BUILDING RESEARCH AND INFORMATION VOLUME 19 NUMBER 11991

NDTStrength of old timberTEST CORES FROM 50-400 YEAR OLD TIMBERINDICATES STRENGTH OF OLD TIMBERS CORRESPONDSTO THAT OF NEW

Wolfgang Rug Axel Seemann

Non destructive testing has proved to be a useful way of determining the strength of oldconstruction timbers, confirm Wolfgang Rug and Axel Seemann of the Bauakademie,Berlin. The authors in a significant number of tests, did not observe the expected decreasein strength of old timbers

Wolfgang Rug et Axel Seemann confirment que les test non-destructifs SOlltune bonne methode pourdeterminer la resistance des vieux bois de construction. Au GOUTSd'un nombre considerable de testsles auteurs n'ont pas constate la diminuition de la resistance des vieux bois de construction a laquelle J

ils s'attendaient.

Definition of the problem

For a lang period, wood was the Olle and only buildingmaterial at people's disposal in adequate abundance.Tirnber structures determined the architecture, engi-neering and technology of building and construction inmany fields for millenniums. In the Middle Ages - withthe erection of roof systems, floors, framework struc-tures and bridges - carpenter's tirnber constructionachieved a workmanlike perfection.

There are a great nurnber of serviceable roof, floor andhall structures in industry, in agriculture and in publicbuildings from the first flowering of timber engineeringat the beginning of this century. The maintenance andpreservation of these buildings and structures has

become increasingly significant. M~~~~.k~:~~_eriencewasgained in the GDRfor aperiod of miiny years in the fieldsof the estimation, evaluation and repair of buildingfailures. The decision to be taken OIltb.e type and extentof the maintenance and preservation measures andactivities being required was preceded by an analysis ofthe structural state of repair, ref. 1.

At the stage of the evaluation of the structural state ofrepair, the responsible engineer or architect would

provide data concerning the existing loadbearing capa-city and structural stability. For this purpose, he or sheneeds reliable information and data concerning theactual strength of the timber and of the connections.

Determination of strength

The determination of the data and particulars concerningthe strength must, in general, be accomplished bymeans of built-in timber and is thus subject to a numberof restrictions which must be taken into account in theselection of suitable methods für determining thestrength. .

The evaluation of the loadbearing capacity of theconnections is even more difficult and requires specialknowledge and experience with regard to the historicaldevelopment of the constructional principles in timberconstruction.

The authors focused their studies and investigationson the partial subject of the strength determination; inparticular the determination of the compression strengthparallel to the grain of new and old timber by means oftest Gares (hereinafter referred to as °BK),and the com-parison with the compression strength as determined bymeans of the standard test specimens (hereinafter

32

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~

'",..;

steel platen

tF

groove eJ 15 mm

Fig. 1. Principle of the testing of timber cores

referred to as aNP),which hitherto have beeil normallyused für strength investigations and tests according toref.2.

The initial findings and information as published inref. 3, have beeil further deepened and emphasized by agreat number of new studies and investigations.

Test eore method

Based on a thorough and careful analysis of the presentstate of development in the field of non-destructive andlow-destructive testing methods and procedures, theauthors have decided on the application of the test Garemethod. The simple practical management and handlingof this method is connected with a relatively smailtechnical expenditure in terms of equipment andappliances. In addition, to determine the strength, otterproperties and parameters mayaiso be determineddirectly, Le. the bulk density or volume weight, themoisture of timber, ref. 2, the pH value, the check ofaggressive media and the flexural strength by adoptingthe Dynstat-type method, ref. 4.

The drill holes resulting from the sampling of test Garescan be put back again immediately. In spite of thesepositive characteristics and features, the method hasonly seldom beeil applied für determining strength,refs. 3, 5-7.

Tests and experiments

Applieability tests eoneeming the test eoremethodInitial studies and investigations are covering. the definition of the geometry of test Gares. the development of a suitable testing device. the determination of the nullteT of sampies required.

The test Gare diameter to be selected should be suchthat the smailest possible reduction of cross section willresult. On the otter hand, however, an adequate sizeshould ensure a timteT structure being approximatelyequal to that of the standard test specimens.

