engineering of Tensioned Formworks for Concrete Shells”, which aims to explore the application prospects and workflow of advanced computational design method and die construction technology in the design and construction of large-scale building components. Besides， we would give special thanks to the members from ‘International Undergraduate Competition” for their persistence and dedication.

Part1：Background

《Graphical Statics》 put forward in 1866 that as an engineer’s language, the structure should be expressed as a geometric image rather than an algebraic formula. Grapic Statics is a method to solve the equilibrium of force as a vector. Its core is the “triangle rule of force”, that is, if an object is balanced under force, its action force will be connected first and last to form a closed polygon.

Graphic Statics in Catenary Model

“Why is the shape found by the catenary inverted to the arch?”Because the “rope” can only bear tension, the force in the rope must follow the rope. By inverting the whole shape and replacing the flexible rope with a rod, the decomposition direction of the force can remain unchanged. Therefore, if there are enough subdividing points in the whole device, it will be closer to the acceptance form under uniform gravity load. This process is called “Form Finding”. The shape found is called “compression only” arch.

Graphics Static in Catenary Mesh

There are two steps in trying to solve statically deterministic problems in space. First, we must ensure that the component of the space vector under tension at a certain point is balanced on the XY plane. This process is called “Horizontal Equilibrium”. After theHorizontal Equilibrium and fixed, the whole device makes “Vertical Equilibrium” , enlarges the force points in equal proportion, and carries out Z-direction shape finding under the premise of maintaining the plane direction unchanged; therefore, no matter the force of each point is 5N or 100N, the plane Equilibrium preserves it after the static setting forever.

Part2：Workflow

（1）Introduction and installation of Compas platform

Compared with Grasshopper, a software which is familiar to only architectural students, relying on Rhino platform for visualization and editing, COMPAS is a python-based language platform which dose not need to attach to CAD, ordering a more basic-language based environment for researchers to collaborate further. The installation of it are showd on website:https://github.com/BlockResearchGroup/WS_digitalfutures/blob/master/windows.md 微信图片_20190731181732

（2）FOFIN (Form Finding)

After the installation of COMPAS, we found that many commands in Rhino are actually written by python text, such as “FOFIN_select”, a command case used to select the structure lines in multiple grid systems at a time. is actually python’s line-by-line read instructions in the background. 微信图片_20190731182208

Pyhon background with “FOFIN_select ” command in Rhino，@Xiao ZHANG

“FOFIN” is a 3D geometry shape finding plug-in which is similar to Kangaroo. But unlike ‘the black box operation’ of Kangaroo, it can output the force of each node and bar. If the command icons are designed for the FOFIN plugin, it becomes similar to the RhinoVAULT, the Python instruction set which can call COMPAS command library (the plugin released by the BRG research team in 2012 and has helped designers build a large number of shell projects). 微信图片_20190731182301

FOFIN workflow，@Ke LIU

Step1: Run the ‘FOFIN_init‘ command to find the FOFIN python library in your computer and call it into Rhino.Step2: Run “FOFIN_from“ to load the object to be typed. The “.jason” file is the most important file format in the work camp. It is the coordinates of each point in the grid system. User organizes Rhino model through the instruction set, forming three kinds of pickable geometries: ‘point, line, and face’.Step3: ‘FOFIN_select‘ and ‘FOFIN_attributes‘ work cooperatly. ‘Select’ is used for fast selection of elements (points, lines, surfaces) on the geometry. Multiple lines and points can be selected at one time separately according to the direction of U/V of the grid and the boundary continuously. ‘Attributes’ can set degree of anchorage at each point and degree of tension in each line.Step4: ‘FOFIN_run‘ is a comprehensive operator, finding statically determinate form of mesh space under the corresponding conditions. After Form-finding, designers can repeat the third step, making iterative optimization.‘FOFIN_settings‘ can show direction and size of the stress on each structural unit, such as anchor point and rope. ‘FOFIN_save“ stores the found form as geometric information with the data, which can be adjusted the next time.

