PREMA EN ANALYSIS AND MODELLING COMPOSITE TIMBER-CONCRETE SYSTEMS : DESIGN OF BRIDGE STRUCTURE

Primena adekvatnog građevinskog materijala na odgovarajućem mestu u konstrukciji jedan je od postulata savremenog građevinskog konstrukterstva i optimalnog projektovanja. Ideja konstrukcijskog sprezanja u širem smislu (u odnosu na kompozitno sadejstvo u užem smislu sprezanje istih vrsta materijala u različitim tehnološkim fazama ili starostima) tj. zajedničkog rada dva ili više različitih materijala s ciljem što boljeg iskorišćenja njihovih mehaničko-fizičkih i drugih svojstava na nivou preseka, elementa i sklopa, najšire je razvijena i ima najveću primenu kod spregnutih sistema beton–čelik. Spregnute konstrukcije drvo–beton primenjuju se u inženjerskoj praksi oko 80 godina, prešavši put od intuitivnog inženjerskog rešenja problema ojačanja drvenih mostova u šumskim predelima Severne Amerike do potpuno prefabrikovanih hibridnih sklopova drvo–beton za suvu ugradnju u današnjoj Evropi. Razvojni put spregnutih sistema drvo–beton bio je uvek praćen ekstenzivnim teorijsko-eksperimentalnim istraživanjima [45], čiji su rezultati uspešno primenjivani u praksi tj. na tržištu, ali su do današnjih dana u nedovoljnoj meri rezultovali konsekventnim i detaljnim građevinskim propisima za široku inženjersku upotrebu.


INTRODUCTION
The use of an adequate construction material in the appropriate position in the structure is one of the basic principles in modern and optimal structural design.The idea of structural composite action in wider sense (relative to composite action in a narrow meaningwhen composite action is achieved with the same type of materials in different technological phases or ages), i.e. the idea of composite action of two or more different materials with the aim to take advantages of mechanical, physical and other properties of each material on the cross-sectional, element or assembly's level, is highly developed and has huge application in the field of composite steel-concrete structures.
Timber-concrete composite (TCC) structures are in use in civil engineering practice over 80 years and went through the intuitive problem solution for strengthening timber bridges in rural zones of North America to the fully prefabricated hybrid assemblies for dry building in today's Europe.
The development path of timber-concrete composites was always followed by extensive theoretical and experimental research [45], whose results were successfully implemented in practice, i.e. on the market, but up to now they still fail to result in modern and detailed designer's codes for wider engineering application.
TCC systems can be successfully applied in buildings and bridges, as newly designed structures or method for rehabilitation and strengthening of existing ones.
The advantages of TCC systems in newly designed floor structures are reflecting in: − Significant increase of bearing capacity and stiffness -reduction of deformation and vibrations, improvement of fire resistance and acoustic barriers, increase of thermal mass, upgrading the seismic performance by improved diaphragm action (related to timber floors); − Reduction of total mass and loads on foundations that leads to improved seismic behaviour, in faster and more efficient construction with timber beams and timber based boards (formwork), as well as in economic and ecological building -reduction of CO 2 emission (related to concrete floors).
The advantages in bridge building are direct and easily recognizable: composite action with concrete adds timber bridges a new dimension when used as road bridges that stimulates the construction of these sustainable and attractive structures [21], [35], especially in Nordic countries.
In our country the notable theoretical and experimental results in the field of TCC structures are achieved by Prof dr B.Stevanovic, that are recognized in worldwide science [41], [42].In practical application of his work he made succesfull rehabilitations of numerous old buildings in Serbia.The reasearch potential and possibility of practical estimation stimulated the younger researchers to expand the problem of application of light concrete in TCC and study vibrations caused by action of human ruch impact, LJ.Kozarić [26].The highcyciclic fatigue, as well as combination of natural athesion and dowel fasteners in timber-concrete connection is also one of important recently condacted research, R. Cvetković, [5].
Besides the recognizable individuals in the field of TCC, it could be concluded that majority of civil engineers in Serbia very rarely decide to apply this efficient method of rehabilitation of old timber floors in practice, as well as to use it in design of new structures.The reasons are multiple, and generally could be summarized in two categories: • Lack of / or understatement of codes, that always reference to experimental work in every untypical case if different fasteners of way of coupling are used; • Software packages that are present at the market fail to directly support TCC with their libraries.The reasonable need of each modern structural engineer is to fully model a structure, whereby, in the circumstances of lack of clear code requirements, he faced with numerous confusions and confirms about acceptable level of problem approximation.
In anticipation of new European code for TCC structures (EN 1995-3), the objective of this paper is to provide a comprehensive review of available standards provisions and recent conclusions from literature that might have practical consequences in design.The key issues are highlighted and illustrated on the example of glulam composite arch bridge structure with concrete deck, designed according the Eurocodes.

