Handcrafted tensile membrane structure with low-tech form finding and construction

Proceedings of the IASS-SLTE 2014 Symposium “Shells, Membranes and Spatial Structures: Footprints”

15 to 19 September 2014, Brasilia, Brazil Reyolando M.L.R.F. BRASIL and Ruy M.O. PAULETTI (eds.)

Copyright © 2014 by the authors.

Published by the International Association for Shell and Spatial Structures (IASS) with permission. 1

Handcrafted tensile membrane structure with low-tech form

finding and construction

Joao Victor CORREIA DE MELO*, José Luiz RIPPERa, Patrick STOFFEL

a,

Laboratório de Investigação em Livre Desenho, Departamento de Artes e Design

Pontifícia Universidade Católica do Rio de Janeiro

Rua Marquês de São Vicente, 225, Gávea, Rio de Janeiro, Brasil. [email protected]

a Laboratório de Investigação em Livre Desenho, Departamento de Artes e Design

Pontifícia Universidade Católica do Rio de Janeiro

Abstract

Tensile membrane structures are used by mankind since the first days of its existence, on the tents of the nomad

hunters and gatherers’ settlements. Those tents were built without the need of big planning and material process.

Nowadays, our technic/scientific society made a lot of improvements on this kind of structures, nevertheless the

design and construction complexity became so higher that turned this objects very expensive and specialized.

Trying to get in a midpoint between these two ages of membrane structures, the research at the Laboratory for

Investigation in Living Design – LILD, attempts to unite the logical academic modern knowledge and the

spontaneous popular ancient knowledge, in order to spread and popularize handcrafted reputable lightweight

structures

In this paper, the construction of an experimental tensile membrane structure is described. Such object was built

in the city of Formoso - in São Paulo state, southeast Brazil - in an open terrain exposed to sun, rain, and winds.

It aims to be a weather shelter covering.

Following the precepts of LILD`s research, this membrane structures was first studied in scale models. These

models were used, at first, for form finding, and, after that, for measurements and constructive method studies.

The object is constituted by two parts. The first one is the main structure. Based on tensegrity technology, with

some adaptations, it is a prestressed freestanding structure composed by four arched bamboo beams, three

bamboo poles - which shape up spatially the object -, and synthetic cables for prestressing the whole system.

The second one is the composite membrane. It has two parts. The first one is the cable net, composed by a

synthetic cable net prestressed over the main structure tied at its beam edges, and sewed at its cable perimeter.

This net describes the basic geometry of the final object and, more than this, it receives and distributes

prestressing and external loads through the whole system.

The other part is the membrane weatherproof roofing. It has basic units which are pre-fabricated composite strips

made of jute fabric and raw earth. Their lengths vary. These strips have superposed zones where they are bound

up and then rested over the cable net. Variations in these overlaps allow the flat strips to discrete the cable net

geometry. At last, the strips are sewed over the main structure cable perimeter, which attach it to the whole

system. After the drying of the raw earth the membrane is painted with castor polyurethane resin for waterproof.

This research is still an initial step in the search for the handcrafted tensioned membrane, which could be made

by layman. The object is still under construction. However, technical matters which came up during its

assemblage, up to this point, have already given hints to the understanding and improvement of this technology.

Keywords: Tensile Membrane, Bamboo, Lightweight Structure, Raw Earth, Low-tech form finding

1. Introduction

The tent, as it is being building since thousands and thousands of years, is an effective and simple covering

system, being adaptable trough the most different physical/geographical environments. It allows constant


Copyright © 2014 by the author(s).

Published by the International Association for Shell and Spatial Structures (IASS) with permission.

assemblage and disassemblage, practical and efficient transport, that fits in most diverse situations, from

traditional nomad cultures, to modern life needs.

Traditional tents were generated in a very similar way as the spontaneous natural process, that means, consuming

the fewer potential energy existent in the environment in which it is constructed. Nowadays, our

technical/scientifical society, in order to concretize megastructures, brought a lot of specialization to this kind of

covering. Nevertheless, these big and heavy load constructions, turned the model to be copied by medium and

low load structures, ignoring the simple solutions from the ancient cultures, bringing unnecessary technical

sophistication, and higher costs, to smaller tents. (BECHTOLD, 2008).

Trying to get in a midpoint between these two ages of membrane constructions, the research at the Laboratory

for Investigation in Living Design – LILD, attempts to unite the logical academic modern knowledge and the

spontaneous popular ancient knowledge, in order to spread and popularize handcrafted reputable lightweight

structures (CORREIA DE MELO, RIPPER, & YAMAKI, 2012).

