KẾT CẤU MỚI ENGINEERING AN INTEGRATED ARCHITECTURE FOR WIDE SPAN ENCLOSURES

Architecture has the purpose of creating and enriching space for human activities. Structure is the means by which space is spanned and enclosed. Structure, then, is an integral and inevitable part of architecture, its form, its function, its economy, and its spirit. Today this simple relationship is often lost, since, for smaller buildings, contemporary structural technology can support almost any chosen form. For large spans structural form is still important, for tensile solutions it is critical. Yet this is not always obvious to architectural designers at a time when new technologies are evolving rapidly and design tools are not yet user friendly.

ENGINEERING AN INTEGRATED ARCHITECTURE FOR

WIDE SPAN ENCLOSURES

Horst Berger Light Structures Design Consultants, White Plains, N Y, USA Professor, School of Architecture and Environmental Studies The City College of the City University of New York

A B S T R A C T

This paper deals predominantly with tensile architecture

whose application for permanent buildings has occupied

this writer for the more than 30 years In tensile

architecture the historic unity of structure and

architecture is maintained and many building functions

are integrated The fabric membrane acts as structure and

enclosure; reflector and transmitter of light, heat, and

sound; generator of the interior space and the exterior

sculpture Using the Denver Airport terminal and other

structures in whose design and engineering the writer

played a critical role, this paper mainly presents principal

tensile structure forms and their impact on function and

construction of the building The examples include the

Hajj Terminal of the Jeddah Airport, Riyadh Stadium,

Canada Place in Vancouver, and the San Diego

Convention Center Their dramatic forms and spaces

consist primarily of minimal surfaces deriving from their

structural tensile order Weight of construction material

is drastically reduced, construction time shortened,

energy saved, maintenance simplified, and life cycle cost

improved Raising technology to an art form lets tensile

architecture add a softer tone to a new vocabulary of

architectural design The paper ends with the new

UniDome roof structure, which replaced the 25 year old

air-supported roof with a combination of an opaque

grid-dome and a translucent fabric structure in its center

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

I N A R C H I T E C T U R E

Architecture has the purpose of creating and enriching space for human activities Structure is the means by which space is spanned and enclosed Structure, then, is

an integral and inevitable part of architecture, its form, its function, its economy, and its spirit Today this simple relationship is often lost, since, for smaller buildings, contemporary structural technology can support almost any chosen form For large spans structural form is still important, for tensile solutions it is critical Yet this is not always obvious to architectural designers at a time when new technologies are evolving rapidly and design tools are not yet user friendly

We live in a period of transition from the relatively settled world of the Middle Ages to a New Age whose outlines are only beginning to become apparent.The last two centuries were marked by a huge population growth (six times world wide, three times in my own life time) and by drastic changes in the way people live as a result

of the innovations of the industrial age and the electronic age The evolving built environment is a critical part of this changing world in which human activity puts a burden on the resources of our planet and exerts pressure

on the delicate balance which maintains an environment friendly to human existence The consequences could be

Fig 1 The Jeppeson Terminal, Denver International Airport

disastrous Therefore, to survive on this planet may make

it necessary to select order systems in which visual form

and structural form are congruent and which respect the

natural balance of the natural environment

It is my belief that our ideas and images of what

constitutes architecture were first formed long before the

tiny fraction of the human evolution which we call

'history' There is evidence that human dwellings of

substantial size and grouped in community settings date

back over 400 000 years More significantly, the form

and structure of these dwellings was most likely similar

to village houses found in Africa and Asia reaching into

the last century and to the American Indian wigwams

encountered by the European settlers Their shape

derived from the process of building the shelters using

available natural means Flexible saplings, would be set

in the ground in a circular or oblong floor pattern

Bending opposite members inward, lacing them together,

and adding horizontal rings, domes were formed Two

principle patterns emerged: radial and orthogonal grids

They are identical with the two principal engineered

dome forms we have today Thatched with grass, leaves,

or reed, they provided protection against rain and wind,

produced ventilation and modified temperature These

enclosures were minimal surface lightweight structures

forming comfortable interior spaces and gracious

exterior building forms The similarity of their geometric

order (Fig.2) to recent air-supported fabric domes (Fig.3)

