Wood-concrete composite

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Wood-concrete composite (wood-concrete composite, HBV) is a construction method for supporting structures . The hybrid supporting structures produced in this way are also referred to as wood-concrete composite. They represent a variant of the composite construction . HBV constructions consist of a wooden and a concrete component, which are connected to one another as shear-proof as possible using a suitable process ( form fit , gluing ) or connecting means (mechanical connecting means). The connection between wood and concrete is also known as a bonded joint, shear joint or joint. When manufacturing the concrete part, a distinction is made between the fresh concrete process and the precast process.

The wooden component can have a linear ( beam , beam ) or a flat (cross laminated timber, board stack) shape. In the case of concrete components, the most common form is a flat concrete slab, as this can act as a stiffening disc and ensures that the load is distributed across the linearly shaped wooden components. A linear formation of the concrete component is called a concrete strip. When constructing a single-span girder that is subjected to bending loads, the concrete component is mainly subjected to compression, while the wooden component is mainly subjected to tension and bending. The composite joint and thus the connection technology arranged here between the wood and concrete component reduces or prevents a relative displacement of the two individual components. The joint is accordingly stressed by a shear force that must be absorbed by the connection technology in accordance with the load and deformation.

Application, areas of application and requirements

Wood-concrete composite structures are used both in building construction (new and existing buildings) and in bridge construction. The dimensions and design of the individual cross-sections and the connection joint are based on the loads and requirements. The combination and arrangement of the two cross-sections (concrete in the upper third - under pressure, wood in the lower third - under tension and bending) make them an efficient and resource-saving alternative to conventional ceiling systems made entirely of wood or concrete.

Refurbishment, existing buildings, revitalization and old buildings

In the renovation, old wood beam ceilings with wood-concrete composite (HBV) are being upgraded and renovated. HBV constructions are used both for monument protection and for urban living space acquisition. Dry storage, attic storeys and mezzanine storeys can be upgraded with a HBV construction and thus designed for higher loads. As a rule, a film is inserted to avoid leaks, then mechanical fasteners are inserted into the wooden component, basic structural reinforcement is laid to limit the width of the cracks, and flowable, self-compacting in-situ concrete (consistency class F6) is inserted.

The bond here is mainly through mechanical fasteners and in-situ concrete, since the use of prefabricated parts is not possible due to the pre-deformation. With screwed shear connectors, old building ceilings between 4 and 7 m span can be upgraded to a maximum live load of around 5.0 kN / m². New construction ceilings can be used with beam spacings of up to 3.5 m and spans of up to 9 m with a traffic load of up to max. 5.0 kN / m² can be produced.

Multi-storey and residential construction

HBV constructions are used in new buildings due to their high rigidity and load-bearing capacity. In addition, fire protection is a decisive parameter, especially for multi-storey wooden buildings. By using HBV constructions, it is now also possible to implement multi-storey wooden buildings and high-rise buildings with a maximum proportion of wood in a constructive and legally compliant manner (Life-Cycle Tower One, Mjøsa Tower).

Bridge building

In bridge construction, pedestrian bridges in particular have so far been made with HBV constructions. However, there are already approaches to use HBV bridges with spans of up to 20 m for heavy goods traffic.

conditions

The requirements for a structure in general have increased steadily in recent years. In addition to the basic requirement that the supporting structure can absorb the loads that occur, the requirements for vibration and fire behavior, the quality of living and the acoustics of the supporting structure are particularly important. In many cases, these requirements cannot be met by purely wooden structures. In order to improve the above-mentioned physical properties, it is therefore usually unavoidable to incorporate more mass into the system and to make the structure heavier. In the past, heavy fillings between joists and mineral cover layers (e.g. cement screed) were often introduced, which lay loosely on the wooden components. However, this can become a problem in terms of load-bearing capacity. In HBV constructions, the mass required due to vibration and sound insulation is applied statically, which enables slimmer ceiling constructions.

Mode of action

The efficiency of the bond between wood and concrete - and thus the bending stiffness of the structure - is primarily dependent on the stiffness of displacement (flexibility and force absorption) of the connection technology used. The characteristic value of the “displacement module K ser ” of the selected connection means provides a statement about the degree of flexibility of the composite joint of the HBV system . Depending on the connection technology used, the load-bearing capacity and flexural rigidity of the entire construction can be increased by 3 to 5 times compared to conventional support systems without composite support.