RUG AND SEEMANN

25-40 mm have proved to be weil suited für the investi-gations. Tests performed by using test Gares with adiameter of 10 mm have produced similar results. Theexperimentaily used testing device is shown in Fig. 1.

In accordance with the minimum quantity of sampiesas required in ref. 2, the number of test specimens wasdefined to be 25 per test series.

Test proeedureAll sampies have beeil treated by subjecting them to astandard climate, with a temperature of 25°C and arelative air humidity of 65% so that the moisture oftimteT was 12% with ail specimens.

The test Gares and standard test specimens weresampled by the authors from timteT discs next to eachotter. The tests have beeil performed by using a ZD 20type tension-compression testing machine. The deter-mination covered the faHure load. With a great nullteT oftest specimens, the load-deformation diagram has beeilrecorded and plotted as weil.

Prior to testing the strength, the bulk density of eachtest specimen was determined according to ref. 2. Thedocumentation and evaluation have beeil implementedin a computerized way. The tests and experiments wereinitially performed by using new timber, oak, beech,spruce.

The subsequently tested old timber specimens havebeeil sampled by the authors from square timber ofbuHding constructions with an age ranging from 60 to200 years.

Results and findingsThe results and findings of 1324 tests, Table 1, wereobtained from a total nullteT of 23 test series performedby means of test Gares (with a diameter of 15 mm) and ofstandard test specimens (sized 20x20x30 mm).

Tests using new timberThe tests performed by means of 3 seiles using oak,beech and spruce timber, demonstrated that with testGares, a clearly discernible faHure is occurring. A faHurepattern is being produced resembling that of 'thestandard test specimens. Same typical examples areillustrated by Fig. 2, refs 5, 8.

A comparison of the mean values of the strengthsdetermined by using test Gares, with these values deter-mined by means of standard test specimens, shows thatthe ratio of the quotients of aBK/aNPamounts to 1.18,1.01, 1.25 für oak, beech and spruce timber, respectively.The scatterings are 8% für oak, 15% für beech, and 12%für spruce.

Analogous ratios were also obtained with regard tothe bulk densities (hereinafter referred to as PBKfür thetest GOfeSand as PNPfor the standard test specimens,respectively) of the investigated series. A comparison ofthe 5%-quantities is producing approximately equalresults. The average quotient foTail three kinds of timberis 1.1.

Tests using old timber. Pille timteTThe investigation included 11 series with pille-timteT

(service life) of 60 years. In this Gase, the ratios of thestrength values and the bulk density are rangingbetween 0.84-1.12 (conceming the series 4,5 and 15 to23 according to Table 1), so that the same results andfindings as the new tirnber were identified. Also, the 3test seiles performed with structural tirnber being 70years old and the 3 seiles für structural tirnber being 140or 200 years old, were producing analogous results.. Spruce timteTSpruce tirnber which has beeil built in atout 140 yearsaga, produced similar quotients (being 1.14 and 1.07,respectively) für the strength and far the bulk density.. OaktimteTThe tests covered two series using oak with an age of140 years, which were showing quotient values as

fellows:

aBKlaNP==1.03 and aBKlaNP==1.01

The strength values of old oak are within the range of thevalues applying to new oak (oak==52, beech==60,spruce==40, pine==40N/mm2 according to ref. 9.

Result of a regression analysisThe linear regression analysis according to refs. 10 and11 was accomplished far the mean values of the strengthand bulk density. Fig. 3 illustrates the correlationbetween the compression strengths depending on thenullteT of the test series.

The ratio of the strength of the test Gares to thestrength of the standard test specimens approximates to

Fig. 2. Typical tailure patterns tor test cores (diameter ot 15 mm) and standard test specimens

, -!

