Timber-steel-hybrid beams for multi-storey buildings

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1 Timber-steel-hybrid beams for multi-storey buildings Wolfgang Winter 1, Kamyar Tavoussi 2, Tamir Pixner 3, Felipe Riola Parada 4 ABSTRACT: For modern multi-storey buildings timber-steel-hybrid elements present a very efficient construction method. The combination of these two materials leads to economic and ecologic benefits as the construction height can be optimized, the earthquake resistance can be improved and the assembling can be executed more efficiently. Steel reinforced timber structures are light, fast and clean. Based on research activities of the Institute of Architectural Sciences, Structural Design and Timber Engineering, presented at the WCTE 2010, some static tests on hybrid timbersteel beams have been carried out. KEYWORDS: Timber-steel, hybrid, beam, multi-storey 1 INTRODUCTION 1234 The application of timber-steel-hybrid-elements for multi-storey buildings will be one objective in the future. Several ideas and details have been carried out. Flitchbeams were the first generation of timber- steel-hybrid beams. A further development of flitch-beams is economically and statically advantageous and should be considered as the future generation of timber-steelhybrid beams. The idea is to optimize the geometry of the timber-steelhybrid beam regarding cost effectiveness and load bearing capacity. A new idea is to use cold formed "U" profiles made of thin steel plates. Based on structural performance requirements, some static calculations of middle-span (4-10 m) timber-steelhybrid beams with loads up to 30 kn/m have been done. First economical calculations and static analyses were presented by the authors at the WCTE 2010 Trento [1]. The good results of the static calculations demonstrate the efficiency of timber-steel-hybrid beams. In comparison to GL-beams, a cost reduction can be expected and steel-steel connections become possible. The benefits of such connections are the possibility to produce semi rigid column-beam joints in a simple way. These moment-resisting frames can resist lateral loads. This concept was presented at the WCTE 2010 Trento [1]. Gun driven nails, which are developed for the connection of steel-steel plates, can also be used for timber-steel connections. One topic of this research project is the suitability of gun driven nails to build composite profiles. Some timber-steel-hybrid beams with different crosssections and timber grades have been tested in order to verify the results of the numerical calculations. 1 Wolfgang Winter, Institute of Architectural Sciences, Structural Design and Timber Engineering, Vienna University of Technology, Karlsplatz 13/259/2, 1040 Vienna, Austria. winter@iti.tuwien.ac.at 2 Kamyar Tavoussi, Institute of Architectural Sciences, Structural Design and Timber Engineering, Vienna University of Technology, Karlsplatz 13/259/2, 1040 Vienna, Austria. 3 Tamir Pixner, Institute of Architectural Sciences, Structural Design and Timber Engineering, Vienna University of Technology, Karlsplatz 13/259/2, 1040 Vienna, Austria. pixner@iti.tuwien.ac.at 4 Felipe Riola Parada, Institute of Architectural Sciences, Structural Design and Timber Engineering, Vienna University of Technology, Karlsplatz 13/259/2, 1040 Vienna, Austria. riola@coag.es

The cold formed section works together with a timber beam (Figure 1a,b).

These plates can be connected with screws or gun driven nails (Figure 1a-d).

2 2 HYBRID BEAMS FEASIBILTY STUDY 2.1 Principle of the assembling The beams were assembled using two cold formed U steel profiles and glue laminated or cross laminated timber cross sections. The cold formed section works together with a timber beam (Figure 1a,b). 2 mirror-inverted timber-steel beams (Figure 1c) are fixed together with some bolts and can be reinforced, if necessary, with steel plates on both sides. These plates can be connected with screws or gun driven nails (Figure 1a-d). a b c d Figure 1: Assembly process of completed timber-steelhybrid beam The elements of the timber-steel-hybrid beam are shown in Figure 2. Figure 3: Reinforcement with thin steel plates on both sides connected with driven nails The fixing of the reinforcement with thin steel plates on both sides connected with screws is shown in Figure 4. Figure 2: Elements of the timber-steel-hybrid beam The fixing of the reinforcement with thin steel plates on both sides connected with driven nails is shown in Figure 3. Figure 4: Reinforcement with thin steel plates on both sides connected with screws

2.2 Samples In the following chapters the shortcut H1 describes the hybrid beams with 2 cold-formed U-steel profiles (S235) with each a depth of section h of 407 mm, with a flange width b of 61 mm,

