QUESTIONS AND ANSWERS
1. Research and tabulate the compressive strength of different wood in the Philippines used in the construction. Indicate the source.
Ø Working Stresses for Selected Species of Structural Light Framing Lumber:
| SPECIES | GRADE | COMPRESSION PARALLEL TO THE GRAIN(“Fc”) | COMPRESSION PERPENDICULAR TO THE GRAIN | MODULUS OF ELASTICITY |
| Southern Pine | Dense Select structural | 2 100 | 475 | 1 900 000 |
| | Select structural | 1 800 | 405 | 1 800 000 |
| | No.1 Dense | 1 700 | 475 | 1 900 000 |
| | No.1 | 1 450 | 405 | 1 800 000 |
| | No.2 Dense | 1 350 | 475 | 1 700 000 |
| | No.2 | 1 150 | 405 | 1 600 000 |
| | No.3 Dense | 1 000 | 575 | 1 500 000 |
| | No.3 | 800 | 475 | 1 500 000 |
| | Stud | 675 | 405 | 1 500 000 |
| Hem-Fir | Select structural | 1 300 | 405 | 1 500 000 |
| | No.1/appeaance | 1 050/1 250 | 405 | 1 500 000 |
| | No.2 | 825 | 405 | 1 500 000 |
| | No.3 | 500 | 405 | 1 400 000 |
| Douglas Fir-Larch | Select Structural | 1 600 | 625 | 1 200 000 |
| | No.1/Appearance | 1 250/1 500 | 625 | 1 800 000 |
| | No.2 | 1000 | 625 | 1 800 000 |
| | No.3 | 600 | 625 | 1 700 000 |
2. Cite the application of the results of this test.
Ø The result of this test can be used in determining which has the greater strength between parallel or across the grain. After the said test the students have found out that the wood parallel to the grain has greater strength
3. Sketch the different types of failure of wood under compressive stress.
Ø
Side view of failures in compression across the grain, showing crushing of blocks under bearing plate. Specimen at right shows splitting at ends.
End view of failures in compression across the grain, showing splitting of the ends of the test specimens.
Problems:
- Complete the table below and rank it according from highest to lowest compressive strength of the material:
| Construction Materials | Load | Area | Stress | Rank |
| Steel | 768.56 lbf | 6.0 in2 | 128.09 psi | |
| Plain concrete | 1497.17KN | 25 cm dia. | 30.50 MPa | |
| Plastic | 365.75 kgs | 0.35m x 0.25m | 41.006 Mpa | |
| Reinforced concrete | 3648 kg | 6.0” dia. x 12.0” | 20 kg/cm2 | |
| Wood | 4567.5N | 114.50 in2 | 345.32 psi | |
Ø
Solution:
- Steel
Stress (δ) = P =768.55 lbf = 128.09 psi
A 6.0 in2
- Plain Concrete
δ = P ; P= δ A P= (30.50 N/ mm2 )(250mm) 2 (π/4) = 1497.17KN
A
- Plastic
Stress (δ) = P = 365.75 kg (9.81 m/s)(1000N) = 41.006 Mpa
A (0.35m x 0.25m)(1000mm2/1m2)
- Reinforced concrete
δ = P ; P= δ A P= (20 kg/ cm2 )(15.24cm) 2 (π/4) = 3648 kg
A
6”= 15.24 cm 12”=30.48 cm
= 152.4 mm = 304.8 mm
- Wood
L = (4567.5 kg. m/s2)(100cm/1m)(1in/2.54cm)(2.2lb/1kg)
= 39539.37 lb.in/s2
345.32 lb/in2 = 39539.37 lb.in/s2
A A= 114.50 in2
2. Determine the compressive strength of a wooden column with 0.3m x 0.3m dimension if the applied load is 1450 kN. The maximum compressive strength of the wood sample is 20 GPa. Discuss if the wooden column can accommodate the given load.
P = 1450 kN
A = 0.09 m2
Solution:
δ = P = 1450 kN (1000N/1kN) = 16.11 MPa
A 0.09 m2 (1000mm2/1m2)
The wooden column can accommodate the given load (1450 kN) because its compressive strength is less than the maximum strength of the wood sample.
ANALYSIS AND INTERPRETATION OF DATA AND RESULT
In the experiment, testing the compressive strength of wood, which is the measurement of the largest compression force the material can withstand before it loses its shape or fails, the students used two samples (wood parallel and perpendicular to the grain). Compression parallel to the grain shortens the fibers in the wood lengthwise. Compression perpendicular to the grain shortens the fibers in the wood crosswise.
The compressive strength of the material would correspond to the stress at the red point shown on the graph. This linear region terminates at what is known as the yield point. Above this point the material behaves plastically and will not return to its original length once the load is removed.
The students observed that during the testing, the wood has shortened. The material tends to spread in the lateral direction and increases the cross sectional area. The first effect of compression across the grain is to compact the fibers, the load is irregularly increasing as the density of the material is increased. If the specimen lies on a flat surface and the load is applied to only a portion of the upper area, the bearing plate indents the wood, crushing the upper fibers without affecting the lower part.
