Evaluation and analysis of the old timber structures
A. Ceccotti Trees and Timber lnstitute, IVALSA-CNR, Florence, ltaly
ABSTRACT: A methodology for evaluating the actualload-bearing performance ofan ancient timber construction is given in this paper based on a simple non-destructive approach. Materiais and methods are illustrated and discussed. Examples are given emphasizing the key points ofthe decision making procedure. Final considerations with particular reference to the "minimum intervention" principie conclude the work.
fNTRODUCTION
Before an ancient timber structure the structural designer is always asked for the fundamental question: what to do with this structure? That actually means: how sound are wood elements and joints? Is this structure capable of standing up for many years more, even under new service loads? Are any strengthening and repair needed?
Having in mind the conservation of the structure as main-guidance tine, in order to take the most appropriate decision, the designer should follow a multi-disciplinaryapproach.
2 A METHODOLOGY FOR EVALUATION AND ANALYSIS
There are two basic and separate questions actually:
• is the wood still sound? • is the wood enough resistant?
In fact resistance is a matter not only of wood soundness but also it is a matter of wood actual stress compared to wood actual strength. Evaluation ofwood conditions and strength and analysis of forces and stresses are two moments of the same processo In Table I a synopsis of these points is given referring to a simple case of a single wooden rectangular cross section (b' original width, h' original depth) under a only bending moment M:
YM and Y! safety coefficients ( > I) cover further uncertainties about possible actual strength values and
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Table I. Evaluation and analysis process (schematic).
Evaluation (Wood Tecllnologist) Decay detection - Residual cross section --> b, li Strength grading, in situ --> limber grade Anticipated strength --> fm ,k(5% )
Uneertainties --> YM , malerial side safety coefficienl
Analysis (Slrl/ctl/ral Engineer) Actions (Ioads, qk) Struetural se hem e for ealculations: qk -> Mk Stresses on elements: M k --> Om.k
Uneertainties --> YI, aClion side safety coefficienl
acting load values (nominally intended for passing from 5th percentile to 0,5th percentile, lower tail for strength distribution and higher tail for load distribution, respectively). kmod is a modif ication factor taking into account service conditions and duration of load effects (see Table 3) .
2.1 Evaluation phase
Evaluation phase consists of:
• evaluation of possible biological decay across the member section and along the member itself. This allows to determine the residual cross-section dimensions along the entire length of the member to be used by the Engineer in his calculations (Bonamini, 1995).
• in-situ grading, i.e. evaluation of the strength of sound timber according to grading rules accepted for that kind of timber.
2.1.1 Important remarks 2.1. J.l Grading For a certain timber population (wood species, location of origin, grade) it will be possible to assign a set
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Figure I a- c. Strength grading. Grading allows to separate better timbers from less resistant ones. With no grading, for the same timber population, strength could range from 15 to 95 Mpa (top, left) like in the case of Swiss pine beams. By grading pieces into strength-quality groups (a,b,c grades, for example) it is possible to classify timbers (top, right) according to their res istance. Grading rules are different over the world because locally calibraled on locally grown timbers. Grade determining defects are usually knots, slope of grain, annual ri ngs thickness et cetera. For each grade, after an extensive testing campaign on that timber population, the relevant characteristic strength values (5th percenli le) can be found (below). Grading rules are not 100% efficient because they do not allow to put ali the best pieces in the upper c1ass, for example, and the worst pieces in the lowest c1ass neither (below), but they are nevertheless essential because they separate limbers according to their characteristic resistance anyway. Please note that in the same grade we have 95% of pieces more resistant than lhe 5% strength.
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ofstrength values fo r different kinds of stress (strength profile). In fac t everywhere in the world there are grading ru les that a llow to grade timbers according to their strength (Figure 1). Every country has its own rules calibrated upon home grown timbers peculiarities. Therefore there are tables available that attribute each timber population to a certain strength profile (strength class), see Table 2.
2. 1. 1.2 Strength It is here necessary to remind the Reader that old ti mber characterist ic strength values (e.g.: };1I ,k(5%» are used as high as for "new" timber. It is actually acknowledged that timber is not loosing its strength over time just because is becoming older (excluding of course any decay due to insects and fungi). In fact there is no real evidence that long lasting pre-Ioading oftimber or timber structures to a limited load levei has produced any damage (Kuipers, 1986), see Table 4.
2.2 Analysis phase
Analys is phase consists of:
• Internai forces analysis conducted by using the simplest structural scheme, at f irst. Then more and more accurate analysis which takes into account as much as possible of the actua l restra ints and of actual mechanical behaviour of materiais. That inc lude, fo r example, semi-rigid behaviour of j oints (s lip), and possible structural gross deformations and disorders. More and more sophisticated models (i.e. anisotropy of wood, II order analysis) should be adopted only when the complexity of the structure requires it. The more the scheme is reality-bound (e.g. considering hyperstatic behaviour, load sharing, et cetera) the more force peaks decrease so that veri fications are fac ilitated, i.e. a better model, though more complex and time demanding, at the end of the entire process will be rewarding (see Figures 3--4).