Slika 1. Tipovi spregnutih nosača: a) greda T-preseka, b) drvena ploča od dasaka i c) drvena ploča od CLT
In our country for coupling the floor girders -"T" beams, the dowel type fasteners are mainly used (nails, bolts, screws, dowels and rods).Their application could be in vertical or inclined position, figure 2, [26], so, as widely used, they are the subject of consideration in this paper.

Elastic coupling in timber-concrete system
The degree of composite action in timber-concrete composite systems could vary from full (rigid) that could be mainly achieved by gluing or by continual mechanical strips, through partial (elastic) that could be mainly achieved by application of discrete mechanical dowel type fasteners, until independent behaviour of two materials is achieved when there is no composite action or the coupling is inadequately reached, figure 3, [31].Elastic (partial) composite behaviour has the practical meaning for most of timber-concrete structures, where the choice of fasteners type, the way and position of their application, has dominant influence / determine the degree of composite action through so called "slip" modulus of connection (fasteners).Slika 3. Kruto i elastično sprezanje drvo-beton [31] Figure 3. Rigid and elastic action in TCC beams [31] Iako se štapasta spojna sredstva smatraju najmanje krutim u domenu delimičnog sprezanja, ona su u širokoj upotrebi kod nas zbog male cene, jednostavne ugradnje i zadovoljavajućeg učinka u prenosu smicanja u odnosu na namenu, te su predmet analize u ovom radu.
Although the dowel type fasteners are recognized as the most elastic ones in domain of partial composite behaviour, they are widely used in our country because of low cost, simple application and satisfactory effects in shear transmission comparing with functional demands, so they are subject of analysis in this paper.

Design methods for timber-concrete composites
The design method that is dominantly used in calculation of TCC systems is so called "gamma" method [3], which is included in Annex B of Eurocode 5 [15], but as the procedure for calculation of timber-totimber and timber -wood based panels' connections.With little modifications, this design procedure could be applied on TCC systems as well [42].The method is initially derived by Mohler (1956), who considered the problem of slip in connections realized with mechanical fasteners.The procedure is based on solving differential equation of fourth degree, equation (1), for simple supported beam with constant and continual connection stiffness along the beam length; under an applied load q(x) that could be presented through sinusoidal function q=q 0 •sin(π•x/l).
It is adopted that "γ -method" could be applied for other types of loading as well, because variation of slip at member interface has greater influences on stressess than differences between exact and aproximative solutions of differential equation.Therefore, this simplified procedure is on the safe side and gives comperable results for timber-concrete composite systems.
Where is: γ 1 , γ 2 -gamma factor (introduces the slip of fasteners in shear plane) for concrete and timber, respectively, E 1 , E 2 -mean value of Young's modulus of elasticity for concrete and timber, respectively, A 1 , A 2 -cross section area of concrete/timber element, respectively, s -spacing fasteners at shear plane, K i -slip modulus of fasteners at shear plane between timber and concrete (K ser -slip modulus for serviceability limit states, K uinstantaneous slip modulus for ultimate limit states), L -span of the beam, a 1 , a 2 -distance between centreline of concrete/timber element and neutral axis of TC composite section, h 1 , h 2 -depth of cross section of concrete and timber element, respectively, EI ef -the effective bending stiffness of TC composite beam.
In cases where formwork exists like one interlayer, figure 4b, distances between centreline of concrete/timber element and neutral axis of TCC section are given by modified equations (7)(8), while the other equations stay unchanged.System where formwork exists is the real circumstance in the construction process of TCC system, and was the research subject in several investigation [19,20].

