Compressive strength parallel to the fiber of spruce with high moisture content
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Abstract
The research aimed to derive substantiated reduction factors for compressive strength and modulus of elasticity parallel to the fiber for structural sized spruce and fir (round) wood with moisture contents at and beyond fiber saturation. Such values, which at present are not given in Eurocodes 5 and 7, are essential in order to perform a reliable design of water/soil immersed piles as well as structural assessment of piles in embankments and foundations of historic, cultural heritage buildings and bridges. The experimental investigations, preceded by a comprehensive literature evaluation, comprised a total of 17 pairs of matched structural sized spruce (Picea abies) pile specimens with midlength diameters of about 200 mm, with each pair being cut successively from individual logs. From each pair, one specimen was tested in the dry state (MC = 12%), and the other in the wet/green state (MC_{mean} = 90%) according to EN 408 (2010). Further, for calibration purposes, matched small clear specimens with two different aspect ratios were sampled close to the sections from which the structural sized piles were cut. At the mean strength level, the pile compression tests parallel to the fiber gave a moisture reduction factor of \(k_{moist, green, mean}\) = 0.57, conforming well with the condensed result (0.54) of the literature evaluation on clear wood data, whereas the conducted matched clear wood reference tests gave a 15 % lower reduction factor. The moisture reduction at the 5 % quantile level of the pile compressive strength \(k_{moist, green, 05}\) = 0.65 was less expressed, and 30 % lower than that obtained for the small clear specimens. The moisture modification factor for the modulus of elasticity in compression parallel to fiber from dry to green state was found to be 0.89 for the pile specimens, which conforms closely to literature data on small clear specimens. Based on the experimental results for the moisture reduction and the present implicit provisions of EC 5 on duration of load, a modification factor \(k_{mod}\;=\;0.65 \times 0.6\;=\;0.4\) is proposed for structural sized spruce/fir piles as well as sawn lumber subjected to permanent load in the water saturated state.
Keywords
Compressive Strength Fiber Saturation Wood Moisture Content Clear Wood Round Wood1 Introduction
Compressive strength represents the strength property of wood which is most affected by moisture content and hence has to be considered more than in the case of other strengths. In modern timber construction the most important practice relevant moisture conditions are in the range of 6 to 20 %. This covers the climate service classes (SC) 1 and 2, specified equally on the European (CEN) and international (ISO) levels (DIN EN 199511 2010; ISO 201521 2010). SC 1 relates roughly to heated indoor conditions and SC 2 to ventilated, sheltered outdoor conditions. In SC 3, which relates to structures which are fully exposed to weather or subjected to longer lasting, very high (>85 %) relative humidities, the average moisture typically does not exceed 22 to 26 % but can reach beyond fiber saturation in specific cases.
Wood was used in past constructions extensively as timber piles in fully water saturated applications. The soil and water submerged wooden piles still form the bases of many historic buildings and cultural monuments worldwide. In Europe, such historic wooden foundations are widespread, especially in the Netherlands, Italy and in Germany. In structural building conservation and renovation, an often occurring task consists in the assessment of the load carrying compression capacity of (historic) water saturated piles. Further design situations for piles with water contact are marinas and embankments.
Today’s European material and design standards do not cover strength and stiffness values of round wood or sawn lumber at fiber saturation level or above. On the national level, the Netherlands have standardized quality requirements for soil and water immersed piles (NEN 5491 1999) and design values for wet piles as well (NENEN 199511 NB 2011). The former German timber design code (DIN 10521 1988) contained provisions on strength depreciation of fully water immersed timber and, on the other hand on strength increases for round wood as compared to sawn timber with rectangular crosssections. The lack of acknowledged design provisions and data on wet timber pile strength in conjunction with a structural safety assessment of a 140 m span historic stone bridge over the Neckar river in Southern Germany, founded on spruce piles, gave rise to the presented investigation.
The paper aims to provide substantiated information on compressive strength and stiffness parallel to the fiber at different dimensions of European spruce (Picea abies) wood in the wet or green state, i.e. covering the moisture content at and beyond fiber saturation. It is reported on coherent tests and evaluations with small defect free (clear) cubic and fullsized round wood pile specimens.