• Stress analysis, it is possible to pass from internai forces to stresses using well consolidated calculation methods, as for example Eurocode 5, 1993 (usually the limit states design methods are more generous than allowable stresses methods). Then a safety check becomes possible referring to strength values given by standards, as said before.
3 DECISION MAKING
If the verification, by applying the previously illustrated method, is positive, this is suff icient to state that the structure is safe enough. However the opposite is not true. In fact it must be said that when using the above approach based on modem calculation codes and characteristic strength profiles and standard
Table 2a-2b. Strength classes and the "Magic" table, assignment of visual grades and species according to CEN/TC 124.215. Table gives coniferous timbers and source combinations matching strength class C24 profile.
Sfrenglh c/asses and charac ferisfic values according lo EN 338. Coniferous species and poplar.
Table 2b.
Grading rule Strength publishing country Species class (Grading standard) Grade commercial name Source
C24 Austria G.BH Spruce, Pine, Fir, Larch CNE Europe (ONORM B 4100-2)
France CF22 Whitewood, Douglas fir France (NFB 52001-4)
Germany SIO Spruce, Pine, Fir, Larch CNE Europe (DIN 4074-1)
Nordic Countries T2 Redwood, Whitewood NNE Europe (INSTA 142)
The Netherlands B Spruce + fir NC Europe (NEN 5466)
UK SS Redwood, Whitewood CNE Europe (BS 4978) SS Douglas fir, Larch, USA +Canada
Hem-fir, S-P-F SS Southern pine USA SS Parana pine Brazi l SS Pitch pine Caribbean
USA + Canada J+P Douglas fir, Larch, USA+Canada (NGRDL + NLGA) Sei Hem-fir, S-P-F
CNE Europe: Central, Norfh & Eastern Europe, NNE Europe: Norrhern & North eastern Europe, NC Europe: Northern and Central Europe.
grading rules, when the verification would fail at first approach, an appeal should be given to the structure.
We do not want to touch here the issue of possible reduced safety coefficients due to the fact that the structure is already existing (modem calculation codes are thought for not-yet existing structures). Therefore
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uncertainlies about the material still to be provided are much less beca use the material is already there and we can see it in place. Moreover, load standards are changed over lhe years increasing snow loads, for example, where our structure does exist from centuries with no interest in human discussions.
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Table 3. Load- duration classes and kmod for so!id ti mber and glulam.
kmod for Service classes
Load-duration Examples
class Duration" of loading 1 &2 3
Permanent More than Self 0,60 0,50 10 years weight
Short-term Less than Snowb 0,90 0,70 one week and wind
Instantaneous Acc idental 1,10 0,90 load
a The Load-duration classes are characteri sed by the effect of a constant load acting for a certa in period of ti me. For variable action the appropriate class depends on the effect of the typical variation of the load in the !ife of the structure. The accumulated duration of the charateristi c load is often very short compared with the total loading time. b In areas wi th a heavy snow load for a prolonged period of time, pari of the load should be regarded as medium-term.
Figure 2 and Table 3. Duration of load effect. Duration of load effect is illustrated here according to EC5, CIB W 18 code and Madison curve (Giordano, 1999) where ti me is in a logarithmic scale (x ax is). Stress ratio as reported on y axis, is the ratio between the actual su·ength and the 5-minute-duration test strength. It is easy to see that under 50% of short term load resistance (so-called creep limit) the time-to-fa ilure is almost infin ite. That means that there is no damage at ali inside the wood. Remember that when loaded under quasipermanent load combinations the stress ratio is 15- 20% of 5 minute strength. Possibility of internai damage due to long lasting action ofloads that may overcome the creep limit fo r a certa in duration of ti me is anyway considered by codes with the modification facto r kmod according to the time of accumulated duration of load at maximum leveI (characteri stic, 5% fracti le). Service conditions are: class 1, indoor; class 2, indoor or outdoor protected; class 3, outdoor not protected.
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We want just to say that more sophisticated structura l schemes, closer to reality, should be used in this case. In addition, grading of timber elements should be done ad hoc on the spot where the safety verification is actually performed (so called criticai sections).
Actuallya timber element is classified into strength grades according to some defects, basically. lt is not possible to know a priori where in si tu that element will be located, how it will be loaded, and where the most stressed section will stay (criticai section). Therefore timber elements are classified by the timber supplier independently from the location of the grade determining defect along the timber e lement itself. Let's consider the case, for example, of an isolated big knot as the grade determining defect. Once the timber element has been put in place, if this knot is just near bearing supports or at the extra-dos of a bent beam, it will have much less importance than if it would be staying at the mid-span intra-dos of a bent beam. Old carpenters were actually used to put the best pieces in the m ost stressed parts ofthe structure! (see Figure 5).