and the most recent grid domes is amazing

F A B R I C T E N S I L E S T R U C T U R E S F O R

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

Tensile structures satisfy at least part of this challenge

The terminal building of the new Denver International

Airport, completed in 1994, illustrates most of the

significant features of a fabric tensile structure It took

less time to build than a conventional roof system and

provided protection during construction of the spaces

below It weighs one tenth of any conventional roof

system Using Teflon coated fiberglass, it cost more than

a conventional opaque roof, but less than any roof with

similar translucency It reduced the cost of supports and

foundations, required less mechanical equipment and simplified the drainage It saves energy because of the use of daylight, the reflection of heat from the sun, and the outward night radiation And there is less general maintenance Therefore, its life cycle cost is lower than that of any comparable roof system Above all, the bright interior (Fig.4 ), with its sweeping tensile shapes offers a great space for the traveler And the exterior sculpture is powerful and distinctive (Fig.l) Architectural form is identical with structural form And the structural form I kept as pure and direct as possible

It is one of a number of significant public buildings using tensile structure as the dominating architectural

feature The roofs of the San Diego Convention Center and of Canada Place in Vancouver have become

landmarks for these two cities The roof structure for the

King Fadh Stadium in Riadh is still the largest stadium

cover(despite its large central opening) The Haj

Terminal of the Jeddah International Airport, now

almost 20 years old, is still by far the world's largest roof cover Amphitheaters, indoor sports facilities, malls, stores, and industrial structures are among the other frequent areas of application

These and many structures by other designers indicate the successful entrance of fabric tensile technology into the world of permanent architecture and the potential of

a larger role in the future when fabric properties will advance and their cost will reduce, and when architects

Fig 4 Jeppeson Terminal, Denver International Airport, Interior View

and engineers will be more familiar with their design,

and when this technology and its forms become more

acceptable to both design professionals and the general

public

P R I N C I P A L C O N S I D E R A T I O N S :

T H E D E N V E R E X A M P L E

As a structural category fabric tensile structures are a

special form of lightweight surface structures which

include shells, grid-domes and cable nets In each of

these the continuous spatially curved surface is a critical

and integral structural element In tensile structures the

surface elements, consisting of structural fabric and high

strength cables, can carry load in tension only

The primary advantage of tensile members over

compression members is that they can be as thin and as

light as their tensile strength permits Consequently the

weight of tensile structures is almost The weight of the

Denver roof, for instance, is 10 kg/m2, which is one tenth

the weight of a light steel truss roof, one thirtieth the

weight of the most intense snow accumulation which this

roof is designed to carry The fabric skin is not only part

of the structure but also the building's enclosure,

requiring no additional dead load for cladding

A further advantage of thin, lightweight tensile

components is that they are easy to ship and erect Their

flexibility allows them to be coiled, rolled or folded into

small packages Cables can be a few hundred meters

long, requiring no splices or internal connections They

can be raised and connected to their end supports by*

cranes, winches or helicopters, requiring no scaffolds In

fact, the erection time for a fabric structure is much

shorter than that for a conventional structure

Form and prestress, rather than gravity and rigidity, are

the basic means of providing the stability and the

strength to carry load Structural form becomes a critical

determinant of architectural form To make a tensile

surface structure work, requires a minimum of four

support points, one more than needed for a rigid

structural system The most basic form, therefore, is a

four point structure (Fig.5) If an orthogonal grid is used,

this is the basic module One of the four points has to be

Fig 5 Four Point Structure

outside the plane defined by the other three to achieve the double curved surface which gives the structure its

stability a n d its capacity t o carry load The alternative

geometry is a radial tent As long as these surfaces are in tension the structure is stable Under external loads part

of the surface can be permitted to go slack in one direction as long as the stability of the support system is not lost in this state

The pattern of surface stresses which is required for the stability and load carrying capacity of the structure results in horizontal forces at the anchors in addition to the customary vertical forces This is the price to be paid for the advantages of a tensile structure The skill and efficiency with which these horizontal forces are

U-Cl—E2U

Fig 6 Denver Section, showing ridge and valley cables, and the building's horizontal anchor elements

anchored or balanced has a large impact on the economy

of the structural system The Denver roof, for instance, is anchored to the conventional sub-structure by supplementing the existing structural frame with diagonals to balance the horizontal forces along the shortest possible path.(Fig.6)

Because of the lack of structural weight, there need to be elements which resist upward loads from wind suction in addition to the elements which carry downward loads In order to generate the structural surface grid which satisfies all these requirements there have to be supports

at the high points of the surface, others at the low points, and still others located around all sides of the periphery The choice of these support points defines the shape of the structure Their geometry combined with the stress pattern assigned to the surface leads to the form of the structural surface New forms can be explored with the help of stretch fabric models which simulate the actual shape rather well and are easy to make The final shape

is determined with the help of a formfinding computer program It puts all the tensile forces in all the elements

in equilibrium For one given configuration of supports and one internal stress pattern there is only one equilibrium shape Form clearly follows structural function Since the surface which is generated in this way is also the enclosure, the structural form defines the sculptural shape of the building on the outside and the form of the space on the inside There is no longer any distinction between engineering and architecture

