10-11 October 2016, Bern, Switzerland. Advanced Building Skins

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1 0- October 06, Bern, Switzerland Advanced Building Skins

2 th Conference on Advanced Building Skins 0- October 06, Bern, Switzerland ISBN: Advanced Building Skins GmbH Hostettstr. 30 CH-606 Wilen (Sarnen) Switzerland VAT: CHE Tel: info@abs.green Copyright: Advanced Building Skins GmbH

3 Reinforced, insulated glazing for large windows Marc Donzé, Maurice Brunner, Urs Uehlinger 3,,3 Research and Development; Architecture, Wood, and Civil Engineering; Bern University of Applied Sciences Solothurnstrasse 0, 504 Biel, Switzerland, Abstract In modern buildings, the ever-increasing size of glazing in the building skin means that stronger and larger frames are needed. At the same time, modern design calls for small frame sections in order to let in as much light as possible through the windows and to allow free view. Both goals - larger glazing surfaces and smaller frames - could be achieved if the glass could be strengthened to take over the structural duty of the window frame. Researchers at the Bern University of Applied Sciences have investigated a new type of window: the insulated glass element is strengthened by gluing on at its edges a U-profile, made of glass fibers and polyester (GFP). The research included simulation modeling to research different parameters such as the window size and the properties of the U-profile, the insulating glass and the adhesive. The subsequent tests confirmed that composite action can increase the rigidity of the glass window by up to 8 times, and the loadbearing capacity by a factor of.5. These positive results are the basis for further research to refine the system with regard to functionality and economical manufacture. Keywords: Reinforcement of insulating glass, glued composite system, structural system, glazing. Introduction Modern architects like windows with small frames because they enhance the transparency of the glazed part of the building skin. In classical window systems, the insulated glass is set in a frame of wood, plastic or aluminium. The cross section of the frame depends upon the window size and the wind forces: the insulated glass is not included in the load-bearing action of the frame. The tendency for larger windows calls for larger frames with a greater load-bearing capacity. Modern window manufacturers now need the assistance of specialists in structural analysis for the design of the window frame. Researchers at the Bern University of Applied Sciences have long observed that glass itself has excellent structural properties. In particular, its E-modulus is seven times higher than that of wood. The idea was to develop a mechanical coupling between the two outer glass layers of the window, and thus to strengthen its load-bearing capacity. Ideally, the strengthened glass could itself take over the structural function of the window frame, which could thus be drastically reduced in size to let in abundant light. The strengthening effect would be particularly interesting for windows with three glass panes and a total thickness of mm. Unlike classical window systems, where a thicker window implies the need for thicker frames, in this new system a thicker window would be stronger and thus have even less need for a load-bearing frame. The aim of the project was to develop the basic scientific knowledge to ignite a major project to develop a new type of window for future buildings.. Theoretical Analysis. Composite action between the glass panes Insulated glass comprises two or three glass panes which are separated from one another with spacers. The spacers are connected to the glass panes with a very flexible adhesive like butyl, which also guaranties the imperviousness of the enclosed air between the glass panes. The air usually the gas argon is not only responsible for the good insulation property of the window, but it also establishes a mechanical coupling between the glass panes (Feldmeier [3], DIN und ). Thus, when a wind force acts on the outer pane, this coupling action makes all the glass panes participate in the load-bearing, as a function of their thicknesses (Figure ).

4 Wind force Figure : Deformation of the two glass panes of an insulated glass under wind forces The very flexible adhesive cannot transfer a significant shear force. Thus under wind forces, all the glass panes will suffer bending, but no normal forces can be mobilized in the outermost glass panes (Figure ). Thus, in the simple case of a glass with two panes, the total bending resistance is the sum of the bending resistances of the different panes: () EA EI - + M EA EI - + M Figure : Bending stresses in a two-pane glass with no shear connection The project idea is to develop an adhesive system which would ensure the transfer of shear forces, and thus stiffen the structure (Figure 3). The transfer of shear would in turn mobilize opposite normal forces in the two outermost glass panes, thus leading to an additional bending moment resistance: EA EI - N - + M - + = M T EA EI + N - + M + Figure 3: Bending stresses in a two-pane glass, augmented with opposing normal forces () (3) The transfer of the shear force can be achieved by gluing a U-profile at the edges of the glass element (Figure 4). The value of the binding moment M V depends on the stiffness of the adhesive used. th Conference on Advanced Building Skins 577