Since the concrete has a very high compressive strength and is also a relatively inexpensive building material, it is particularly suitable for use in a composite structure. The wooden beam is subjected to a combination of bending and tension. As a result, both materials are used according to their strengths and optimal use occurs in terms of system-related load-bearing capacity , usability and resource utilization.

Connection technology

There are various techniques or design variants for the execution of the composite joint, which can be divided into three categories with regard to their load-bearing effect. When using special fasteners such as composite screws, the selected variant must be approved for use in a wood-concrete composite system by the highest German building authority, the DIBt and by institutes recognized by the European Organization for Technical Assessment .

Mechanical fasteners

Mechanical fasteners are screws, dowels and expanded metals that are either inserted mechanically by hand or glued in. The shear forces in the composite joint are mainly transmitted via local tensile components (pulling the connector out of the wood) or by shearing (plasticizing the metallic connector).

Wood-concrete composite screws

HBV screws are among the most tried and tested and most common types of bond. There are systems in which the screws are screwed in at an angle of 45 ° to the respective support. The screw is loaded upon pulling it out of the wood and the tensile strength F ax of the screw is activated. Other screw systems are screwed in vertically, in pairs or crossed against each other at an angle of 45 °, 90 ° or 135 °. Since only one of the two screws is subjected to tension in the case of screw pairs, they usually have lower load-bearing capacities. According to the current status of the approvals, a maximum of 50 mm thick formwork may be installed between the wooden beam and the concrete slab. The associated reduced embedment length of the screw in the wood must be taken into account in the static calculation.

Wood-concrete bonded anchor

Wood-concrete composite dowels represent a mixed construction between mechanical fasteners and form-fitting connections. The dowels are screwed onto the wooden component using vertically inserted screws. The steel dowel is encased in fresh concrete. Wood-concrete composite anchors are being researched at the University of Padua , among others .

Expanded metals

Expanded metal (perforated sheets), which are glued into the wooden component in strips in the direction of the fibers, are mainly used in new construction and bridge construction. A circular saw must be used to create a groove in the wooden component into which the expanded metal is glued. The protruding part of the metal is poured with fresh concrete. Bonded expanded metals are researched at the RheinMain University of Applied Sciences , among others . The so-called flat steel locks are a special case of expanded metals. They are inserted into a sawn groove across the grain. It is not necessary to glue the metal into the wooden component.

Form fit

The design of the bonded joint is called form fit, in which the shear forces in the joint are mainly transmitted via contact and local compressive stresses. These include so-called mouths and cams or cylindrical steel pipes.

Kerve

A Kerve (notch, transverse groove) is a recess in the wood that is embedded across the grain and filled with concrete. The thrust is transmitted via the wedging effect between wood and concrete. To prevent it from being lifted off, vertically screwed screws must also be inserted in each bird mouth. Birds' mouths are produced in different geometries, with the rectangular or trapezoidal shape being the most common. Birds are currently the subject of research at the University of Stuttgart .

Cams, steel tubes, cylindrical connectors

The form fit and the wedging effect can also be produced by steel pieces, cams, steel cylinders filled with concrete or other cylindrical connectors. The composite variant of the cam is approved by ETAs from an institute that has been recognized by the EOTA. Due to the high load-bearing capacities of approx. 35 kN per connector and high displacement modules of approx. 19,300 N / mm, the system is often used in highly stressed sections (e.g. near the support) or in confined spaces, which is often the case when renovating old buildings . The composite variant can be used with any other method, such as. B. screws, can be combined. It is installed by milling a hole in the wooden component, into which the cylindrical connectors are hammered vertically.

Flat composite

In contrast to the bond through mechanical fasteners or form fit, the shear stresses in the flat bond are not transmitted locally as contact, embedment or tensile stresses, but continuously over the entire bond length. From a mechanical point of view, the two-dimensional bond achieves the maximum degree of bond, which makes it the most effective static solution.

In the renovation, wooden components are reinforced by pouring reactive resin concrete or polymer mortar into strips. The composite is based on the effect of bonding. A pane effect or a continuous plate is not achieved here, it is a local reinforcement measure.

The use of precast concrete elements is being researched in a large-scale network at the University of Kassel and the Fraunhofer Institute for Wood Research in the new storey and bridge construction . After successful investigations with sandblasted concrete surfaces and cold-curing, 2-component epoxy resin adhesives, full-scale tests were carried out on bridge girders and projects with hot-curing adhesives.

Manufacturing

When producing a wood-concrete composite structure, a basic distinction can be made between fresh concreting on the construction site ( in situ ) and production with precast concrete parts.

