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ENROOT

Enroot: Enhancing the robustness of timber structures

Enroot is a project funded by Generalitat Valenciana and hosted at the ICITECH of the Universitat Politècnica de València.

The overall aim of Enroot is to enhance the robustness of timber structures through the design and evaluation of improved inter-element connections to meet structural robustness requirements.

The project started in 2023 and spans a total duration of 2 years.

KEY CHALLENGES

Timber building structures are considered to be the construction of the future, given the environmental advantages that this type of construction offers over concrete or steel structures. In fact, using wood from sustainably managed forests allows more CO2 emissions to be removed from the atmosphere than can be generated by its use in the construction sector, reaching consumptions of up to one tonne per cubic metre of material used. In contrast, concrete, steel or masonry structures generate CO2 emissions with average values of 385 kgCO2/m3, 12200 kgCO2/m3, and 375 kgCO2/m3, respectively. This main advantage is followed by the outstanding characteristics of energy efficiency, thermal and acoustic comfort, lightness and even fire resistance, as well as the economic and temporal advantages of industrialised construction. The use of wood also requires little energy or water and is 100% renewable.

However, this reality and the current promotion of timber structures contrast drastically with the scant amount of research carried out in the field of structural robustness in relation to other materials. This contrast is particularly relevant today, as extreme events (e.g. storms, floods, vehicle impacts, explosions or terrorist attacks) that cause local-initial failures of structures become more frequent and unpredictable due to climate change and emerging conflicts. Robust buildings that resist initial damage without triggering progressive collapse are becoming increasingly important. In this context, state-of-the-art structural design standards and guidelines indicate that structures must have sufficient structural continuity, redundancy and ductility. This will prevent the propagation of failures because the structural system will be able to activate alternative load paths in case of failure of any element.

In the case of timber structures, continuity and efficient activation of alternative load paths are considered critical because of how the connections between elements are made.

Collapse of the Siemens Arena in Ballerup (Denmark)
The Bad Reichenhall Ice Arena collapse (Germany)

THE GAP

To date, very few experimental studies have been carried out in the field of structural robustness of timber structures, with clear limitations in terms of: connections between elements, construction typologies studied, three-dimensional behaviour and activation of alternative load paths after local-initial structural failures. Therefore, there is still a long way to go, especially in the case of modular structures, CLT-Platform type structures and frames made up of beams and columns (post-and-beam structures).

There is a clear need to make progress in this area, primarily through experimental testing, to bring the structural robustness of timber structures up to the level of concrete or steel structures.

Number of publications in the progressive collapse field of timber, concrete and steel structures.

OBJECTIVES

The overall aim of Enroot is to enhance the robustness of timber structures through the design and evaluation of improved inter-element connections to meet structural robustness requirements. This overall aim will be achieved through three specific objectives:

  • Objective 1: Develop beam-column connections that improve structural robustness, validating their correct functioning through tests on substructures.
  • Objective 2: Evaluate the beam-column connections’ behaviour in situations other than those tested.
  • Objective 3: Apply the developments achieved to other connections widely used in the sector and develop simplified computational models.

METHODOLOGY

Enroot is organised into three Work Packages (WP).

WP1. Developing and testing robust connections.

This WP will experimentally evaluate the behaviour of timber substructures against column removals. The strength and rotational capacity of connections in such structures will determine the feasibility of activating the last defence mechanisms (e.g. catenary) after column failure. To date, tests on some of the most commonly used connections have shown severe limitations regarding their rotational capacity or strength. In this WP, two substructures (e.g. three columns and two beams) will be tested using two existing connection typologies different from those previously tested by other authors. In addition to these two tests, two further tests will be carried out with similar characteristics but improving the design and response of the connections in terms of strength and rotational capacity to enhance the structure’s robustness. In total, four substructures will be tested against column removals.

WP2. Assessment of the structural performance of connections.

In this WP, computational models that fully reproduce the behaviour observed in the tests will be developed. This validation will be carried out by calibrating some parameters of the models previously identified. From here on, these validated models will be used to analyse situations other than those tested. The computational simulations in this block will be carried out using the finite element method and the applied element method. The former will be used, either by implicit or explicit calculation, when the analysis focuses on the structural behaviour under low loads (e.g. load combinations in accidental situations according to current codes and recommendations). The applied element method shall be used when analysing collapse.

WP3. Implementation in new designs.

This WP will consist of the application of the knowledge acquired in the previous WPs through: i) the use of the new optimised connection designs, extrapolating them to other types of beam-column connections that can be improved by robustness condition, and ii) the development of structural models of connections that are simplified and can be easily introduced in commercial structural software (e.g. Tekla, SAP2000). In this WP, the tools previously used in relation to the finite and applied element methods, as well as the use of commercial software, will be necessary. The team’s experience modelling structures and their simulated and real behaviour will also be relevant to achieving applicable simplified computational models.

IMPACT

Enroot can be expected to have a considerable impact in scientific, technological and social fields:

  • Scientifically, the project will generate knowledge applicable to increasing the safety of timber structures, directly impacting the resilience of buildings constructed with structural timber.
  • Technologically and societally, the project will indirectly strengthen the competitiveness and growth of timber building structures, bringing the project’s innovation to the market and increasing its current technological offer. The results will bring timber constructions and the companies associated with this type of structure closer to the level of robustness of other structures, such as in-situ concrete, precast concrete or steel structures, making this type of structure more competitive in terms of reliability and cost.

Funded by Generalitat Valenciana – Conselleria de Innovación, Universidades, Ciencia y Sociedad Digital under the GE 2023 call (CIGE/2022/120).