5 Insulated glass Adhesive U-profile Figure 4: Drawing of an insulated glass cross-section showing the glued on U-profile.. Structural system: distribution of the wind force on the window frame The wind induces pressure or suction forces (kn/m ) on the building façade. In this paper we assume that the forces are concentrated as trapezoidal forces (KN/m) on the stiffer elements of the window, i.e. the insulated glass-u-profile system. We further assume a simply supported beam as statical system (Figure 5). q v [kn/m ] q v [kn/m ] Support Section L (span) w L U-Profile b Support a) b) Figure 5: a) Elevation of window with distribution of wind force; b) Glass-U-profile system as simple beam.3 The adhesive properties In the window and façade industry, two main adhesive groups are used. The first group comprises soft adhesives like silicones, MS-polymers (Gyso 444) and polyurethanes, with a layer thickness - 0mm, depending on the adhesive and the window type. The second group comprises semi-stiff adhesives like the two component acryl adhesives (SikaFast 55) with a thickness - 3 mm. The bending of the glass-u-profile system induces a slip between the two glass panes, which in turn can lead to shear forces if there is an adequate shear connection. With the help of standardised shear tests (SN EN 4869-), it was possible to determine the shear modulus and the shear resistance of the adhesive system (Figure 6). The tests also revealed that some adhesives like «SikaFast 55» exhibit elastic-plastic behaviour: the force-slip curve is initially constant, and then flattens as the maximum force is approached. Other adhesives like «Gyso 444» exhibit purely elastic behaviour, followed by sudden, brittle failure. 578 th Conference on Advanced Building Skins

6 F F F Fu Fu lc lc bc Fel Fel Ac = bc * lc el K el = tan() u (Slip) el K el = tan() u (Slip) F a) b) c) Figure 6: a) Force-slip-curve of an adhesive with elastic-plastic behaviour; b) Force-slip-curve of an adhesive with purely elastic behaviour; c) Test set-up for tensile shear The shear modulus K el is the tangent of the angle α of the force-slip-curve from the shear test (Figure 6): K el tan Shear modulus [N/mm] () : Angle of the force-slip-curve of the shear test. The stiffness k el of the connection between the glass and the U-profile can be calculated from the shear modulus of the adhesive K el as follows: k el K A el c b j k b el el () j K A k el : Stiffness of the connection [N/mm ] A c : b j : c Area of the adhesive layer of the test specimen (A c = *4 mm, d j = mm) Width of the adhesive layer (4mm).4 Load-bearing behaviour of the composite system The differential equations which describe the load-bearing behaviour of the composite beam are quite well documented in various publications. This paper referred in particular to the following: Aicher [], Kenel [4] and Schlänzlin [5]. The complex calculations can be readily solved for a simple beam under a sinusoidal load distribution. This often used ansatz yields quite accurate results for beams with constant distributed loads or with many concentrated loads. The following assumptions were also used: The glass cross section comprises two glass panes of float glass. The calculation is valid for purely elastic adhesives. The calculated shear slip occurs between the U-profile and the glass panes. Hook s law applies: E (3) Although the modulus of elasticity of glass is different for compression and tension, we assume the same modulus as for bending. Under loading, the deflection and the curvature of the two glass panes are affine: the deflection and the curvature of their axes are the same at each position x along the beam. x w x w x x x (4) bzw. w x th Conference on Advanced Building Skins 579