2 Literature review on relationship of compressive strength and wood moisture content
Investigations into the moisturestrength relationship of wood have been one of the major research efforts since the use of wood in civil and aircraft engineering. Hereby, as indicated in the introduction, the main research interest focused on the effect of moisture in the hygroscopic range, below about 22 % moisture content. A lesser number of primarily older studies cover the whole wood moisture content range from ovendry material (\(u\;=\;0\;\%\)) to high degrees of moisture content at and above fiber saturation (30 to 80 %). The technical term—(wood) fiber saturation—is understood as the water content where in the absence of any free water in the cell lumina, the cell walls are saturated with chemically and physically bound water. Fibre saturation is not a discrete point, but rather a moisture range which may vary considerably depending on species, sap and heartwood, age, local soil and moisture conditions (Glass and Zelinka 2010). The fiber saturation range of spruce and fir stretches from 30 to 34 % (Kollmann 1955).
It is state of the art to assume that strength and stiffness properties do not change any more beyond fiber saturation, i.e. are independent of any free water in the cell lumina. Although this assumption holds true for most mechanical properties, literature results (see below) show that in case of compression parallel and perpendicular to fiber direction there is still some quantitatively nonnegligible strength decreasing effect up to moisture contents of about \(u= 50\;\%\). According to a literature review, the underlying reason for the further strength decrease is unknown. Here, in line with textbooks (Glass and Zelinka 2010) and design codes, the transition from the hygroscopic vs. the water saturated state of wood, also termed green state, is assumed in a rough simplification as a discrete point at 30 % moisture content, marking the lower bound for spruce/fir. As evident from values given below, a further strength degradation of about 7 % occurs between \(u=30\;\%\) and \(u=35\;\%\), roughly marking the upper fiber saturation limit of spruce/fir species. Between 35 to 50 % a further strength decrease of about 5 to 7 % can be obtained.
2.1 Present code provisions
2.1.1 European timber design code
2.1.2 US code provisions
The National Design Specification (NDS) for Wood Construction (ANSI/AWC NDS 2015) refers to ASTM D2899—12 (2012), which converts strengths and stiffnesses related to clear wood specimens to reference design values of piles and poles that are graded per ASTM D3200—74 (Reapproved 2012) (2012). The clear wood material properties in ASTM D2555–06 (Reapproved 2011) (2011), for 33 softwood species grown in the USA are given with mean values and standard deviations for the moist, i.e. “unseasoned” (\(u >\) 30 % = green) state. The compressive strengths parallel to the fiber for spruce and fir wood species, range from \(f_{c, 0, 50}=\) 15.0 MPa for Engelman Spruce to \(f_{c, 0, 50}=\) 19.6 MPa for Black Spruce.
In addition to the compressive strength and MOE properties in the green state, the ratios of green to dry (u = 12 %) clear wood properties \(\frac{f_{c, 0, green, clearwood}}{f_{c, 0, dry, clearwood}}\) and \(\frac{E_{0, green, clearwood}}{E_{0, dry, clearwood}}\) are given. For spruce and fir species grown in the United States, the ratios of compressive strengths vary from 0.47 for Subalpine Fir to 0.56 for Grand Fir. The MOE reduction factors—pointedly higher—span from 0.78 for White Fir to 0.86 equally for Balsam Fir and Black Spruce.
 \(k\;=\;1.645\)

5 %quantile of the normal distribution
 \(C_{cdol}\)

for duration of load (10 years) and safety
 \(C_{hv}\)

for height and for reduced variability
 \(C_d\)

for density (only Douglas fir and Pine)
 \(C_g\)

for grade characteristics
3 Review of published tests
3.1 Small and medium sized clear specimens
Figure 1 shows experimental relationships of compressive strength parallel to the fiber versus wood moisture content of European spruce and fir wood derived from literature. The moisture dependant strength values are given normalized to the strengths at moisture content u = 12 % (compare Eq. 8b). The majority of the curves is based on pioneering investigations performed in the middle of the 20th century (Roš 1936; Kühne et al. 1955; Kollmann 1955), where the effect of wood density and specimen size on strength and stiffness has not been considered as relevant as it is today and hence had not been reported throughout. Although the specimen dimensions are not specified explicitly in all above cited references, it can be stated that the crosssectional dimensions were in the range of 20 to 40 mm and the specimen height parallel to wood fiber and load direction was about 2 to 4 times the smaller crosssectional edge length. The specified graphs are mostly based on figures given in the references; for details see legend of Fig. 1 and Table 1.
