A database model for the analysis and assessment of historic timber roof structures

Abstract

Visual inspection assisted by well-structured forms allows experts to collect homogeneous data in order to report typical damage/vulnerabilities of structures. This is the basis for deriving vulnerability factors to predict failure mechanisms and identify urgent interventions. A database model with an associated structured form for the assessment of historic timber roof structures has been developed during a two-week Short Term Scientific Mission (STSM) in May 2015 at CNR IVALSA Institute in San Michele All’Adige (Italy). The aim is to assist during inspection in recording all the necessary information and later in analysing data from several inspections, allowing to identify typical damage and its causes. The database model, starting from the work of COST Action FP 1101 and further developed and digitalised during the STSM, has been initially populated with data previously collected by the University of Strathclyde through visual assessment of 29 historic timber roofs in Scotland.

Keywords:

• Timber
• Roof
• Structures
• Historic
• Database model
• Structured form
• Assessment
• Diagnosis

Introduction

Visual inspection is the basis of any structural assessment, which is a prerequisite for further analysis, assessment and intervention. Different factors and their interdependency must be considered during the assessment, such as: form, material, loads and environment. The original characteristics and any later alteration/damage must be examined and their causes identified. There are many assessments of single case studies in literature, but their methodology is heterogeneous. The use of structured forms to guide visual inspections can overcome this problem: it provides a uniform approach in the assessment, even when different professionals are involved, thus facilitating comparison of different case studies and identification of vulnerabilities.

Structured forms have been used since 1980s for vulnerability surveys and post-event damage recognition surveys. Benedetti and Petrini’s form (1984) was developed to assess the seismic vulnerability of masonry buildings in Italy. More recent research has produced different types of forms to assess seismic damage (Baggio et al. 2007), to assess causes of failures in twentieth-century timber roof structures (Frese and Blas 2011; Hansson 2011; Toratti 2011) and to assess the seismic vulnerability of traditional Italian timber roof structures (Parisi et al. 2013). In all of these studies the first step was a visual examination of the structures according to a codified procedure: a series of characteristics that determine or influence the specific aspect studied were identified and then a classification criteria was defined to be able to compare different cases, identify urgent interventions and make a plan for risk reduction on the territory.

Toratti’s form (2011) was developed with a two-fold purpose: to help the experts in the assessment, making them aware of the relevant questions that need answers; to produce uniform results that can be analysed in order to draw conclusions on typical damage and related causes. The form developed by Task Group 1 (TG1) of Working Group 1 (WG1) of COST Action FP1101 (Riggio et al. 2015) had the same purpose but focused on traditional timber roof structures, which had not been addressed so far. Starting from this work a database model and associated form have been developed during a two-week Short Term Scientific Mission (STSM) in May 2015 at CNR IVALSA Institute in San Michele All’Adige (Italy). This paper describes the outcome of this STSM.

The database model and associated structured form

Approach

TG1 identified a series of objectives that the form should fulfil (Riggio et al. 2015):

• Allow collecting a significant amount of data reporting typical damage/vulnerabilities;

• Support multi-level analysis (visual inspection, in-situ tests, lab-tests, etc.);

• Define structural types (considering the European built asset) and a damage taxonomy to provide a unified approach to the evaluation of structural damage;

• Distinguish between material degradation and mechanical damage.

Before the STSM the TG1 form was in word format, which did not allow for the analysis of a big amount of data and was not suitable for quick inspections, due to the considerable amount of pages involved. Moreover, a tree-like organisation had been implemented but structure types and damage taxonomy for each level (system, unit, element, connection) had not been clearly identified. The inclusion of Visual Strength Grading (VSG) regulations (such as UNI 2004) had to be defined too, and graphics were needed to support the relevant terminology. Moreover the form needed to be validated by applying it to real case studies. The STSM work focused on the reorganisation of the contents of the TG1 form in both format and structure using Microsoft Access software and data from visual inspections carried out in summer 2014 by ADCRU (Architectural Design and Conservation Research Unit at University of Strathclyde) in 29 timber roof structures in Scotland. The software was chosen because it allows managing and multiple-querying a big amount of data and also because it would allow an easy integration with a mobile phone application being developed in the frame of Working Group 2 of COST Action FP1101 to assist professionals in the selection of interventions on timber roofs (Harte et al. 2015).

Since the data available was related to seventeenth- and eighteenth-century timber roofs in Scotland, it was decided to focus on developing a form for the inspection of historic timber roof structures. Once validated, it will be possible to extend the work to the development of a form for other roof structure types and other load-bearing systems.