Procedure with concreting in situ

The production of an HBV structure with on-site concreting has been state of the art for years. It is carried out primarily in the renovation of old buildings, in the acquisition of urban living space in attics, as well as in the production by means of a form fit. In old buildings, this is often the only option, as the existing wooden structures usually have deformations and curvatures that cannot be compensated for with prefabricated parts.

During the execution, the existing wooden components are usually covered with a film to prevent the fresh concrete from escaping from crevices and cracks. If no foil is used, the wooden component should be cleaned dust-free before concreting with the cement slurry. The reason for this is that water could otherwise be withdrawn from the fresh concrete due to the suction effect of the wood and there would no longer be enough water for hydration in the concrete edge area. With regard to a chemical interaction between fresh concrete and wood, it could be proven in historical buildings that the basicity of the concrete does not damage the wood. Since the concrete is primarily subjected to pressure, low concrete strength classes are usually sufficient.

The installation of shear connectors is done by slotting and gluing, screwing in or drilling and hammering, depending on the system. It is carried out in accordance with the specifications from the associated building authority approval of the shear connector used with a minimum structural reinforcement (e.g. reinforcing steel mesh Q 188 A, or equivalent steel bars). In the renovation, the panel thickness is usually between 60 and 120 mm.

The concrete slab is usually given basic reinforcement with a diameter of 8 mm, every 20 cm in both directions, due to structural requirements (e.g. crack width limitation when the concrete initially shrinks). However, since the concrete slab is mainly subjected to compressive stresses, reinforcement for static requirements is usually not necessary.

Process with precast concrete parts

Systems with precast concrete elements are increasingly being used, especially in new buildings. The state of the art is the production of the flexible bond by setting plastic sleeves in concrete and subsequently inserting the HBV screws. A rigid, two-dimensional bond can be created by gluing precast concrete parts and wood.

Alternatively, entire wood-concrete composite elements can be prefabricated in the factory. All approved fasteners can be used to create the bond.

Calculation method

The bond joint can be verified taking into account the design standards for wood (currently EC5) and concrete (currently EC2), taking into account the mechanics. Mechanical fasteners require a national technical approval or an ETA. The stiffness of the connection joint (represented by the connection technology) is decisive for the load-bearing capacity and the flexural rigidity of the entire system.

Calculation method for determining the load-bearing capacity

There are various methods for calculating the normal and shear stresses in the cross section. The planning civil engineer is responsible for choosing a suitable design method. The values ​​of the connecting means required for dimensioning, such as the modulus of elasticity, the displacement module K ser and the characteristic value of the shear strength T k, can be found in the respective valid building approval (ETA or DIBt). The minimum distances between the individual fasteners are also regulated, parallel to the grain, perpendicular to the grain (in the case of multiple-row installation) and to the edge of the timber cross-section. Some providers provide their own software for calculating HBV structures. Since the calculated internal forces change with the assumed number of bolts, the calculation is optimized through repeated iterations.

EC5 - Appendix B

For simple calculations, the so-called γ method (also known as the “Möhler method”) according to EC 5 Appendix B is recommended. However, this method can only be used under certain boundary conditions. Multi-span girders or concentrated loads, such as those that occur in a roof in the form of posts or struts, cannot be calculated using the γ method.

The γ factor provides information about the degree of bond. A degree of bond of 100% corresponds to the γ factor of 1.0. A 100% network is i. d. R. only by the bonding of the individual cross-sectional portions (. Eg bonded laminations, the so-called. Glulam , laminated veneer lumber , plywood board or glued wood-concrete composite components) reaches and generally referred to as "rigid composite".

If the γ-factor is less than 1.0, the structure is called a "flexible bond". The bending stiffness of these jointly acting cross-sections is largely determined by the stiffness (represented in approvals by the so-called K ser value) of the fasteners used.

FE modeling

FE modeling can also be carried out to determine the stress. The structure can be modeled with truss members and springs ("framework model"), with disc elements or with volume elements.

Consideration of long-term behavior

The long-term behavior of HBV constructions is an important part of the design due to the interaction of time-dependent parameters such as creep, shrinkage and relaxation. Some modifications are necessary here for a correct calculation.

standardization

There is currently no separate standard for wood-concrete composite structures. If an ETA approval or a building authority approval from the DIBt is available for the respective fastener, these may be used in all areas of construction. Scientific institutions from all over Europe are currently working on a technical specification from which a future standard to supplement EC 5 - dimensioning and construction of wooden structures - is to be created.

