# Effect of Interlayer and Inclined Screw Arrangements on the Load-Bearing Capacity of Timber-Concrete Composite Connections

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

**s**with screws have become since the 1980s a widespread solution for floor-building and retrofitting, representing an option to the typical “timber plank” additional layer [1].

## 2. Background Experiments

_{est}, which represents the maximum load expected based on preliminary evaluations, the load was increased to 0.4 F

_{est}and maintained for 30 s, then was decreased to 0.1 F

_{est}and maintained for the same time period. Subsequently the load was increased further, until the failure of the specimen or a maximum deformation of 15 mm (whichever came first).

## 3. FE Numerical Investigation

#### 3.1. Modelling Assumptions

^{th}of the nominal geometry also required the application of constraints for ${u}_{z}={u}_{r,x}={u}_{r,y}=0$ on the xy symmetry plane and ${u}_{x}={u}_{r,y}={u}_{r,z}=0$ on the yz symmetry plane, respectively.

#### 3.2. Material Properties and Constitutive Models

^{2}and ${E}_{90,m}=300$ N/mm

^{2}, respectively. The shear modulus and the rolling shear modulus were also quantified in ${G}_{v}=650$ N/mm

^{2}and ${G}_{r}=65$ N/mm

^{2}, respectively. Poisson’s ratios for the local directions of interest were set to $v=0.4$, according to the extensive literature research in [27] for softwoods.

^{2}and ${f}_{90,m}=3.5$ N/mm

^{2}. The shear strength and the rolling shear strength values were defined in ${f}_{v}=3.5$ N/mm

^{2}and ${f}_{r}=1.2$ N/mm

^{2}, respectively.

^{3}[28,29]. The corresponding mechanical properties were thus used for the definition of an orthotropic material. According to [28], the elastic modulus and the strength in the main and secondary directions were assumed as ${E}_{0,m}=3500$ N/mm

^{2}and ${E}_{90,m}=1400$ N/mm

^{2}and ${f}_{0,m}=18$ N/mm

^{2}and ${f}_{90,m}=9$ N/mm

^{2}. According to [29], finally, the shear moduli and shear strength values were defined in ${G}_{v}=1080$ N/mm

^{2}and ${G}_{r}=50$ N/mm

^{2}, ${f}_{v}=7.0$ N/mm

^{2}and ${f}_{r}=1.0$ N/mm

^{2}.

^{2}and:

^{2}and yielding stress such as to guarantee the same yield moment declared in the ETA-19/0244 [30], that is ${f}_{y}=1195$ N/mm

^{2}.

#### 3.3. Screw-Members Interaction

- the high initial withdrawal stiffness guaranteed by the direct timber-screw interaction through the screw thread;
- the progressive degradation of this interaction due to damage to the interface at the attainment of a limit stress;
- the possibility of separation on the screw-timber interface;
- the actual axial and flexural stiffness of the screw.

^{2}and ${G}_{a-r}={G}_{a-t}={G}_{r-t}=650$ N/mm

^{2}). The chosen value of the elastic modulus in the axial direction of cylinder was calibrated in such a manner as not to affect the axial stiffness of the fastener (${E}_{a}=50$ N/mm

^{2}).

^{3}, ${f}_{n}=0$ N/mm

^{2}) were set to zero, in order to allow free separation of the two surfaces. With these positions and considering a decoupled behaviour, the stiffness matrix of the interaction was hence reduced to:

#### 3.4. Model Updating

^{3}and ${f}_{s1}=7.5$ N/mm

^{2}.

## 4. Elaboration of Push-Out Experimental Results

#### 4.1. Mechanical Performance Indicators

_{s}and K

_{u}, which are the slip moduli used for design at serviceability limit states and ultimate limit states. In present study, both these parameters were evaluated according to UNI EN 26891:1991 [25] as:

_{0.1}, ν

_{0.4}and ν

_{0.6}represent the displacements recorded under loads of 0.1 F

_{est}, 0.4 F

_{est}and 0.6 F

_{est}, respectively, whereas ν

_{24}is the displacement measured under a load of 0.4 F

_{est}in the second loading branch.

#### 4.2. Load-Bearing Capacity and Failure Mode

_{s}and K

_{u}with regard to the whole specimens are presented in Table 2 and Table 3. The characteristic value F

_{u,k}of the maximum load was evaluated by means of the logarithmically normal distribution provided by EN 14358:2016 [35]. With reference to the failure mechanisms, the prevailing failure modes were determined by timber embedment combined with withdrawal of screws from the timber element as indicated in Table 2 and Table 3, respectively. Only two specimens of the clc8240 group evinced a different failure mode, characterized by withdrawal of screws from the concrete slabs, and three specimens of the same group showed a partial splitting of the glulam element near the midline of the specimen in addition to the recurring mechanism previously described, as shown in Figure 7.

