SAHC2014 – 9th International Conference on Structural Analysis of Historical Constructions

F. Peña & M. Chávez (eds.) Mexico City, Mexico, 14–17 October 2014

INVESTIGATIONS ON LOAD SHARING EFFECTS IN TIMBER-CONCRETE COMPOSITE CONSTRUCTIONS

Hubertus Kieslich 1 and Klaus Holschemacher 2 1 Leipzig University of Applied Sciences

Karl-Liebknecht-Strasse 132, 04277 Leipzig, Germany e-mail: hubertus.kieslich@htwk-leipzig.de

2 Leipzig University of Applied Sciences Karl-Liebknecht-Strasse 132, 04277 Leipzig, Germany

e-mail: klaus.holschemacher@htwk-leipzig.de

Keywords: lateral load sharing effects, timber-concrete composite, retrofitting of timber beam ceilings, bending tests, lightweight concrete, slab tests

Abstract. A lot of researches were carried out in the last years to investigate different prob-lems of timber concrete composite (TCC) constructions, particularly full-scale short-term col-lapse tests, in order to evaluate the load bearing behavior of different TCC systems. Nevertheless, some questions are still very few explored; among them are the load sharing effects of TCC constructions. They are able to distribute loads along and perpendicular to the span of the timber beams. A concentrated load applied to one beam of the TCC construction will deflect it due to bending. Because of the concrete slab adjacent beams will deflect as well, although no load is applied to them directly. These beams contribute to the load bearing of the loaded beam and relieve it partly from the loads reducing stress in this beam. This topic shall be focused on in this paper. It will report on short-term bending testes of TCC slabs car-ried out at Leipzig University of Applied Sciences. Several slabs had been tested. Each of them had a span of 3.9 m, consisting of three timber beams (10/20 cm) and a connecting con-crete slab (thickness 6 cm). Only the middle beam was loaded and deflections as well as strains were recorded at different points. Additionally, the wood and concrete properties were determined and push-out tests were carried out. Overall seven TCC slabs were manufactured and tested. With the slabs several parameters had been varied, e.g. the type of concrete, the stiffness of the connection system and the interjoist of the timber beams. As reference a timber beam ceiling, without concrete topping, was tested. The experiments will be described in de-tail in the paper.

Hubertus Kieslich and Klaus Holschemacher

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1 INTRODUCTION The technique to connect timber beams or decks with an upper concrete flange, using dif-

ferent types of connectors, is well known since the 20th of the last century. At that time the rare recourses may have been the motivation for this idea. Nowadays, we are in a quite similar situation. There is a decrease in natural resources, whereas the worldwide population is in-creasing, pursuing a high level of prosperity. That is why the focus is more and more concen-trated on sustainability. Often the replacement of the existing infrastructure is not the only way. Retrofitting of existing buildings is getting more and more important. Currently, timber-concrete composite (TCC) structures are mainly used as refurbishment method for existing timber beam ceilings. Often today’s requirements concerning bearing capacity and servicea-bility (deformation, vibration behavior and sound insulation) cannot be achieved. The reasons reach from the change of use to the biological damage of wooden structures. Often a combi-nation of several reasons is the causation for a required reconstruction. Among various ways (differing strongly in complexity, effect, and price) the TCC construction method can be one alternative. Thereby, a concrete slab is added to the timber beams. Both parts of the construc-tion are connected, using special metallic connection members. So it is possible to maintain principal parts of the ceiling. A TCC ceiling represents a special case of flexibly connected bending members. The properties of the structure are influenced by the components of the composite construction as well as by the properties of the connection itself. If there is no con-nection between timber and concrete, both parts of the cross-section are loaded in bending, they can move against each other. The increasing stiffness of the connection causes an in-creasing part of normal forces in the composite cross section. In a rigid connection no move-ments are possible any more. The fault in the distribution of stress of the cross-section, regarding the change-over from concrete to timber, results from the different modulus of elas-ticity of both construction materials. Possible tension forces in the concrete slab, in or perpen-dicular the direction of span, have to be taken by reinforcement. Several metallic connection members are used currently. They differ strongly in effectiveness, required labor input, and price. In Germany and many other European countries the use of fasteners, like nails or screws, is only allowed for bonding timber or timber with steel. If they should be used to con-nect timber and concrete, it is necessary to apply for an approval in individual case or a na-tional technical approval. The registered systems used in Germany are summarized in Holschemacher et al. 2013 [1].

