PSI - Issue 11

Roberto Scotta et al. / Procedia Structural Integrity 11 (2018) 282–289 R. Scotta et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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1. Introduction

The intrinsic characteristics of wood as construction material, i.e., lightness, mechanical resistance and thermal insulation, make it an effective material to realize mid-rise buildings, even in Countries where the climatic characteristics and seismic susceptibility are not favourable. These properties have led to the development and spread of new technologies, in the perspective of reducing costs and time for on-site assembly, by taking advantage of prefabrication, and of overcoming main limitations of traditional timber systems, as height of the buildings and spans of residential floors. The use of massive timber panels as floors and shear walls proved to be an effective strategy in the realization of strong, stiff and safe structures also in the case of accidental actions, as fire or earthquake (Ceccotti, 2008; Ceccotti et al., 2013). Nevertheless, the hygroscopic behaviour of timber and its susceptibility to moisture absorption are the main problems worsening its durability, when moisture content exceeds a critical limit (about 20%). Therefore, discussion on possible details for critical building zones such as flat roof, balcony and wet rooms is of utmost importance (Ringhofer and Schickhofer, 2014; Silva et al., 2016). Another critical detail for timber constructions in terms of moisture protection is the base of the walls at first storey in contact with concrete foundation (Teibinger and Matzinger, 2013). Inaccurate waterproofing or wrong details may lead to timber decay and relative damages, difficult to be restored, in particular when moisture is absorbed and trapped at the wall base, Fig. 1. However, few research is available, which studied the proper support of timber walls at foundation, to avoid deterioration due to moisture absorption, rising dampness or fungicides attack (e.g., Ridout, 2000; Teibinger and Matzinger, 2013; Merotto, 2017). Calculations and tests were performed to determine the interaction between concrete foundations and timber-frame buildings (Ridout, 2000; Tariku et al., 2016; Toyumi et al., 2010) or Cross-Laminated Timber (CLT) structures (McClung et al., 2014). For example, Tariku et al. (2016) investigated three different scenarios: a case with direct wood-concrete contact and two cases with different moisture barriers between the two materials. They confirmed the need for a moisture barrier at the wood sill plate and concrete foundation interface: in the case with no moisture barrier, the moisture content in the sill plate remained high (about 20%) for most of the experimental period. McClung et al. (2014) tested sixteen 0.6m by 0.6m CLT panels made of five different wood species, initially wetted with the moisture content in the surface layers approaching or exceeding 30%. They demonstrated that low-permeance materials such as polyethylene and non vapour permeable water-resistive barriers caused slower drying of CLT and should be used with more caution; the low-permeance materials may have a more deleterious effect due to the reduced drying capacity. Finally, guidelines, e.g., DIN 68800 (2012) and ÖNORM 2320 (2017), suggest particular precautions to ensure the wood preservation and provide constructive details.

Fig. 1. Example of damaged internal timber wall due to impeded transpiration

In this work, traditional techniques for supporting timber walls at foundation are described, presenting some details for wood preservation and possible examples of incorrect realization of waterproofing or concrete surfaces. An innovative aluminium bottom rail for anchoring timber wall buildings to foundation is described and main details are given. The correct installation process in new buildings is discussed, and a proposal for possible restoring intervention to replace the damaged timber at the base of the wall is described. Three case studies of new buildings are also presented. Finally, main results from latest experimental campaigns and numerical simulations are given to characterize the system in terms of load-bearing capacity.