Long term performance of contnuous wood-concrete-composte systems Leander A. Bathon Professor, Dept. of Archtecture and Cvl Engneerng, FH Wesbaden Unversty of Appled Scences, Wesbaden, Germany, bathon@t-onlne.de Olver Bletz Research Assstant, Ph.D. Student, Dept. of Cvl Engneerng, TU Munch and FH Wesbaden, Germany, holzbaulabor@fab.fh-wesbaden.de. Summary The short-term performance of contnuous wood-concrete composte systems has been tested, analyzed and successfully used n varous long span structures under constant clmate condtons. However, the nfluences of sgnfcant clmate changes have not been looked at so far. Ths paper ntroduces recent work n Germany that has been done on long-term performances of contnuous wood-concrete-composte systems usng the HBV-shear connector under varous clmate condtons. Intal test results showed a sgnfcant nfluence of creep wthn the concrete secton and therefore ndcate a closer look towards the ndvdual fractons of concrete verses wooden cross sectons. Therefore the studes nclude varous cross-sectons and loadng condtons.. HBV-System The success of a wood-concrete composte system depends solely on the performance of the shear connector. It s most advantageous from a structural standpont to use a shear connector that provdes ) a stff connecton between wood and concrete whle undergong stresses n the elastc range and ) a ductle response whle undergong stresses n the plastc range. Consderng the relatvely non-ductle falure modes of the concrete and wood n tenson, t s most logcal to desgn for falure to occur n the shear connector where mproved stffness and ductlty can be acheved through an elastc perfectly-plastc steel falure. Ths desgn consderaton was ntegral to the composte system developed n ths study. Ensurng falure ntates n the steel component also mproves system relablty due to the consstent nature of steel n comparson to wood or concrete. Fg. shows the structural elements of the HBV-System. A tmber secton s conjoned to concrete slab through glued-n steel meshes. One half of the connector s glued nto a channel wthn the glue lam plate element. The other half s reachng nto the concrete and acts as a support for the renforcement. A comparson of load-dsplacement-curves of wood-concrete-composte specmens usng varous shear connectors s llustrated n Fg.. Whle many wood-concrete shear connector systems n use today are nherently soft n terms of the latter slp plane the HBV-shear connector provdes a stff connecton between wood and concrete.
Fg : Structural elements of the wood-concrete composte system. Comparson of loaddsplacement curves of dfferent shear connectors. 3. Bendng tests The system was tested n shear and bendng condtons. The shear tests were performed on a number of shapes under varous clmate condtons. The short-term and long-term bendng tests were performed on full scale members wth a maxmum span of 0 Meters. The testng was conducted at the Materal Testng Laboratores (MPA) Wesbaden [3] [4]. The structural elements of the wood-concrete-composte specmens n bendng tests are shown by Fg.. The vew shows an optmzed desgn of the overall system to account for the ndvdual producton cost of the materals used. Both components wood and concrete are conjoned through two contnuous outer rows as well as one dscontnous (50 cm, 50 cm, 50 cm, 50 cm) nner row of HBV-shear connectors. The composte system was desgn to provde a fre resstance of up to 80 mnutes. The weght of the wood-concrete-composte specmen s 473 kg and therefore creates a dead load of 4,7 kn. The desgn of the cross secton shows a π-shaped concrete secton on top of a wooden plate made out of a glue-lam beam (00*960mm). The ar-pockets between the concrete and wooden layers are created by styrofoam blocks. Fg. 3 shows an optmzed desgn of a woodconcrete-composte beam used n the short term bendng test. Fg. : Structural elements of the full-scale wood-concrete composte system.
Fg. 3: Test set-up of the full-scale wood-concrete-composte specmen. In order to nvestgate the bendng performance of the wood-concrete-composte-system a 4 pont bendng test was conducted. As shown n Fg. 4, the center-to-center span was 9.86 m. The gluelam beam was prestressed by mdspan before the concrete was appled. The prestressng was done through a camber of the wooden beam plate of 3.3 cm at md-span. Two load lnes were appled.60 m apart centered about md-span usng a 000 kn capacty hydraulc actuator mounted to a system of steel spreader beams. Fg. 4 llustrates the load-dsplacement-dagram of the specmen. The fgure shows almost a lnear performance up to falure (steel falure of the HBV-shear connector). The ultmate falure load was approxmately 57,5 kn wth a md-span deflecton of approxmately 9,6 cm. Load [kn] 80 40 00 60 0 80 40 0 0 0 40 60 80 00 Dsplacement [mm] Fg. 4: Set-up and results of the short term bendng test In order to determne the long-term performance of the full scale wood-concrete-composte system under natural clmate condtons, a 4 pont bendng test was conducted accordng to the lab test. Fg. 5 shows the test set-up showng a free span of 9.86 m wth a bearng length of 40 cm at both supports. The specmen was placed outsde the MPA Wesbaden to be exposed to nature but covered. The gluelam beam was prestressed by mdspan before the concrete was appled. The prestressng was done through a camber of the wooden beam plate of 3.3 cm at md-span. Two load lnes were appled.60 m apart centered about md-span usng the dead load of a second woodconcrete-composte specmen whch was tested pror to that. Vertcal beam dsplacement was montored at md-span.
