KẾT CẤU MỚI ON FREI OTTO''S PHILOSOPHY OF WIDESPAN LIGHTWEIGHT STRUCTURES

BENDING AND LATTICE STRUCTURES The scaffolding lattice system for roofs was devised to avoid the volume of material that would have been required of a ''bending'' structure. Small diameter compression or tension tubes in a three dimensional lattice transfer the roof loads back to a few columns for the 102m x 52m membrane covered Interbau Buildings Berlin 1961. This was a ''first'' for Mero and in a way the precursor to the many lattice space frame structures by Kenzo Tange at Osaka 1970 (fig 1).

ON FREI OTTO'S PHILOSOPHY OF WIDESPAN

LIGHTWEIGHT STRUCTURES

Michael Dickson

This is a text prepared in collaboration with Professor

Frei Otto and based on a presentation given on his behalf

at the inaugural session of the Bath University

Symposium on Widespan Enclosures in April 2000

Frei Otto's long career in lightweight structures includes

the development of stressed tensile sails for the Lausanne

EXP064, the distinguished German Pavilion membrane

and pre-stressed cable nets for EXP067 in Montreal, the

Munich Olympic Roofs in 1972, and the Gridshell at

Mannheim in 1975 The conceptual design studies for

these and many other projects were carried out at the

Institut fur LeichteFlachentragwerke (I.L.) which Frei

founded at the University of Stuttgart Between 1967

and 1995 he worked often with Ted Happold, a friend

and fellow spirit, on such projects as the 120m x 90m

cable net structure for Jeddah University and the

Diplomatic Club, Riyadh, (Aga Khan prize for

architecture 1998) Professor Otto's current work

includes consultancy on the Venezuelan and Japanese

pavilions for the EXPO at Hannover 2000, and

conceptual design for the new railway station for

Stuttgart 21 He is one of the leading innovators in the

development of lightweight structures and has recently

received the following international prizes:

• Honda prize for ecological technology 1990, Tokyo

• 77ie* principle prize of the German Institution of

0 Architects and Engineers in Berlin, 1996

• The Wolf Prize for Arts, Jerusalem, 1997

He is an Honorary Fellow of the Institution of Structural

Engineers and of the Royal Institute of British Architects,

and Dr of Science honoris causa at the University of

Bath

I N T R O D U C T I O N

In both the developed and the developing world,

widespan enclosures are increasingly required to house

and facilitate many of the collective activities of society

Such enclosures need to do this without drawing down

excessive quantities of scarce construction material or

drawing upon unnecessary quantities of energy in their

operation To ensure such aims requires an efficacy of

construction, a delight in their occupation and

appropriateness to their location Beauty of architecture,

efficiency of structural form and appropriateness of

material usage are the fundamentals in securing this aim Yet in the solution of this theme the use of large spans is not just a game to make the Guinness Book of Records but a search for real solutions for mankind To know about large spans also opens opportunities for advances for shorter spans In the absence of scale factors on short spans it is possible to use material less effectively Conversely for the larger spans, it is necessary to seek out fundamental 'absolutes' of performance and to recognise the significance of 'scale' and the problems of enclosure

Part of this search is the recognition of optimal performance and benefits of different structural forms in ascending order of opportunity - so this paper tackles the fundamentals of performance of successive structural types - bending structures for smaller spans, lattice structures, gridshells and compression vaults, tension structures and finally the opportunities for pneumatic structures These structural systems are discussed and illustrated principally through a wide variety of projects

B E N D I N G A N D L A T T I C E S T R U C T U R E S

The scaffolding lattice system for roofs was devised to avoid the volume of material that would have been required of a 'bending' structure Small diameter compression or tension tubes in a three dimensional lattice transfer the roof loads back to a few columns for the 102m x 52m membrane covered Interbau Buildings Berlin 1961 This was a 'first' for Mero and in a way the precursor to the many lattice space frame structures by Kenzo Tange at Osaka 1970 (fig 1)