7 Only the bending component of the deflection of the composite beam is taken into account. The deflection component of the shear force in the U-profile is neglected. Figure 7 shows a longitudinal segment of length dx of a composite beam with two glass panes. The internal forces which are needed for equilibrium with the external loads are shown. The two U-profiles (one on each edge of the glass plate) are not shown. Figure 8 shows the corresponding slip γ between the two glass panes induced by the shear stresses between the U-profile and the panes. q(x) u *M 3 M N *V 3 M N V V T(x)dx V +dv a M +dm M +dm N +dn V +dv N +dn *(M + dm ) 3 3 *(V +dv ) 3 3 a u. h. h. h dx Figure 7: Equilibrium in a longitudinal segment of length dx of a composite plate with two glass panes Figure 8: Slip between the two glass planes at the beam supports A differential equation was developed to determine the slip as a function of the longitudinal position x, taking into account the geometrical values of the system and the corresponding E-moduli. Only the key equations will be discussed in this paper. The equilibrium conditions yielded the following relationships: N V V N 0 thus N N (5) V V3 Mel 3 N M M M N a (7) (6) The indices & refer to the two glass panes, whilst index 3 refers to the two U-profiles at the glass edges. h h h (8) a In the horizontal direction, the equilibrium conditions will yield the following relationship between the axial forces N in the glass panes and the shear flow between the U-profile and the glass: T x dn (9) dx The sum of the partial moments in the beam segment of length dx in figure 7 yields: V V V dx Tx a dx dm dm dm3 3 (0) The mechanical behaviour of the glued connection is described by the shear flow, which is linearly proportional to the slip as follows: 580 th Conference on Advanced Building Skins

8 x x T kel () The axial connection force can be obtained from the integration of the shear flow: x Txdx C k x N el dx C () The slip is equal to the tangent () of the deflection curve, multiplied with the distance between the axes of the two glass panes (a) minus the two deflections induced by the axial connection force N: x a u x u x with x 0 x u, (3) The following general form of the differential equation was obtained by integrating equation 3 twice and substituting the relationships above in equations 3: a '' x kel x qx dx C EA EA EI EI E3I3 EI EI E3I (4) 3 a The solution of equation 4 yields the following solution for the slip: a L q0 EI EI E3I3 x x sin (5) a L k el L E A EA EI EI E3I3 The axial connection force N(x) as well as the curvature (x) of the beam can be readily calculated from the slip determined in equation 5 above. Thus the total moment M el (x) and the beam deflection w(x) can also be determined. The effective flexural stiffness (EI eff ) is the relationship between the moment (M el ) and the curvature (): EI eff x x Mel (6) The effective flexural stiffness (EI eff ) is also a function of the geometry, sizes of the glass panes and the U- profile, as well as the stiffness of the shear connection. Equation 6 can be simplified in analogy to the socalled gamma-method as a function of the bending stiffnesses EI,,3 of the component parts, a connection factor () as well as the «Steiner-portion» (S): EIeff EI EI E3I3 S (7) th Conference on Advanced Building Skins 58

9 The «Steiner-portion» (S) is determined solely from the properties of the glass panes as follows. The U- profile is neglected in this calculation. For E = E : S E a, (8) h h b h h The factor of the strengthened system: k L (h h ) el (9) kel L h kel L h E, b h h The equation for can be further simplified as follows: h h x with E, h h h h k x el L x (0) b For a symmetrical glass construction (h = h ): E, h, () x k el x The simplified equation 4 is very interesting because it makes it possible to illustrate the performance of a composite system with a simple calculation. Figure 9 illustrates the factor as a function of L /b for a composite system, where the glass panes are 6mm thick and have an elastic modulus of 70'000 N/mm. The factors are shown for two different adhesives: the soft adhesive (Gyso 444) with k el = 0 N/mm and the semi-stiff adhesive (SikaFast 55) with k el = 00 N/mm. The adhesive properties were determined in tensile shear tests (Donzé []). The curves illustrate that a stiffer adhesive can increase the connection factor from 0. to 0.6. The curves also illustrate the influence of the glued width: for a given beam length, the smaller the width, the greater will be 58 th Conference on Advanced Building Skins