Specimen | F_{u,max} | K_{s} | K_{u} | Failure Mode |
---|---|---|---|---|

[kN] | [kN/mm] | [kN/mm] | ||

1 | 55.9 | 69.6 | 77.0 | c + wt |

2 | 51.7 | 42.7 | 43.2 | c + wt |

3 | 36.3 | 28.3 | 27.2 | c + wt |

4 | 46.8 | 37.2 | 37.5 | c + wt |

5 | 41.6 | 32.1 | 32.5 | c + wt |

6 | 48.9 | 38.1 | 38,6 | c + wt |

7 | 41.8 | 64.8 | 51.2 | c + wt |

8 | 49.4 | 41.6 | 41.8 | c + wt |

9 | 49.8 | 43.1 | 42.0 | c + wt |

10 | 52.2 | 44.9 | 46.1 | c + wt |

max | 55.9 | 69.6 | 77.0 | |

min | 36.3 | 28.3 | 27.2 | |

mean value | 47.4 | 44.2 | 43.7 | |

σ_{y} | 0.13 * | 13.2 | 13.5 | |

F_{u,k} | 35.9 | - | - |

Specimen | F_{u,max} | K_{s} | K_{u} | Failure Mode |
---|---|---|---|---|

[kN] | [kN/mm] | [kN/mm] | ||

1 | - | - | - | wc |

2 | 76.3 | 62.6 | 73.8 | wc |

3 | - | - | - | c + wt |

4 | 66.7 | 45.8 | 48.0 | c + wt |

5 | 63.5 | - | - | c + wt |

6 | 67.8 | - | - | c + wt + s |

7 | 58.7 | - | - | c + wt |

8 | 69.1 | 45.6 | 45.3 | c + wt + s |

9 | 74.0 | 53.6 | 54.9 | c + wt + s |

10 | 67.2 | 57.3 | 46.9 | c + wt |

max | 76.3 | 62.6 | 73.8 | |

min | 58.7 | 45.6 | 45.3 | |

mean value | 67.9 | 53.0 | 53.8 | |

σ_{y} | 0.08 * | 7.4 | 11.7 | |

F_{u,k} | 67.7 | - | - |

## 5. Results of FE Numerical Parametric Study

#### 5.1. Screw Inclination and Interlayer Thickness

#### 5.2. Interlayer-Timber Member Interaction and Interlayer Mechanical Properties

- “particleboard”: interlayer made of particleboard (i.e., structural panels for use in wet areas, as defined in EN 12369-1:2001);
- “glulam”: interlayer made of the same material as the main timber member;
- and “gap”, that is with an interlayer consisting of a physical gap, but being still capable of keeping the timber and concrete members separated (with appropriate kinematic constraints) in the direction perpendicular to the sliding plane.

#### 5.3. Friction Contribution and Test Setup

- the share of force term which is transferred by friction is lower on the interlayer-glulam sliding plane (SL2) with respect to the concrete-interlayer sliding plane (SL1), due to the lower static friction coefficient;
- for the configuration E1, which is similar to the G5 configuration in terms of inclination angle and interlayer thickness but is characterized by a different penetration length ${l}_{p,E1}/{l}_{p,G5}=0.61$, the FE numerical analysis shows a negligible increase in the share of force transferred by friction on the sliding plane SL1. Conversely, on the sliding surface SL2, the share of force transferred by friction slightly decreases;
- as the thickness of the interlayer increases, with constant inclination and length of penetration for the screw into the glulam member, the share of force transferred by friction clearly increases;
- finally, as the angle of inclination of the screw increases with respect to the normal to the sliding plane, for constant thickness of interlayer and length of the screw penetration into the glulam member, the share of force transferred by friction decreases.

#### 5.4. Perfectly Glued Fastener

## 6. Discussion

## 7. Conclusions

- the inclination of screw and the thickness of the interlayer have a modest influence on the resistance of a given TCC connection but strongly affect the expected failure mode;
- the insertion of an interlayer, even of limited thickness, produces a significant reduction in stiffness, which slightly increases with its thickness;
- the interlayer type and mechanical capacity is also an important parameter, since it further affects the TCC connection stiffness;
- the type of friction/contact interaction of main load-bearing components on the sliding plane was found to be the most significant parameter. The transition from frictionless (“$\mu =0$”) to bonded (“glued”) numerical configurations could lead to increases of 84% and 81% in terms of strength and stiffness, respectively, for TCC systems. A reduction factor of strength and stiffness of 0.66 can be adopted when friction between timber and concrete is not guaranteed, whilst whenever the interlayer is glued a coefficient of 1.25 can be taken into account;
- friction alone contributes between 24% and the 56% to reference mechanical performance parameters, depending on the considered sliding plane and the specific geometric configuration;
- the push-out and inclined test configurations, finally, introduce additional forces perpendicular to the sliding plane, and this phenomenon affects the contribution of friction to the overall ultimate force and stiffness of TCC systems. In this regard, strength and stiffness correction factors equalling 0.95 and 0.90, respectively, are suggested to normalize inclined shear and push-out test results to the direct shear results.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**Reference push-out specimens for the experimental tests: (

