an innovative prefabricated timber-concrete composite system...in order to achieve composite action...

An innovative prefabricated timber-concretecomposite system

Roberto Crocetti, Tiziano Sartori, Roberto Tomasi, Jose L. F. Cabo

1 Abstract

A novel type of timber-concrete composite floor, consisting of longitudinal glulambeams with a fibre reinforced concrete (FRC) slab on the top is proposed. In orderto check some relevant mechanical properties of such a floor, full-scale laboratorytests along with numerical analyses were carried out. The shear connector systemused in the investigation consisted of self-tapping screws driven at an angle of 45◦ tothe grain direction of the glulam beams. The manufacture of the structure occurredaccording to the following steps: (a) the screws were inserted on the top of theglulam beams; (b) the beams were rotated 180◦ about the longitudinal axis andplaced in a concrete formwork; (c) the FRC was cast into the formwork; (d) aftercuring of the FRC, the composite floor was again rotated 180◦ about the longitudinalaxis into its right position, i.e. with the FRC slab on the top side. Long term tests andquasi-static bending tests were performed. It was found that the proposed connectionsystem showed a very high degree of composite action both during the long-termtesting and at load levels close to the failure load. Furthermore, the assembly of theprefabricated timber-concrete composite system revealed to be very fast and easy.

Roberto CrocettiDepartment of Structural Engineering, Lund University, Sweden, e-mail:[email protected]

Tiziano SartoriDepartment of civil, environmental and mechanical engineering, University of Trento, , Italye-mail: [email protected]

Roberto TomasiDepartment of civil, environmental and mechanical engineering, University of Trento, , Italye-mail: [email protected]

Jose L. F. CaboETS of Architecture, Polytechnic University of Madrid (UPM), Spain e-mail: [email protected]

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2 Roberto Crocetti, Tiziano Sartori, Roberto Tomasi, Jose L. F. Cabo

2 Introduction

Timber-concrete composite structure consists of timber beams effectively intercon-nected to a concrete slab cast on top of the timber members. Most of the studiesperformed to date have focused on composite systems where wet ordinary concretewas cast on top of timber beams with mounted shear connectors. Even though suchsystems have proven to perform very well from the point of view of statics and dy-namics, in-situ concrete casting has some clear disadvantages, for example, wasteof time due to concrete curing, low stiffness and high creep, concrete shrinkage ef-fects on the composite beam, high cost of cast-in-situ concrete slabs, etc. Recently,composite systems where the concrete slab is prefabricated off-site with shear con-nectors already embedded and then connected to the timber beams on site have beeninvestigated ([4] and [7]). The research presented herein focus on the use of com-posite structure with very high prefabrication level, good performance and shortconstruction time. Such composite structures are ”floor modules” consisting of twoglulam beams with a a concrete slab on the top.

3 Materials and methods

In order to investigate the behaviour of the proposed composite system, three full-scale floors were built at the laboratory of Structural Engineering, Lund University.The main dimensions of the floor system are reported in Table 1.

Table 1 Geometry of the A, B and C composite system (dimensions in [mm]). See also Fig. 1.Span (l) Slab width Slab thickness(h1) Beam width (b2) Beam depth (h2) Beam spacing (i)

7200 800 50 115 360 585

Fig. 1 Geometry of the floor system

The geometry of the three tested floors was nominally identical. The timber usedfor the manufacture of the floors was glulam GL30c. The moisture content of the

An innovative prefabricated timber-concrete composite system 3

beams was approximately 12%. For the production of 1 m3 of fiber reinforced con-crete, 45 kg of steel fibres and 480 kg of cement were used, which gave a meanvalue of compression strength fc = 51 MPa for the concrete. In the following text,the three tested specimens will be referred to: A, B and C. Specimens A and B weretested on short term bending, whilst specimen C was tested on long term bending.During the short-term bending tests, the load was applied by an actuator in a dis-placement controlled manner. The load was distributed on four lines perpendicularto the longitudinal direction of the floor in order to induce stresses and deformationsin the floor similar to those induced by a uniformly distributed load q, see Fig. 2.

The total load applied to the specimen, the mid-span deflection, and the relativeslip between slab and beam at the supports were continuously measured during test-ing. For the long-term bending test, a uniformly distributed load of 1 kNm2 was appliedon the slab by means of sacks of cement. The mid-span deflection was measuredover time in order to investigate the creep effects of both timber and concrete.