Parameters of fits and the relative strength values at different moisture contents normalized to strength at reference moisture content (MC) = 12 %
Reference details^{a}  Reference strength  Moisture modification factors \(k_{moist, mean, green} (u)\)  

Species –  Moisture %  Density kg/m\(^3\)  \(\rho _0\) kg/m\(^3\)  A MPa  C MPa  \(B=b\) %  a –  c –  \(f_{c,0,dry}\) MPa  18 % –  24 % –  30 % –  50 % – 
Roš (1936), Fig. 20.  
sp^{b}  0;18; >30  300–500  400  54.5  16.1  0.076  1.342  0.423  38.1  0.79  0.66  0.57  0.46 
wf^{c}  0;18; >30  300–500  450  65.9  18.0  0.077  1.492  0.408  44.2  0.78  0.64  0.56  0.44 
Roš (1936)), Fig. 22.  
sp^{b}  0;18; >30  300–500  –^{d}  29.6  19.2  0.064  0.898  0.583  33.0  0.87  0.78  0.71  0.62 
Kollmann (1955), Fig. 652.  
sp^{b}  0–100  –^{d}  –^{d}  111.4  19.7  0.117  2.367  0.419  47.1  0.71  0.56  0.49  0.43 
Kühne et al. (1955), Fig. 9a.  
sp^{b}  0–160  400–450  400  61.1  18.1  0.090  1.571  0.465  38.9  0.78  0.65  0.57  0.48 
wf^{c}  0–160  400–450  450  88.6  24.0  0.118  1.948  0.528  45.5  0.76  0.64  0.58  0.53 
sp^{b,e}  6–22  330–660  400  95.0  19.8  0.126  2.329  0.485  40.8  0.73  0.60^{f}  0.54^{g}  0.49^{g} 
sp^{b,e}  6–22  330–660  450  100.7  18.7  0.104  2.116  0.393  47.6  0.72  0.57^{f}  0.49^{g}  0.40^{g} 
Average^{h}  –  –  425  75.9  19.2  0.097  1.758  0.463  41.9  0.77  0.64  0.56  0.48 
3.2 Full sized pile specimens
Results of comprehensive compressive strength tests with 57 watersaturated European spruce pile segments, investigated at the Dutch Houtinstituut (Timber Institute) TNO, are reported in Buiten and Rijsdijk (1982). The tests did not comprise matched dry pile specimens, which would allow a direct assessment of the moisture influence. The diameter (without bark) of the spruce specimens ranged from 265 to 540 mm, with an average of 460 mm. The length was throughout 900 mm resulting in a very stout specimen with an average slenderness of 2.0. The moisture content of the spruce specimens as evaluated from the report ranged from 60 to 180 %, and was therefor throughout (average: 110 %) well beyond fiber saturation.
The wood/pile quality was assessed by measurement of maximum knot diameters, and knot area ratio according to NEN 5491 (1999), i.e. sum of knots (largest diameter) over a length of 150 mm divided by the circumference. The maximum knot diameters were between 5 and 60 mm and the knot area ratio was in the range from 0.2 to 0.6. As a considerable number of specimens did not fully comply with the pile grade requirements specified in NEN 5491 (1999) where single knot diameter is the most relevant criterion, the strength evaluation was subdivided into two samples comprising (1) all specimens and (2) all standard conforming specimens. However, the strength differences between the full sample and the respective subsample were revealingly marginal. Hence, no correlation between the growth characteristics and the strength values was found.
The mean water saturated compressive strength (total sample \(=\) 57) thus became
\( f_{c, 0, mean, green}\;=\;{20.0\pm 2.2}\) MPa (COV = 11 %).
A characteristic wet (green) compressive strength value was then derived as (van de Kuilen 1994) \(f_{c, 0, k, green}\;=\;x_{mean}  t_{student} s \sqrt{1+n^{1}}\;=\;20.0  1.7 \cdot 2.2\;=\;16.3\) MPa. An evaluation according to EN 14358 (2007) would result in a very similar value of 16.0 MPa.
