# Compressive Strength Characteristic Values of Nine Structural Sized Malaysian Tropical Hardwoods

^{1}

^{2}

^{3}

^{4}

^{*}

## Abstract

**:**

^{3}. A verification of the equation used in EN 384 to determine compressive strength characteristic value yields a different equation, ${f}_{c,0,k}=2.2{\left({f}_{m,k}\right)}^{0.7}$. This shows that the EN 384 equation is not suitable to be used with hardwood timber with a density more than 700 kg/m

^{3}, since it will underestimate the strength value.

## 1. Introduction

## 2. Materials and Methods

#### 2.1. Materials Preparation

#### 2.2. Compressive Strength Properties Evaluation for Parallel and Perpendicular to the Grain

_{max}, was reached within 300 ± 120 s. Each configuration’s time to failure was documented and reported. To measure the deformation, two (2) Linear Variable Displacement Transducers (LVDT) were placed at a central gauge length four times the smaller cross-sectional dimension for specimen parallel to the grain, and at the 0.6 h gauge length located centrally in the specimen’s height for specimen perpendicular to the grain. Universal Testing Machine (UTM) (AUTOMAX-T, CONTROLS, Milan, Italy) with a capacity of 2500 and 450 kN was utilised to evaluate structural size specimens parallel and perpendicular to the grain, respectively. Figure 2 and Figure 3 illustrate the test setup’s specifications to evaluate the compressive strength properties for samples both parallel and perpendicular to the grain.

_{max}= maximum load (N), A = cross sectional area (mm

^{2}).

_{1}= Gauge length for the determination of MOE, f

_{2}− f

_{1}= Increment of load on the straight-line portion of the load-deformation curve (N), w

_{2}− w

_{1}= Increment of deformation corresponding to f

_{2}− f

_{1}(mm).

_{c}

_{,90,max}= maximum compressive strength determined using the iterative process, b = Width of the cross section or smaller dimension of specimen (mm), ℓ = Length of cross section or larger dimension of specimen (mm).

_{40}− f

_{10}= Increment of load on the straight-line portion of the load deformation curve (N). f

_{10}is the 10% and f

_{40}is the 40% of f

_{c}

_{,90,max,est}, w

_{40}–w

_{10}= The increment of deformation corresponding to f

_{40}− f

_{10}(mm), h

_{0}= gauge length (mm), b = Width of the cross section or smaller dimension of specimen (mm), ℓ = Length of cross section or larger dimension of specimen (mm).

#### 2.3. Evaluation of Compressive Strength Characteristic Value

#### 2.4. Statistical Analysis

## 3. Results and Discussion

#### 3.1. Compressive Strength Properties

^{2}= 0.62 and R

^{2}= 0.78 for compressive strength parallel and perpendicular to the grain, respectively). According to the findings, the relationship between density and compressive strength perpendicular to the grain is stronger than the relationship between density and compressive strength parallel to the grain. As the density of the wood increased from 400 to 1200 kg/m

^{3}, the compressive strength parallel and perpendicular to the grain increased three-fold and nine-fold, respectively. The results show that compressive strength perpendicular to the grain is more sensitive to the changes in wood density, which is likely due to its inherent low value when compared to compressive strength parallel to the grain. As a result, even minor changes in density have a significant impact on compressive strength.

#### 3.2. Characteristic Value of Compressive Strength Properties

_{c}

_{,0,k}and MOE, E

_{c}

_{,0,k}which are 43.9 and 22,647 MPa, respectively, even though Kempas does not possess the highest characteristic density (716 kg/m

^{3}). This was followed by Balau (38.9 MPa), Resak (37.6 MPa), Kapur (32.3 MPa), Kelat (32.2 MPa), Keruing (28.1 MPa), Mengkulang (26.9 MPa), Light Red Meranti (19.7 MPa) and Geronggang (18.2 MPa). The characteristic value of MOE varies between 10,795 to 22,647 MPa with the highest MOE for Kempas and the lowest for Geronggang. For characteristic density, Resak shows the highest value while Light Red Meranti has the lowest, which are 813 and 361 kg/m

^{3}respectively.