3.1 Shear connectors and manufacture of the floor systems

In order to achieve composite action between the timber beams and the concreteslab, self-tapping screws with dimensions d = 11mm and l = 250mm were driveninto the timber beams before the concrete was cast. The screws were driven at anangle of 45◦ to the longitudinal directions with a spacing of 200 mm close to the

Fig. 2 Test setup


supports and 300 mm in the middle part of the floor respectively (Fig. 3). The mainfunction of the inclined screws is to transfer the shear force from the concrete slabto the timber beam both by shear in the direction parallel to the slip interface andmainly by tension in the direction of the screw axis. The ultimate tensile strength ofthe screws was approximately fu = 1250MPa.

Fig. 3 Screw positions

The screws were inserted on the top of the glulam beams, then the beams wererotated 180◦ about the longitudinal axis and placed in a concrete formwork. TheFRC was cast into the formwork, see Fig. 4. After curing of the FRC, the compositefloor was again rotated 180◦ about the longitudinal axis into its right position, i.e.with the FRC slab on the top side.

4 Test

4.1 Preliminary tests

Preliminary tests were performed in order to obtain modulus of elasticity of timberbeam, strength of concrete, withdrawal resistance of screw to concrete connectionand strength of steel screws.Standard compression tests were carried out on three concrete cubes. The geometryand the compression strength of the tested specimens are resumed in Table 2.

Screws with different penetration length were inserted in the concrete cubes. Thevalue of withdrawal tests on these specimens are reported in Table 3.

Non destructive bending tests were performed on two timber beams in order toestimate the modulus of elasticity. The results are reported in Table 4.


(a)

(b)

Fig. 4 (a) Timber beam with inserted screws- (b) ”upside down” floor system (including the form-work) directly after concrete casting

Table 2 Geometry and compression strength of the tested concrete cubesID Side Side Depth Compression

strengtha b h σm

# [mm] [mm] [mm] [N/mm2]A 150 150 150 51,11B 150 150 150 50,89C 150 150 150 51,11

Table 3 Ultimate withdrawal capacity of the screws inserted into the concrete with different pen-etration lengths.

ID specimen penetration lengths [mm] Ultimate tensile capacity [kN]

A 50 27,49B 50 19,75C 50 23,90

Mean value 23,71A 75 40,03B 75 40,04C 75 38,32

Mean value 39,46A 100 40,68B 100 42,92C 100 40,58

Mean value 41,39


Table 4 Modulus of elasticity and density of timber beamsID Lenght [mm] Depth [mm] Width [mm] ρm [kg/m3] Em [MPa]A 7198 355 112 477 12480B 6595 356 111 449 12189

4.2 Short-term bending tests

Load-deflection curves and load-slip curves are shown in Fig. 5 and Fig. 6 respec-tively.

0 20 40 60 800

20

40

60

80

Deflection at mid-span f (mm)

Equ

ival

entu

nifo

rmly

dist

ribu

ted

load

q(k

N m2

)

Floor AFloor BEImaxEImin

Fig. 5 Equivalent uniformly distributed load vs mid span deflection

The curves in Fig. 5 show the relationship between the equivalent uniformly dis-tributed load q (i.e. the total load applied divided by the slab area) and the deflectionf at mid-span. As it ca be observed, the behaviour is linear up to a load level of ap-proximately 80 kNm2 , which is well above the design load used in design of commonfloor structures. The stiffness of both composite floors shows, after a slightly non-linear initial part, a constant trend up to the failure of one of the two timber beams.The curve of Fig. 6 shows the slip at the support related to the equivalent distributedapplied load. Failure of floor A occurred at a load q ' 84 kNm2 , with the propagationin one of the two beams of two large cracks in the direction parallel to the grain.The failure of the floor type B, on the other hand can be attributed to local failure ofa finger joint of the lowest lamination located close to mid span of one of the twobeams. The collapse of floor A occurred at q=80 kNm2 firstly due to bending failure ata finger joint in one beam and secondly due to a shear failure located along a linerunning through the tips of the screws used as shear connectors.


0 20 40 60 800

0.5

1

1.5

2

2.5

Equivalent uniformly distributed load q ( kNm2 )

Slip

atth

esu

ppor

t(m

m)

Floor AFloor B

Fig. 6 Load slip deflection vs equivalent uniformly distributed load

4.3 Long term bending test

For the long-term bending test, a uniformly distributed load of 1 kNm2 was appliedon the slab by means of sacks of cement(see Fig. 7). The purpose of the long-termtest was to investigate the time-dependent behaviour of the prefabricated timber-concrete composite system. The long-term test results for the specimen B is pre-sented in Fig. 8 in terms of time vs mid-span deflection. The variables monitoredduring the entire test were the mid-span deflection through 2 inductive transducers,positioned at the mid-span of each glulam beam.

Fig. 7 Long term test setup


Fig. 8 Increase in mid-span deflection of the floor C with time

Also the temperature and the humidity in the laboratory was continuously moni-tored. The value of these parameters are presented in Fig. 9.