In this paper, the construction of an experimental tensile membrane structure is described. Such object was built

in the city of Formoso - São Paulo state, southeast Brazil - in an open terrain exposed to sun, rain, and winds. It

aims to be a weather shelter covering.

2. Methodology

The Laboratory for Investigation in Living Design (LILD) from PUC-Rio’s Department of Arts and Design –

together with LASE (Structural Systems Laboratory – UFMG) and Bambutec (a partner company which works

with design, assemblage, and renting of bamboo structures for events and fairs) – is developing appropriated

techniques for the use of raw, or nearly raw, natural materials, in lightweight, shallow foundations and low

energy constructions. These objects results from the combination of the so called “game units” (MOREIRA &

RIPPER, 2014) - made of bamboo culms and strips, natural fibers, and raw earth - looking for the development

of shapes with high strength due to its form/material conjugation. In resume systems which are feasibles in an

economic standpoint, and permits assemblages, disassemblages, and reassemblages, with the possibility of a

total reuse of its elements. These systems should be realized in convivial practices, where the users of the objects

are involved in their constructions, which guarantee the correct use, maintenance, and discard of it [1, 2, 3].

Other important point refers to the obtaining of the structural forms that should be adequate to the mechanical

specificities of the materials that are used. The forms of these structures are not those produced by man’s

imagination. To describe the geometry of the objects, shapes derived from those unveiled from the nature, and

those obtained by devices which were putted in interaction with some natural phenomena - like, the gravity, the

hydrostatic pressure, the liquid surface tension, among others [4, 5, 6, 7]. Some other existent reputable forms,

recognized by their high resistance grades because of its geometries, are used, such as regular polyhedrons (and

their geodesic derivation), spheres, pressure curves, etc..

This methodology is a result of the lab’s working process, which is subjected to the technical development and

the constant learning from the natural formation, and, as a process, is in a continuous transformation. It is being

demonstrating its effectiveness not only in LILD’s research, but too, in LASE’s research, as showed by Moreira,

Ripper & Caliman [8].

2. Self-standing behaviour

At LILD, the works about self-standing bamboo structures originate from equipment intended to people with

motor disabilities [Fig.1]. These devices are lightweight, bamboo vehicles, powered by human strength,

handcrafted and custom-made, achieved and in continuous improvement with techniques specially focused on

the structural conditions of bamboo.

The bamboo walkers help children that have balance issues caused by cerebral paralysis. They have been made

in various types and with different techniques. The joints of the devices showed are based on the ones that

Santos Dumont used as his balloon basket, and that was the joint solution, combined with the tensegrity, that

later made possible the bamboo bicycles produced in Denmark. Those joints have tensioned cables along the

inside of the culm. Another type of vehicle displayed is the amphibious sliders, which allow people in

wheelchairs and other motor disabled people to move in places where the wheel does not work.




The observation of those devices´ functioning affirms, not only the general potential of the employed techniques,

but also improve the condition of structural self-standing resulted from the, at times violent, movements of the

vehicles [9].

Figure 1 – Walkers and Amphibian vehicles.

The acquired knowledge with those experiments, performed in the real social environment, opens the way to

new formulations on structural typology that are favorable to self-standing. This has been achieved following the

same structural self-standing conditions of the previously mentioned devices, that refers to shallow foundations,

that is, the ones just rested on the surface of the terrain.

These structures work as structural cores, and is being applied on membrane covering infrastructures and support

elements for structural shells – specially made of fibered raw earth (a natural composite) [10]– without the need

of the usual foundations, or big and heavy anchorage blocks.

3. Preceding self-standing textile covering objects

Objects with a small size, in general, brings more information and understanding to the researcher than others

which have bigger sizes, or are very bigger compared with the man. It happens because related to the small ones

the researcher is a giant and dominates it, can manipulate it with their own body. In an opposite side the bigger

ones dominates the researcher because their body are too small compared to the object. Some structures which

were developed with this logic, starting from small to understand and dominate the bigger one, is being

concretized and are research precedents of the structure threated in this paper.

3.1. Self-standing Tents made of Bamboo Culms

As showed earlier, the research on bamboo structure has its begining in vehicules toa id people with moviment

disabilities. Those objects utilized the same joint as the Brazilian inventor Alberto Santos Dumont used in its

airplanes and Balloons. The joint works very well; nevertheless it needs an assembler with a high level

technique.