Content

The aim was to organise the form in order to fulfil a two-fold purpose, as in Toratti (2011):

• Assist professionals during inspections in recording and organising all the information needed, pointing out critical aspects that need special attention;

• Assist the analysis of data from several inspections allowing to draw conclusions regarding construction history and typical damage and causes.

The tree-like organisation of the TG1 form has been retained and enriched with a classification of types of structures and damage for each level (Fig. 1). The first part of the form deals with general information on the BUILDING, then each building can have one or more roofs (SYSTEMS), composed of a PRIMARY and a SECONDARY structure; each of these can be composed of UNITS (groups of elements repeated with the same arrangement), and of ELEMENTS and CONNECTIONS between elements.

1 The tree-like organisation of the form, with structural types and damage effects

Each part of the form has a first section on geometrical and typological aspects (Fig. 2), and a second section on damage. Structural types have been grouped into families according to their structural behaviour rather than their geometrical arrangement. Clearly, not all local variations have been considered. Past interventions (alterations, repairs, etc.) can be recorded too. As highlighted by Parisi et al. (2013) the effect of past interventions is particularly critical to evaluate, as they can be localised or involve the whole structure, and differently affect the original behaviour/concept of the structure. Thus the form includes only a description and picture/drawing of past interventions, not their evaluation.

2 Form screenshot (section related to roof geometry, typology and material characteristics) with list of terms included in the drop-down menus

Building

General information on the building’s name, location, property, protection status, use, interventions, references and geometrical aspects can be recorded in this section. X and Y coordinates and the geo-referencing system have been included in order to be able to plot the results. Values, rather than categories, have been used for seismic, snow and wind loads, and the protection status has been simplified to ‘UNESCO’, ‘National’ and ‘Regional’, to suit every country. Inspections can be linked to each building, specifying: the date, the inspector’s name, the parts of the building inspected. Foreseeing the development of forms for the inspection of other load-bearing systems (vertical, floors, foundations), the load-bearing system inspected can be specified too.

Roof

The shape, construction period, roofing material, height, span(s), structure type of each roof can be recorded (Fig. 2). The roof structure can be composed of elements only (common rafter roofs, common rafter roofs with a ridge support, purlin roofs), or units too, in one direction (2D framing), and in two or more directions (3D framing). Other information includes whether the roof space is in use and if there are accessibility limitations, which can affect the possible interventions. The Eurocode 5 Service Class can be estimated.

Unit

Units have been divided into trusses and frames, as defined by Yeomans (1992, pp. 26–28). It is important to record if the original structural concept is different from the present behaviour: e.g. a unit might have been designed as a king-post truss but is actually behaving as a king-post frame, because of poor design, poor construction, etc. A series of data can be recorded if homogeneous in the whole unit: the timber dressing (rustic or civil, which affects the kind of intervention that can be done), the timber species (grouped in softwoods and hardwoods), the section shape (round, rectangular, rectangular with wanes, trapezoid), the timber treatment, carpentry marks, signs of reused timber.

Element

Elements have been classified as working in compression, in tension, in bending or shear, or a combination of the above (Fig. 1). The dating of the element can be specified, if different from the rest of the unit, as well as all the other information related to the timber dressing, treatment, section shape, etc. Strength affecting defects/natural features (knots, checks, etc.) and material degradation (fungi attack, insect attack, metal corrosion) have been included in this part only, as they cannot be homogeneous for a whole unit or roof. Since most countries have regulations for VSG of new timber but not for VSG of in-situ historic timber, it was decided to include only the natural defects that can significantly alter the structural behaviour or reduce the strength of an element: the position of the pith, the slope of grain, knots in the tensile area and horizontal deep checks which alter the section inertia. The type, extent and status of material degradation can be estimated.

Connections and secondary structure

The part on connections and secondary structure is organised in the same way, the only difference being the types of connections, bracing systems and damage (Fig. 1).

Damage

An important distinction has been made between damage effects and causes. During an assessment, we record the visible effects of damage and estimate their causes. The form guides the inspector in doing that: lists of damage effects (deformations, displacements, rotations, cracks, etc.) are suggested for each level (system, unit, element, connection), and for each of the effects selected the damage status and role can be estimated. Tampone’s work (1996, 2016) was very useful to build the damage taxonomy (Fig. 1), as well as the work of Task Group 3 of WG1 of COST Action FP 1101 on traditional carpentry joints (Sobra et al. 2015). Regarding the status, a damage can be ‘active’ or ‘non-active’, but it is not always possible to judge. Therefore, a simple ‘intervened on’ can be chosen, meaning that an intervention has been done to repair the damage but no evaluation on its effectiveness is given. Regarding the role, a structure is often affected by a sequence of damage rather than a single one, and it is important to identify the first one that caused all the others. This first one is defined as ‘primary’ damage, while all the others are defined as ‘secondary’ ones.