To take into account the partial composite action, resulting from the flexibility of shear connection, two approaches are most common. Möhler derived the first one in 1956 [3]. It is a linear-elastic method (γ-procedure). Thereby, the Steiner’s dues of the plane-area moment are reduced because of the flexibility of connection members. The reduction factor γ ranges be-tween 0 (used for no connection action between timber and concrete) and 1 (full or rigid con-nection between both parts), which is practically not possible with mechanical connection members. This method is also part of the timber standard (Eurocode 5) in Europe [4]. Fur-thermore, this procedure can be well used, particularly for TCC beams with stiff connectors such as notches or glued in connections [2]. A second method (elastoplastic solution) was presented by Frangi and Fontana [5]. It is applicable for TCC-structures with connectors showing a low stiffness and high ductility [2]. Yeoh et al. [2] gave an overview of different full-scale short-term collapse tests in order to evaluate the load bearing behavior of different TCC systems. In actual structures, slabs are not only loaded by equally distributed loads, but also by single loads, for example because of heavy parts on the ceiling or because of trimmers, needed in order to realize openings in the ceiling. Often these parts of the structure are critical due to high deflections.

Investigations on Load Sharing Effects in Timber-Concrete Composite Constructions

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Figure 1: Lateral load-bearing behavior.

TCC ceilings can be considered as 3 dimensional plane structures, being able to distribute loads along and perpendicular to the span of the timber beams. A concentrated load applied to one beam of the TCC construction will deflect it due to bending (Figure 1). Because of the concrete slab, all beams are connected with each other. For this reason adjacent beams also deflect, even if no load is applied to them directly. These beams contribute to the load bearing of the loaded beam and relieve it partly from the loads, reducing stress in this beam. The de-scribed behavior, of distributing loads perpendicular to the span, is called lateral load bearing behavior. Taking this effect into account when designing a TCC ceiling may contribute to an economic way of design. This question is very few explored, and therefore the following ex-perimental results are presented. They are part of the researches at Leipzig University of Ap-plied Sciences.

The lateral load-bearing behavior of TCC and timber beam ceilings was investigated by Resch [6], using linear-elastic FE-models. Several factors influence the level of lateral load distribution [7], e.g.:

the thickness of the concrete slab,

the distance between the timber beams,

the kind of concrete (modulus of elasticity, strength),

the elastic properties of the timber (also the difference between adjacent beams),

the level of connection respectively type,

and distance of the connection member. To use the TCC construction, the consideration of the lateral load bearing behavior is of

particular interest, especially regarding economical aspects. Experimental investigations were started at the University of Applied Sciences Leipzig, to determine these influences. The pri-mary results will be presented below. This topic is rarely investigated so far. Some experi-mental tests are described in Dias et al. [8] and Skinner et al. [9].

2 EXPERIMENTAL PROGRAM

The experimental program consists of several parts, influencing each other. The principal part were the tested slabs. Secondary tests were dealing with wood, concrete and interface properties.

2.1 Principal tests – Slab tests

Basically, the specimens were single-spanned slabs, spanning in direction of the timber beams (Figure 2). The slabs consisted of three beams (10/20 cm) aligned in one direction and had a distance between each other of 60, 75 or 90 cm (compare Table 1). The span of the sys-tem was 3.90 m.

Hubertus Kieslich and Klaus Holschemacher

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Table 1: Specimen types.