Slip modulus in timber-concrete connections
A stiffness of timber-concrete connection i.e. interface member slip due to mechanical fasteners is introduced into "gamma" method through γ shear coefficient which is defined by the value of slip modulus K. Slip modulus of fastener is usually determined on the basis of experimental tests, and it is given by force/displacement ratio, defining the connection stiffness by force that has to be applied in order to get unitary displacement: gde je: K -modul pomerljivosti veze; P -sila; δ -pomeranje.
On the basis of numerous experimental researches about TC interface stiffness with different fasteners and under the standardized loading conditions (for example SRPS EN 26891 [40]), it is evident that P-δ diagram is not linear so the value of slip modulus is not constant also, figure 5.
Slip modulus could be defined as secant or tangent in relation to certain load level or adopted connection's displacement, figure 6. Important facts for engineers working in practice are: if experimental results are available -slip modulus K ser for serviceability limit states (SLS) has to be determined as secant for load level of 0.4F est, while slip modulus K u for ultimate limit states (ULS) has to be determined for load level of 0.6F est , [2], [3], [40].If there are no experimental results, it is possible to use expressions suggested in EC5 [15] when fasteners are not new at the market.
Eurocodes 5 [15], defines the values for slip modulus K ser for fasteners and connectors in interface shear plane for timber-to-timber and wood-based-panels-totimber connections according to the expressions given in table 1, that are based on wood density ρ m [kg/m 3 ] and fastener's diameter d (d c ) [mm].The expressions are easy to use, but limited to vertical application of dowel type fasteners and for system without presence of formwork.

Slika 7. Pomeranje spojnog sredstva u spregnutom preseku: a) drvo-drvo, b) drvo-beton
U spregnutim konstrukcijama drvo-beton često se može naći upotreba oplate koja se postavlja prilikom izlivanja sveže betonske mešavine, slika 8. Ovaj slučaj je vrlo čest kod ojačanja, sanacije i rekonstrukcije starih drvenih tavanica [19], kao i kod mostovskih konstrukcija gde je postavljanje skele u vodi neizvodljivo.perpendicular to own axis i.e. when fastener is applied perpendicular to grain direction, while load is acting parallel to shear plane between members.Also, suggested values are applicable only in cases when formwork is absent.If dowel fasteners are applied in inclined positions to the grain direction and/ or the formwork is present, the value of slip modulus K ser is necessary to obtain by tests, what could be quite a problem for engineers in practice.

Slika 8. Spregnuta konstrukcija drvo-beton (a) bez prisustva oplate (b) s prisustvom oplate
Za uzimanje u obzir uticaja oplate na veličinu modula pomerljivosti spojnog sredstva može se primeniti tzv.Gelfi model [19].Teoretska podloga ovog modela, za In figure 8a), the formwork is positioned in such a way that concrete slab has direct contact with timber beam, so there is no influence of interlayer on connection stiffness i.e. on slip modulus K ser. .In case where the formwork is placed as one interlayer, figure 8b), its presence has significant influence on K ser value.The problem of valuation of slip modulus in TC system with formwork interlayer is very complex, and dependent from several variables, such as formwork thickness t, fastener's diameter d, mechanical properties of constitutive materials (timber, concrete and fastener), etc.The analysis in this paper is limited on cases of dowels as fasteners, because they are significant for Serbian engineering practice.
Winkler's elastic foundation, figure 9b).It is considered that dowel parts are supported on elastic foundation in concrete and timber, while the dowel part in interlayer is considered as free, because the stiffness of formwork is negligible.Winkler elastic stiffness of timber as foundation is determined experimentally and approximately its value is about k w = 1300 N/mm 2 , where the dowel diameter has insignificant influence on timber foundation stiffness.Concrete stiffness foundation k c , [20] is enclosed through ratio k c =Ec/β, where E c is the modulus of elasticity of concrete, while coefficient β=2.5-3.3 is in function of ratio of fastener diameter and mutual distance of dowels.Slika 9. Spojno sredstvo (trn) kao greda na elastičnoj podlozi [20] Figure 9. Fastener (dowel) as a beam on the elastic foundation [20] Rešenje teorijske analize, u pojednostavljenoj formi za potrebe prakse sa zanemarivom razlikom u odnosu na tačno, dato je sledećim izrazom: The theoretical problem solution, in simplified form with neglect differences and suitable for use in everyday practice, is given by the following expression: gde je: E s -modul elastičnosti trna; I s -moment inercije trna; l * -idealna dužina trna, predstavljena na slici 9-d), i data izrazom (18).
increase of formwork thickness.The similar conclusion is possible to derive for fastener's bearing capacity, figure 10 (right).Only for particular thickness of formwork and the same dowel diameter, the intersection of EC straight lines and Gelfi's curves indicates the real (same) solution of TCC.

