Approved Concept for Inspection of Modern and Historic Timber Structures

Frank Rinn

More than 2.5 million historic half-timbered buildings and more than 5 Million buildings with wooden ceiling beams have to be preserved in Germany in as good as possible condition due to regulations on historical monuments as cultural heritage. More than 200 Billion Euro is spent on buildings in Germany every year. Approximately 60% of this building budget is spent for restoration and repair of existing buildings.

However, the education of architects and engineers still mainly focuses on design of new buildings.
Between 1 and 3 Billion Euro is spent each year for preservation of historical monuments, mostly at least partially financed by taxes or lottery funds.

Approximately 5% of new houses are built with structural timber (+10% p.a.). The inspections of ‘new’ timber buildings (built > 1950) is growing, due to poor quality of design, misuse and lack of maintenance. As a consequence, the need for non-destructive timber inspection increases.However, specific boundary conditions have to be regarded:

● Architects and engineers are still mostly paid a percentage of the total costs (thus are not primarily interested in saving costs, especially if public money is involved).

● If a building is 100, 200 or even more years old and does not show significant deformation, the timber structure is supposed to be strong enough and then only decayed parts of the structure have to be replaced. That’s all! There is no need for structural analysis and calculation of load carrying capacity.

● If there are significant obvious deformations or the future use of the building will bring in more load (less than 5% of all cases) then stiffness and strength of beams have to be determined.

Based on the market, the client’s needs and the given boundary conditions, we developed a concept how to inspect timber structures in a fast, efficient and reliable way.

Major objectives and tasks of our inspections

Non-destructively creating an easy to understand and clearly visible status report on timber structure condition without harming other (historic) fabric:

• create/modify a sketch of construction covering all relevant timber beams
• determine dimensions of timber cross-sections and connections
• reliably identify decayed and intact parts of beams and connections
• (sometimes: determine MOE and estimate MOR)
• visualize results in color inventories (rather than writing long text reports)

The inspection concept was developed in cooperation with architects, engineers, carpenters and administrations. Several thousand buildings have been successfully inspected.

The major steps of the inspection are:

1. Create new or modify existing sketches of the construction.

2. Visually inspect all accessible parts (condition, external defects, dimensions of beams, previous repairs) – often a major part of total work to be done.

3. Technical measurements:

3.1 Moisture content measurement.

3.2 Resistance drilling (with calibratable machines only, such as Resistograph^®).

3.3 Stress wave timing (for example using impulse hammer or Arbotom^®).

4. Documentation of all results and all relevant information about timber condition into a few graphical sketches by avoiding long text reports.

 Basic sketch for colored inventory

All relevant beams have to be shown in at least one of the sketches. A coordinate system reliably identifies each beam and connection.
The demands made for an inventory regarding the timber construction can be easily formulated:

1. All pieces of timber relevant for the statics of the construction must be drawn in at least one plan.
2. When possible, no piece of timber should be drawn on top of any other.
3. The relative position of the beams to each other must be correct.
4. Significant deformations relevant to the static construction must be included.

Unfortunately, it can be determined that existing inventories often do not fulfill these conditions. The common ground plans at approx. 1 meter height over the ceiling beams are useless for this purpose. It has proven better to draw up a new schematic sketch of the construction according to the above-mentioned conditions instead of trying to correct existing plans. In some cases it makes sense to examine the hidden timbers in the ceiling by means of thermography. Up until now, this technique was predominantly used in winter, because great differences in temperature are required.

Visual (ordinary) inspection

This first part of the inspection is often the majority of the total working time and usually consists of the following steps:

1. Look for external decay (by fungi or insects).
2. Knock on all beams and connections with ordinary hammer
3. Use handcraft tools to check in all suspicious holes/connections
4. Take samples from insect or fungal decayed parts and determine decay species.
5. Check quality of previous repairs and replacements.
6. Look for and measure major deformations.
7. Try to find reasons for decay and other defects.
8. Document all results in colored sketches.
9. Determine points where technical inspection is required.

At the end of the visual ( ‘conventional’ ) inspection, the colored sketch already contains a lot of information but has many white spaces where the condition of the corresponding beams is still unknown.

Technical inspection

When visual inspection is unable to clear all questions or hidden beams have to be evaluated, technical methods are used in order to answer the remaining open questions :

1. Relative moisture content.
2. Drill resistance measurements
   a. find hidden beams behind stucco or below flooring
   b. assess depth of obvious outside decay or cracks (e.g. in glue-lam)
   c. check internal condition (of visible and hidden beams and connections)
   d. determine gross density (after calibrating drilling machine)
3. Stress wave timing: speed of sound, detect hidden cracks or connections
4. Combine (not only numeric) results, e.g. density * speed² = MOE
5. Document all measurement points and all results in colored inventories.