With regard to the vibration behavior of HBV constructions as ceiling construction, DIN 1052: 2008-12 has higher requirements compared to DIN 1052: 1988 or EC 5, which makes the wood-concrete composite particularly interesting for the renovation of old wooden beam ceilings, as this requirement often cannot be fulfilled in any other way. The susceptibility to vibrations in the composite construction is much lower than in a simple wooden structure.

Individual evidence

  1. ^ Kiefer, I .: Elastic bond with surface ready for covering - renovation of the Konstanz tax office . Ed .: leobraun architects. 2014.
  2. ^ Frohnmüller, Jens: Wood-concrete composite in the renovation, conference proceedings Hanseatische Sanierungstage 2019 . Ed .: Bundesverband Feuchte & Altbausanierung eV 2019.
  3. ↑ Wood-concrete composite Wood-concrete composite ceiling manufacturer, this is how you renovate your wood beam ceiling | Elascon. Retrieved May 7, 2020 .
  4. ^ BauNetz: Life Cycle Tower One in Dornbirn | Sustainable building | Office | Baunetz_Wissen. Retrieved May 7, 2020 .
  5. ↑ BROTHER PUBLISHING Albert brother GmbH & Co KG, Rudolf Müller Media Group Cologne: Mjøsa Tower in Norway: tallest wooden skyscraper in the world. Retrieved May 7, 2020 .
  6. ↑ Miebach, Frank: wood-concrete composite bridges: experiences and perspectives . Ed .: Quality Association Holzbrückenbau eV 2018.
  7. ↑ European Technical Assessment (Ed.): ETA-18/0264 Elascon SFix System, wood-concrete composite with pin-shaped connecting means .
  8. ↑ European Technical Assessment (Ed.): ETA-18/0264 Elascon SFix System, wood-concrete composite with pin-shaped connecting means .
  9. ↑ Elascon GmbH: STABEKO TFuse Nailed Shear Connector . Ed .: European Technical Assessment.
  10. ^ Bathon, Leander: Wood-concrete composite as a rigid and ductile connection . Ed .: 10th International Wood Construction Forum. 2004.
  11. ↑ Kudla, Katrin: Kerven as a connecting means for wood-concrete composite road bridges . 2017.
  12. ↑ Schänzlin, Jörg: On the long-term behavior of stacked concrete composite ceilings . Ed .: University of Stuttgart - Institute for Construction and Design. 2003.
  13. ↑ European Technical Assessment (Ed.): ETA-18/0264 Elascon SFix System, wood-concrete composite with pin-shaped connecting means .
  14. ↑ General building approval (publisher): Z-10.7-282: Type of construction for reinforcing wooden components with reaction resin concrete . 2014.
  15. ↑ Building maintenance and timber construction: Economical production of high-quality wood-concrete composite elements using an innovative quick-bonding technique and the use of hardwood - SpeedTeCC. Retrieved May 7, 2020 .
  16. ^ Eisenhut, Lars; Seim, Werner: Long-term behavior of bonded components made of wood and high-strength concrete in a natural climate . 2016.
  17. ↑ SpringerProfessional (Hrsg.): Innovative hot-bonding of load-bearing wood-concrete composite elements, Adhäsion KLEBEN & DICHTEN . 2019.
  18. ↑ European Technical Assessment (Ed.): ETA-13/0029 Würth ASSY plus VG screws, self-drilling screws for wood-concrete composite structures .
  19. ↑ Elascon GmbH: STABEKO TFuse Nailed Shear Connector . Ed .: European Technical Assessment.
  20. ↑ European Technical Assessment (Ed.): ETA-18/0264 Elascon SFix System, wood-concrete composite with pin-shaped connecting means . 2018.
  21. ↑ Elascon GmbH: STABEKO TFuse Nailed Shear Connector . Ed .: European Technical Assessment.
  22. ↑ Schänzlin, Jörg: Outlook on the future dimensioning of wood-concrete composite floors . Ed .: Techn. Ber. HBC. Hochschule Biberach University of applied sciences. 2017.
  23. ↑ Schänzlin, Jörg: On the long-term behavior of stacked concrete composite ceilings . Ed .: University of Stuttgart - Institute for Construction and Design. 2003.
  24. ^ Dias, A .; Schänzlin, J .; Dietsch, P .: Design of timber-concrete composite structures . Ed .: COST Action FP1402 / WG 4. 2018.
  25. ↑ Schänzlin, Jörg: Outlook on the future dimensioning of wood-concrete composite floors . Ed .: Techn. Ber. HBC. Hochschule Biberach University of applied sciences.

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