Demountable buildings - a new design strategy? P. S. Westbury Euro Happold, Camden Mill, Lower Bristol Road, Bath,

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1 Demountable buildings - a new design strategy? P. S. Westbury Euro Happold, Camden Mill, Lower Bristol Road, Bath, Abstract This paper discusses the design and detailing of demountable buildings. By looking at two recently completed projects as case studies, we can define a design strategy that differs from the one used on more traditional permanent installations. The paper concludes that by suitably redefining the framework for the design of mobile structures to address the additional construction, erection and transportation problems over and above those that are traditionally encountered on permanent buildings, we can be confident that it is possible to construct successful demountable buildings for multiple use and fast turnaround. 1 Introduction Historically, there has always been a demand for the construction of touring event facilities. The circus and carnival companies have been successfully touring for many years and have staged thousands of performances worldwide by using their simple 'Big Top' style canvas tents and integrated seating. These organisations recognised that the only way that their businesses could succeed was to take each production to as wide an audience as possible. It is now quite clear that this touring philosophy is being adopted by a greater number of organisations worldwide. The increasing demand for more aggressive and higher profile marketing strategies has forced many people to look at new and more exciting ways to promote their events or products.

2 420 Mobile and Rapidly Assembled Structures The development of the touring facility has perhaps been the most recognisable in the USA where we regularly see large scale product launches or theatrical events which appear for only a few days and then pass seamlessly to the next location. As designers it is essential that we embrace this demountable building philosophy and tailor our design strategy to help provide the efficiencies in use and design which are essential to the success of any touring or temporary event. It is clear that the design strategy applied to a demountable style of building is different to that adopted for a traditional permanent installation. A demountable structure must provide flexibility to the end user. The system must be capable of being transported within a minimum number of standard size units, it must be quick and easy to erect and dismantle, it must involve as few dedicated site personnel as possible and it must be deployable on any realistic site. The technical challenges cannot be separated from the aesthetic and the final building must enhance the overall event by providing an excitement or a sense of drama in its own right. Both structure and fit-out must be fully integrated and custom designed for the ultimate flexibility. To help define this revised design strategy, we have developed two case studies. which analyse the way we have approached the demountable building and highlight the ways in which the final solutions differ from the more traditional. 2 Carlos Moseley Music Pavilion, New York, 1991 In 1965, Carlos Moseley, the Chairman of the New York Philharmonic initiated the concept of free summer concerts in New York City parks. This idea was developed in 1991 when a new $3.4 million orchestra stage and acoustic band shell was designed by FTL Associates, Euro Happold and MG McLaren P.C. to allow thirty performances to take place in sixteen park locations every summer. Provided that each site has suitable access and that the ground is not too unfavourable, the entire system of stage, 3,120 sq.ft acoustic membrane, lights and speakers can be assembled and erected from seven standard size trailers in a total of just 6 hours. After each performance, the system can be just as easily dismantled and transported to a second site for a next day performance. The arrangement of the erected system is indicated on figure 1.

3 Mobile and Rapidly Assembled Structures 421 Figure 1: Plan view and front elevation on the fully deployed structure The key to the success of this project was in designing a system that could be deployed quickly and easily with as little site or structural preparation as possible. The solution was to use the standard trailers as part of the structural system itself. By positioning each trailer in a pre-determined location, levelled to the site by hydraulic out-riggers, pre-packaged items could automatically fold out of each trailer and interconnect to form the support structure, stage and acoustic tensile membrane. Figures 2 to 7 indicate this auto-erection procedure from first arrival to completion. Figure 2 Five of the seven trailers park in pre-determined locations and are levelled and supported by hydraulic out-riggers

4 422 Mobile and Rapidly Assembled Structures Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 The hinged eight panel marine plywood 'accordion' folds out of the central trailer to form a 40' x 78' stage. The two centrally hinged side trusses are folded out to their full 86' length. The full length side trusses are pinned to the top of the still folded central rear truss. Hydraulic rams open the third truss to form the main 68' tall structural support tripod which is firmly fixed into position by a hydraulic pin. The folded fabric 'bundle' is opened out on the stage and is attached back to the structure and winched towards the apex. The final shape is fully tensioned. The acoustic tensile membrane flies above the ground; all structural fixings are made to the trusses and trucks and there is no requirement for the time. consuming installation of ground anchorages. Each of the main trailers is strengthened to carry the truss loads and is fitted-out with additional undercarriage ballast to resist uplift or overturning forces. The site is left untouched and the natural environment of each location is unharmed.