Prefabricated standardised galvanised Delta units and

'bolted' cross nodes made up the 42 cm deep

intermediate viewing platforms for the German Pavilion

at Montreal (1967) Engineered by Leonhardt and Andra

the cruciform head units positioned diagonally across the

grid concentrate loading from the floor grid onto the

column top, each element as in the human body prepared

for its particular duty (fig 2)

Fig 2

In the 24m high Bell Tower for Berlin (fig 3), the

architectural form of the virendeel truss is retained while

the plate thickness is aggregated from 10mm at the top to

50mm at the bottom in order to restrain the drift of the

Tower to 16mm under the ringing action of the 3 bells

-function following form:

Fig 3

D I R E C T F O R C E S T R U C T U R E S

Inescapably, the most efficient way to transfer load back

to foundations is by a 'direct' way - an inclined straight spar Early investigations with the mushroom support 'spars' to the 'humped' tent at Koln 1957 led to studies for radiating 'fan' systems for the Transrapide Maglev viaduct system (figure 4a) It should be noted in passing that the alternate form to the nose of the model capsule

on the bridge is itself a holistic proposal to reduce wind

resistance at speed, hence the required magnet power and

therefore also the weight to be supported by the bridge

itself

The purpose of the viaduct design for the Maglev was to

reduce the impact of the linear induction Transrapide

support system on the countryside of Northern Germany

from Hamburg to Berlin The Transrapide Maglev is a

light multicar system capable of travelling at up to 450

km/hr using the technology of aircraft systems Breaking

loads from emergency deceleration are more critical to

the support structures than vertical loading Structural

continuity and close accuracy of construction, allowing

also for thermal distortions, is essential for ride comfort

- hence the structural concept of a minimal triangulated

tubular network casting little shade on the ground below

and supporting loads onto simple foundations

Fig 4b

A further development of these thoughts has led to the

fan pedestrian bridge system for Gelsenkirchen 1999 (fig

4b) In line with earlier studies of bamboo structures at

the I.L., the various spans of this radiating system are to

be made from kit form of solid 70mm galvanised bars

and 4-bolt cup-clamp systems Individual buckling

lengths are to be reduced by an internal criss cross of

stabilising bars - also 70mm 0 The 1960 study at Yale

for a thin roof did so by dividing the individual spars to

form a "triangulated" network of stable compression

elements (fig 5) For the Council of Ministers project in

Riyadh (1978) the loading from the 3 ' D ' hexagonal grid

shell for the seating bowl is gathered by irregular

triangulated configurations of tubes of successively

increasing diameter These match the buckling length

restrictions to the requirements of increasing load back

onto a single composite column of 3 individual braced

tubes (fig 6)

Fig 6

But the aesthetic of a design in its surrounding is also of great importance - the tree fountain on its well in Warmbronn drips its water carefully into the well (fig 7) Sometimes a symbol will be sufficient For the one day meeting of the Evangelic Church at the Berlin Olympic Stadium (1961), only a single 40m high guyed cross structure was needed to express the enclosure (fig 8)

Fig 7

I

1 L ^ A

1 i 1

''ii'-tnnMi''

K '

Fig 8

F U N D A M E N T A L S O F M A T E R I A L A N D

F O R M

In terms of the 'absolutes' of measurement, illustrations

on a logarithmic scale relate the basic forms of stability

of everything from mountains to grasses and hairs - a i m

high grass has a H/D of 100 or more (fig 9) Of

consideration too, for all enclosure tension studies, are

the fundamental rupture lengths of different materials

under their self-weight - wood is the leader

1

f ! QB'«a»

/ «f

1

1 '

1 i« m

r \ f

| * 1

L ,J » f; 7h ? i

—

J s

I *

'j

-r-' -r-' j

J s

I *

'j

-r-i

Fig 9

I

Other studies have also shed light on fundamentals of performance:

• What stable forms does sand create when allowed to run away? (fig 13)

• What are the laws of form for spine structures (fig 10) and for hanging vaults? (fig 11)

Fig 13

• Based on the historic shells from Harran (fig 12) what crucial forms from local brickwork can resist the lateral forces of an earthquake? - as measured on the tipping table, the cone of make 0.3g (30°)