STEENGTH OF OLD TIMBER 33

Table 1. Summary ot the test results and findings

Test Series Kind of Age n (JBK PBK n (JNP PNP (JBK/p PBKI PNp

group timber (N/mm2) (kN/m3) (N/mm2) (kN/m3) (-) (-)

1 oak new 25 49.92 6.80 25 42.43 6.17 1.18 1.102 beech new 25 58.88 6.94 25 58.12 7.33 1.01 0.953 spruce new 25 3.11 3.94 25 24.87 3.48 1.25 1.13

I 4 pille 60 48 50.55 4.35 17 44.32 4.49 1.14 0.975 pille 60 25 49.84 5.42 25 66.23 5.22 1.08 1.046 pille 70 43 43.96 5.16 25 39.68 5.10 1.11 1.017 pille 70 54 60.56 6.26 132 62.04 6.18 0.98 1.018 pille 70 28 53.47 4.24 27 49.03 4.24 1.09 1.00

9 pille 140 16 60.89 5.82 25 60.59 5.42 1.00 1.0710 pille 140 25 51.11 5.06 25 50.87 5.01 1.00 1.0111 pille 208 35 43.13 4.74 50 43.28 4.36 1.00 1.09

II 12 spruce 138 22 46.53 4.37 36 40.48 40.75 1.14 1.0713 oak 138 20 5.53 6.68 21 53.80 0.65 1.03 1.0314 oak 140 32 54.85 0.71 50 54.40 6.06 1.01 1.17

15 pille 60 25 5.68 6.65 20 50.56 6.04 1.12 1.1016 pille 60 25 50.61 5.26 20 49.45 5.05 1.02 1.0417 pille 60 25 50.45 5.26 20 47.65 5.58 1.06 0.9418 pille 60 25 46.09 5.37 20 54.01 5.19 0.85 1.04

III 19 pille 60 25 50.95 4.96 20 48.00 4.76 1.05 1.0420 pille 60 25 5.41 0.55 20 54.87 5.33 0.99 1.0321 pille 60 25 5.02 5.61 20 51.60 5.30 0.97 1.0622 pille 60 25 48.13 5.71 20 54.37 5.37 0.89 1.0623 pille 60 23 50.57 5.30 20 61.74 5.24 0.82 1.01

34

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RUG AND SEEMANN

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Fig. 3. Comparison oi the eompression strength oi the test eores with the eompression strength oistandard test specimens

the value of 1/0.97=1.03 with an increasing number oftest series (see Fig. 4).

The simplified conversion formuIa reading

aBK=1.0 aNP

is within the confidence limit of 95% (see Fig. 4).Concerning the pine-timber specimens sampIed framstructural components being 60 to 200 years eid, thedependence of the strength of the test Gares and of thestandard test specimens on the bulk density (see Fig. 5)is obtained as fellows:

aBK=4.12xPBK+29.21 (r=0.51)

aNP=6.66xpNP+16.65 (r=0.54)

The bulk density of the standard test specimens isobtained fram bulk density of the test Gares by means ofthe equation

PNP=0.9xPBK+0.33 (r=0.94)

Within the confidence limit of 95%, the simplified

formula (see Fig. 6) is sufficient and reads as fellows:

PNP=0.9XPBK (5)

(1)A comparison of the dependence of the strength on thebulk density für oId timber with that appIying to newtimber is verifying that oId timber has no Iess strengththan new timber (see Fig. 6).

A decrease in strength of timber having beeil sub-jected to stress and strain für many years - whichhitherto was assumed für sound (i.e. faultless) timber -couId not be determined.

With a view to recording the influence of the bulkdensity on the regression line (according to Fig. 4), thequotients of PBKlpNP-UBKluNPhave beeil plotted into adiagram (see Fig. 7).

In addition to this, the frequency distributions andnormal distributions of the quotients are being calcu-lated. With a direct influence being exercised by thequotients of the bulk density on the quotients of thestrength, the gradient of the regression line ought to beabout 1.0. However, instead of this the calcuIatedregression line reads as fellows:

(2)

(3)

(4)

UBKlaNP=0.36xPBKI PNP+0.66 (r=0.20) (6)

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STRENGTH OF OLD TIMBER

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Fig.4. The quotient (JNP/(JBKdepending on the number oi the test series

number of tt-eseries

y

[kN/~

I

II,

3,0 ',0 r:p

~BK CI!P

60 ~o fW Je

[kN/m3 ]bulk density

Fig. 5. Comparison oi the bulk density oi the test cores with the bulk density oi thspecimens

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1000 Ö:I>

750c41..c-

500 0'-aJ..Cl

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36

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(Nimm) 1 spruce u= 158/8. EMPA. Switzerland20 ~ aa;;J 51. new timber2b ~aa:.toI Sldd timbero(300 years otd)

3a pine old timber 160 to 200 years I

}stcnDrd fest spE!(;imens gur own

3b p;ne oId Umber 160 to 200yeors I teststest cores " 1Smm

1.0 Swedish plne acc. t 0 Kollmonn.u=6...108/8, new timber

4b Mecktenburg pine ace. to KoUmann,new timber

90

80

70

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0.35 0/00 0,45

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RUG AND SEEMANN

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12b

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Fig. 6. Compression strength parallel to the gram depending on the bulk density tor new timber andold timber

This line is considerabIy deviating flom the ideal line.Thus, the strength of the test cores being somewhathigher cannot fully be explained by the determined bulkdensity being higher as weIl.