5 Erection Sequence Mobile and Rapidly A<scmhlrd Structures 423 Figure 2 Figure 6 Figure 7

6 424 Mobile and Rapidly Assembled Structures 3 Cadillac 'Exceleration' Centre, USA, 1995 In 1995, Visual Services Inc. a Los Angeles based event management company, approached FTL Happold to design and construction manage a touring entertainment facility on behalf of the Cadillac Car Company Inc. A key factor in the Cadillac marketing strategy was to tour a stage managed 'event' around their dealerships in North America to coincide with the launch of the latest range of cars. The touring facility had to be fully air-conditioned and had to be capable of providing kitchen and dining facilities and a blackout performance theatre for a total of 500 people. During the course of the event, guests would be introduced to the latest revisions to the Cadillac products and would be presented with an exciting stage managed production in the theatre space. The entire facility had to be demountable and had to tour a large number of venues over a 2 month period at the end of the summer. Once the tour was completed, the theatre building was to be re-used for future as yet unknown events over the following years. The most onerous condition associated with the project, was that the design and construction of the facility had to be completed within a thirteen week period to meet the first tour dates. It was immediately recognised that the entire building could not be custom designed in this short time. In order to accommodate the tight schedule, it was decided that the dining and reception halls would be built from standard rental tent units but that the theatre space would be custom designed to provide the required drama for the external appearance and internal performance. The arrival car ports and the dining/reception space would be linked to the theatre through simple walkway style connector tents - refer to figure 8. After the successes of the Carlos Moseley Pavilion, it was clear that the simple structural tripod with its easily definable geometry was the most effective method for support. It was also recognised that the packaging and transportational efficiencies, and the dramatic aesthetics which were offered by a fabric membrane were applicable to this project as well, but unlike the band shell a full enclosure and total blackout facility was required. The solution was to generate a conic shaped envelope by joining three back-to-back fabric hypars and to suspend it from the apex of the support tripod; this form generated a three way symmetric, 50' high internal space which could be orientated between support trusses to provide a floor area of approximately 100' diameter - refer to figures 8 & 9. A key feature in the design was that the walling was included as an integral part of the main roof skin. This allowed the membrane enclosure to continue to the ground and provide full weatherproofing and the all important blackout without adding additional procedures to each installation. Interestingly, this overall form was not chosen directly, it was created by solving the three-

7 Mobile and Rapidly Assembled Structures 425 dimensional problem that was defined by the key boundary conditions of geometry, internal space and constructional simplicity. Figure 8: Plan view on fully deployed facility - standard rectangular rental units form reception & dining areas and are linked to the custom designed theatre space with covered walkways - total footprint 250' x 180'

8 426 Mobile and Rapidly Assembled Structures Figure 9: Front elevation on the theatre structure In contrast to the self-contained solution that was adopted for the Music Pavilion, the nature of the theatre was such that anchoring the tensile membrane to the ground was unavoidable. This had the potential to create an immediate problem with time consuming ground anchor installation; as each site on the tour was rented on a specific date, it was not possible to send an advanced crew to prepare anchorages. The solution was three-fold; reduce the total number of anchor points by using each for multiple functions, set-out the remaining anchor points on an easy to define geometry and use an easy to install system of selfsetting 'buried plate' tension anchors. Figures 10 and 11 indicate the method that was used to install each anchor; hydraulic hammers drive the plates 6' below the surface and simple hydraulic tension jacks load lock' each anchor and proof test its capacity. The graph in figure 12 indicates the load-extension characteristics of a typical anchor; when correctly specified and installed, the operation of the anchor should remain within the linear region of the curve. By providing two crews to install anchors, each site could be prepared in a total of two hours. This time was used by a third crew to unload and prepare the superstructure for the final erection and fit-out.

