Assessment methodology for historic timber roof structures
Alexandra Keller1, Marius Mosoarca 2
Faculty of Architecture and Urban Planning, Politehnica University of Timisoara, Romania1,2
alexandra.keller@upt.ro1; marius.mosoarca@upt.ro2
Abstract
Historic timber roof structures are complex systems with a close connection to the building they belong to and to their context. The shape of the roof, the importance of the building in the surrounding area, urban planning principles from the period in which the building was built, architectural and structural features and ultimately the knowledge of the craftsman, all influence the choice of structural type and the complexity of the structure.
However, recent assessment methodologies developed for historic timber roof structures evaluate them strictly from a structural point of view, by taking only their state of conservation and structural behavior into consideration, without evaluating also the link between the structure and all surrounding elements.
Therefore, based on historic roof structures from Timisoara, the study aims to provide a more complex assessment methodology which can be used to evaluate roof structures from a multidisciplinary point of view.
Keywords: assessment methodology, heritage timber structures, roof structures, multidisciplinary assessment
1 INTRODUCTION
In recent years, various historical studies on roof structures have been carried out in different historical cities across Europe to better understand historic roof structures [1–3]. The primary objective of the studies was to find a way to understand them and ultimately preserve them. In the same time, assessment guidelines have been developed in order to simplify the assessment of historic roof structures [4,5], which highlight criteria that have to be considered in order to adequately assess timber roof structures.
2 CRITERIONS FOR ROOF STRUCTURE ASSESSMENT METHOLOGYS
The study of historic timber structures from Timisoara, has shown that roof structures have a significant value that is not always associated with their structural features. The position of the building in the city [6], the architectural value of the building, the link between the roof and its context [7] and complex details and proportions [8,9] can influence the features and ultimately the value of a roof structure. In the same time the influence of the roof structure on the behavior of the building during seismic events [10,11] and the state of conservation of the timber elements composing the roof structure, should also be considered (see Fig. 1). Therefore, a complex assessment procedure for timber roof structures has been developed, which takes all these factors into account.
Fig. 1 Proposed assessment methodology features
2.1 Urban planning principles
The evolution of urban planning principles in time led to different relations between the building and surrounding urban space. In Timisoara, in the historic part of the city, buildings are placed on a rectangular grid of narrow streets which makes the perception of roofs almost impossible. Still, in the main squares of the historic city, roof have a more important role, completing the building and help defining their shape.
At the beginning of the 20th century, after the old fortress was destroyed, urban planning principles strongly influenced the architecture of the new buildings. In this period, roofs have a significant height and are meant to highlight the building and increase its monumentality (see Fig. 2) which is ultimately raising importance of the roof in defining urban space (Andreescu & Keller, in press). These roofs create a strong link between the building and the street, highlighting the fact that urban planning principles were of great importance in the shaping of roofs and roof structures.
Fig. 2 Roof structure with a significant role in defining the urban context
2.2 Architectural principles
Architectural principles, styles and functional features also play a significant role in defining the shape of the roof and the relation between the building and the roof. Complex shapes of the building or additional special needs like introducing natural lighting of the attic can lead to changes of the roof structure (see Fig. 3). In the same time, imposing roof structures were used for buildings which should have a monumental appearance, highlighting the importance of the building and making it look more imposing.
Fig. 3 Complex roof structure influenced by the geometry of the building
2.3 Symbolic elements
Harmonic ratios play a significant role in defining architecture and the surrounding space of buildings. According to assessed case studies in Timisoara [8,9] harmonic principles were used to define the height of the roof in relation to the height of the building as well as the position of main structural joints. The studies also show that the used ratios change in time from complex ones, mainly golden ratios, used in the 18th century, to dynamic ones (√2, √3 and √5) in the 19th century and ultimately to static ones (1/1, 1/2, 2/3, …) and mix of ratios in the 20th century (Fig. 4).
Fig. 4 Harmonic ratios analysis of 18th century roof structures
2.4 Structural principles
The structural features of roof structures can be linked on one side to the evolution of the urban planning and architectural features which imposed certain shapes of the roof but also to the continuous search to develop more efficient structures (both technical and economical), suitable to cover buildings and protect them from environmental factors.
Starting with the tent-like roofs, roof structures got in time more and more complex, being able to cover larger spans, gaining significant height during the gothic period and becoming more structurally-efficient towards the 20th century. Since the architectural styles of the beginning of the 20th century imposed monumental roofs, complex new structural types were developed, composed of a mix of previous styles (see Fig. 5).