A direct conclusion on the moisture impact cannot be drawn from the test results. Nevertheless, the specified characteristic green pile strength value has been used to derive pile design compressive strength values according to the EC 5 format (Eq. 2). In order to derive a dry single pile compressive strength value, a multiplication factor of 1.21 was applied (van de Kuilen 1994), resulting in \(f_{c, 0, k, dry}\;=\;19.8\) MPa. The former Dutch single pile wet/green design compressive strength value of \(f_{c, 0, d}= 9.9\) MPa (NEN 6760 2008) was then derived by applying \(k_{mod}=0.6\) and \(\gamma _M\;=\;1.2\).
As mentioned previously the validity of the moisture modification factor of \(k_{moist, green}=1/1.21=0.82\) should be questioned, especially if this factor was indiscriminately applied to adjust dry compression strength parallel to grain values to wet values. In the van de Kuilen (1994) study, the reversed adjustment from wet to dry compression strength parallel to grain values yielded an overly conservative dry characteristic strength.
This is evident from the absolute level of the characteristic dry pile strength, which is lower than the characteristic compressive strength value \(f_{c, 0, k}\;=\;21\) MPa for sawn coniferous lumber of strength class C24 which assumingly was reached in the reported test sample.
The present Dutch design compression value for wet/green piles of \(f_{c, 0, d}\;=\;9.8\) MPa (NENEN 199511 NB 2011) for load combination permanent, which is slightly lower than the preEurocode value of NEN 6760 (2008), was validated (Jorissen 2007) by use of a calculated characteristic wet pile compressive strength value of \(f_{c, 0, k, green}\;=\;15.4\) MPa, which is slightly lower than the reported test value of 16.3 MPa. Hereby, the wet strength value was derived from the dry pile value by multiplication with factor \(k_{moist}=0.6\) derived from Eq. (7).
The above illustration of the different approaches on compressive strength design values of water saturated piles, irrespective of the similar final results, reveals the obvious necessity to resolve the issue of a \(k_{moist, green}\) modification factor applicable to structural sized timber including piles.
4 Materials, specimens, and test program
The conducted test program was designed to establish a consistent verification of the factor \(k_{moist, green}\) for small clear and fullscale round wood piles, at the mean and 5 % quantile level. In order to reveal the effect of moisture on strength between the 12 % moisture content and the green state, matched specimens (cut in a specific manner from individual logs, see below) were used at the small and the fullscale pile level.
4.1 Log material
The test material comprised 2 \(\times \) 21 log/stemsegments from individual trees which were purchased from a selected forest stand in Southern Germany, close to Stuttgart. The grade of the logsegments conformed to NEN 5491 (1999) with maximum knot diameters of 19 to 42 mm and knot area ratios of 0.11 to 0.31, hereby also conforming to Güteklasse II according to DIN 40742 (1958) and to Güteklasse B according to RVR (2015). The log segments, hereby known as “logs” only had a length of 3.5 m each and the average bottom and top diameters were \(d_{bottom, mean}\;=\;230 \; {\text{mm}}\) and \(d_{top, mean}\;=\;185 \; {\text{mm}}\), respectively. The felling of the trees had been undertaken one month prior to purchase; between felling and delivery to the Institute, the trees were stored unsheltered at the sawmill. The logs were received in the green state. After grading, 4 pairs were excluded due to excessive knots/bow, so 2\(\times \)17 pairs remained for full sized compression testing. One of the rejected logs was used for detailed moisture measurements.
4.2 Pile specimens
The aspect ratio of the \(2\times 17\) fullscale pile specimens tested in compression parallel to the fiber was chosen according to the respective provisions in the European test standard for structural sized lumber, EN 408 (2010). The length of the pile specimens was nominally 6 times the smallest diameter of the employed conical log sections. The diameters of the dry and wet pile specimens varied in the range of 170 to 255 mm at the bottom end and 162 to 241 mm at the top end. The average midlength diameters of the dry and wet tested specimens were 213 and 197 mm, respectively (see Table 4). The specimens tested in the wet state were taken throughout from the thinner, tree toporiented part of the stem section. This selection procedure, slightly violating the otherwise rigorously pursued matching procedures, is owed to the limited diameter of the pressurevacuumvessel that was used to increase the moisture of the wet logs (see below) before they were tested.
4.3 Small clear specimens
Two different geometries and slightly different sizes were investigated with regard to effect of moisture on strength. One set of small clear specimens conformed to the aspect ratio provisions given in EN 408 (2010) where the crosssectional dimensions were 20 mm \(\times \) 20 mm perpendicular to the fiber direction and the length was 120 mm. The other set of specimens conformed to the previous German standard DIN 52185 (1976) for small clear specimens where the much smaller aspect ratio of 2 resulted from a crosssection of 30 mm \(\times \) 30 mm and a length of 60 mm.