The causes of primary damage have been grouped in:

• Poor design (insufficient dimensions, inefficient joints, inefficient overall arrangement);

• Poor construction (poor material quality, poor seasoning, poor treatment, poor detailing, alterations of design);

• Material degradation (fungi attack, insect attack, metal corrosion);

• Poor maintenance;

• Interventions (increase of dead loads, repair/consolidations);

• Extreme actions (wind, fire, earthquake, snow, impact loads, settlements).

The specific type of causes vary in each part (roof, unit, element, connection) but the groups remain the same. The forms previously discussed (Frese and Blas 2011; Hansson 2011; Toratti 2011) have helped in defining the possible causes of damage.

Poor design, poor construction and all the other, can also be recorded as potential causes of damage, called ‘unfavourable conditions’. This means they have not caused damage yet but might be a problem in the future, which is why they should be recorded nonetheless. Other unfavourable conditions have been included too, such as poor ventilation and the presence of bats/wasps/pigeons.

While the geometrical/typological aspects remain the same, damage can change in time, and its evolution and development must be recorded. Thus the date of inspection has been included in the damage part.

Form

A main menu and linked forms have been created, to allow for an easier use and visualisation of the form. Instructions have been included in the main menu, explaining how the form is organised and the nomenclature used.

Drop-down menus with fixed lists have been included to help users in both filling the data, as they give suggestions, and in analysing it afterwards, as data filled in as free text is always less homogeneous and less easily searchable (Fig. 2). Items can be added to the drop-down lists and more than one choice can be selected where appropriate. ‘Other’ and ‘unknown’ have been included as choices in every drop-down menu, to not force users in choosing something that does not match their judgment. The possibility of adding descriptions and pictures/sketches has also been provided.

The form is suitable for both quick and more thorough assessments. Some fields have been repeated (such as the timber dressing, the timber species, etc.) at different levels, so that if they are homogeneous for the whole roof, or a whole unit, they do not need to be repeated for each element. Moreover, while providing space for all the information that one might want to record, all the fields have been left as ‘non-required’, so that they can be left blank: only the information that is available and useful for the specific purpose of that inspection should be recorded. Data can be changed and added any time later on.

There are many terminology issues, as types of roof structures, elements and connections have different names in different countries, and even the terminology associated to different damage/failures types is not always clear for English-as-a-second-language speakers. It was decided to solve this problem with drawings/sketches, which work as legends for the drop-down menus and other terms that might be misinterpreted (Fig. 2). Many terminology issues remain though: a work on terminology is a complex and long activity that was outside the scope of this work.

Future developments

The database model and form were presented and discussed at the COST Action FP 1101 meeting held in Trondheim, Norway, on 18th/19th June 2015, and at the COST Action FP 1101 final conference in Wroclaw, 8th September 2015. The feedback received was used to revise it. The form has been distributed among TG1 members for further revision and validation. There is also the possibility of it being implemented in an online version, in collaboration with a research group who has developed a semantic database model for the assessment and diagnosis of historic structures (Cacciotti et al. 2015).

Future developments include extending the methodology to the development of forms for other load-bearing systems, incorporating data also from non-destructive/minor-destructive testing in order to quantify the damage in addition to characterising it.

The data collected through the form could also be useful to associate each type of damage with the most suitable repair intervention (Harte et al. 2015).

Conclusions

A database model and associated form for the assessment of historic timber roof structures has been developed based on the work of COST Action FP1101 and ADCRU at the University of Strathclyde. The aim is to guide visual inspection through a structured digital form, collecting a relevant amount of homogeneous data recording and reporting typical damage/vulnerabilities of structures. This is the basis for deriving vulnerability factors to predict failure mechanisms and identify urgent interventions. The development of this data collection and analysis procedure will support professional practice and increase knowledge on existing timber structures. Once validated, the approach used (the tree-like organisation of the form, the flexibility for its use in both quick and in-depth inspections, the graphics explaining the relevant terminology, etc.) could be applied to develop forms for the analysis and assessment of other structural systems.

Acknowledgements

The authors are grateful to COST Action FP1101 for funding this STSM and to CNR IVALSA Institute for hosting it in May 2015, and acknowledge the contribution of TG1 of WG 1 of COST Action FP1101. We are also grateful to the 29 owners of the timber roof structures in Scotland for agreeing to use the data in the research.

ORCiD

A. Serafini http://orcid.org/0000-0001-7547-4639

Additional information

Funding

This work was supported by COST Action FP1101 [COST-STSM-ECOST-STSM-FP1101-180515-058826].