The proposals for the naturally light and ventilated forms

for Islamic University at Uzbekistan, constructed solely

of bricks is the outcome of such studies (fig 14) To

optimise bridging, theoretical studies show that you can

bridge 10 miles so probably at least 1/10 of that can be

achieved in reality How do vaults really work? In the

vousoir model, it is to be noted that the zig-zag string

transmits the 'shear' for stability (fig 15)

Indeed the study of arch forms led directly to the form of the openings in the supporting walls of the Diplomatic Club, Riyadh [now CasaTuwaiq] (fig 18)

Fig 15

On the tipping table, lower arch forms are more stable

than high forms Even arches can be curved in plan,

(figures 16 & 17)

Fig 16

Fig 18

S H E L L S , G R I D - S H E L L S A N D VAULTS

In 1958, with the help of students at Washington University, a rubber membrane weighted with nails was used to investigate forms "without bending" Such forms were then stiffened with plaster and inverted into a shell form

There followed the single layer timber gridshells for Essen (1962) (fig 19) and that by students at Berkeley (1962) constructed out of steel rods and washers from the hardware store (fig 20)

The bending free grid-shell form is really a low cost

construction method for creating complex forms for

public space An early example is the auditorium of the

German Pavilion, Montreal (1967) prefabricated in

Germany and drawn out into its final form on site (fig

21) This was a forerunner to the minimal energy house

designed for Ted Happold Here oak lath gridshell, turf

covering, south facing glass wall, pv cells and wind

generation are all part of a holistic approach to design

(fig 22)

Fig 22

With the help of the computer, there are now forms

which are difficult/impossible to model physically - the

naturally light and ventilated workshop in Dorset for

John Makepeace of roundwood spruce trees formed by

green bending the tapered green debarked trees is one

(fig 23) Another, the Japanese Pavilion at Hannover

with Shigeru Ban is in reality only "findable" on the

computer even though here physical modelling gets

close to the final form (fig 24) An originally flat grid of

12cm diameter paper tubes banded together in a 1 metre

grid is pushed up to form a bended amphora form

subsequently stiffened by the cable - undertied timber

ladders and diagonally braced cable formed honeycomb

end walls In turn these were used to attach the paper membrane All components including the "sand box" foundations were designed to be easily recyclable and so give an enclosure which specifically "touches both the 'planet' and ground lightly" (fig 25)

Fig 25

The double layer gridshells for the Bundesgartenschau, Mannheim designed by Frei with Mutschler, Langer, Happold and Liddell were most courageous enclosures and extremely inexpensive (fig 26) So inexpensive that Kikutake followed them with a much larger 'shell' for the Japanese Silk Road Exhibition in 1988 (fig 27)

'Inversion' of the tension eye for Montreal and the I.L (fig 28) led to the development of the compression forms for the new below-ground naturally light station beneath the Schlossgarten, Stuttgart with Ingenhoven and Buro Happold/LAP (fig 29) The inverted forms modelled in plaster span a grid of 60m x 30m using only a concrete vault 35cm thick at the crown thickened to 65 cms around the eye Indeed each pier supports of the order of 35,OOOkN of loading from the landforms above

Recent form models for Stuttgart 21 envisage inexpensive construction techniques using propped timber grid shell forms (remember the bending free forms of Mannheim) to create the free vaulted form from the plasticity of wet reinforced concrete

H A N G I N G S T R U C T U R E S A N D D E A D

W E I G H T F O R M S

Simple hanging forms are able to exploit the

effectiveness of the long rupture lengths of tension fibres

- especially if they can be stabilised against disturbing

loads by self weight, damping or enclosure Early

studies for a pagoda roof 1983 previewed the prototype

house at Hooke Park with Richard Burton The hanging

roundwood spruce thinnings curved down under dead

weight are opposed by A frame compression spans (fig

30) The elegant Wilkhahn factory with Gestering

architects and Speik und Hinkes engineers for timber

products in its agricultural landscape uses a similar

philosophy but employs square sawn timber (fig 31)