Specimen [Unit]

T_75

T_N

C_7

5_12

T_LC

1_75

_12

T_LC

2_75

_12

T_LC

2_75

_6

T_LC

2_75

_18

T_LC

2_90

_12

T_LC

2_60

_12

Center distance beams [cm] 75 90 60

Concrete type [-] - NC LC1 LC2 Spacing of con-nection members [cm] - 12 6 18 12

A fixed support was located on one side, preventing any translation. A flexible support was

used on the other side of the beam, allowing translation. By using these types of supports, constraints were prevented. An interlayer with a thickness of 3.0 cm was installed between timber beams and concrete slab. A foil, to prevent any water exchange between them, separat-ed concrete slab and interlayer (wooden boards). For connecting the concrete slab with the timber beam, hexagonal timber screws 12 x 120 mm were used. The screws were drilled into the wood in such a way that 3 cm of the screws protruded into the concrete, providing an in-terlocking between concrete and timber. The distance between the screws varied between 6 cm, 12 cm and 18 cm.

Furthermore, the type of concrete, used for the concrete flange, was varied. For one TCC slab (T_NC_75_12) a normal weight concrete (NC), with a maximum grain size of 16 mm, was chosen. The other TCC slabs were characterized by the use of structural lightweight con-crete (LC), with a maximum grain size of 8 mm (the maximum grain size of the lightweight aggregate was 4 mm). The lightweight concretes 1 (LC1) and 2 (LC2) differ in their composi-tion. For reinforcing the concrete slab, a mesh reinforcement in the middle of the slab was chosen, with a reinforcement ratio of 2.57 cm²/m in each direction (Ø 7/15 cm). The concrete slab always had a thickness of 6 cm. All specimen types are summarized in Table 1.

The test arrangement was set up in a way that the load was applied directly to the beam in the middle of the slab, while the adjacent beams received the load applied indirectly by the concrete slab. The load was applied by the help of a hydraulic cylinder in the third part points of the middle beam. At first the load was increased up to 32 kN for the TCC slabs (13 kN for the timber beam ceiling). Than it was hold constant for approximately 30 s and degreased again down to 0 kN (30 s). This procedure was repeated one more time. Finally, the load was increased up to the failure of the specimen.

For measuring the deflection and strain at the tested slabs LVTDs and strain gauges were applied. The measuring points are shown in Figure 2. Nine LVTDs were used to observe the deflection for each specimen. All transducers where located on top of the concrete slab. Six of them were arranged near the supports (A1-A6) and three in the middle of the slab (D1-D3). Nine strain gauges (SG) were adhered. They were located in the middle of the slab, three at the bottom side of each beam (1-3) and six on top of the concrete slab (4, 6 and 8 in load car-rying direction; 5, 7 and 9 in lateral direction).

Investigations on Load Sharing Effects in Timber-Concrete Composite Constructions

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Figure 2: top: measuring device, bottom: Experimental set-up of the tested timber beam ceiling (left) and the TCC ceiling (right).

2.2 Secondary tests The secondary tests were addressed to the determination of all properties connected to the

composite slabs. Several tests were carried out. Following are the most important: hardened concrete properties (compressive strength, oven-dry density, Young’s modulus),

wood properties (bending strength and Young’s modulus, compressive strength in and perpendicular the grain direction),

interface properties (push-out tests for the load-slip relationship and the maximum load of the connection members)

Figure 3: Test arrangement for the bending test of wood (left), determination of hardened concrete properties

(middle and right).

Hubertus Kieslich and Klaus Holschemacher

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The samples for the determination of the wood properties were taken from every beam of each tested slab (near the support of the slab) after the bending test. The tests concerning the bending strength of wood were based on German standard specification DIN 52186 [10]. The specimen was an error-free timber bar with a cross section of 20 x 20 mm2 and a length of about 360 mm. The span was 300 mm. The load was applied with a constant velocity of 0.1 mm/s in the middle of the timber bar (compare Figure 3 on the right). The compressive strength of wood was tested on cubes (50 x 50 mm2). The bending strength of the beams will be concentrated on in this publication.