Bearing capacity of dowels according to Eurocode 5
A bearing capacity of dowel type fasteners is mainly determined by three parameters: embedment strength of timber f h ,, fastener's modulus of plasticity M y and rope effect of applied dowels F ax , that could be experimentally tested by standards EN 1380 [39], EN 383 [37] and EN 409 [38].
where is: M y,Rk -the characteristic value for the yield moment, in [Nmm], When dowel type fasteners are subjected in shear, different failure modes, depending on mutual relation between the wood characteristic embedment strength, fastener's yield moment and thickness of timber element, could occur.Johansson (1949) was the first one who dealt with fasteners bearing capacity in relation with possible failure modes ("Johansen yield theory"), identifying three general failure modes: very stiff fastener and crashing wood grains, formation of one or two plastic hinges in fastener with slight crushing of timber.Mode I occurs in timber when timber embedment strength is exceeded and there is no plastic hinges in dowels, figure 11 (a,b,c).Mode II is determined by appearance of one plastic hinge in enough tick element, figure 11 (d,e).Mode III occurs when two plastic hinges appear what usually happens when all the parameters are balanced, figure 11 (f).From the aspect of fastener's ductility, the third type is the most favourable one.[31] Figure 11.Failure modes for timber and panel connections, single shear [31] U zavisnosti od tipa loma, Evrokod 5 daje izraze za proračun nosivosti na osnovu Johansenovih jednačina, a za veze drvo-drvo i čelik-drvo.Kako EC 5 ne sadrži proračun nosivosti spojnih sredstava za veze drvobeton, na osnovu rezultata različitih istraživanja mogu se koristiti predloženi izrazi za veze čelik-drvo, primenom izraza za debele čelične ploče (izrazi 23a, b, c), slika 12.
On diagrams, figure 13, it is shown that for specific widths ratio of composite members n•b c /b t , there is a particular heights ratio h c /h t for which the max effective stiffness will be generated in the case of rigid coupling [46].With adequate choice of dimensional proportions maximal bending stiffness EI max can be achieved which can be even four times greater than stiffness of EI min for structures with zero composite action.
The conclusion about desirable dimensional proportions is also valid in the case of elastic coupling.From diagrams, figure 14, where the beam span is L, fastener spacing is s and widths ratio is n•b c /b t , it can be noticed that heights ratio h c /h t that provides optimal effective stiffness, is rather constant in the case of elastic coupling, regardless of the slip modulus values.For analyzed spans (4m, 6m, and 8m), the effectiveness of composite action significantly arise between 10-300 kN/mm.Above that value, the increase of slip modulus fails to lead to further increase of composite action, while below the tested limits the composite action is insignificant [46].
It can be concluded that dimensional proportions have crucial impact on effective bending stiffness of TCC systems with partial composite action.As the slip modulus K also affects effective stiffness, figure 14, it is possible to find maximum ratio (EI eff /EI min ) max related to efficiency of composite action η, (expression 26), figure 15.