Fig. 5 Complex roof structure made of multiple typologies
2.5 Influence on the seismic behavior of masonry structures
During the study, finite element simulations were performed, using SCIA Engineer, on a 18th century building from the city center of Timisoara with and without roof structure in order to better understand the influence of the roof structure on the behavior of the masonry structure during seismic events [10,11]. The performed simulations supposed placing 3 different types of roof structures (form the 18th, 19th and 20th century) on the masonry building. Connected subsequently hinged, rigid or sliding to the masonry walls, the simulations showed that the presence of the 18th century roof structure can reduce to top horizontal displacement with up to 40%, while 19th century ones reduce it with 30% and 20th century roof structure with up to 20%.
In the same time, it was observed that the roof structure not only influences the top horizontal behavior of the masonry building, but also changes the displacement of every floor (see Fig. 6) and reduces the damage level of the masonry structure after the seismic event.
Fig. 6 Horizontal displacement of a 18th century building during seismic events with and without roof structure
2.6 Climate change
Changing climatic factors are significantly influencing the state of conservation of roof envelopes and roof structures.
High wind velocities and hail stones are some of the most serious threats for the exterior appearance of roofs, being able to cause extensive damage to the roof coverings and even the entire roof structures [12,13]. These exterior climatic threats affect the aesthetic and historical value of historic structures.
Humidity on the other hand is a threat for the timber elements of the roof structures, influencing the mechanical properties of the timber and the state of conservation of the entire structure.
Fig. 7 Roof structure damaged by wind (after [14])
3 THE PROCEDURE
The developed assessment procedure was divided into 5 levels: “Urban value of roof structures”, “Architectural value of roof structures”, “Symbolic value of roof structures”, “Structural value of roof structures” and “State of conservation of roof structures” (see Table 1).
Table 1 Assessed criteria of the assessment metholodology
I.
Urban value
(25%) 1. Value of the urban area
2. Urban analysis
3. Geometry
II. Architectural value
(25%) 4. Historic analysis of the building
5. Building analysis
6. Functional analysis
7. Aesthetic analysis
8. Geometry of the roof structure
9.Exterior appearance
III.
Symbolic value
(15%) 10. Ratio between the roof and the building
11. Ratio between structural elements
12. Symbolic aesthetics
IV. Structural value
(35%) 13. Roof structure
14.Structural elements
15. Joint typology
V.Value reduction
(-15%) 16. Decay visible from the outside
17. Decay of the roof structure
Each level was subsequently divided into a tree of key features and a list of possible answers for each level provided, based on the observation made during the study. In order to simplify the procedure, for each answer a certain score was assigned, which automatically increases the objectivity of the evaluation [2].
By using this procedure, it is possible to assess the roof structure as a whole, based on a visual inspection of the structure, the building, its urban surrounding and a historical analysis of the context in which the building was built and how it evolved in time.
4 RESULTS
After completing the form and choosing all the answers to every assessed feature, the procedure offers information about:
• The main valuable feature of the roof and roof structure; whether the roof plays part in shaping the urban context, if it is rather influenced by architectural principles, if the symbolic elements are predominant or if it is a complex structure (see Fig. 8).
Fig. 8 Predominant value of the roof structure
• The ideal value of the roof structure by making a weighted sum of all the main assessed criterions, without taking the decay into consideration, or any other vulnerability:
Videal=0.25*VUrban+0.25*VArchitectural+ 0.15*VSymbolic + 0.35*VStructural
• The real value of the roof structure is considering the ideal value of the roof structure, but this time is subtracting 15% of the obtained sore of the value reducing factors.
• The vulnerability of the roof structure which considers the ideal value of the roof structure and is subtracting the decay and the seismic and climate change vulnerability (see Fig. 9).
VVulnerability=0.65*Videal+0.20*DDecay+0.10*SSeism+ 0.05*CClimate
Fig. 9 Vulnerability of the roof structure
5 CONCLUSIONS
The study presents an attempt to develop a complex assessment procedure useful to assess historic roof structures from a multidisciplinary point of view based on a preliminary visual survey.
Because the methodology was developed like an form offering a clear overview of evaluated features and a list of possible answers it represents a fast and easy way to evaluate roof structures from various points of view, starting from their context down to every valuable detail.
Still despite its efficiency, the methodology is a tool useful for a general assessment for roof structures and a guide to prioritize future interventions and should not be considered a tool to evaluate the historic timber structure.
References
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[7] A. Keller, M. Mosoarca, A complex assessment of historic roof structures. In: Arun G, editor. 4th Int. Conf. Struct. Heal. Assess. Timber Struct., 2017, p. 157–68.
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[12] A. Keller, M. Mosoarca, The impact of climate change on historic roof structures and roof coverings. Proc. 7th Struct. Eng. Wold Congr., n.d.
[13] M. Mosoarca, A. Keller, C. Bocan, Failure analysis of church towers and roof structures due to high wind velocities, Engineering Failure Analysis, n.d.
[14] T. Bălan, Religious buildings are suffering [in Romanian], Banatul Azi, 2017. https://www.banatulazi.ro/lacasele-de-cult-sufera-impreuna-in-fata-stihiilor-foto/.