The hardened concrete properties were tested 28 days after placing the concrete. The con-crete compressive strength (fcm,cube) and oven-dry density (ρcm) were measured on cubes with an edge length of 150 mm in each instance. The modulus of elasticity (Ecm) was measured on cylinders with a diameter of 150 mm and a height of 300 mm.

Furthermore, push-out tests (according DIN EN 26891) [11] were carried out to investigate the properties of the connection system. For each concrete type six push-out specimens were tested. Figure 4 shows one possible experimental set-up of the push-out test. Concrete is add-ed on one side of the timber connected with the fasteners chosen for the TCC construction (screws, diameter 12 mm). The maximum load has to be estimated and the loading regime has to be implemented in the test procedure. At first the load has to be increased (force-controlled) by a loading rate of 20 % of the estimated maximum load per minute up to 70 % of the maxi-mum. Within this span the load is kept constantly for 30 sec at 40 % and 10 % of the estimat-ed maximum value. From this point on up to the failure of the specimen or a displacement of 15 mm the force is increased displacement-controlled. For measuring the displacement be-tween the timber beam and the concrete slab 2 LVTDs are applied as shown in the picture (on both sides of the specimen).

Figure 4: Test arrangement for the push-out test.

3 RESULTS

3.1 Accompanying tests (concrete, wood and interface properties)

The hardened concrete properties are presented in Table 2. The normal weight concrete (NC) exhibits twice higher values for the E-modulus and approximate 30 % higher strength results compared to LC1 and LC2. Especially the E-modulus has a direct influence on the per-formance of the slabs in serviceability limit stage, due to higher stiffness of the concrete slab. Using lightweight aggregate concrete (LWC) help to reduce the self-weight of the construc-tion. The used LWC offers an approximate 35 % reduced density.

Investigations on Load Sharing Effects in Timber-Concrete Composite Constructions

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Table 2: Hardened concrete properties.

Specimen [Unit]

T_75

T_N

C_7

5_12

T_LC

1_75

_12

T_LC

2_75

_12

T_LC

2_75

_6

T_LC

2_75

_18

T_LC

2_90

_12

T_LC

2_60

_12

fcm,cube [N/mm2] - 37.50 23.31 26.10 25.58 24.77 21.56 23.95 Ecm [N/mm2] - 25574 11884 14141 13100 13534 12760 13773 ρcm [kg/dm3] - 2.22 1.54 1.53 1.51 1.53 1.42 1.49 The main results of the push-out tests are summarized in Table 3. By the use of normal

weight concrete (NC), the highest load in the push-out test could be achieved. Unexpectedly, the push-out tests showed higher stiffness values for the lightweight series compared to the normal weight concrete. Both lightweight concrete series showed similar results.

Table 3: Results of the push out tests (per screw).

Specimen [Unit]

T_75

T_N

C_7

5_12

T_LC

1_75

_12

T_LC

2_75

_12

T_LC

2_75

_6

T_LC

2_75

_18

T_LC

2_90

_12

T_LC

2_60

_12

Fu [kN] - 7.5 5.5 5.7 ks [kN/mm] - 2.1 3.2 3.5 The results of the bending tests of the error free wood samples are summarized on Table 4.

The average value of at least 10 samples is given. The samples were stored under equal cli-matic conditions that can be found in normal office rooms.