The shape of the Denver roof consists of fabric spanning

between alternating ridge and valley cables, with the

periphery defined by edge catenaries Fig 7 shows the

entire form of the 320 m long roof This image is based

on thee writer's iterative geodesic formfinding system

Fig 7 Denver membrane grid

The photo of Fig.9 shows the completed structure My

initial proposal for the shape was to keep all interior

fabric units identical The concern was the simplicity

and economy of the structure The visual impact would

be naturally enriched by the deep perspective caused by

the large scale, an effect seen in medieval cathedrals The

architects' desire to emphasize the two main entrance

points which also divide the terminal into three

functional sections, led to the use of four larger units with

higher masts.( See Fig.l, Fig.7, and Fig.9) This resulted,

of course, in a tremendous variation of shapes due to the

continuity of the stress pattern The impact on cost was

considerable but probably worth it

Fabric as the surface element in a tensile structure is

critical in maintaining the hierarchy of materials which

makes the system compatible Fabric stretches more than

cables, they stretch more than rigid structural elements

Rigid surface elements instead of fabric cause

compatibility problems unless frequent expansion joints

are provided or the surface is regarded a rigid shell and

included as such in the analysis There is no expansion

joint in the 320m length of the Denver roof

Fig 8 Clerestory with inflated tube closure

Fig 9 Aerial View of Denver terminal roof

Translucent fabrics further define the character of the spaces they enclose by bringing in daylight High reflectivity and low absorption of heat greatly moderate the interior climate And the surface geometry, together with characteristics of the fabric or of an inner liner control the acoustics in the space The sound dissipating geometry of tent shapes combined with the sound absorbing surface of the inner liner acts as a "black hole" for internal sound Users of the Denver airport, which has

an acoustic inner liner, comment on the quiet atmosphere inside this busy terminal

A feature of critical importance in a permanent building .with a fabric structure roof is the treatment of the connection between the flexible membrane and the rigid periphery wall Clamping the components of the roof structure directly to the top of the wall requires the wall

to be designed for substantial horizontal forces If the membrane forces are anchored separately, a connection has to be found which allows for the substantial differential movement between fabric and roof membrane

In the case of the Denver roof with its high, cable supported cantilevering glass walls and the big fabric roof overhangs, a workable solution was the introduction

of an inflated fabric tube which allows roof movements

in the order of 0.65 m at the clerestory windows (Fig.8) and around 1 m at the south and north walls Simple spring operated valves let the air escape and the tube flatten out or elongate A small pump keeps the tube inflated The inner fabric liner, connected directly to the top of the periphery glass walls, hides the tubes from the inside Fig.8 shows the tube before installing the inner liner

Fig 10 Construction of Denver roof

M A S T S U P P O R T E D S T R U C T U R E S

The example of the Denver terminal building shows the

principle structural features of a mast supported tensile

structure The upper support points are formed by pairs

of masts which are spaced 46 m apart Ridge cables are

draped over these masts and anchored to the adjacent

lower roofs similar to the main cables of a suspension

bridge They occur every 18.3 m along the length of the

building and are designed to carry the downward loads,

Fig 11 Denver, main fabric, stressed

mainly snow in the case of Denver Valley cables are

placed between any two ridge cables and run parallel,

taking on the form of an arch They carry the upward

load from wind suction and are tied to lower roof

anchors The edges of the roof are formed by edge

catenaries outside the window walls which are anchored

against the building Construction progressed linear

(Fig 10), a bay at a time, starting at the north end , and

ending at the south, where external anchors complete the

structure The exterior fabric was stressed by pulling

down on the main connectors right outside the clerestory

walls (Visible in Fig 11 at the far end of each valley

cable) This photo shows the main fabric, stressed and

before installation of the inner liner The cables running

parallel to the fabric seams are redundancy cables which

act as rip stops and as replacement of fabric stresses in

case of a rip or during replacement

An interesting and integrated part of the Denver enclosure are the cable supported glass walls around the entire periphery of the terminal space The south wall itself is one of the largest glass walls built, being up to 20

m high and 67m long The upper edge anchors the inner liner The deflection of the top of this wall under wind load is only 8 cm

