Stacking glass elements, how to build a glass tower
Architectural Challenges and Solutions Stacking glass elements, how to build a glass tower F.A.Veer*, W. Lia*, P.M.J. van Swieten*, G. Hobbelman*, F.P. Bos*, P.C.Louter*, T. Romein&, J. Belis# * Delft University of Technology, www.zappi.bk.tudelft.nl & Van Noordenne glas groep, www.noordennegroep.nl # Ghent University Keywords 1= glass structure 2= glass tower 3= stacking technology 4= structural design Abstract In recent years several designs of large glass beams have been introduced in Germany and Scotland. At Delft University of Technology a project to build an 18 meter reinforced glass beam is in progress. This of course raises the engineering question of supporting such a beam. If the beam is used only to support the glass roof of a concrete building there is no problem, but an all glass atrium with 18 meter roof beams will need 15 to 20 m high glass supports. This can only be achieved by prefabricating glass sections and stacking them. As this has not been done before, a feasibility study has been conducted to build an 20 meter high glass tower composed of stacked segments. The functioning of this tower has been analyzed by structural calculations of several alternatives. Two models have been made and tested in a wind tunnel. Figure 2 Cross section of rectangular tower Introduction The last twelve years significant work has gone into designing glass based spanning structural elements and structures,[1,2,3,4]. Although this work has showed that large and inherently safe spanning structures are possible, the question is what will support the spanning element. Several modern glass roofed atrium structures have been made in recent years, notably, [5 ,6]. But the glass roof is here supported by a normal concrete structure. The ultimate evolution of glass atria requires large vertical glass structures that will support large glass spanning elements and roofs. Although tubular column technology has been developed and tested before ,[7,8], columns of this type are not very suitable for the 10 to 20 meter heights that need to accompany 15 to twenty meter spans. Thus a different vertical structural glass technology is required. After consideration of alternatives it was decided to work out a concept for a 20m high glass tower. If it can be proven that this design is workable, it will also include the technology for different vertical glass structural elements. One student, W. Lia , used the glass tower concept for his MSc work. 166 Figure 1 Design alternatives Design alternatives In designing a glass tower there are several considerations. The shape of the cross section is one of the most important. The cross sectional shape/ size/area determines the bending stiffness and thus is critical for how much glass needs to be used. In the early phase of the project the shapes shown in figure one were considered. All have advantages and disadvantages. The round tower has the lowest resistance to wind loading but is the most complicated to manufacture in glass. The triangular tower has good bending and torsion stiffness but is also difficult to manufacture. The square section tower has many advantages but needs to be very wide to accommodate stairs and floors. The rectangular Figure 3 Selected design shape consisting of two large U shape profile with floors at different heights and connecting stairs, shown in figure 2, combines the flat and rectangular nature of glass with an efficient shape and good efficiency. This was selected as the basis for further work. This resulted in a preliminary design shown in figure 3. Figure 4 shows this design from different angles. Essentially the GLASS PERFORMANCE DAYS 2007 | www.gpd.fi Architectural Challenges and Solutions Figure 4 View of design form different angles design is composed of stacked U profiles facing each other on the open side. Structural design Figure 5 Figure 6 Different structural alternatives A U profiles connected by balustrades B Tubes using floors as stiffening elements C U profiles using central fin and stairs Wind pressure on structure A conforming to Dutch building regulations B simplified but conservative model Figure 7 Figure 8 Modelling for loads A dead load plus live load per floor conforming to Dutch building regulations B dead load plus extreme wind load Dependency of wind load on angle GLASS PERFORMANCE DAYS 2007 | www.gpd.fi The basis design concept shown in figure 3 and 4 shows a good general concept. This needs to be worked out to a integrated architectural/structural design. Using the general concept three main structural alternatives are possible. These are : A.Independent U profiles connected by the balustrades B. Rigid tubes using the floors as a stiffening element C.U profiles stiffened by the floors using a central fin and the staircases to provide stability. These alternatives are illustrated in figure 5. The dead load in the structure is considerable but the wind load is the most important factor. Figure 6 shows the most extreme wind load according to Dutch regulations. Although this pressure pattern can be used in finite element calculations for simplicity a conservative approach was used were the maximum pressure was applied universally. As it was intended from the beginning to make the segments exchangeable where possible allowing for uniform pressure is also logical as optimization of wall thickness at different heights is thus not required. In the modelling two main load cases were considered, the dead load plus the live load from visitors and the dead load plus the