Table 4: Average bending strength of the error-free samples (taken from every beam)

Bending strength [Unit]

T_75

T_N

C_7

5_12

T_LC

1_75

_12

T_LC

2_75

_12

T_LC

2_75

_6

T_LC

2_75

_18

T_LC

2_90

_12

T_LC

2_60

_12

Beam 1 [N/mm2] 77.80 82.96 92.83 66.14 77.60 98.88 88.66 71.46 Beam 2 [N/mm2] 71.10 90.01 73.47 70.66 57.79 73.49 69.31 60.66 Beam 3 [N/mm2] 66.66 83.35 107.2 68.74 80.04 65.05 92.04 68.72

3.2 Bending tests of Timber and TCC-slabs In Figures 5 and 6 the load-deflection respectively load-strain curves are presented for all

tested specimens (slabs). The diagrams show the load in kN (ordinate) in relation to the de-flection in mm respectively the strain in µm/m (abscissa). The deflection or strain because of dead load of the construction was not measured. The deflection in the middle of the slab (D1-3) was reduced by the average displacement at the supports (A1-6). The measured results only corresponded to the additionally applied load.

Hubertus Kieslich and Klaus Holschemacher

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Figure 5: Load-deflection-curves of the slab tests.

Investigations on Load Sharing Effects in Timber-Concrete Composite Constructions

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Figure 6: Load-strain-curves of the slab tests.

Hubertus Kieslich and Klaus Holschemacher

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Table 5: Selected test results.

Value

[Uni

t]

T_75

T_N

C_7

5_12

T_

LC1_

75_1

2 T_

LC2_

75_1

2 T_

LC2_

75_6

T_

LC2_

75_1

8 T_

LC2_

90_1

2 T_

LC2_

60_1

2

Load at 7.8 mm (l/500) [kN] 13.1 42.3 29.9 25.1 32.4 20.8 29.4 31.5 Deflection adjacent beam 1 [mm] 3.57 8.15 5.88 2.54 4.63 5.01 4.85 6.52 Deflection adjacent beam 3 [mm] 4.17 4.14 6.28 6.28 5.90 4.52 5.04 6.85 Ratio (average adjacent / middle) [%] 49.9 79.4 78.6 57.6 68.5 62.4 63.4 85.9

Load at 13.0 mm (l/300) [kN] 20.4 57.4 46.2 42.6 46.4 36.5 47.3 48.3 Deflection adjacent beam 1 [mm] 5.55 12.6 9.66 6.76 8.08 9.08 7.64 10.5 Deflection adjacent beam 3 [mm] 6.30 7.98 10.0 10.4 9.7 9.0 8.2 10.9 Ratio (average adjacent / middle) [%] 45.7 79.7 76.8 66.4 69.3 69.8 61.1 82.2

Maximum Load [kN] 70.5 140 121 130 92.8 96.4 124 130 Deflection at Maximum load beam 2 [mm] 53.6 70.0 66.2 58.1 35.8 47.0 48.1 63.3

Deflection at Maximum load beam 1 [mm] 18.4 63.6 47.8 40.7 23.1 30.8 23.8 43.7

Deflection at Maximum load beam 3 [mm] 19.9 38.8 45.7 44.7 27.3 34.7 26.5 51.5

Ratio (average adjacent / middle) [%] 35.8 73.2 70.7 73.4 70.5 69.6 52.4 75.1

Connecting the timber beams to different types of concrete, the maximum load compared

to the timber beam ceiling was significantly increased. Also the stiffness of the construction could be improved. The highest stiffness and the highest maximum load (140.1 kN) were ob-served for the use of the normal strength concrete (the maximum load could nearly be dou-bled). In every case the timber beams finally cracked. The timber beam ceiling was almost 3.7 times softer than a TCC slab, regarding the normal weight concrete (T_NC_75_12). Figures 5 and 6 illustrate that loads were distributed in lateral direction of the ceiling in a relevant pro-portion. It could be observed that all timber beams collapsed at a strain of more than 3 000 µm/m, except of T_LC_75_6; T_LC_75_18. The properties of the timber beams seemed to have an important influence on the load bearing behavior, and therefor were investigated.