Fig 12 Shoreline Amphitheater, during constrution

A few notes on a number of other mast supported structures, pointing out features of special interest:

The roof of the Shoreline Amphitheater shows a mast supported structure in its simplest form and largest scale The two masts are 45 m high, spaced 61 m apart, supporting a roof with 8,000 m2 of plan area The front edge catenary spans 140 m between two pile supported abutments The fabric spans between ridge cables and edge catenaries with only a few internal cables placed within the fabric surface for sectionalizing the membrane and reinforcing it along a few critical lines The fabric was stressed by jacking the masts at the ground level

In the roof design for Canada Place (Fig 13) in Vancouver the masts are placed at the ends and are anchored back with external tie-down cables The tent units have a 45o skew in plan, orienting them parallel to the city streets This arrangement made the patterning

1

-Fig 13 Canada Place

complex But it gives the building the sail-like character

for which it has become known The large external

moments created by the position of the high masts at the

ends was balanced by engaging two floor levels of the

building and utilizing the building's structural

components Pairs of cables are used for the external

anchorage to provide for redundancy and to make it

possible to replace them

In the earlier design for the Haj Terminal of the Jeddah

Airport, completed in 1982, central mast supports were

avoided by suspending the 46 m span square tent units at

their peaks Eight suspension cables carry the load of

each unit up to the top of the 46 m high pylons, which

consist of single masts in the interior and of rigid frame

double pylon structures along the periphery of each

module as well as between modules The roof covers a

total of 420,000 m2 or 105 acres of plan area, by far the

largest roof cover in the world

The roof's purpose is to moderate the climate by

simulating the functions of a forest in the desert The

translucent roof provides shade and reduces the effect of

the heat and light from the sun to about 10% It avoids

the heat storage in the ground and its subsequent

radiating back into the space It allows warm air to rise

up and escape through the center ring openings

The construction of this very large project made use of its

repetitive design, which becomes visible in Fig 14 The

210 tent units are arranged in 10 modules, each three

units wide and seven units long The 21 units of one

module were assembled close to the ground The support

ring in the center of each ring was split in a top and

bottom section The top ring, hanging from the main

support cables, contained winches and jacks, which

could be operated from one central control space on the

site The winches lifted all 21 units simultaneously

within about one meter of the top ring Four screw jacks

each were then installed Again, simultaneously jacking

all 21 units the rings were docked, the structure fully

stressed and the rings bolted to each other In the photo

the five modules of one side of the structure (Modules A

to E) are completed The first module of the other side

Fig 14 Jeddah Airport Roof: Construction

(Module F) has been raised and is being stressed Module

G, next to it, is being installed near the ground, soon to

be raised

It should be noted that this process was tested on two full scale test modules which were also instrumented with stress sensors to check the accuracy of the computer analysis The test results deviated from the analysis output by less than 5%, giving us confidence in the reliability of our analysis process Because of the tremendous scale of this nearly 20 year old structure it is becoming a test for the reliability of fabric tensile construction

The Riyadh Stadium extends the concept of mast

supported tent units to create the largest span roof structure to date (The design could have been adjusted without difficulty to cover the area formed by the central opening which is only one quarter of the total plan area Functionally this was not desired) This 247 m diameter

Fig 15 Riyadh Stadium Roof

span is achieved by arranging 24 units in a circle with an outer diameter of 290 m, covering an area of 49,000 m2

In each unit a main vertical mast and a smaller sloping mast combine with triangulated peripheral tie downs to provide the rigid supports which hold the structure out and up On the interior the horizontal forces are balanced

by a large ring cable with 130 m diameter Again, ridge

1 ' : * :