10 Factor as a function of L /b for two different adhesive stiffesses (k el ) Factor [-] kel = 0 N/mm (Gyso 444) kel = 00 N/mm (SikaFast 55) L /b L = Span b = Width Figure 9: Factor γ as a function of L /b and k el, for h = h = 6 mm and E, = 70'000 N/mm 3. Experimental work 3. Four point bending tests The aim of the experimental part of the project was to determine the bending stiffness (EI g ) of the insulated glass before, and after it has been strengthened with the U-profiles. The results would make possible to estimate the statical performance of the glass element as a function of the adhesive used. The test results could thus be compared to the theoretical results, in order to verify the accuracy of the calculation modelling. In a first phase, the glass elements were tested without the U-profile and the bending stiffness (EI g ) and the slip () was measured. In the next phase, the glass elements were strengthened with the U-profiles and then tested till failure to yield data on the performance of the composite system, in particular the bending stiffness (EI eff ), the maximum load (F max ), the slip () and the deflection at midspan (w max ). 3. Description of the test specimens The test specimens were all insulated glass elements of the Swiss company «Verres industriels SA à Moutier». The elements comprised two glass panes and were 8mm thick, 700mm wide and 000mm long. The official name of the company for the product was: Glass (): 6-6EA (Argon)-6Low-E, t total = 8 mm. The E-modulus was N/mm. Two different adhesives were used to glue the U-profile to the glass elements: MS-polymers: Gyso 444, K el = 00 N/mm two-component-acryl-adhesive: SikaFast 55, K el = 900 N/mm The U-profiles used in the tests were made of glass fibres and polyester (GFP). Because there was no other information available, tensile tests were carried out to determine the following values (Donzé []): Tensile strength (average value): f t,90 = 800 N/mm (Number of test specimens n = 5) Modulus of elasticity (average value): E t,90,mean = 30'500 N/mm (n = 5) The adhesive was applied with a thickness of mm. The width of the glued surface was 5 mm; the length of the element was 000 mm. th Conference on Advanced Building Skins 583

11 Table gives an overview of the test series which were carried out. Test series Nr. Description Number of test specimens (n) Drawing A. A. Insulated glass alone B. B.5 Insulated glass strengthened with GFK-U-profile; adhesive: Gyso444 5 Profile U 0x40x0/4 B.6 B.0 Insulated glass strengthened with GFK-U-profile; adhesive: SikaFast55 5 Table : Overview of the four point bending tests 3.3 Test set-up The bending tests were carried out in accordance with the Swiss standards SN EN 88- and -3. A steel frame and the testing machine «Zwick 50 kn» were used. In this machine, the loading was applied horizontally, so that the effects of gravity (self-weight) were neutralized. The testing machine measured the force applied and the deflection of the test specimen with the help of two electronic dial gauges (Position in Figure 0). The slip between the glass panes and the U-profile was measured with a dial gauge (position in Figure 0) placed at the supports. Load introduction Insulated glass a) c) F/ F/ A L/3 L/3 L/3 L B. Positions of the two electronic dial gauges. Position of the digital dial gauge b) d) Figure 0: a) Surface view of the test specimens; b) statical system of the four point test; c) Photo of a test specimen; d) The specimen was placed vertically in the testing machine and loaded horizontally. 584 th Conference on Advanced Building Skins

12 The four point tests yielded the following mechanical properties: ) Failure force (F max in N) ) Deflection at failure (w max in mm) 3) Bending stiffness (EI g in Nmm ) 3.4 Test results and discussion The tests results were analysed with relatively simple methods. An elaborate statistical analysis was not possible because the number of test specimens was too little. However, the test results were adequate to demonstrate the potential of the new strengthening method and to assess the accuracy of the calculation models; hence they could fulfil the aim of performing the tests. Table gives an overview of the test results of the four point bending tests with the strengthened glass elements. The failure force of the strengthened glass averaged 3800 N for both adhesives, though the scatter was greater for the soft adhesive Gyro 444. In a previous study (Donzé []), the failure force for the same glass elements without the U-profile had been estimated to be 500 N. Thus the strengthening effect of the composite was by the factor.5. The adhesive had a marked influence on the failure deformation: the specimens with the semi-stiff adhesive averaged 38 mm, those with the soft adhesive averaged 68 mm. Failure force F max [N] Maximal deflection w max [mm] Failure force F max [N] Maximal deflection w max [mm] Series N 300 B. B.5 B.6 B.0 Average (x) x-s x+s Median Series N 0 B. B.5 B.6 B.0 Average (x) x-s x+s Median a) b) Table : Results of four point bending tests on strengthened glass elements: a) Failure force (F max in N); b) Maximum deformation at failure (W max in mm) The bending stiffness is a central indicator of the study. In Table 3 below, the EI g -values are listed for the test series «A. - A.7» with the naked glass elements: the variation coefficient is less than 3%. The test series «B. - B.5» were for the strengthened glass elements, whereby the soft adhesive Gyso 444 was applied: the variation coefficient was 4%, and the average EI eff -value was 3,-mal higher than the EI g values of the naked glass elements. In the case of the semi-stiff adhesive SikaFast 55, the test series «B.6 - B.0» exhibited a variation coefficient of %, and the EI eff value was 7.8 higher. The test results were compared to the calculated results: the good agreement confirms the accuracy of the calculation model. th Conference on Advanced Building Skins 585