live load from extreme wind loading. It was not considered necessary to combine live load from extreme wind loading with live load from visitors. This is shown in figure 7. The live load due to wind is variable and depends on the orientation as shown in figure 8. Regarding final failure two main modes were 167 Architectural Challenges and Solutions Figure 10 Figure 11 Wind flowing through open tower Model in wind tunnel Figure 12 Figure 13 Wind resistance of closed tower (line) and open tower (shaded area) Bending moment on tower due to extreme wind loading, for closed tower (line) and open tower (shaded area) Figure 14 Figure 15 Structural elements in design Stresses in tower A tensile stresses B compressive stresses Figure 9 Failure modes A segments shifting B Failure at support of tower considered, shifting of the segments and failure at the base of the tower, as shown in figure 9. Wind tunnel testing As the wind loading is extremely varied two alternatives were considered. A closed tower and an open tower. Orientation is also important, as shown in figure 10, as the pressure on the structure is dependent on how the wind moves through the structure. A computational fluid mechanics approach was considered, but it was decided that wind tunnel testing would provide better insights. Models of the open and closed tower were manufactured from acrylic using laser cutting techniques and adhesive bonding. The test setup is shown in figure 11. Figure 12 shows the measured wind resistance at different orientations for the open and closed designs. The open tower has a slight advantage when the wind is to the side of the tower, but in general the differences are minor. From the wind tunnel test data the bending moments were calculated for the different orientations. These are shown in figure 13. The open tower generally is more favourable. As this would also be climatically better and psychologically better for the visitors the open tower variant was considered the best. Dimensioning Using the data produced so far a final structural design was produced. This is composed of the components shown in figure 14. The required glass thicknesses were calculated. A thickness of 50 mm for the walls of the U sections and the floors should be sufficient. Some problems are introduced by stress concentrations, as shown in figure 15. This mainly affects the balustrades and the joint between balustrade and U section. Although several solutions are possible, this was not worked out due to time constraints. 168 GLASS PERFORMANCE DAYS 2007 | www.gpd.fi References Although the tower could be build of glass and adhesive alone with a metal joint to connect the central fin with the floors such as used in [9], this would not be a very safe approach. If an accidental collision took place such as shown in figure 16 part of the support would be damaged. At this point it is essential to limit the damage and to allow in the design for a secondary path to carry the loads. One way of doing this which the authors have tried in beams, [1,2,3,4], is to use stainless steel reinforcement. Placing reinforcement at the corners of the u profiles such as seen in figure 17 and the edges of the fin and the free ends of the floors would limit any damage and allow for loads to be transferred form damaged regions to undamaged regions. [1] Veer, FA Gross, S Hobbelman, GJ Vredeling, M, Janssen, MJHC Berg, R van den Rijgersberg, HA: Spanning stuctures in glass. Glass processing days / educational glass conference, Tampere, Finland 2003 [2] Bos, FP,Veer, FA,Hobbelman, GJ, & Louter, PC: Stainless steel reinforced and post-tensioned glass beams (cd-rom). In C Pappalettere (Ed.), International conference on experimental mechanics / icem12 / advances in experimental mechanics (pp. #1-#9). Bari: Politecnico di Bari, 2004 [3] Reinforced Glass Cantilever Beams, Christian Louter, Freek Bos, Fred Veer & Gerrie Hobbelman: Reinforced Glass Cantilever Beams. Glass processing days, Tampere, 2005 [4] Louter, PC, Herwijnen, F van, Schetters, L, Romein, T, & Veer, FA: Experimental research on scale 1:8 models of an 18 m reinforced glass beam. In G Siebert (Ed.), International symposium on the application of architectural glass ISAAG 2006 : conference proceedings (pp. 215-222). Munchen: Universitat der Bundeswehr. [5] Wolfson building of the medical faculty of the university of Glasgow [6] IHK building , Munich [7] F.A.Veer, J.R. Pastunink: Developing a transparent tubular laminated column. Proceedings 5th international glassprocessing days , Tampere Finland, 1999 [8] E.J. van Nieuwenhuijzen, F.P. Bos & F.A. Veer: The Laminated Glass Column. Glass processing days Tampere: GPD, 2005 [9] F.A.Veer, J. v.d. Meer, A. Borgart, T. Romein, J. Zaman: Building a glass staircase to heaven while keeping both feet on the ground. This conference [10] F.A.Veer, W.Lia, J. Belis, G. Hobbelman, F.P. Bos, P.C.Louter, W.Kamerling, T. Romein: Stacking glass elements, building a glass tower. This conference Conclusions The feasibility study shows that it is feasible from a materials and structural mechanics point of view to create a 20 m high glass tower constructed out of prefabricated stacked segments. The engineering feasibility should be tested next. This dealt with in the following article, [10]. Architectural Challenges and Solutions Reinforcement Figure 16 Accident scenario Figure 17 Location of reinforcement Red Main reinforcement Blue Reinforcement to create secondary path for loads GLASS PERFORMANCE DAYS 2007 | www.gpd.fi 169
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