Fig 16 Riyadh Stadium : start of fabric erection

and valley cables form the main elements to which the

fabric membrane is attached with the valleys forming the

downward anchors The ring cable, suspension and

stabilizing cables provide redundancy and make a simple

erection feasible Fig 16 shows one step in the erection

process The entire cable system is in place Fabric is laid

out on the ground, ready to slide into position

Note in both photos that only two fabric panel shapes

were required to make up the entire roof and give it its

dramatic shape

Fig 17 San Diego Convention Center, exterior

The roof of the San Diego Convention Center provides a

91.5 m clear span by suspending the masts They rest on

the main suspension cables placed 18.3 m apart, which

carry the load to triangular concrete buttresses whose

dominant forms give the building its character The roof

structure is again formed by stretching the fabric between

ridge cables, valley cables, and edge catenaries A

special feature of this roof design is a horizontal flying

pole with forked ends which has the purpose of resisting

the tensile forces of the two open ends (Fig 18) This

makes it possible to keep the end openings totally free of

supports, giving the roof its sense of floating

weightlessness A visually delightful feature is the

so-called rain-fly, a closure structure on top of the main roof

which covers the ventilation openings of the main roof

In 1997, Light Structures!Horst Berger were engaged to

provide an enclosure design for for the area under this roof The schematic design proposed a convertible enclosure to include a free standing, cable supported glass wall at the 91.5 m long open end similar to the south wall at the Denver airport Movable wall panels were to convert the space from naturally ventilated to fully air-conditioned, curtains and fabric baffles from bright daylight to a shading level permitting video presentations to 6,500 people A different scheme by a design/construct team is presently under construction

Fig 19 Mitchell Amphitheater, near Houston

A - F R A M E S U P P O R T E D S T R U C T U R E S

Tent shapes require a support at the peak of each tent unit Architectural spaces most often need to be free of interior supports Of the examples above, at Canada Place this was resolved by moving the supports to the edge The result is a space which is high at the ends and low in the center, and a structure which is not very efficient At Jeddah the masts were placed at the corners and extended upward to be able to suspend the tent units from them, again a structurally inefficient solution At San Diego the masts ride on support cable which transfer the load to the perimeter requiring heavy anchors there

One way to resolve this problem is to replace the mast by

an A-frame One of several such structures is the roof of

the Cynthia Woods Mitchell Center of the Performing

Arts at the Woodlands outside of Houston, Texas It

covers 3000 fixed seats Three A-frames form the support system together with the stage house structure Horizontal anchors are avoided by introducing compression struts which link the support columns and edge cable anchors to the stage house, thereby balancing the horizontal components of the membrane forces The supports of the A-frames form low points of the membrane which function as drainage locations for the rain water The trussed columns supporting the A-frames contain the rain leaders and support platforms for the follow spot lighting of the theater

Fig 18 San Diego Convention Center, interior

Fig 20 Mc Clain Practice Facility

A R C H - S U P P O R T E D S T R U C T U R E S

For spans of rectilinear structures of up to 100 m arch

supported fabric roof systems can be highly efficient

For domes with circular, elliptic or super-elliptic edge

shapes spans of more than 200 m can be an efficient

solution, as long as the arch components remain within

dimensions which are shippable by trucks

A number of structures have been built using

prefabricated steel sections, often with a triangular cross

section The largest one using such prefabricated steel

arches is the McClain Indoor Practice Facility of the

University of Wisconsin in Madison This building

covers a football practice field Arches of 67 m length,

spaced 18.3 m apart, span the the full width between

rigid concrete abutments They are 2.1 m deep Shop

fabricated in 12 m long sections they were bolted

together in the field to form half-arches These were

lifted by cranes, pinned in the center and braced against

the adjacent arch, requiring no temporary support

elements It took 10 days to assemble the entire arch

system The outer quarters of the roof are covered by

standing seam, stainless steel roofing Only the middle

half is covered by fabric membrane which spans between

the arches and is held down by valley cables This

arrangement provides excellent natural lighting

conditions for sports by concentrating vertical light in the

center Also the combination of the insulated opaque

roof sections with the translucent, reflective fabric roof

help reduce thermal energy consumption Up-lighting

against the reflective underside of the roof make for good

lighting conditions in the night

One of the many other arch supported designs was for the

tennis practice facilities of the AELTC in Wimbledon It

uses exterior, exposed precast concrete arches from

which the fabric is suspended This provides a neutral

geometry of the translucent ceiling which is essential for

playing tennis It was completed in 1988

Fig 21 Bayamon Baseball Stadium Roof Design

S T A D I U M D O M E S

A single arch spanning 168 m was proposed to support the cover for an existing baseball stadium in Puerto Rico This dramatic design illustrates one of many ways of spanning a full size stadium facility The arch, rising over the middle of the field, supports two cable reinforced fabric membranes, one anchored to a horizontal edge beam behind the outfield, one connected to two cable stayed masts located in front of the stadium

Fabric structures entered the world of permanent buildings with large and super-large spans Geiger Berger's low profile air-supported roof design for the US Pavilion at the 1970 World's Fair in Osaka led to eight stadium-size roofs built in the United States and Japan between 1973 and 1985 All followed David Geiger's special geometry, consisting of a superelliptic ring and a cable net with cable lines parallel to the diagonals of the superscribed rectangle The economy and speed of erection of these domes together with the attraction of high levels of daylight of the new Teflon coated fiberglass fabric made them win out over conventional structural systems They became the engine that drove the new train of fabric structure technology