Furthermore, in Table 5 selected test-results are pointed out, e.g. load values at a deflection of the middle beam of 7.8 mm (l/500) and 13 mm (l/300). This corresponds to the recom-mended boundary value regarding the serviceability limit state for wooden structures. The highest loads at these deflections were observed for T_NC_75_12 also the influence of the level of connection (T_LC_75_6; T_LC_75_12; T_LC_75_18) can be determined. Addition-ally, the deflection of the middle beam (D2) and the two adjacent beams (D1 and D3) are shown. Furthermore, the ratio between the average value of the two adjacent beams compared to the deflection of the middle beam is presented. Even for the reference slab T_75 the adja-cent nearly deflect 50 % of the middle beam, only because of the boarding. This ratio can be regarded as a level of load transfer and is significantly increased by adding a concrete topping. The highest ratio was observed for T_LC2_60_12. In general, the ratio between middle beam

Investigations on Load Sharing Effects in Timber-Concrete Composite Constructions

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and adjacent beam deflections seem to degrease with increasing load due to plastic defor-mation.

4 CONCLUSION

TCC construction is a practicable possibility to toughen up existing timber beam ceilings. In the past various researches, to develop this kind of construction method, were execut-ed.

By connecting timber and concrete, it is possible to use the typical advantages of both construction materials. The load carrying behavior of timber beam ceilings can be im-proved.

The maximum load and the stiffness (deflection) were significantly increased when a concrete slab was added to the timber beam ceiling. The highest stiffness and the highest maximum load (140.1 kN) were observed for the use of the normal strength concrete (the maximum load could nearly be doubled).

But the question of the lateral load bearing behavior of TCC ceilings is not investigated sufficiently so far.

Even for the reference slab T_75 (without concrete topping) the adjacent beams nearly deflect 50 % of the middle beam, only because of the boarding.

Adding a concrete slab to a timber beam ceiling, effect a lateral load distribution in a rel-evant proportion (up to 80 %).

The ratio between the average value of the two adjacent beams compared to the deflec-tion of the middle beam decreases, due to plastic deformation, when the serviceability and ultimate limit state (maximum load) is compared.

The mechanical properties of wood seem to have an enormous influence on the perfor-mance of the TCC slabs. They should be focused on in later researches.

Several parameters, e.g. the thickness of the slab, the distance between the timber beams, the kind of concrete, the level of connection respectively type, and distance of the used connection members, have to be investigated in further researches.

The mechanical properties of the tested timber beams and the concrete slab are of partic-ular interest, especially concerning FE-simulations, which are part of the experimental program to take various parameters into account.

The aim is to build up and calibrate a FE-model. It shall help to examine the lateral load bearing behavior using finite element software instead of actual tests. With the finite el-ement model the lateral load distribution can be considered in more detail by varying several parameters in an effective way. For example, the span, the stiffness of concrete slab, the stiffness of interlayer and the interjoist of the beams could be varied easily by using software.

The FE-analysis has been started. The first results show promise and will be presented in later publications. Especially the non-linearity of the material properties of wood, con-crete and inter face have to be taken into account.

Hubertus Kieslich and Klaus Holschemacher

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ACKNOWLEDGEMENT The study, which is the base for results presented in this paper, was part of the research

project: “PVA-faserbewehrter Trocken-Fertigleichtbeton“ (support code 1763X09). It is sup-ported by the Federal Ministry of Education and Research of the Federal Republic of Germa-ny. We thank the Federal Ministry of Education and Research for the financial advancement as well as the project executing organization “Arbeitsgemeinschaft industrieller For-schungsvereinigungen (AiF)” for the cooperative collaboration and assistance. Furthermore, the research is part of a PhD-project supported by the EFS (Europäischer Sozialfond) and the SAB (Sächsische AufbauBank).

At the end of this paper the authors want to thank our colleagues Yvette Klug, Stefan Käseberg, Torsten Müller, Sebastian Melzig, and Chenyu Wang for supporting our work at the Institute of Concrete Construction at Leipzig University of Applied Science.

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principles for the determination of strength and deformation characteristics , Beuth-Verlag, Berlin, 1991.