3 2.2 Samples In the following chapters the shortcut H1 describes the hybrid beams with 2 cold-formed U-steel profiles (S235) with each a depth of section h of 407 mm, with a flange width b of 61 mm, with a thickness of 1 mm additionally reinforced on both sides with 2 steel plates (S235) with a width b of 120 mm and a thickness of 2 mm. The shortcut H2 describes the hybrid beams with 2 coldformed U-steel profiles (S235) with each a depth of section h of 329 mm, with a flange width b of 100 mm and with a thickness of 2 mm. The shortcut GL stands for the glue laminated timber (GL28h). The shortcut CLT stands for the cross laminated timber working as a beam with a width of 5,7 cm (2 vertical layers and one horizontal layer each of 1,9 cm made out of C24). The shortcut S describes the connection with screws (Hilti, S-MD01Z). The shortcut N describes the connection with shooting nails (Hilti, XU47MX) Sample 1 and 2 (H1-GL-S / H1-GL-N) The first sample (H1-GL-S) consists of 2 glue-laminated beams and 2 cold-formed U steel profiles, reinforced by 2 steel-plates (each 2 mm) on both sides. The connection advices are screws. This first option proved to be very laborious because of the requirement of pre-drilling the 2 mm thick reinforcement steel plates. The second sample (H1-GL-N) consists of 2 gluelaminated beams and 2 cold-shaped U steel profiles, reinforced by 2 steel-plates (each 2 mm) on both sides. The connection advices are Hilti-nails. The properties of the components and of the assembled hybrid beams are illustrated in Table 1. Table 1:Characteristics of sample H1-GL-S and H1-GL-N H1-GL-S H1-GL-N Both parts of the beams were joined together using four steel bolts with a diameter of 30 mm. These bolts were located at the resting and loading sections of the beam. A gap of 5 mm in the bottom part of the all these beams was provided for the tolerance dimensions of both elements. This gap was filled using steel plates in the resting and loading sections of the beam. Steel Cross sections Timber A= 15,34 cm² A= 480 cm² G= 12 kg/m G= 24kg/m EI= kncm² EI= kncm² EI/EI tot = 0,51 EI/EI tot = 0,49

2.2.2 Sample 3 (H1-CLT-N) The third sample consists of 2 cross-laminated beams and 2 cold-shaped U

2.3 Sample 4 (H2-GL) The fourth sample consists of 2 glue-laminated beams and 2 cold-shaped U

The properties of the components and of the assembled hybrid beams are illustrated in Table 3.

4 2.2.2 Sample 3 (H1-CLT-N) The third sample consists of 2 cross-laminated beams and 2 cold-shaped U steel profiles, reinforced by 2 steelplates (each 2 mm) on both sides. The connection advices are Hilti-nails. The properties of the components and of the assembled hybrid beam are illustrated in Table 2. Table 2: Characteristics of sample H1-CLT-N Sample 4 (H2-GL) The fourth sample consists of 2 glue-laminated beams and 2 cold-shaped U steel. Because of the thicker U cold formed steel plates (t=2 mm) no reinforcement was provided. The properties of the components and of the assembled hybrid beams are illustrated in Table 3. Table 3: Characteristics of sample H2-GL H1-CLT-N H2-GL Steel Cross sections Timber Steel Cross sections Timber A= 15,34 cm² A= 456 cm² G= 12 kg/m G= 19,2 kg/m EI= kncm² EI= kNcm² EI/EI tot = 0,58 EI/EI tot = 0,42 A= 21 cm² A= 640 cm² G= 16,5 kg/m G= 32 kg/m EI= kn/cm² EI= kncm² EI/EI tot = 0,50 EI/EI tot = 0,50

5 2.3 Simulation-based calculation Before starting the test, some simplified simulations have been carried out. The aim was to estimate the load bearing behaviour of the hybrid beams. The interconnection between the timber beam (element below) and the steel profile (element above) was simulated by 2 simple rigid links. According to the flexural stiffness, each beam has to carry the own part of the loading forces. The separated bending moments for the hybrid beams H1-GL-S and H1-GL-N are shown in Figure 5. Figure 8: Deflection H1-CLT-N The separated bending moments for H2-GL are shown in Figure 9. Figure 5: Bending moments and support reaction H1-GL The deflection for the hybrid beams H1-GL-S and H1- GL-N is shown in Figure 6. Figure 9: Bending moments and support reaction H2-GL The deflection for the hybrid beam H2-GL is shown in Figure 10. Figure 10: Deflection H2-GL Figure 6: Deflection H1-GL. The separated bending moments for H1-CLT-N are shown in Figure 7. Figure 7: Bending moments and support reaction H1- CLT-N The deflection for the hybrid beam H1-CLT-N is shown in Figure 8