Efektivna širina betonske ploče
Efektivna (sadejstvujuća) širina betonske ploče u spregnutom preseku drvo-beton zavisi od where is: h c -depth of concrete slab, h t -depth of timber beam, b c -effective width of concrete slab, b t -width of timber beam, n -ratio of the Young's modulus of elasticity of concrete and timber, E c /E t .
The particular attention should be paid on beam span L that affects effective stiffness of TCC.Considering the mutual relation between span L and the degree of composite action η, it can be noticed that for spans less that 2m, the increase of slip modulus have insignificant influence on effective bending stiffness, figure 16.At the other hand, it is possible to achieve the same level of composite action with lower slip modulus, adopting larger beam spans for structure, figure 16, [34].
Faktor redukcije širine betonske ploče pri dejstvu u momentu savijanja: The research by Natterer and Hoeft [33] was focused on the analysis of behaviour of TCC beam systems subjected to common load cases (concentric, uniform and sinusoidal loads).The authors have derived and suggested a method for effective slab width based on reduction factor's definition.Applying the theoretical approach in consideration of axial force and normal stress in variable slab width on simple beam under different load cases, several types of reduction factors have been derived [36].The influence of loading type on concrete slab width is introduced through reduction factors due to internal forces i.e. bending and axial stresses.Relevant expression for reduction factors are given by equations 29-32.
Where is: b -mutual spacing of wooden beams, L -span of the beam.Finally, the effective concrete slab width in TCC system can be calculated according to expressions (32 and 33), that depend on loading type (uniformly distributed or point concentric load).

MODELLING TIMBER-CONCRETE COMPOSITE STRUCTURE
Problem formulation and modelling TCC system is a difficult task due to its complexity and lack of generalized experimental data, especially when there is no consequent structural codes [29].The number of available software packages for structural analysis is immense.Some of mostly widespread software on domestic market is Tower, AxisVM, SAP2000, etc., but these packages fail to contain particular libraries with relevant data for TCC modelling i.e. they are not particularly developed for this purpose.Therefore, relevant mechanical, physical and slip properties are usually inserted by "hand" and on the basis of poor and half-done code requirements.Data about slip modulus of dowel type fasteners in inclined / crossed position do not exist in EC5 (despite expressions for bearing capacity), so modelling of this type of timber-concrete connection in this moment is unclear and on the safe side without experimental results.Modelling vertically positioned dowel type fasteners with existing code's provision is efficient and could be done through several options.
Modelling TCC system is possible through the application of Virandel's model, figure 17.This model is easy to use for TCC with vertically positioned dowels because it directly imitates the behaviour of real system.When EC5 suggested (conservative) expressions for slip modulus are applied, the analysis leads to classic design given by "γ" method.Virandel's model is based on verticals that connect two members of similar or opposite physical and mechanical properties.The verticals represent mechanical dowel fasteners with stiffness [EI] defined through slip modulus for particular fastener type K, as it is shown in figure 17.
In certain software where "link" elements exist, it is possible to define and model timber-concrete connection in a more direct way."Link" elements are implemented in structural software in order to model and connect each set of joints between the beam and the plate as links between two nodes or two lines through unique interface, [25]."Link" elements have 6 stiffness components, 3 translations and 3 rotations that may have a nonlinear behaviour.A good way to connect all the links is to generate grid members or to draw the first one as referent and then copy it along the length of the member.If any of stiffness components is zero, the corresponding force or moment fails to transfer from node to node.In some software, e.g SAP2000, there is a "link" element between two joints in which it is possible to define nonlinear, plastic etc. behaviour what leads to more precise definition of partial composite behaviour.At the other hand, such modelling is suitable for research activities, when experimental data are available, but makes a lot of problems in everyday practice and it is inadequate when there is no clear code provisions.The stiffness of "link" element could be simply defined as slip modulus i.e. each link element represents dowel fastener in a structure with partial composite behaviour, connecting the centrelines of composite members.
Modelling composite behaviour of TCC system by 'link" elements is shown in figure 18, where timber and concrete members are linked at distances of real fasteners.The translator stiffness component of link element in direction parallel to shear plane is equal to slip modulus of fasteners K [N/mm].
Besides the modelling methods of timber-concrete composite behaviour that have both -advantages of being fast to model and limitation in application and accuracy, there are numerous sophisticated procedures that are convenient in theoretical research and suitable for real presentation of experimental work [29].Slika 18. Primer spregnute konstrukcije drvo-beton modelirane link-elementom [46] Figure 18.Modeling of TCC structure by "link" elements, example [46] 5 PRORAČUN MOSTOVSKE KONSTRUKCIJE OD LLD SA SPREGNUTOM BETONSKOM KOLOVOZ-NOM PLOČOM PREMA EN STANDARDIMA