_{c}

_{,0,k}, MOE, E

_{c}

_{,0,k}and density, ρ

_{k}of the timber species with those specified in EN 338 [11] and MS544: Part 3 [6] for the strength classes assigned to each timber species. Strength classes for timbers from many parts of the world, including five species of tropical hardwood from Southeast Asia, namely Balau, Kempas, Kapur, Merbau, and Keruing, are provided in EN 1912 [43], but the strength properties are provided in EN 338 [11], with compressive strength properties in this standard derived from the equation provided in EN 384 [15]. These test results were also compared to the values provided in MS544: Part 3 [6], which provides structural strength data for many Malaysian hardwood timber species, including Mengkulang and Light Red Meranti. Because the strength data in MS544: Part 3 [6] are in grade stresses that adopt the data of tropical hardwoods accessible in BS 5268: Part 2 [5], hence the characteristic values were taken from EN338 [11] but followed the similar strength class in MS544: Part 3 [6]. MS 544: Part 3 [6] strength data for Balau, Kempas, Kapur, and Keruing are equivalent to EN 338 [11], which was preceded by BS 5268: Part 2 [7]. Due to lacking of published data, the comparison can only be done on six of the nine species: Balau, Kempas, Kapur, Keruing, Mengkulang, and Light Red Meranti. The properties of the other 3 species are not available in the standards. Therefore, only the six available timber species are listed for comparison purposes and to show the discrepancy among of strength values. It is to highlight the importance to derive the characteristic compressive strength value of other Malaysian hardwoods from experimental results.

^{3}, respectively, which are 33, 36, and 2% greater than the published values in EN 338 [11], and these values are the highest among the five species. The typical compressive strength, MOE, and density of Balau, which is categorised as D50, are 27, 17, and 30% greater than EN 338 [11]. This finding is consistent with Hannouz et al. [44], who investigated the mechanical properties of European hardwood ash wood (Fraxinus excelsior L.) and discovered that the value obtained through experimental work is greater than the value obtained using the formula in EN 338 [11]. Obinna Osuji and Inerhunwa [45] and Gamper [46] similarly stated that the test value is bigger than the values derived using the EN 338 [6] calculation. The typical strength (32.3 MPa) and MOE (17.4 GPa) for Kapur demonstrate no significant difference with EN 338 [11] values of 33 MPa and 17 GPa, respectively. Keruing’s characteristic strength of 28.1 MPa is somewhat lower than the 30 MPa stated in EN 338 [11], although it has 19% and 9% higher values in the EN 338 [11] for its characteristic MOE and density, respectively.

^{3}were significantly understated in EN 338 [11]. As a result, the data acquired in this study are more relevant to be used in order to build safer and more economical timber structures, as this strength data are derived from actual structural size specimens rather than an approximated value using an equation. The compressive strength properties of the species with densities less than 700 kg/m

^{3}indicate no significant difference.

#### 3.3. Verification of Equation in EN338

_{c}

_{,k}can be calculated using the Equation in Table 2 [15] using the bending strength properties, f

_{m}

_{,k}which are called the “basic values” in determining the characteristic values of other mechanical properties and the strength class of timber given in EN 338 [11]. The equation given in EN 384 [15] to derive the characteristic compressive strength parallel to the grain is ${f}_{c,0,k}=4.3{\left({f}_{m,k}\right)}^{0.5}$. However, this equation might not be appropriate to be used for tropical hardwoods, specifically Malaysian timbers, as it is derived from softwoods and some European hardwoods which are temperate timbers with their densities ranging from 200 to 1000 kg/m