Trying to make the connections between the bamboo bars simpler, the research turns to the tensegrity

technology. Developed by Kenneth Snelson and theorized by R. B. Fuller, this structural typology divides in its

elements the tension field of the object: the rigid bars are responsible to bear the discontinuous compression

field; and the cable net will bears the continuous tensile field. These conditions are very suitable for the

mechanical properties of the integral bamboo culm, and are very effective in the construction of functional,

lightweight, self-standing objects, as can be demonstrated by a tent suspended by a tensegrity geodesic dome

assembled as a research device to the thesis “Non-visual sensorial aesthetics: the haptic beauty” [11] [fig.2].




Figure 2 – Tensegrity geodesic tent

Other application of this structural principle is the Tensegrity Tent an under graduation project presented by the

designer Mario Seixas (PUC-Rio 2011). Intended to be a device for the Rio de Janeiro’s public markets, it shows

some tensegrity structure benefits: versatility, energetic efficiency, lightweight and mechanical simplicity .[fig.3]

Recent developments on tensegrity structures in being carried out by the lab’s research partner Bambutec, a

company established by LILD’s former researchers which works renting equipment for fairs and exhibitions. An

example of Bambutec’s developments is a tent with a tensegrity structure made of bamboo culms and synthetic

cables which was assembled as a commercial exhibition booth at the Organic Product Fair in São Paulo. The

conjugation of the rigid bars and the cable net was especially studied to permit an assemblage with a maximum

of twelve hours and only four people working on it. In this way, the assemblage plans should be continuously

optimized and the self-standing tensegrity structures is being demonstrating a great value for this kind of

application. A structural analysis of the object was made by LASE [8], which shows an intense partnership and

research exchange between LILD, LASE and Bambutec, looking for the development of the complete cycle of

an object: concept, analysis and cultural application.

Figure 3 - Self-tensioned tents – up: Tensegrity Tent (Mario Seixas, PUC-Rio, 2001) down: Bambutec’s Tent

(Bambutec, Rio de Janeiro, 2010)




The “Sombrinha” is another object that demonstrates these precepts, but different from the other ones, it has a

clear separation between the main structure and the covering. Circa 2003, the lab’s research was looking for a

covering for the bamboo geodesic dome, which, in that moment was understood, not only as an emblematic

structure, but too as a structural core, capable of bearing external loads. A millenary object was studied in order

to solve this problem: the Chinese Umbrella – made of bamboo, rice paper – which adapted to the dome, could

cover the inner space. The chinese umbrella are very noticeable because of its delicacy, structural simplicity and

functionality. At first, to demonstrate the easy way it could be assembled over a dome, a regular umbrella was

attached to a dome model, and after some studies on it, the main structure and the membrane paneling could be

find and two objects in real size were constructed one with five parts and one with eight parts. These objects, in

“in use state”, were made of bamboo culms, cotton fabric, and an iron central joint.

The “Sombrinhas” were installed in the garden of PUC-Rio’s campus and was placed in the air by means of

tension cables anchored in the trees. This way the objects could be tested in their usability conditions and

aerodynamics behavior. Left in their place for a few months, the “Sombrinhas” resisted to strong winds,

especially because of their anchor points which permitted strong vertical movements without affecting the

structure [12, 13].

Figure 4 – “Sombrinhas” made of bamboo and cotton fabric, a look at the millenary chinese umbrella

The idea to construct the structure shown in this paper, was originated from the necessity to cover an old shelter

constructed in the lab’s advanced experimental field located in the small town of Formoso, in São Paulo state,

southeast Brazil. The first experiment tried in order to cover this space was done several years ago. In the

occasion a tensegrity structure was assembled and covered with a plastic canvas [fig.5].

Figure 3 – First covering of the shelter. Tensegrity Structure.




The structure was removed because of environment actions. The wind effect tear down de canvas, nevertheless

the tensegrity main structure was kept intact, which brought a lot of data to the planning and execution of a new

covering proposed by the researcher Patrick Stoffel.

4. Handcrafted tensile membrane structure with low-tech form finding and construction

Following the precepts of the lab`s research, this membrane structures was first studied in scale models [5, 6, 7].

These models were used, at first, for form finding and structure development, and, after that, for measurements

and constructive method studies.

The object is constituted by two parts. The first one is the main structure. It is a prestressed freestanding structure

composed by arched bamboo beams, bamboo poles - which shape up spatially the object -, and synthetic cables

for tying and prestressing the whole system.

Figure 4 - Miniature showing the infrastructure and miniature with the membrane

The structural principle of this main structure is very similar to the tensegrity structures. Its basic structural

elements are two: compression loaded elements and tensile loaded elements. The joint’s level of freedom is the

main conceptual difference between the tensegritys and the structure proposed here. In the tensegritys, the

contact among the bars is avoided, in order to guarantee only compression loads in the bar, and tensile loads in

the cables. In a different way, the proposed structure has some tied bar joints, nevertheless, in general, the

bamboo beams are still working under compression and the cable net still works under tensile loads.