The specimens were cut as shown schematically in Figs. 2 and 3 from each intermediate log slice in such a manner that matching was best possible between dry and wet specimens of each specific specimen type (DIN 52185 or EN 408). Further, the spatial arrangement of the specimens should allow for the assessment of the effect of the position within the stem crosssection on the moisture strength relationship. In order to enable this, one group of the specimens was cut from the heart wood area at a distance of 20 mm from the pith and the other group was cut from the sapwood region with a distance of 20 mm from the periphery. The matching of the dry and wet specimens according to EN 408 and DIN 52185 was performed as shown schematically in Figs. 2 and 3. Due to the shorter length of the DIN specimens (60 mm), the matching with regard to effect of moisture was performed by cutting both specimens from one longer stick (\(l\;=\;125 \; {\text{mm}}\)), enabling the highest possible similarity with regard to annual ring orientation within the crosssection. The matching of the longer sized EN 408 specimens with regard to the moisture effect was performed by cutting the specimens at the same radial distances \(r_i\;=\;20 \; {\text{mm}}\) and \(r_a\;=\;D/220\;{\text{mm}}\) at a circumferential angle distance of \(\pi /2\). The 21 logs together with the applied cutting scheme delivered for each geometry configuration (DIN 52185 and EN 408) 42 dry and 42 wet small clear specimens, whereby 50 % were from the heartwood and sapwood, respectively.
5 Conditioning and moisture content of specimens
The conditioning of the specimens with regard to the target moisture conditions was performed as follows. The small specimens used for tests in dry condition (nominal MC = 12 %), were stored for 12 weeks at a climate of 20 \({^{\circ }}\)C and 65 % rh, resulting in an average moisture content of 13.9 ± 0.3 % at test time. The dry pile specimens were first stacked in a climate chamber at 20 \({^{\circ }}\)C and 65 % rh for 25 weeks and subsequently subjected to a dryer (35 % rh) climate in the testing hall for 4 weeks. The pile specimens dried to an average MC of 12.5 ± 0.8 %, determined after testing from slab 5 at midlength (see below), whereby a moisture gradient of about 4 % along the crosssectional diameter remained.
The small and the pile specimens tested in wet condition, though having been cut from green stems, were vacuum pressure treated with cold water (10 \({^{\circ }}\)C) to obtain a higher degree of moisture uniformity over the crosssection. The water treatment comprised 20 successive cycles whereby each cycle consisted of a vacuum (absolute pressure of 20 kPa) held for 20 minutes, followed by 35 min of pressure treatment (absolute pressure: 500 kPa). This procedure resulted in very uniform moisture contents for the small specimens, with an average value of 192 % with variations of ±20 % over crosssectional dimensions.
In summary, it can be stated that the minimum moisture content of the pile specimens was in majority along pile length and crosssection beyond the chosen fiber saturation limit of 30 %. However, since the saturation level might be higher for some specimens and clear wood tests indicate that compressive strength drops after reaching the upper realistic saturation level of 35 %, the obtained strengths of the wet log specimens should be rather higher than lower compared to longtime water submerged piles.
6 Compression tests parallel to the fiber
6.1 Small clear specimens
Prior to testing, the weights and dimensions were measured to determine the density \(\rho _u\) at test time. The axial compression tests parallel to the fiber with the small clear specimens following the provisions in EN 408 (2010) and DIN 52185 (1976), were performed displacementcontrolled with a constant rate of crosshead motion of 4 mm/min in a computer controlled electromechanical test machine (type: Zwick/Roell Smart.pro 100 kN). The modulus of elasticity was not measured. The tests were conducted at about 20 \({^{\circ }}\)C; relative humidity (rh) was in the range of 40 to 55 %. Immediately after testing, all specimens were oven dried for determination of moisture content and densities \(\rho _0\) and \(\rho _{12}\).
6.2 Pile specimens
For all wet and dry pile specimens, the axial compression displacement was measured over a constant length of 660 mm in the center section of the specimen length by four LVDTs mounted at circumferential distances of \(\pi /2\) (see Figs. 7, 8a). Depending on the slightly varying log diameters, the displacement measurement length was in the range of about 2.5 to 3 times of the specimen diameter at midlength.