^{3}, while the densities of Malaysian hardwoods ranged from 300 kg/m

^{3}to more than 1200 kg/m

^{3}. Due to limited data of hardwood timbers from other countries, this standard has limitations for hardwoods where some clauses specially mention that the modification is only applicable for softwoods and European hardwood timbers. According to the results in Table 4, the compressive strength characteristic values obtained from the experimental work for timbers with a density of 700 kg/m

^{3}are higher than the values given in EN 338 [11]. As a result, the equation supplied in EN 384 [15] must be tested to determine if it is suitable to be used with tropical timbers, particularly high-density timbers. In order to determine the relationship with the compression characteristic, this equation must be verified using the characteristic bending strength. Thus, the typical bending strength was adapted from Baharin [47], who investigated the bending strength attributes of the same Malaysian tropical hardwood species.

^{3}. On the other hand, the experimental characteristic values for the species with densities of less than 700 kg/m

^{3}, namely, Mengkulang, Light Red Meranti, and Geronggang, demonstrate an insignificant difference between their values and the EN 338 [11] of 7, 3, and 7%, respectively.

^{3}because it underestimates the strength of the timber, making structural timber design uneconomical.

## 4. Conclusions

- The compressive strength of the timber specimens was influenced by their size. In some cases, the compressive strength of the specimens decreased as the size of the samples increased from 75 × 150 × 450 to 100 × 150 × 600 mm.
- The grain direction has a substantial influence on compressive characteristics, with all specimens examined parallel to the grain having a higher compressive strength and MOE than specimens tested perpendicular to the grain. The compressive performance of Kempas (SG2) is the highest than the other species studied in this study.
- Compressive strength and stiffness were positively correlated. Meanwhile, density also exerts substantial effect on the compressive strength of the timber specimens.
- With exception of Keruing, the compressive characteristic values for other species are higher than the values stipulated in EN 338 [6] for corresponding strength classes, notably for hardwood timber, with a density greater than 700 kg/m
^{3}. - An equation that differed than the one given in EN 384 [15] was developed in this study for the determination of compressive strength characteristic values. The equation developed was ${f}_{c,0,k}=2.2{\left({f}_{m,k}\right)}^{0.7}$. Based on this equation, it was revealed that the equation in EN 384 [15] is only suitable for low-density timber such as Mengkulang, Light Red Meranti, and Geronggang. The equation stipulated in EN 384 [15] is unsuitable for timber with densities higher than 700 kg/m
^{3}because it underestimates the strength of the timber, making structural timber design uneconomical.

## Author Contributions

## Funding

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

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**Figure 4.**Relationship between compressive strength parallel to the grain and modulus of elasticity for selected Malaysian tropical hardwood timber.

**Figure 5.**Relationship between compressive strength perpendicular to the grain and the modulus of elasticity for selected Malaysian tropical hardwood timber.

**Figure 6.**Relationship between compressive strength parallel to the grain and density for selected Malaysian tropical hardwood timbers.

**Figure 7.**Relationship between compressive strength perpendicular to the grain and density for selected Malaysian tropical hardwood timbers.

**Figure 8.**Relationship between experimental and derivative compressive and bending strength characteristic values.

Species | Air-Dry Density (kg/m^{3}) ^{1} | Strength Group (SG) ^{2} | Grain Direction | Dimension (mm) | Number of Specimens | Loading Rate (mm/s) |
---|---|---|---|---|---|---|