The beams are arranged in two directions, longitudinal and transverse to the object and can be separated into

primary, the first case, and secondary, the second case. In order to take advantage of the bamboo culm natural

flexure, pre-arched beams were chosen, in a similar way as the indian bow. It would be impossible to reach the

necessary arching to the system using only a large diameter culm; hence, it was chosen to make beams with

bamboo culm stripes of the species bambusa tuldoide – available locally – with a diameter between 3 to 4 cm,

already tied-up and attached to the curve.

In order to obtain the curve projected to the arches – a symmetric one – the process employed was the bamboo

compensation due to its conic form, that is, the bamboos were organized so to always have a bamboo in each

direction, the basis of a culm is on the tip of the other one.

Natural imperfections to the linearity of the culms when displayed in bundles are attenuated by the whole.

Another interesting factor is that by composing the bundle, in case of any bamboo is deteriorated as by insects,

for example, this is compensated by the others and so it continues to contribute geometrically to the performance

of the element it works with.




Figure 5 – Beams bundles preparation

The main beam – which is in a longitudinal position related to the object – was encapsulated with a composite

made of castor oil resin and natural fibers. This composite, besides of general strength improvement, was applied

because of the direct contact between the arch and the membrane which brings a proper environment for insects

and fungus, which could attack the bamboos if they were open. The secondary beams received bandages with the

same composite, but only as reinforcement in specific points where there is interaction with other in the system,

as the membrane and masts.

The cable net is composed of polypropylene cables with 7 cm of diameter. Those cables are ties-up and the

extremities of the beams, creating a perimeter that afterwards is tensioned. Secondary cables are also tied-up and

tensioned perpendicular to the secondary beams as to create a net of vertical tension between them and the main

beam.

Figure 6 - Main cable net assemblage




The vertical volume and the necessary space which will give tension to the whole system are given by some

masts made of bamboo pieces from the species bambusa vulgaris with twelve centimeters in diameter. These

elements, are also called “tension elements”, are placed between the main and the secondary beams, fixing the

main geometry.

Figure 7 - Infrastructure.

Over the infrastructure, covering the system, natural composite strips are placed. This composite is made of

natural jute fabric and raw earth. The length of this strips varies and the overlap of them permits the

discretization of the intended geometry, in a similar way as IL used to obtain the cutting pattern for tension and

pneumatic structure the so-called “comb method” [14]. When the composite is dry, the membrane is painted

with a castor oil resin, which gives waterproof to the covering.

Figura 8 – Composite Membrane




5. Conclusions

It is known that this research is still an initial step in the search for the handcrafted tensioned membrane which

could be made by layman. The object is still under construction. However, technical matters which came up

during its assemblage, up to this point, have already given hints to the understanding and improvement of this

technology.

One of the findings is that the covering strips can be lighter, which could improve the structure efficiency and

appliance on bigger objects. Another important conclusion is that the strips direction also influences the

structural quality of the membrane. Therefore, a new arrangement of strips is needed, so it could improve the

load distribution, as well as rain water paths, on the surface and through the main structure.

Looking for future optimizations of the object, it is proposed a restudy of the obtained shape, and the cable net

geometry. For that, it is intended to use, as a base theory, the soap film’s minimal surfaces. Some preliminary

experiments is being realized, indicating promising paths towards the reduction of materials, weight, labour and

membrane tensions.

Figure 9 – Preliminar soap film model

References

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[2] CAMPOS, D. M. e MELO, J. V., Square-based Bamboo Dome Based on Geodesics, em IC-NOCMAT-2011

Book of Abstracts, Changsha, China, (2011).

[3] CORREIA DE MELO, J. V., RIPPER, J. L. M. e YAMAKI, R. T., Formfinding Process for Bamboo

Structures, World Bamboo Congress, Antwerp, (2012).

[4] RIPPER, J. L. M. e MOREIRA, L. E., Métodos de Ensino de Design de Produtos e sua Aplicação às

Estruturas da Engenharia Civil, em Conngresso Brasileiro de Ensino de Engenharia, Brasília, (2004).

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[7] YAMAKI, R. T., O uso da miniatura no desenvolvimento e passagem de formas técnicas: subjetividade e

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[14] OTTO F., et al, IL 15 - Lufthallenhandbuch - Air Hall Handbook, Stuttgart: ILEK, (1982).

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Handcrafted tensile membrane structure with low-tech form finding and construction