Fg. 5: Test set-up for nvestgaton of the long-term performance. The prelmnary load / md-span dsplacement curve of the beam s depcted n Fg. 6. After a tme of 430 days a deflecton of nearly 4,60 cm has been occurred. The test results show a sgnfcant effect of the cross secton geometry, the degree of constant loadng and the clmate condtons towards the long term deflecton. The elastc deflecton of 3,3 cm based on the loadng condtons of two specmens (total of 9 kn) s derved from the short term load vs. dsplacement curve llustrated n Fg. 4. Therefore the elastc deflecton of 3,3 cm elmnates the camberng of the composte beam n the same magntude. Therefore the deflecton n Fg. 6 starts at 0 cm. 5 Deflecton [cm] 4 3 calculated measured 0 0 50 00 50 00 50 300 350 400 450 tme [d] Fg. 6: Long-term performance of the specmen 4. Analyss The analyss of the wood concrete composte system was done through a truss model. The model s shown n Fg. 7. It consders the varable cross sectons of the π-shaped concrete secton as well as the tme dependent non lnear reducton of elastcty modulus for wood, concrete as well as the shear connectors. The reducton of the of elastcty modulus based on European Code for wood DIN 05, European Code for Concrete DIN 045 as well as the Code approval for the wood-concretecomposte system wth glued-n HBV-shear connectors. The model provdes a tool to predct the short-term test perfomance as well as the long-term behavor of the system.
Fg. 7: Truss model The analyss shows that the wood concrete composte system provdes a nearly full composte acton. The specmen s nfluenced by a number of parameters: the cross secton geometry, the degree of constant loadng as well as the clmate condtons towards the long term deflecton. The calculated deflectons of the partcular parameters are shown n Fg. 8. For example the dead load of the specmen produce a md-span deflecton after t = 430d of,00 cm. The loadng of the specmen through the second wood-concrete-composte specmen produces a deflecton of,5 cm after t = 430d. For t = 430d a deflecton of approxmately, cm results from the nfluences of clmate. Therefore the summaton of these values results after t = 430d nto a calculated total of 4,6 cm. A comparson wth the measured values of Fg. 6 show a good correlaton. Parameter t = 0 t = 50 d t = 70 d t = 35 d t = 430 d Dead Load [cm] 0,00 0,5 0,7 0,89,00 Loadng [cm] 0,00 0,77,0,34,5 Clmate [cm] 0,00 0,65,7,85, Σf [cm] 0,00,93,99 4,08 4,63 Fg. 8: Results of analyss based on the truss model 5. Desgn Proposal A prelmnary desgn based on the European code for wood, DIN 05 has been done. The code proposes a desgn for t = 0 as well as t =. The desgn of the wood-concrete-composte-specmen for t = has been done wth reduced elastcty modulus for wood, concrete as well as the shear connectors. In accordance wth ths code, the effectve bendng stffness (used n calculatng stffness and stress) of a b-materal system wth non-rgd shear connectors s calculated as: ( EI) ef = ( EI + γ EA a ) = γ = ; γ = π EA + Kl γe A ( h + h ) a = γ E A = [] [] [3]
These equatons were used to develop an analyss program that allows the desgn of wood-concretecomposte systems usng the HBV-shear connectors. Usng ths tool one can determne an nfnte md-span deflecton of 4,3 cm (7,6cm 3,3cm) based on an modulus of elastcty of the gluelam plate of nearly 750 kn/cm², an modulus of elastcty for the concrete secton of nearly 00 kn/cm² as well as a slp modulus for the HBV-shear connector of nearly 45 kn/mm (creatng a effectve bendng stffness of nearly 5,7 MNm²). 6. Concluson The long term performance (so far 430 days) of the wood-concrete-composte beam usng contnous HBV-shear connectors has been tested and analyzed. A truss-model has been developed consderng varous load and clmate condtons. In comparson wth the short term test results and the desgn approach of the DIN 05 a desgn program has been developed whch allows a good predcton of the system performance over tme. The work done allows for a predcton of the long term performance of wood-concrete-composte systems. In order to compensate the long term deflecton of a wood-concrete-composte system one should desgn a suffcent negatve deflecton (chamberng of the wooden secton) pror to the assembly of the concrete. A recent measurement n Aprl of 006 (t = 50 days, at 0 C) on the test specmen showed a reducton of the md-span deflecton of 0,5 cm down to 4, cm. Ths ndcates that the relatve elongaton of the concrete secton under warm clmate condtons takes some overall md-span deflecton out of the system. The montorng of the test specmen contnous. 7. References [] Bathon, L.; Bletz, O. 004. Holz-Beton-Verbund als starre und duktle Verbndung. IN: 0. Internatonales Holzbauforum 004, Garmsch-Partenkrchen [] Bathon, L.; Bletz, O. 005. Holz-Beton-Verbundsystem mt engeklebten HBV- Schubverbndern. IN: Bautechnk 8 (005), No. 5, P. 3 37 [3] Bathon, L.; Clouston, P. 004. Expermental and Numercal Results on Sem Prestressed Wood-Concrete Composte Floor Systems for Long Span Applcatons. IN: Proc. of the 8 th World Conference on Tmber Engneerng, Lath, Fnnland, June 4 7, 004, Volume : page 339 344 [4] Bathon, L. & Graf, M. 000. A Contnuous Wood-Concrete-Composte System. IN: Proc. of the 6 th World Conference on Tmber Engneerng, Whstler, B.C. [5] Blaβ, H. & Schlager, M. 996. Trag- und Verformungsverhalten von Holz-Beton- Verbundkonstruktonen. IN: bauen mt holz. 5/96, 395-399. [6] Ceccott, A. 995. Tmber-Concrete Composte Structures. IN: Informatonsdenst Holz. Holzbauwerke nach Eurocode 5. STEP. Lecture E3. [7] Köng, G.; Holschemacher, K.; Dehn, F. 004. Holz-Beton-Verbund. Innovatonen m Bauwesen. Beträge aus Praxs und Wssenschaft. Bauwerk Verlag GmbH, Berln [8] Kreuznger, H.; Hartmann, H.; Mohr, B. 997. Holz-Beton-Verbundecken. Dokumentaton der Ingeneur-Akademe Bayern 4.0.997, TU München