Problems with snow melting and removal, the cost and inconvenience of operating mechanical devices to maintain the stability of the roof structures, and the limitation and expense of a highly pressurized building led owners to return to static structural systems

This writer's first opportunity to respond to this development with a fabric tensile roof came in 1983 with his initial design for the St Petersburg Sundome, for which he was the partner in charge He called the system cable dome The main principle of this patented system came from the idea of spanning suspension cables from opposite points of the ring beam and supporting sets of

flying poles on them, similar to the basic arrangement of

the San Diego roof Integrating these elements leads to

this simplest of all cable dome systems, where each cable

carries two poles, each pole is supported by two

intersecting cables Again, one, two, or several layers can

be used, whereby each layer is added like a cantilever

Erection needs no temporary supports

Fig 23 Cable Dome for Sundome by David Geiger, built 1986

In the final design, carried out by David Geiger (after the

dissolution of Geiger Berger Assoc in 1983), the

configuration was changed to a system consisting of

concentric rings and radial cables ( Fig 23) There

fabrication and erection is difficult A number of other

cable dome structures have been executed, most

prominently the roof of the Georgia dome in Atlanta,

designed by Weidlinger Associates, using a triangulated

configuration

Cable domes of this type are not efficient in heavy snow

areas because of the multiplying effect which this

geometry has in transferring loads from the center to the

periphery This leads to very high cable quantities

accompanied by very large deflections To avoid these

problems this writer's cable dome patent includes a

version with arch-shaped compression members at the

top These carry gravity loads in the most direct way to

a peripheral ring The cable system below the arches becomes very light as its function is reduced to carrying part of the unbalanced roof loads, stabilizing the arches and allowing the roof to be erected without a scaffold and

a minimum of interference with the space below

In studying the replacement of the air-supported

UniDome roof at the University of Northern Iowa, a

cable-dome proved to be impractical It was not possible

to adapt its radial configuration to the existing orthogonal geometry and the first row of flying struts interfered with the sight lines from the upper seats which is a common shortcoming of all cable dome structures It was also not economical for Iowa's heavy snow loads

The answer evolved from taking advantage of the special nature of the existing geometry in which the horizontal forces from the cable grid are in perfect funicular balance with the shape of the ring beam The initial concept was

a grid of compression elements following the same plan configuration as the existing cable net but located above

it The compression members were assembled from shop fabricated, three dimensional truss sections which were connected by vertical ties to the old cable net re-installed below This combination offered the most direct force flow for downward or upward loads for the 15 000 m2

Fig 25 New UniDome, Arch and cable grid The cable net is that of the former air structure

Fig 26 New UniDome Hybrid roof, computer image superimposed

on photograph

dome spanning 140 m across the diagonal The cable net

stabilizes the grid dome and provides sufficient bending

capacity to accommodate eccentric load cases for snow

and wind

In the final version of the design the center section was

replaced by an arch-supported fabric tensile roof which

reduced the dead load where it is most critical and

provided translucency where it is most desired The rest

of the roof surface is enclosed with a stainless steel

standing seam roofing on metal deck and bar joists

Fig.26 shows the roof design in a computer generated

image superimposed on an existing aerial photograph

The concrete ring beam which on this structure was

made of rather thin precast sections was prestressed with

tendons rapped around its exterior face to give it the

capacity to become a tension ring

Fig 27 Prestressing tendons applied to the out

side of the existing ring beam

The construction began with the prestressing process in

the winter of 1997/98, while the stadium was in full use

(The air-supported roof had failed in a sudden snow fall

two winters before and had been repaired with PVC

coated polyester fabric, a process which took Birdair

only weeks to complete) Parallel to prestressing, shop

fabrication of structural components took place.The

stadium remained in use until the middle of March 1998

The new roof was completed and the first football game

took place in the stadium in October of 1998

Fig 29 Beginning of steel erection

Though a support free erection was studied, the use of four construction masts under the intersection points of the four continuous arches proved most practical and economical (See Figs 28 and 29) The arch sections (1.2m X 1.8m ) were shop fabricated in up to 17m straight lengths, bolted on site into sections ready for installation The four long arch sections were strengthened by tie cables Bar joists, spanning between arch ribs, and metal deck, spanning between joists, followed Insulation and stainless steel roofing was installed parallel to the center fabric structure and the cable net below

Fig 30 The UniDome with its new roof