Design task and preliminary analysis
In order to illustrate the theoretical issues from structural building codes and accompanying research, the design project of glulam arch bridge structure with composite concrete carriageway deck was done according to European codes [31].
As idea for structural solution of the bridge, the "Montmorency south forest bridge" Québec, was adopted, figure 19, which was modified with concrete deck instead the existing wooden one.
The three variant solutions of the bridge disposition and number of structural members are shown in figure 20.The variants are results of iterative analysis where the vehicle load positions and all relevant load combinations according to EN were considered [22,23,24] in order to rationalize the bridge structure.
Model 1 is a bridge structure formed by 22 glulam two-hinge arches as substructure and 21 glulam longitudinal continuous beams with concrete composite deck as superstructure.In Models 2 and 3 the number of structural elements is reduced, so the number of arches is 16 and 20, while the number of longitudinal beams is 11 and 10 respectively.The variant solution Model 2 represents the rational choice in structural design considering number and disposition of structural elements, as well as total structural behaviour under the proposed bridge actions.The model 2 is adopted for further analysis.

Analysis of actions and load combination according to EN
For adopted model, the analysis of actions according to EN 1991 [7,8,9,10] (self-weight, imposed traffic load -model LM1, with combinations gr1 and gr2, wind action -general method, when structure is insensitive to dynamic induces, snow) as well as analysis of seismic action according to 1998-1 [17] and 1998-2 [18] (seismic action through modal spectral analysis and linear elastic time analysis) were made.The combinations of actions are made for ULS and SLS, [6], [1], [44], for each relevant construction phases -design situations for bridges [13], [14], [16].
For timber glulam structure, the adopted material is the strength class GL28h.Concrete carriageway slabs, as well as all other concrete parts of the bridge, are designed in concrete class C35/45.Reinforcement steel and steel for dowel type fasteners is B500.

Slika 21. Isometric view of bridge structural model
The modelling of composite timber-concrete partial behaviour was made using the Virandel's girder, with substituting dowel diameter.Dowel fasteners are positioned into two lines with constant mutual distances of e=20cm along the bridge span.Pairs of dowels are modelled with substituting dowel with slip modulus K ZAM =2•K i .Slip modulus was determined according to the "Gelfi" model, taking into account the presence of formwork of thickness d=24mm.One model was made for each limit state, because slip modulus are different, as well as substituting dowels' diameters.In table 2 the main positions and labels of bridge structural elements are presented, together with transversal and longitudinal cross-sections.Tabela 2. Oznake pozicija i dimenzije elemenata konstrukcije mosta, s presecima [31] Table 2. Labels of elements, basic dimensions and cross-sections of the bridge structure [31]  U tabelama 3-4 dati su statički uticaji po pozicijama LLD elemenata.Za elemente sprezanja -trnove u tabeli 5 prikazane su maksimalne i minimalne vrednosti smičućih sila za najopterećeniji zamenjujući trn (POS T) za sve merodavne kombinacije dejstava, [31].

Effects on structure, dimensioning, dowel's check and ULS/SLS in elements
In tables 3-4 the actions' effects -internal forces in glulam elements are given.The maximum and minimum shear forces values in fasteners for substituting dowel (POS T) for all relevant load combinations are given in table 5, [31].
Relevant actions combinations were made for checking SLS and adequate static analysis was performed.In table 6 the max deformations due to relevant action combinations are given for main structural members (arch girders and composite carriageway deck).Maximum deflections are limited between intervals L/400 -L/500 for characteristic imposed traffic load.Analyzed deflections are among permissible limits.For checking ULS i.e. checking of stress distribution in cross-sections of main structural positions indicates the stress level of 70% for arches and about 80% for longitudinal beams.
Composite action between longitudinal glulam beams and concrete deck is realized by smooth steel (B500) dowels with diameter Φ=22mm and with length l=300mm.Minimum spacing and edge distances are adopted according to EN 1995-1 [15] recommendations.Pairs of dowels for coupling concrete slab are applied vertically across the shear plane at the constant distance of e=20cm along the longitudinal beams.
U ilustrativnom primeru, komparativna analiza seizmičkog dejstva na konstrukciju mosta pokazala je da Designed bearing capacity F v,Rd for dowels Φ=22mm is: Application of dowels with minimum spacing and edge distances could provoke practical difficulties at the site.The possible problem can be overcome with alternating rows that allow more precise application of dowels into the timber beam.
In the case when vertical elastic dowels are applied, the possibility for increasing connection stiffness can be found in their combination with notches in timber, when natural adhesion with concrete increases the total stiffness.Domestic experimental research [5] show that keeping the equidistant spacing among vertical dowels and having notches on the place of every second one, the slip modulus can multiply and composite degree could significantly arise.The results of similar worldwide investigation about combined application of fasteners and notches -natural adhesion, indicate that connection stiffness can be increased from 50 till 300% [27], [35].