Really this was a focussed study in the use of minimum

embodied energy and of minimum operational energy in

the industrial context - built in the countryside

Fig 31

Both are predated by the aluminium covered, heavily

insulated auditorium for Mecca with

Gutbrod/Arup/Happold The 22mm 0 spiral cables

hanging from the central steel portal are cross connected

by double angles that support and distribute the loads of

the insulation and cladding and contain the enclosed air

volume (1968) (fig 32)

Fig 32

More daringly, the wind tunnel at Teddington was used

to investigate the stability of the proposed hanging roof for covering the Berlin Olympic Stadium Solid steel rods supported on tension cables add sufficient weight to the plexiglass forms (1973) (fig 33)

Fig 33

P R E S T R E S S E D T E N S I O N E D R O O F S

At the heart any discussion of prestressed tensile roofs are the many studies that contrast tents with a central support point and radial cutting patterns to those with double curved saddle and sail surfaces into which eyes, rings or mushroom supports are introduced The Riyadh Heart tent (1986) with its radial spider net of stainless steel cables supporting painted glass panels onto a central mast (fig 34) is diametrically different to the 40 x 30m Berlin humped tent of deformed cotton canvas tied down

at the edges but supported on a series of mushroom supports (1957) (fig 35) The 55m radial patterned squares of the Hadj tents by Fasler Khan of S.O.M supported on central cable-hung rings of ptfe glass fabric

by Chemfab/Birdair are forerunners to the triple layer central supported tensile enclosure for Storek Furniture

in Leonberg (2000) (fig 36)

Fig 34

The 36m 0 wave tent for Dance Stage at Koln 1957 is now also a protected structure for its six exceptionally slim supporting batten masts, each externally guyed to separate foundations (fig 38) To save on foundations, the high points of the waveform for the Biennale at Venice (1996) use A-frame masts to transfer loads to foundations shared with the tie down (fig 39)

Fig 35

Fig 38

Fig 36

The first wave form system had 3 parallel spans of 15m

and was patterned by overlapping cotton canvas to create

the enclosure (fig 37) for a flying priest, Pater Schulte

Multipurpose, it doubled by day as a translucent place of

worship and at night as a covering to his small aircraft!

Fig 39

Tension structures offer a huge opportunity for very

longspan lightweight structures The recently restored

building of the Institut fur LeichteFlachentragwerke (IL)

is now also a listed building (fig 40) Originally this was

the prototype eye structure for the many masted

free-form translucent white pvc enclosure for the German

Pavilion at Montreal (1967) The patterned membrane

was hung underneath the cable net of 12mm galvanised

cables at 500mm centre on springy bretzels (fig 41)

This remarkable cable net construction brought the skills

of Gutbrod architect, Fritz Leonhardt engineer, and Peter

Stromeyer, tent maker and manufacturer together with

Otto to create a longspan building that brought with it a

paradigm shift in cable net technology This technology

was then transferred to the 120m x 90m double

membrane cable net enclosure on eight masts for the

Sports Centre, King Abdul Aziz University, Jeddah

(1978) (fig 42) Clamped anchorages and chizel point

masts and 'teller'plate membrane supports were

introduced here

Fig 41

The plan of the multimasted Voliere at Munich (1980) is

reminiscent of Montreal but the doubly-curved snow

supporting stainless steel woven mesh gets its form from

the earlier humped tents at Berlin and Dyce (Aberdeen)

1975 (fig 43) Here computer visualisation enabled

development of the zigzag eye form required to support

Fig 42

the mesh over an existing ash tree The particular form for this Voliere was devised to facilitate flight and resting patterns for the ornithological occupants within a natural landscape The architect for Miinchen Tierpark was Jorg Gribl

Fig 43

Another protected structure is the Olympic Roof at Munich (1972), by Behnisch, Otto and Leonhardt These structures with their first use of the flying mast laid the corner stone to the wide-ranging research (SFB64) on long span structures directed by Leonhardt, Otto and others in many departments of the University of Stuttgart (1975-1985) (fig 44)