Balau | 850–1155 | SG 1 | Parallel | 100 × 150 × 600 | 100 | 0.023 |

75 × 150 × 450 | 100 | 0.02 | ||||

Perpendicular | 45 × 70 × 90 | 200 | 0.009 | |||

Kempas | 770–1120 | SG 2 | Parallel | 100 × 150 × 600 | 100 | 0.023 |

75 × 150 × 450 | 100 | 0.02 | ||||

Perpendicular | 45 × 70 × 90 | 200 | 0.009 | |||

Kelat | 495–1010 | SG 3 | Parallel | 100 × 150 × 600 | 100 | 0.022 |

75 × 150 × 450 | 100 | 0.023 | ||||

Perpendicular | 45 × 70 × 90 | 200 | 0.008 | |||

Resak | 655–1155 | SG 4 | Parallel | 100 × 150 × 600 | 100 | 0.023 |

75 × 150 × 450 | 100 | 0.02 | ||||

Perpendicular | 45 × 70 × 90 | 200 | 0.008 | |||

Kapur | 575–815 | SG 4 | Parallel | 100 × 150 × 600 | 100 | 0.023 |

75 × 150 × 450 | 100 | 0.019 | ||||

Perpendicular | 45 × 70 × 90 | 200 | 0.008 | |||

Keruing | 690–945 | SG 5 | Parallel | 100 × 150 × 600 | 100 | 0.022 |

75 × 150 × 450 | 100 | 0.018 | ||||

Perpendicular | 45 × 70 × 90 | 200 | 0.009 | |||

Mengkulang | 625–895 | SG 5 | Parallel | 100 × 150 × 600 | 100 | 0.022 |

75 × 150 × 450 | 100 | 0.018 | ||||

Perpendicular | 45 × 70 × 90 | 200 | 0.009 | |||

Light Red Meranti | 385–755 | SG 6 | Parallel | 100 × 150 × 600 | 100 | 0.02 |

75 × 150 × 450 | 100 | 0.016 | ||||

Perpendicular | 45 × 70 × 90 | 200 | 0.01 | |||

Geronggang | 350–610 | SG 7 | Parallel | 75 × 125 × 450 | 100 | 0.02 |

50 × 125 × 300 | 100 | 0.016 | ||||

Perpendicular | 45 × 70 × 90 | 200 | 0.008 | |||

Total number specimens | 3600 |

**Table 2.**Compressive strength properties at 12% moisture content of selected Malaysian tropical hardwood timber with different sizes loaded under different grain directions.

Species | Grain Direction | Size (mm) | n | Compressive Strength (MPa) | MOE (MPa) |
---|---|---|---|---|---|

Balau (SG1) | ∥ | 100 × 150 × 600 | 100 | 55.3 (15.2) ^{c} | 16,809 (14.2) ^{f,g} |

∥ | 75 × 150 × 450 | 100 | 54.4 (13.7) ^{d,e} | 16,118 (15.7) ^{h,i} | |

⊥ | 45 × 70 × 90 | 200 | 14.7 (20.3) ^{n} | 1377 (31.6) ^{l} | |

Kempas (SG2) | ∥ | 100 × 150 × 600 | 100 | 62.6 (14.7) ^{b} | 21,985 (12.4) ^{b} |

∥ | 75 × 150 × 450 | 100 | 63.4 (14.0) ^{a} | 22,580 (17.8) ^{a} | |

⊥ | 45 × 70 × 90 | 200 | 12.1 (23.3) ^{o} | 1167 (30.8) ^{l,m} | |

Kelat (SG3) | ∥ | 100 × 150 × 600 | 100 | 46.4 (11.0) ^{f} | 18,388 (15.9) ^{d} |

∥ | 75 × 150 × 450 | 100 | 43.3 (14.8) ^{g,h} | 17,853 (24.0) ^{e} | |

⊥ | 45 × 70 × 90 | 200 | 10.1 (26.0) ^{p} | 1371 (45.1) ^{l} | |

Resak (SG4) | ∥ | 100 × 150 × 600 | 100 | 54.1 (15.2) ^{e} | 22,300 (17.2) ^{a,b} |