**a**) clc8160 and (

**b**) clc8240 series.

**Figure 3.**Example of a typical FE assembly of TCC connection (1/4th of the experimental geometry, with hidden mesh).

**Figure 5.**Experimental and FE numerical load-displacement curves for (

**a**) clc8160 and (

**b**) clc8240 specimens.

**Figure 6.**Typical deformed shape and Von Mises stresses for (

**a**) E1 (clc8160) and (

**b**) E2 (clc8240) models.

**Figure 7.**Failure mechanisms for the examined TCC connections in push-out experimental tests: examples of (

**a**–

**c**) recurring mechanisms or (

**d**–

**f**) infrequent failure modes. Red arrow: failure.

**Figure 9.**FE numerically predicted trends for (

**a**) ${F}_{u}$ and (

**b**) ${K}_{ser}$ for different values of inclination angles and interlayer thicknesses.

**Figure 10.**FE numerically predicted trends for (

**a**) ${F}_{u}$ and (

**b**) ${K}_{ser}$ for different interlayer-to-members interaction and interlayer mechanical properties.

**Figure 11.**Analysis of ${F}_{u}$ percentage transferred on the sliding planes through the screw and by friction. Key: SL1 = sliding plane between the concrete member and the interlayer; SL2 = sliding plane between the glulam member and the interlayer.

**Figure 12.**Evidence of (

**a**) common test configurations and (

**b**) FE numerical estimation of ${F}_{u}$, ${K}_{ser}$ and ${K}_{u}$ performance indicators. Key: DS = Direct shear; PO = Push-out; IS = inclined shear.

**Table 1.**Specimen geometric data, FE numerical results per screw and corresponding percentage scatters.

Arrangement | Mechanical Performance | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|

Model | d | l_{p} | t_{i} | θ | F_{u,max} | S_{FEM-exp} | K_{s} | S_{FEM-exp} | K_{u} | S_{FEM-exp} | Failure Mode |

[mm] | [mm] | [mm] | [°] | [kN] | [%] | [kN/mm] | [%] | [kN/mm] | [%] | ||

E1 (clc8160) | 8 | 128 | 44 | 45 | 11.4 | −4 | 11.3 | 3 | 10.0 | −9 | wt |

E2 (clc8240) | 8 | 78 | 22 | 45 | 17.5 | 3 | 12.2 | −8 | 10.8 | −20 | wt |

**Table 4.**Parametric analysis on screw inclination and interlayer thickness, with evidence of corresponding failure modes.

Model | t_{i} | θ | Failure Mode |
---|---|---|---|

[mm] | [°] | ||

G1 | 0 | 30 | phs |

G2 | 22 | 30 | wt |

G3 | 44 | 30 | wt |

G4 | 0 | 45 | wt |

G5 | 22 | 45 | wt |

E2 * | 44 | 45 | wt |

G7 | 0 | 60 | wt |

G8 | 22 | 60 | c + wt |

G9 | 44 | 60 | c + wt |

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## Share and Cite

**MDPI and ACS Style**

De Santis, Y.; Sciomenta, M.; Spera, L.; Rinaldi, V.; Fragiacomo, M.; Bedon, C.
Effect of Interlayer and Inclined Screw Arrangements on the Load-Bearing Capacity of Timber-Concrete Composite Connections. *Buildings* **2022**, *12*, 2076.
https://doi.org/10.3390/buildings12122076

**AMA Style**

De Santis Y, Sciomenta M, Spera L, Rinaldi V, Fragiacomo M, Bedon C.
Effect of Interlayer and Inclined Screw Arrangements on the Load-Bearing Capacity of Timber-Concrete Composite Connections. *Buildings*. 2022; 12(12):2076.
https://doi.org/10.3390/buildings12122076

**Chicago/Turabian Style**

De Santis, Yuri, Martina Sciomenta, Luca Spera, Vincenzo Rinaldi, Massimo Fragiacomo, and Chiara Bedon.
2022. "Effect of Interlayer and Inclined Screw Arrangements on the Load-Bearing Capacity of Timber-Concrete Composite Connections" *Buildings* 12, no. 12: 2076.
https://doi.org/10.3390/buildings12122076