9 Mobile and Rapidly Assembled Structures 427 PLATE ANCHOR AND TIE-ROD INSTALLED WITH HYDRAULIC JACK HAMMER ANCHOR LOAD LOCKED BY PULLING AT GROUND LEVEL Figure 10: Typical ground anchor installation VARYING SOIL \CONDITIONS Q < O / / / COMPACTION / / / AND LOAD MECHANICAL SOIL STRENGTH EXCEEDED (SHEAR FAILURE) LOAD LOCK EXTENSION Figure 11: Load / Extension characteristic for typical ground anchor

10 428 Mobile and Rapidly Assembled Structures The three trussed legs of the steelwork support tripod are lifted from the trailers by fork lift, unfolded to their full length and laid on the ground. The final erection sequence is indicated on figures 12 to 15. Figure 12 Figure 13 Figure 14 Figure 15 All trusses are connected to their baseplates and to the apex. The two rear trusses are anchored to the ground to form a stiff horizontal 'A' frame and the third extends out of the building footprint. A rental crane picks up the apex of the tripod and lifts vertically while a rental fork lift holds the baseplate of the third truss off the ground and drives it in towards its final position. When in place and fully fixed, the three bases are tied at ground level by restraining cables and the crane is removed. The three fabric panels are laid out on a ground cloth below the tripod and are simultaneously site jointed together and lifted by a winch mounted in the third leg. When 3/4 of the way up, the aluminium perimeter poles are pushed into position and tied back. The membrane is winched to the apex and captured by a hydraulic fixing pin. The membrane skin and walls are fully tensioned back to the ground anchors. As the location of future sites were unknown, the key to the success of this project was in recognising that the structure had to operate on uneven ground as well as on level parking lots. As the 'A' frame of trusses rotate into position, each truss base must accommodate a wide range of rotations. These rotations are further complicated when the plane of any individual baseplate is not coincident with that which is defined by the tripod feet, due to local undulations. Full rotational freedom was designed into each truss base to accommodate these movements. The design of the apex connection joint allowed the steelwork system to rotate into position, it accommodated the winching cable and pulleys and it incorporated a set of guide plates and hydraulic fixing pins which were used to secure the membrane into position. Figures 16 & 17 indicate these two key details.

11 Mobile and Rapidly Assembled Structures 429 Erection Sequence Figure 12 Figure 13 Figure 14 Figure 15

12 430 Mobile and Rapidly Assembled Structures Figure 16: Detail at truss baseplate - full rotational freedom accommodates ground imperfections Figure 17: Structural tripod - apex connection detail

Mobile and Rapidly Assembled Structures 431 4 Discussion Although the Cadillac theatre space was fundamentally different in both form and function to that of the Carlos Moseley Pavilion, the same

13 Mobile and Rapidly Assembled Structures Discussion Although the Cadillac theatre space was fundamentally different in both form and function to that of the Carlos Moseley Pavilion, the same basic principles of design were applied to each. Key features were consistent through each design solution. The choice of materials was an important factor; each element is subjected to a very specific repeated use and must be designed to be both structurally adequate and durable. The lightweight nature and long span capabilities of tensile fabric membranes offer many advantages; large areas can be covered with a structural skin that can be folded and packaged for easy transportation and storage. A PVC coated polyester fabric was used to form the tensile membranes and each had permanently attached aluminium membrane plates and polyester sleeved kevlar ropes. These materials offer a durability which is suited to the repeated folding associated with multiple use demountables, and each can be easily repaired or replaced if damaged. The integration of plates and ropes into the membrane removes unnecessary site work from each installation. Where required,- aluminium is used to minimise weight to allow simple components such as perimeter masts to be handled and installed by hand. An attention to overall logistics and construction detail is essential. Every aspect of each operation must be addressed, from the transportation and equipment packing to the basic structure set-out and an accommodation of non-standard site conditions. When required, structural anchorages and perimeter supports must be minimised and located on an easily defined geometry. Structure and equipment must be designed and specified to fit standard sized trailers or trucks to avoid the requirement for official approval and escort for each journey. Basic structure and connections must be simple and where large or difficult items are jointed on site, guides must be designed to make this process as fast and efficient as possible. Both projects required custom designed solutions. An important factor in each was that the structure itself was considered to be an essential feature within the event; the visual drama enhanced the sense of occasion. However, as already discussed, the final form was never imposed, it was created from the basic principles of geometry, function and simplicity. This highlights a key issue; the architecture and engineering of these structures cannot be separated. It is this combined and fully co-ordinated approach to the design that leads us to the most applicable solutions.

14 432 Mobile and Rapidly Assembled Structures 5 Conclusion The successes of these projects were dependent upon the use of innovative design solutions that would not normally be associated with more conventional buildings, but which provide the end user with operational and functional efficiencies that would not otherwise have been available. The financial benefits of a fast turnaround between performances or uses are significant and are an important driving force in the design of truly demountable buildings. The design strategy associated with demountable buildings is different from the traditional, and it is only by adapting our methods of design and problem solving to embrace this revised approach that we can expect to further develop this form of construction.

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