In conclusion: if first safety check is not positive, before to make a life sentence (demolition or strengthening) a second chance should be g iven to the element, making an ad hoc re-classification of the timber element right around the criticai section aiming for an up-grading of the element.
More: we have to say that even the ad hoc grading, refers to 5% lower fractile strength characteri stic values, that means there is sti ll a high chance that our element will be more resistant than that value.
Advanced research in wood mechanics fie ld, for example using N on Destructive Testing methods coupled with ana lytical tools, could help to guess an actual strength value (see Table 4 and Figure 6).
4 CONCLUSIONS
Wooden cultural heritage's most dangerous enemy today is insensibility, and lack of maintenance, of course. However there is another risk equally frightening that appears j ust when conservation works have been launched.
This sneaking enemy is the "Do something, anyway" philosophy. "This wood is toa old, it has lost its strength!" or "thi s element is going to fali down on us, it does not satisfy the last co de on loads! " and so 011, are typical examples, but many others could be given, that may lead to unnecessary reinforcement to the detriment of cultura l authenticity.
Wood technologists and structural engineers have the privilege ofmastering evaluation tools and analytical models that can help professionals in making the best decision in various circumstances, so that a new philosophy, with more respect and more knowledge wi ll take over.
Figure 3a.
Figure 3b.
TIMPANO LATO EST
Figure 3c.
DEFORMAZIONE E SFORZO NORMALE PER VENTO DA NORD NELL'IPOTESI DI MURATURA NON COLLABORANTE
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Figure 3a- 3e. Simple models are very important in determining rapidly, even in a rough way lhe load distribution wilhin the structure. This approach is usually conservative, because does not take into account possible re-distribution effects. More advanced models allow a finer evaluation of load distribulion and give a better load-path wilh a reduction of peak forces.
47
EVOLUZIONE DELLA MODELLAZlONE
T1MPANO LATO OVEST
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Figure 4a-4e. In some cases when the mechanical behaviour is very complicated to determine, tests should be performed, andlor highly sophisticated models used. This does faci litate verification and allows the adoption ofvery essential strengthening solutions. Tables 5a and 5b are a synopsis Df the Master thesis Df Ms Angela Bevilacqua on "Strengthening Df 16th Century wood-masonry building in Wismar", 1998, University Df Florence, Italy.
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Figure 5a. Figure 5b.
Figure 5c. Figure 5d.
Figure 5a- d. Panchia (Trento, Italy) bridge. Accurate mode lling coupled with in situ load tests allowed to identify the best way to preserve the cultural authenticity ofthe bridge without compromising users safety. To reduce car traffic induced bridge vibration the deck planks were simple inclined at 45° respect to traffi c direction.
Figure 6a. Figure 6b.
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Table 4.
Density Eo Fm Failure determining Beam (kg/m3) (N/mm2
) (N/mm2 ) defect(s)
T I 480 8689 25.0 Ring shake, checks T2 483 8273 28.0 Ring shake, checks T3 516 18266 39.6 Checks T4 464 13048 25 .8 Ring shake, checks T5 487 12906 44.8 Slope of grain, knot T6 505 10355 33.9 Localised decay T7 464 8 102 30.9 Knots, diffuse decay T8 5 13 1401 3 47.0 Checks T9 498 11 747 38.5 Knots T IO 478 12243 29.4 Checks Ti l 469 7104 15.3 Di ffuse decay Tl 2 449 11 630 30. 1 Checks
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Table 4 and Figure 6a- 6b. Large cross sections beams show an interesting semi-ductile behaviour beca use knots are collapsing one afier the other allowing the beam to recover partially (if load sharing is permitted). In a series of results from Dr Marco Togni is shown that only I over 12 beams, discarded from the building site because considered not reliable, gave a strength low as 15 Mpa. Marco Togni (1995); "Elasticity and strength of large cross-section old timber beams: mechanica l evaluation with NDT in situ", Doctaral thesis, University afFlarence, Italy (in Italian).
REFERENCES
Kuipers,1. 1986. Effect of Age/or Laad on Timber Strength. In Proceedings ofCIB W 18 meeting, Paper 19-6-1. Florence, ltaly.
Ceccotti, A & Uzie lli , L. 1989. Reliabil ity of Ancient Timber Structures. In G. Tampone (ed) Proceedings ofIl National 1talian Congress on Wood Restauration. Firenze: Nardini (in Italian).
Uzielli, L. 1995. Restoring ti mber structures - Repair and strengthening. STEP 2 Timber Engineering, lecture D4. Centrum Hout, The Netherlands.
Bonamini , G. 1995. Restoring timber structures - Inspection and evaluation. STEP 2 Timber Engineering, lecture D3. Centrum Hout, The Netherlands.
Giordano, G. & Ceccotti, A. & Uzielli, L. 1999. Timber Engineering. Milano: Hoepli (in ltalian).