Fig. 9 Relative humidity and temperature observed in the laboratory during the period 01-02-2013/ 30-11-2013


As it can be seen in Fig. 8 the mid-span deflection increase to about three timesthe instantaneous deflection after three months. In March the floor has been down-loaded for two days. When the floor was reloaded it achieved again the same levelof deflection as before unloading. This operation was performed again in the firstdays of May and similar results were obtained.

5 Efficiency of the composite beams

Deformations at the shear connectors generate horizontal movement, i.e. slip at theinterface between concrete and timber. Such a behaviour is due to as partial com-posite action and, as the slip increases it reduces the efficiency of the cross section.The efficiency of a shear connection for a composite beam can be estimated usingthe following equation, see [9] and [10].

η =EIreal−EIminEImax−EImin

(1)

where η is the efficiency, EImax is the bending stiffness of the floor with fullcomposite action, EImin is the bending stiffness of the floor with no composite actionand EIreal is the actual bending stiffness of the floor. At load levels comparable tothose at the serviceability limit state (i.e. 1 kNm2 ) the efficiency η is approximately1.0. The efficiency at a load of 20 kNm2 or more remains constant,i.e. η ' 0.85.

6 Conclusions

This paper presents the main results of a research project conducted on a novelprefabricated timber-concrete composite system. The system provides several ad-vantages compared to cast in-situ concrete slabs, e.g. reduced time of constructionand considerable reduction of the effects of concrete shrinkage. Experimental testswere carried out on three 7.2 m long strip floor specimens to investigate on stiff-ness and the strength and stiffness of the prefabricated systems, of which two teststo failure and one long-time test. The principal observations from the experimentalinvestigations are:

• The tested system showed considerably higher stiffness and strength propertiesthan a similar system with concrete deck not able to transfer shear stress

• The load carrying capacity was very high. The equivalent uniformly distributeload at failure was approximately 80 kN/m2, which is considerably larger thancommon designs load for floor structures.

• The stiffness of the system was also very high. This depends primarily on theability of the shear connectors to transmit shear without (or with minor) slipping.In the tested specimens the efficiency of the system was approximately 1 for


loads well above common design loads. For extremely high loads (q≥ 20kN/m2)the efficiency of the system was approximately 0.85, which is also very highcompared to similar composite system with more traditional shear connectors,i.e. screws or bars inserted perpendicularly to the plane of the slab.

• The instantaneous deflection of the floor increased roughly by a factor 3 after aperiod of approximately 7-8 months. This relatively large increase in deflectionis believed to be due mainly to the creep of the concrete slab and of the timberbeam and - in some minor extent - to the long-term deformation of the shearconnectors.

• Last but not least, the easiness of manufacture of the proposed system shouldnot be underestimated, since it allows for a quick construction whit a reducedpossibility of human errors.

7 Acknowledgements

The authors wish to gratefully acknowledge the Mr. Franco Moar who has per-formed his Master’s thesis on this topic. The timber material was supplied by theglulam mill Moelven Töreboda AB, Treboda, Sweden. The screws were suppliedby Rotho Blaas srl, Cortaccia, Italy. The fibers for the FR concrete were supplied byBekaert Svenska A.B. All the suppliers are kindly acknowledged.

References

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2. Blass, H. J., Bejtka I. Screws with continuous threads in timber connections. InternationalRILEM Symposium on Joints in Timber Structures: 193 - 201.

3. Moar, F. Prefabricated timber-concrete composite system Master thesis, Lunds TekniskaHgskola, Lunds Universitet (2012).

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6. Lukaszewska E. Development of Prefabricated Timber-Concrete Composite Floors. PhD the-sis, Department of Civil, Mining and Environmental Engineering Division of Structural En-gineering, Lule (2009).

7. Lukaszewska, E., Johnsson, H. and Sthen L. Connections for Prefabricated Timber ConcreteComposite Systems. Proceedings of World conference timber engineering, Portland - Oregon-USA . (2006).

8. A. Sjöström, J. N.-Montero, D. Bard, R. Crocetti Vibratory investigation of a fiber reinforcedconcrete floor supported by wooden beams: part I. Proceedings of Joint Baltic-Nordic Acous-tics Meeting, Odense - Denmark. (2012).


9. M. Piazza, G. Turrini Sulle strutture composte legno-legno. Proceedings Italian Workshop onComposite Structures, Department of Mechanical and Structural Engineering, University ofTrento, Villa Madruzzo, Trento, Italy, 1993: pp. 349-370.

10. M. Piazza Restoration of timber floors via a composite timber-timber solution. RILEM Work-shop Timber: a Structural Material from the Past to the Future, Trento, 1994: pp. 167-187.

an innovative prefabricated timber-concrete composite system...in order to achieve composite action...

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