The tests were performed displacementcontrolled in accordance with EN 408 (2010) at a constant piston speed of 20 mm/min. Failure was obtained within 300 ± 120 s.
7 Test results for small clear specimens
7.1 Data and observations
Compilation of results for matched small clear spruce specimens tested in compression parallel to the fiber in dry and green state according to two different test standards (DIN 52185 and DIN EN 408)
Physical, mechanical property or ratio  Statistical quantity  Small scale clear wood specimens  

Dry (MC = 12 %)  Green (MC \(>\) 30 %)  
Specimen type  Specimen type  
DIN 52185  EN 408  DIN 52185  EN 408  
No. of specimens tested  42  42  42  42  
No. of specimens used for evaluation  35  38  35  38  
Moisture content in %  \(x_{mean}\) (COV in %)  13.9 (1.9)  13.9 (2.5)  200 (16)  181 (18) 
Compressive strength \(f_{c, 0}\) in MPa  \(x_{mean}\) (std)  44.3 (9.9)  41.6 (8.5)  21.9 (5.4)  20.1 (4.4) 
COV in %  22.4  20.3  24.7  22.0  
\(x_{min}\)  30.2  29.0  14.6  12.5  
\(x_{max}\)  72.4  60.7  36.4  31.2  
\(x_{05}\)  29.3  28.4  13.9  13.0  
Density \(\rho _{12}\) in kg/m\(^3\)  \(x_{mean}\) (std)  442 (75)  442 (58)  439 (64)  442 (60) 
COV in %  16.9  13.0  14.7  13.6  
\(x_{min}\)  349  334  353  348  
\(x_{max}\)  653  580  659  604  
\(x_{05}\)  326  347  337  344  
Compressive strength ratio \(f_{c, 0, EN}/f_{c, 0, DIN}\)  at \(x_{mean}\) level  0.94  0.95  
at \(x_{05}\) level  0.97  0.94 
The density results reveal that all test series had a closely conforming density distribution characterized by an average density \(\rho _{12}\) of about 440 kg/m\(^3\) and 5 %quantile values in the range of about 330 to 350 kg/m\(^3\). The compressive strength ratios \(f_{c, 0, EN}/f_{c, 0, DIN}\) reveal on average (dry and wet, \(x_{mean}\) and \(x_{05}\)level) roughly 5 % lower strengths for the EN specimen shape. This reflects the superimposed bending and buckling effects of the more slender EN specimens. For both specimen types (DIN, EN) the mean compressive strength of the specimens that were taken from the inner part of the crosssections was 83 % of the compressive strength of the outer specimens.
7.2 Evaluation of moisture impact
The revealed high correlation of \(f_{c, 0, green}\) and \(f_{c, 0, u=12 \; \%}\) of the almost defect free specimens suggests that the strength decreasing influence of the high moisture content is rather unaffected by the density of the timber. This is substantiated by a regression of the strength ratio \(f_{c, 0, green}/f_{c, 0, 12}\) vs. density \(\rho _{12}\) which gives no (\(R^2\;=\;0.2\)) correlation (Note: the EN 408 specimens revealed a slightly different relationship of \(f_{c, 0, u=12\%} = 9.32 + 1.14 f_{c, 0, green}\) with a considerably worse coefficient of correlation of \(R^2=0.67\), which is presumably due to the less closely matched specimens).