13 Comparison of the test values of EI g and EI eff [Nmm ].50E+0 Comparison of the test values of EI g and EI eff [Nmm ].00E+0.50E+0.00E E E+00 Series N A. A. B. B.5 B.6 B.0 Moyenne (x).67e+09 8.E+09.08E+0 x-s.6e E+09.85E+0 x+s.74e E+09.30E+0 Theoretical value 8.6E+09.40E+0 a) b) Table 3: Comparison of the EI g -values of the naked glass elements (test series A. A.) with the EI eff values of the glass elements strengthened with U-profiles attached with two different adhesives. 4. Conclusions Modern architecture has a marked preference for large glass openings in buildings. The classical reaction of window manufacturers has been to employ the services of structural specialists to design massive frames for the increased loading. The authors have presented a radical new proposal: the glass construction should be strengthened by composite action so that it can itself bear the wind forces. From the structural point of view, the frame construction will be redundant and will be replaced by a slim PVC U-profile which has only one structural task, namely to bear the shear forces necessary for the two glass panes of the window to act in composite action. The research work comprised calculation modelling of the composite action, which was backed up by experimental work with two different adhesives to transmit the shear forces between the glass panes and the U-profiles. The tests confirm the theoretical predictions that composite action can increase the rigidity of the glass window by up to 8 times, and the load-bearing capacity by a factor of.5. The encouraging research results have paved the way for the initiation of extensive projects to refine the methods used and to help to develop new glass windows for the market. 5. References [] Aicher S., von Roth W. (989): Ein modifiziertes gamma-verfahren für das mechanische Analogon: dreischichtiger Sandwichverbund zweiteiliger verschieblicher Verbund (A modified gamma-method for the mechanic analogon: three layer sandwich composite two parts with elastic connection) Bautechnik /987, Wilhelm Ernst & Sohn Verlag für Architektur und technische Wissenschaften, Berlin, Germany. [] Donzé M. (00): Méthode de renforcement statique d un verre isolant. Rapport projet HESB-ABGC (Reinforcement method for an insulated glass construction. Project report, HESB-ABGC), Bern University of Applied Sciences, Bienne, Switzerland. [3] Feldmeier F. (995): Belastung des tragenden Randverbundes von Isolierglas bei Structural Glazing durch klimatische Einflüsse (Climate influence and loading of the structural frame of insulated glass glazing), Institut für Bautechnik, Rosenheim, Germany. [4] Kenel, A. (03/000): Zur Berechnung von Holz/Beton-Verbundkonstruktionen, Entwicklung und Vergleich verschiedener Berechnungsmethoden (On the calculation of timber-concrete-composite structures: development and comparison of different calculation methods); Forschungs- und Arbeitsbericht 5/4, EMPA Abteilung Holz, Dübendorf, Switzerland. 586 th Conference on Advanced Building Skins

14 [5] Schänzlin J. (0/003): Zum Langzeitverhalten von Brettstapel-Beton-Verbunddecken. (On the longterm loading behaviour of composite floors comprising timber stacked boards and concrete) PhDthesis, Institute for Structural Design, University of Stuttgart, Germany. Relevant standards: [6] DIN and : German industrial standard for glass structures. [7] SN EN 88- and -3: Swiss standard (in line with European standard) Glass in buildings: Determination of the bending strength of glass. th Conference on Advanced Building Skins 587