Having inspected a timber structure visually and technically may lead to great results but does not help to preserve historic fabric or make repairs efficient if the experts planning and executing the repair work do not understand the results. Based on the success of the application of resistance drilling for inspecting timber starting 1986, we then developed a concept of how to document inspection results that provides more precision and reliability but is, at the same time, easier to understand for both engineers and carpenters.

Documentation concept
The first step forward from black and white sketches of timber structures, with shading for marking decay, was to use colors. But, in order to make the drawings as easy as possible to read, the number of main colors had to be as small as possible, at most three or four.

At the time we developed our concept (late 1980’s/early 1990’s), color copies were still quite expensive, especially if printing in larger than standard letter sizes. The colors thus had to be selected in a way that allows black and white copies to provide the major information about decay and condition (Fig. 3). Consequently, we selected red (extensively decayed), orange (mean decay), and yellow (intact) as the major colors – because they can be differentiated easily and because black and white copies still show the three colors reliably in differentiated types of grey (Fig. 4).

The traffic light color scheme, (green for intact, yellow for partially decayed, and red for strongly decayed), was not an option for several reasons: in a black and white copy, green was commonly darker than red, leading to a wrong impression about the condition of the corresponding parts. In addition, structural engineers in Germany commonly used green for marking structurally relevant, local aspects and symptoms, such as cracks.

The biggest step forward was introducing a color for marking parts of timber that were inspected (either visually, by tapping and/or resistance drilling) and were found to be intact and sound. This means, if a beam was tested and no sign of decay was found, this beam is marked with a certain color.

For the first time, this way it was possible to distinguish between the sections of a timber structure that were not inspected (no color) and the parts that were inspected without finding damage (yellow). This may sound like a tiny little aspect but it changed [inspection] a lot, because from then on, later planning and working steps knew what parts of the structure they can rely on without doubting whether these parts had been checked or not (because there was no decay marked).

Another big step forward was combining as many parts of the usually many individual sketches of a structure as possible into one single overview drawing: this reduced the total number of sketches representing the condition of a structure from as many as 10 to 1 or 2 – making it much easier for engineers and architects as well as for carpenters to get an overall impression about the condition of the bridge or structure as a whole. In addition, the overview given by a single sketch with a color coded condition inventory allows the identification of connections between sources and reasons for different spots or areas of decay. That means, these overview inventories provide a base for a much deeper understanding of the structure as a whole instead of only working locally on repair of individual parts.

Figure 5: A typical timber bridge inspected for decay. (Constructed from Tropical Hardwood)
Above: Conventional black and white damage map of a timber bridge. Originally it was common to mark decayed parts with a certain kind of shading and a label that refers to the text list position of the corresponding description of the found damage. Such a drawing usually consisted of 10 individual sketches of each axis and was accompanied by a many-page report.	Coloured version of an inventory map showing wood condition in different colors. The colors not only reveal where decay was found, but furthermore show what parts of the structure were found and proven to be intact. Because colors allow the reader to more easily identify damaged areas, a combined sketch replaces many conventional drawings.

Practical working steps
Commonly we prepare the basic drawings of structures before the technical inspection starts. Such structural sketches have to show all relevant timber parts that belong to at least one plane of the structure or are connected with this plane. While doing that, we try to avoid showing different beams in one sketch that in reality overlay each other and represent different planes – because it is impossible to show correct colors if these beams have different conditions and thus would have to be characterized by different colors overlaying each other.

Usually, the sketches are prepared in a larger size and scale to enable the inspector on site to put in all relevant information while inspecting – as one of our major goals was to avoid text notes but still display all relevant aspects in the sketch. All evaluations should be able to be done on the spot without having to go back to the office to work on profile analysis and come to a conclusion that, for example, additional assessments are required. This is time consuming and inefficient.
Our goal was to enable an inspector to always come to a final conclusion about the condition of timber on site. While on site you can just tap or drill once more in another spot in order to confirm unclear results or suspicious symptoms.
The highest (cost and time) efficiency was always achieved when the inspection came to a final conclusion while on site, and when all relevant results were documented in the color coded inventory map on site. This drawing then only has to be reproduced in the office and surrounded by a short text note.

The reproduction of the colored on-site drawing is usually done by a reduction factor of 4. These squeezed sketches then represent the most significant part of the report. In addition, the report usually contains some illustrating pictures and a short text summary with recommendations. Even the recommendations for repair work can be partially included in the color coded sketch because lines may be implemented indicating where and how damaged beams should be cut and/or replaced. All this fits to the traditional German saying: “A good drawing is the language of a good engineer”.