Ratios of compressive strength and modulus of elasticity parallel to the fiber at water saturated green state (\(u > 30\;\%\)) vs. dry state (\(u\;=\;12\;\%\)) of small defect free and structural sized pile/round wood specimens
Specimen size  Specimen type  MOR and MOE ratios  

\(k_{moist, green}\;=\;\frac{f_{c, 0, green}}{f_{c, 0, dry}}\)  \(\dfrac{E_{c, 0, green}}{E_{c, 0, dry}}\)  
\(x_{mean}\)  \(x_{05}\)  \(x_{min}\)  \(x_{mean}\)  
Small defect free  DIN 52185  0.49  0.47  0.48  – 
EN 408  0.48  0.46  0.43  –  
Structural sized  Round wood  0.57  0.66  0.60  0.89 
8 Test results for the pile specimens
8.1 Data and observations
Statistical evaluation of test results MOR, MOE and density of structural sized spruce pile specimens tested in axial compression at dry and wet (water saturated) state
Physical, mechanical property or ratio  Statistical quantity  Moisture state of specimen  Ratio  

Dry  Wet  Wet/dry  
No. of specimens tested  17  17  17  
Moisture content in %  \(x_{mean}\)  12.5  89.4  
Pile diameter at failure plane \(d_f\) in mm  \(x_{mean}\)  214  196  
Bottom/top diameter \(d_{bottom, top}\) in mm  \(x_{mean}\)  220/205  204/190  
Compressive strength \(f_{c, 0}\) in MPa^{a}  \(x_{mean}\)  30.8  17.6  0.57 
std(x)  6.1  2.3  
COV in %  19.8  13.1  0.66  
\(x_{05}\)  20.3  13.4  0.66  
\(x_{min}\)  21.9  13.3  0.60  
\(x_{max}\)  39.9  21.0  0.53  
Density \(\rho _{12}\) in kg/m\(^3\)  \(x_{mean}\)  453 (48)  447 (34)  0.99 
COV in %  10.6  7.5  0.71  
\(x_{min}\)  398  389  0.98  
\(x_{max}\)  566  550  0.97  
\(x_{05}\)  370  387  1.05  
MOE E _{ c } _{,0} ^{a} in GPa  \(x_{mean}\)  12.1  10.7  0.89 
std(x)  1.9  1.6  
COV in %  15.9  14.9  0.93  
\(x_{05}\)  8.6  7.9  0.92  
\(x_{min}\)  8.9  7.7  0.87  
\(x_{max}\)  15.0  14.3  0.95 
The values obtained for the 5 % quantile of compressive strength, mean modulus of elasticity and characteristic density of the dry round wood specimens \(f_{c, 0, 05}\;=\;21.5 \;{\text{MPa}}\), \(E_{c, 0, mean}\;=\;12.1 \;{\text{GPa}}\) and \(\rho _{12, 05}\;=\;350 \; {\text{kg/m}}^3\) conform very well with the numbers specified for strength class C24 of the European strength class standard EN 338 (2010) for structural timber, which designates values of \(f_{c, 0, k}\;=\;21 \;{\text{MPa}}\), \(E_{0, mean}\) = 11 GPa, \(\rho _{mean}\;=\;420 \; {\text{kg/m}}^3\) and \(\rho _k\;=\;350\) kg/m^{3} (Note: The standardized values refer implicitly to sawn timber with rectangular crosssections; for round wood no European strength class standard exists).
8.2 Evaluation of moisture impact
It should be kept in mind, however, that even higher reduction factors could result from future research.
9 Conclusion
The conducted work confirmed that a moisture modification factor of 0.8 as specified implicitly in the current Eurocode 5 for service class 3 conditions is too little of a reduction for the compression strength parallel to the fiber of spruce and fir in the very wet, fiber saturated state. The test results for clear wood—together with a comprehensive literature evaluation—gave a moisture reduction factor in the range of 0.5 to 0.55 at the mean strength level. The investigations with well matched wet and dry structural sized pile specimens revealed a somewhat less pronounced strength decrease of about 35 % vs. the dry state (MC = 12 %) on the 5 % quantile level. Hence, for design of wet, water saturated piles and sawn softwood lumber, a pure moisture modification factor of \(k_{moist, green}\;=\;0.65\) seems appropriate.
The superimposed impact of moisture and loading time on strength is considered in several design codes productwise. Based on the time effect for permanent duration of load, which can be derived from Eurocode 5 as \(k_{time}=0.6\), at present irrespective of moisture level, a modification factor \(k_{mod}\;=\;0.65 \cdot 0.6\;=\;0.4\) would result for lumber and piles subjected to permanent duration of load in the water saturated state. A value of this order of magnitude is proposed as a safe adjustment factor for a revised version of Eurocode 5. A more moderate value of \(k_{mod}\) in the range of 0.5 might be justifiable if proven by adequate experimental and theoretical approaches i.a. including FORM/SORM (First/Second Order Reliability Method) calibration.
Although the above is derivationwise rigorously confined to European spruce and fir, it may be assumed to hold for other softwood species like larch and Douglas fir as well. In the case of modulus of elasticity of fiber saturated clear and structural sized softwood timbers loaded in compression parallel to fiber, a reduction factor of 0.85 is recommended.
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