Summary of the results and findings

The high linear defineability of the compression strengthof the standard test specimens by means of the strengthof test cores with a diameter of 15 mm verifies the testcore method for determining the compression strength'parallel to the grain.

Thus, the test core method is weIl suited to determinein situ the strength of built-in timber.

A number of ether studies and investigations can beaccomplished by using test cores as weIl.

An expected decrease in strength with old timbercould not be observed. The high correiation between thebulk density and the strength enables the provision ofquantitative data and particulars conceming thestrength by the application of non-destructive methodsand procedures for measuring the bulk density.

Studies and investigations to be accomplished in thefuture will be focused on the following principal aspectsand topics:. developmentof a data basis concerningall investi-

gations into the strength of old timber. determination of partial safety factors for the calcula-tion (design) of old timber construction.

J:; 600,CGI....iii

c500

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9 6

STRENGTH OF OLD TIMBER

I-J y

1,3

0.9

n= 2

37

1,2 \3 x[- )

3

x = 1°'' ' '

Fig. 7. Linear regression, frequency and normal distribution of the quotients (JBK/(JNPand P BK/ P NP

References

1. Rug, W. and Krüger, K. Erhaltung alter Holztragwerke(Maintenance and preservation of old timber load-bearingstructures). (To be published.)

2. GDR Standard Specification: TGL 25 106/01 - Prüfung vonHolz (Testing of Timber), 1979.

3. Schwab, E., Warchau, A. and Willeitner, H. (1982) 'Bohr-kerne zur Beurteilung der Festigkeit hölzerner Rammpfähle'(Test Gares tor determining the strength of wooden impact-driven piles). Bauen mit Holz, Karlsruhe 84 (9), 457.

4. GDR Standard Specification: TGL 0-51222 Dynstat-verfahren (Dynstat-type method).

5. Seemann, A. (1988) 'Durchführung von Versuchen zurFeststellung der Tragfähigkeit von eingebautem Bauholzund Auswertung der dazu vorhandenen Literatur' (Imple-mentation of tests far determining the loadbearing capacityof built-in structural timber and review of the pertainingavailable publications). Weimar College of Architecture andConstruction, University Graduation Paper, Weimar.

6. Steller, S. and Lexa, J. (1986) 'Zur Problematik derLebensdauer von Holz und Holzwerkstoffen' (On the prob-

leIDs connected with the service lire of timber and tirnberengineering materials). Holztechnologie, Leipzig 27 (6),303-05.

7. Schweer, H. and Volk, U. (1986) 'Bauzustandsuntersuchungdurch Endoskopie, Thermographie und Bohrkernentnahme'(Investigation into the structural state of repair by means ofendoscopy, thermography and test care sampling). Bauenmit Holz; Karlsruhe 88 (3), 142-44.

8. Rug, W. and Kreissig, W. (1988) 'Die Weiterentwicklung desIngenieurholzbaus in der DDR' (The turther development oftimber engineering construction in the GDR). Bauen mitHolz, Karlsruhe 90 (11), 758-66.

9. Halasz, R. and Scheer, C. (1986) 'Holzbautaschenbuch,Band l' (Timber Construction Pocket-Book, Val. 1). Berlin(West).VerlagErnst & Sohn. '

10. Stürm, R. (1979) 'Wahrscheinlichkeitsrechnung, mathema-tische Statistik und statistische Oualitätskontrolle' (Prob-ability theory, mathematical statistics and statistical qualitycontral). Leipzig, GDR.

11. John, B. (1979) 'Statistische Verfahren für technischeMeßreihen' (Statistical methods far technical series of meas-urements). Carl Hanser Verlag, München & Wien.

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