6 2.4 Execution of static tests At first, two glulam beams and one CLT beam were tested. In the case of the glulam beams the cross section was 80x400 mm. In the case of the CLT beams the cross section was 57x400 mm. These two reference beams were obtained from the same original pieces used to assemble the hybrid beams. The idea was to find out the flexural stiffness of the used timber beams. In the second step, a series of four different types of timber-steel-hybrid beams were tested. All of them had a span of 6 meters. All the timber steel hybrid beams were tested in a four point static bending test. The beams were single span and simply supported over pin joints. Figure 12: Deflection of H1-GL-S under load The two loading points were located at the one third points of the span (at 2 and 4 meters). The load was gradually applied and the results of the deflection of the beam were recorded every 2 kn of applied load. Three recording devices were located under the beam, one at the middle and the other two under every loading section. In the following load-deformation diagrams only the maximum deflection in the middle is diagrammed. Steel tubes and timber struts were connected in the compression zone of the beams to avoid the buckling problem of the thin steel plates, see Figure 11. When reaching high loads this measure could not avoid the buckling effect. Figure 13: Deflection of H1-GL-N under load Figure 11: Measure for avoidance of buckling in the compression zone From Figure 12 to Figure 15 the deflection under load for all tested hybrid beams is shown. Figure 14: Deflection of H1-CLT-N under load

7 Figure 18: Load-deformation diagram H1-GL N The failure pattern for H1-GL-N is shown in Figure 19. Figure 15: Deflection of H2-GL under load Results of the static tests Every beam was loaded till the state of failure of the timber elements. Because of technical and security reasons the deformations were not measured till this state. Figure 16, 18, 20, 22 show the load-deformation diagrams including the ultimate load for all tested hybrid beams. Figure 19: Failure pattern under ultimate load H1-GL-N Figure 16:Load deformation diagram H1-GL-S The failure pattern for H1-GL-S is shown in Figure 17. Figure 20: Load deformation diagram H1-CLT N The failure pattern for H1-CLT-N is shown in Figure 21. Figure 17: Failure pattern under ultimate load H1-GL-S Figure 21: Failure pattern under ultimate load H1-CLT-N

8 The weakest beam, H1-CLT-N failed because of lateral torsional buckling. The weakening of the cross-section by connecting nails affected this kind of beam much more than H1-GL-N or H1-GL-S. The reason is the existence of only two (of totally three) load bearing (vertical) layers. The reason of failure for H1-GL-N, H1-GL-S and H2- GL was the fracture in the tension area of the timberbeams. Figure 22: Load deformation diagram H2-GL The failure pattern for H2-GL is shown in Figure 23. Figure 23: Failure pattern under ultimate load H2-GL 2.5 Analyses The assembling of H2-GL was the easiest one. The higher use of material was compensated by faster assembling and by reduction of problems with buckling of the compressed upper flange. The evaluation of the results is still in process. 3 OUTLOOK In future research projects following aspects have to be analysed. The long term behaviour of such hybrid beams concerning the creeping and the load distribution. The structural design calculation for hybrid beams concerning the safety factors, the deformation and the modification factors. For necessary reinforcements with additional steel plates the nailing pattern should be optimised to reduce the assembling time and the weakening of the cross-section, especially in the tension area. REFERENCE [1] Kamyar Tavoussi; et al.: Steel reinforced timber structures for multi storey buildings; paper nr. 820: Vienna University of Technology; Proceeding of WCTE For the other three beams, the positioning and the number of used nails (or screws) should be optimised to reduce the assembling time and the weakening of the cross-section, especially in the tension area. The load-deformation behaviour of all tested beams is combined in Figure 24. Figure 24: Load deformation diagram for all beams

Steel reinforced timber structures for multi storey buildings

Steel reinforced timber structures for multi storey buildings Kamyar Tavoussi 1, Wolfgang Winter 2, Tamir Pixner 3, Michael Kist 4 ABSTRACT: For modern multi-storey buildings timber-steel-hybrid elements