The specific steps are shown in the figure below: 微信图片_20190731182308

Workflow of FOFIN plugin，@Xiao ZHANG 微信图片_20190731182316

Setting interface of FOFIN_attributes ＆ FOFIN_settings，@Xiao ZHANG

（3）Cable network system analysis

After the form-finding, students began to write the code under guidance of the professor, calling the COMPAS command library to manipulate the geometry. The first step is to generate a tension cable network system with mechanical significance, and secondly they generate a vertical screw at the intersection of the cable net system and to vent the reserved holes on the PVC film cloth.

Schematic diagram of the mold，@Philippe Block

Cable net and screw，@Xiao ZHANG @Dan LUOThe cable network system is established according to the seaming position of the mesh and place a vertical screw at each intersection. The description of the “vertex_normal“instruction relates to computer graphics, which can be referred to description of the COMPAS official website.For Example:

As“vertex_normal”command for example ，This command is to find the roemal vector for vertex on mesh，Here is how it works!

Step1: Let’s open COMPAS website

（https://compasdev.github.io/main/tutorials/networks.html）

微信图片_20190731182702

Step2: Through the website, we search “vertex_normal”, and it shows：Return the normal vector at the vertex as the weighted average of the normals of the neighboring faces. Here，the weighted average means the length of the vector has something to do with the area of the faces，The bigger the area of the face is, the more it will pull the vector of the vertex .

微信图片_20190731182707

Step3:Here, we take the four meshes on the model as an example. Firstly, we find thecenter points of four meshes in the blue part, whose coordinates are “average of four vertex coordinates”. Secondly, four sides are divided into four triangles according to the center point (Fig. 2). Four face plane normal vectors are solved, and their area quadrature with normal vectors (in Figure 3, the length of the normal vectors of each triangle is different), and the final sum of the vectors is displayed at the vertex of the mesh (Figure 3).

微信图片_20190731182714 微信图片_20190731182719 微信图片_20190731182726

微信图片_20190731182734

As shown in the code above, the points where ‘is anchor’ is true in ‘FOFIN_attributes-vertices’ are firstly colored. Then designers make sure the starting point, end point and the scale of vector. The figure below is the result after solving.

微信图片_20190731182745

（4）Production and construction of PVC film

Cable-net system is the core of whole mold and the double-layer PVC is container for concrete pouring. Its generation logic is to connect midpoints of cable net, reconstituting dual mesh and flattening fabric in UV direction. Designers renewed thickness of the two-layer film, according to the normal vector offset on the mesh. Here we simply show the instructions for the mesh offset and other commands displayed in the Github folder which students can download and study themselves. 微信图片_20190731183535

Generation and construction diagram，@Ke LIU

Name two offset faces ‘edos’ and ‘idos’ and give each vertex a new coordinate which is sum of ‘up/down’ vector with the ‘xyz vector’ by using the “set_vertex_attributes” command in the red box. 微信图片_20190731183545

△ Find the connected point for cables，@Kan LIN，Yumeng ZHNAG

The picture below shows the PVC fabric after cutting and sewing. In this experiment, kevlar line is used for suture combination and 650g/㎡ double layer PVC is used for fabric. 微信图片_20190731183601

（5）Pouring simulation and construction

Three rounds of concrete pouring were tested in this workshop. This part of the program is completed by ChaoYu DU, HaoYu LIU, Eleni Maria Skevaki, Keerthana Udaykumar and other students, which is an innovation attempt based on the COMPAS platform. 微信图片_20190731184144

Simulation of Casting 微信图片_20190731184211

△ Prototype 2 Construction process failed ,The proportion of glass fibre is too much, the fluidity of concrete is poor, and the irrigation is not smooth.，@Group1

The final proportion of concrete :1. Cement: 50kg2. Sand: 50kg

3. Water: 20.0-24kg

4. 2mm Glass Fiber: 1000g（1kg）

5. Secret White Powder: 25g

6. Plasticizer：1000g（1kg）

（6）Concrete watering and demoulding

Group1 students began concrete demoulding after 30 hours of concrete watering. Due to the insufficient vibration of the watering process and the problem of horizontal stitching of the local mold, the mold was broken. Fortunately, the team members manually sutured and blocked them in time (@Cong WANG), and finally completed the work successfully. 微信图片_20190731190539