∥ | 75 × 150 × 450 | 100 | 55.8 (14.0) ^{c,d} | 20,422 (19.2) ^{c} | |

⊥ | 45 × 70 × 90 | 200 | 17.7 (26.7) ^{m} | 1638 (47.5) ^{l} | |

Kapur (SG4) | ∥ | 100 × 150 × 600 | 100 | 41.4 (8.8) ^{i} | 17,646 (22.1) ^{e} |

∥ | 75 × 150 × 450 | 100 | 43.9 (12.6) ^{g} | 16,953 (19.1) ^{e,f} | |

⊥ | 45 × 70 × 90 | 200 | 5.3 (24.3) ^{r} | 532 (40.4) ^{m,n} | |

Keruing (SG5) | ∥ | 100 × 150 × 600 | 100 | 42.5 (17.1) ^{h.i} | 16,401 (16.5) ^{g,h} |

∥ | 75 × 150 × 450 | 100 | 44.5 (16.9) ^{g,h} | 16,791 (19.7) ^{f,g} | |

⊥ | 45 × 70 × 90 | 200 | 6.3 (31.9) ^{q,r} | 615 (61.0) ^{m.n} | |

Mengkulang (SG5) | ∥ | 100 × 150 × 600 | 100 | 39.1 (11.1) ^{i} | 16,401 (21.3) ^{i} |

∥ | 75 × 150 × 450 | 100 | 37.4 (14.4) ^{j} | 16,791 (25.6) ^{h,i} | |

⊥ | 45 × 70 × 90 | 200 | 7.5 (22.5) ^{q} | 691 (43.3) ^{m,n} | |

Light Red Meranti (SG6) | ∥ | 100 × 150 × 600 | 100 | 31.3 (16.6) ^{k} | 11,057 (15.6) ^{j} |

∥ | 75 × 150 × 450 | 100 | 28.1 (12.4) ^{l} | 10,768 (13.5) ^{j,k} | |

⊥ | 45 × 70 × 90 | 200 | 3.3 (21.9) ^{s} | 251 (34.0) ^{n} | |

Geronggang (SG7) | ∥ | 75 × 125 × 450 | 100 | 28.1 (11.1) ^{l} | 10,180 (20.2) ^{k} |

∥ | 50 × 125 × 300 | 100 | 26.1 (14.1) ^{l} | 11,013 (15.6) ^{j,k} | |

⊥ | 45 × 70 × 90 | 200 | 3.4 (36.9) ^{s} | 377 (60.1) ^{n} |

**Table 3.**Characteristic values of compressive strength parallel to the grain, modulus of elasticity and density for nine species of selected Malaysian Hardwood timber.

Compressive Strength (MPa) | Modulus of Elasticity (MPa) | Density (kg/m^{3}) | ||||
---|---|---|---|---|---|---|

Species | f_{c}_{,0,12} | f_{c}_{,0,k} | E_{c}_{,0,mean} | E_{c}_{,0,k} | ρ_{mean} | ρ_{k} |

Balau | 54.7 | 38.2 | 16,439 | 16,786 | 912 | 805 |

Kempas | 62.9 | 43.9 | 22,180 | 22,647 | 879 | 716 |

Kelat | 44.9 | 32.2 | 18,109 | 18,491 | 887 | 731 |

Resak | 54.1 | 37.6 | 21,132 | 21,567 | 992 | 813 |

Kapur | 43.0 | 32.3 | 17,383 | 17,749 | 782 | 655 |

Keruing | 43.5 | 28.1 | 16,588 | 16,937 | 868 | 674 |

Mengkulang | 37.5 | 26.9 | 15,698 | 15,563 | 663 | 541 |

Light Red Meranti | 29.5 | 19.7 | 10,913 | 11,143 | 488 | 361 |

Geronggang | 26.9 | 18.2 | 10,572 | 10,795 | 557 | 445 |

_{c}

_{,0,12}= Mean compressive strength at 12% moisture content; f

_{c}

_{,0,k}= Characteristic value of compressive strength, E

_{c}

_{,0,mean}= Mean modulus of elasticity at 12% moisture content; E

_{c}

_{,0,k}= Characteristic value of modulus of elasticity; ρ

_{mean}= Mean density at 12% moisture content; ρ

_{k}= Characteristic value of density.