CONCLUSION
The basic principles of analysis and design of timberconcrete composite systems are presented in this paper.The highlights are given on the recommendations of Eurocodes for structural design and on the results of accompanying theoretical and experimental research that have great importance on enlargement of knowledge basis and improvement of existing codes.
In anticipation of particular codes for designing timber-concrete composites, it is very helpful to use indirect and conservative recommendations from existing codes, especially when there is lack of experimental data.They have to be considered as basis for calculation of essential parameter for TCC designslip modulus.Important thing is that these recommendations are limited on vertically applied fasteners with direct contact between timber and concrete (no formwork in the cross-section).Besides the slip modulus, correct disposition and choice of members' proportions have also dominant influence on degree of composite action and overall effective stiffness.
ZAHVALNOST: Ovaj rad je proistekao iz istraživačkog projekta TR 36043 "Razvoj i primena sveobuhvatnog pristupa u projektovanju novih i proceni sigurnosti postojećih konstrukcija u cilju smanjenja seizmičkog rizika u Srbiji" koji je finansiralo Ministarstvo nauke Republike Srbije.upper position of carriageway deck, the simplified elastic response spectra analysis for two orthogonal directions could be applied.Through seismic analysis, the correct disposition of elements was achieved that led to more rational structure.
Progress in modern technology of timber elements prefabrication provokes arising worldwide interest for timber bridges.Building TCC bridges in last few decades has become trendy in the world for two reasons: structural efficiency with low costs, and favourable impact on environment.The aim of the paper is to present this type of composite structures to civil engineers through analysis and design of one arch bridge structure according to Eurocodes.
ACKNOWLEDGEMENT: This paper is supported by the research project TR 36043 "Development and application of a comprehensive approach to the design of new and safety assessment of existing structures for seismic risk reduction in Serbia", financed by the Ministry of Science of Serbia.

ANALYSIS AND MODELING OF COMPOSITE TIMBER-CONCRETE SYSTEMS: DESIGN OF BRIDGE STRUCTURE ACCORDING EN
Dragan MANOJLOVIC Tatjana KOCETOV MISULIC Timber-concrete composite structures are already applied more than 80 years in engineering practice, went trought the intuitive problem solution to the fully prefabricated hybride assemblies for dry building.The development path of timber-concrete composites was always followed by extensive theoretical and experimental research, whose results were successfully implemented in practice, i.e. on the market, but till presence didn't result in modern designer's code.In expectation of new europian codes for timber-concrete composites, the objective of the paper is to provide a comprehensive review of available standards provisions and recent conclusions from literature.The key issues for practical design are highlighted and ilustrated on the example of glulam composite arch bridge structure with concrete deck, according the Eurocodes.
Relevant shear forces in dowels Φ=22mm are determined through static analysis on Virandel system.The relevant load combination that gives max effect in substituting dowel is combination 3. [G+A Ed (RS)].Max calculated shear force in dowel for combination 3 is: F v,Ed = F v,Ed,ZAM /2=18.23/2=9.12kN.

ANALIZA I MODELIRANJE SPREGNUTIH SISTEMA DRVO-BETON: PRIMENA NA PRORAČUN MOSTOVSKE KONSTRUKCIJE PREMA EN ANALYSIS AND MODELLING COMPOSITE TIMBER-CONCRETE SYSTEMS: DESIGN OF BRIDGE STRUCTURE ACCORDING TO EN
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