**Table 4.**Comparison of strength classes specified in EN 338, MS 544: Part 3 with experimental characteristic values.

Strength Class | f_{c}_{,0,k} (MPa) | E_{c}_{,0,k} (GPa) | ρ_{k} (kg/m^{3}) | |
---|---|---|---|---|

EN 338 [11] | ||||

Balau | D50 | 30.0 | 14.0 | 620 |

Kempas | D60 | 33.0 | 17.0 | 700 |

Kapur | D60 | 33.0 | 17.0 | 700 |

Keruing | D50 | 30.0 | 14.0 | 620 |

MS 544: Part 3 [6] | ||||

Mengkulang | D40 | 27.0 | 13.0 | 550 |

Light Red Meranti | C22 | 20.0 | 10.0 | 340 |

Experimental Value | ||||

Balau | 38.2 | 16.4 | 805 | |

Kempas | 43.9 | 22.2 | 716 | |

Kapur | 32.3 | 17.4 | 655 | |

Keruing | 28.1 | 16.6 | 674 | |

Mengkulang | 26.9 | 15.7 | 541 | |

Light Red Meranti | 19.7 | 10.9 | 361 |

_{c}

_{,0,k}= Characteristic value of compressive strength; E

_{c}

_{,0,k}= Characteristic value of modulus of elasticity; ρ

_{k}= Characteristic value of density.

**Table 5.**Experimental characteristic values of compressive and bending strength with the values in EN 338 [11] for the respective strength class.

Species | Strength Class | f_{m}_{,k} (MPa) | EN 338: 2016 | f_{c}_{,0,k} (MPa) | ρ_{mean} (kg/m^{3}) |
---|---|---|---|---|---|

Balau | D55 | 55.0 | 32 | 38.2 | 912 |

Kempas | D50 | 50.4 | 30 | 43.9 | 879 |

Kelat | D40 | 44.6 | 27 | 32.2 | 887 |

Resak | D45 | 46.8 | 29 | 37.6 | 992 |

Kapur | D45 | 46.8 | 29 | 32.3 | 782 |

Keruing | D45 | 45.4 | 29 | 28.1 | 868 |

Mengkulang | D35 | 44.6 | 25 | 26.9 | 663 |

Light Red Meranti | C20 | 23.9 | 19 | 19.7 | 488 |

Geronggang | C16 | 17.9 | 17 | 18.2 | 557 |

_{m}

_{,k}= Experimental bending characteristic strength; f

_{c}

_{,0,k}= Experimental compressive characteristic strength; ρ

_{mean}= Experimental mean density.

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**MDPI and ACS Style**

Azmi, A.; Ahmad, Z.; Lum, W.C.; Baharin, A.; Za’ba, N.I.L.; Bhkari, N.M.; Lee, S.H.
Compressive Strength Characteristic Values of Nine Structural Sized Malaysian Tropical Hardwoods. *Forests* **2022**, *13*, 1172.
https://doi.org/10.3390/f13081172

**AMA Style**

Azmi A, Ahmad Z, Lum WC, Baharin A, Za’ba NIL, Bhkari NM, Lee SH.
Compressive Strength Characteristic Values of Nine Structural Sized Malaysian Tropical Hardwoods. *Forests*. 2022; 13(8):1172.
https://doi.org/10.3390/f13081172

**Chicago/Turabian Style**

Azmi, Anis, Zakiah Ahmad, Wei Chen Lum, Adnie Baharin, Nurul Izzatul Lydia Za’ba, Norshariza Mohamad Bhkari, and Seng Hua Lee.
2022. "Compressive Strength Characteristic Values of Nine Structural Sized Malaysian Tropical Hardwoods" *Forests* 13, no. 8: 1172.
https://doi.org/10.3390/f13081172