The gathered data on the table show that the compressive strength of the wood being tested along the grain is greater than the compressive strength of the wood being tested across the grain. In the wood being tested along the grain, the deformation is smaller than the deformation of the wood across the grain; even it has greater loads applied on the wood.
The graph is also one of the proofs where in parallel, has lesser value of strains than in the perpendicular. A material is strong and tough if it ruptures under high forces and has high strains while materials with limited strain values are not tough. The first effect of compression across the grain is to compact the fibers, the load is irregularly increasing as the density of the material is increased. If the specimen lies on a flat surface and the load is applied to only a portion of the upper area, the bearing plate indents the wood, crushing the upper fibers without affecting the lower part.
CONCLUSION
The students conclude that the strength of wood parallel to the grain subjected under compressive load is greater than that of the strength of wood perpendicular to the grain subjected under compressive load.
SAMPLE COMPUTATIONS:
I. PARALLEL
Sample no.1
L= 151.15 mm
a= 78.25mm A= 5795 mm2
b= 76mm
FOR STRESS (MPa)
δ = force (P)/area (A)
δ 1= 9.64(1000)N/5424.32 mm2 (1000)
δ 1= 1.66 MPa
FOR STRAIN (%)
Є = deformation/length (100)
Є 1= 0.13 mm/151.15 mm (100)
Є 1= 0.09 %
I. PARALLEL
Sample no.2
L= 149.1mm
a= 76 mm A= 6011.6 mm2
b=79.1 mm
FOR STRESS (MPa)
δ = force (P)/area (A)
δ 1= 24.51 KN/6077.78 mm2 (1000) δ 1= 4.03 MPa
FOR STRAIN (%)
Є = deformation/length (100)
Є 1= 0.47 mm/149.1mm (100)
Є 1= 0.32 %
I. PERPENDICULAR
Sample no.1
L= 75.4mm
a= 149.45 mm A= 5835.96 mm2
b= 77.4 mm
FOR STRESS (MPa)
δ = force (P)/area (A)
δ 1= 5.23 KN/5835.96 mm2 (1000)
δ 1= 0.89
FOR STRAIN (%)
Є = deformation/length (100)
Є 1= 0.15 mm/75.4 mm (100)
Є 1= 0.20 %
I. PERPENDICULAR
Sample no.2
L= 76 mm
a= 151` mm A= 6011.6 mm2
b= 79.1 mm
FOR STRESS (MPa)
δ = force (P)/area (A)
δ 1= 18.40 KN/6011.6 mm2 (1000)
δ 1= 3.06 MPa
FOR STRAIN (%)
Є = deformation/length (100)
Є 1= .85 mm/76 mm (100)
Є 1= 1.12 %
SET-UP OF APPARATUS
Figure 1: The Universal Testing Machine while testing the wood sample
of its compressive strength along the grain.
b
a
Figure 2: (a) wood sample perpendicular to the grain subjected under compressive load.
(b) Wood sample parallel to the grain subjected under compressive load.
FINAL DATA SHEET
MATERIAL TESTING No. 3
COMPRESSIVE STRENGTH OF WOOD
I. Parallel or Along the Grain
SAMPLE 1
Length = 151. 15 mm Width = 78.25 mm Breadth = 76 mm Area = 5795 mm2
| Trial No. | Load (KN) | Deformation (mm) | Stress (MPa) | Strain (%) |
| 1 | 9.64 | .13 | 1.66 | .09 |
| 2 | 52.67 | .35 | 9.08 | .23 |
| 3 | 104.14 | .55 | 17.97 | .36 |
| 4 | 132.83 | .64 | 22.92 | .42 |
| 5 | 158.14 | .74 | 27.29 | .49 |
| 6 | 184.30 | .84 | 31.80 | .56 |
| 7 | 210.45 | .97 | 36.32 | .64 |
| 8 | 233.23 | 1.28 | 40.25 | .85 |
| 9 | 234.92 | 1..88 | 40.54 | 1.24 |
| 10 | 245.89 | 2.53 | 42.43 | 1.67 |
SAMPLE 2
Length = 149.1 mm Width = 76.00 mm Breadth =79.1 mm Area = 6011.6 mm2
| Trial No. | Load (KN) | Deformation (mm) | Stress (MPa) | Strain (%) |
| 1 | 28.55 | .47 | 3.06 | .32 |
| 2 | 57.01 | .57 | 9.91 | .38 |
| 3 | 84.55 | .66 | 5.26 | .44 |
| 4 | 113.92 | .74 | 6.57 | .50 |
| 5 | 142.38 | .82 | 7.92 | .55 |
| 6 | 170.83 | .91 | 9.19 | .61 |
| 7 | 200.21 | 1.02 | 10.5 | .68 |
| 8 | 227.74 | 1.13 | 11.8 | .76 |
| 9 | 264.48 | 1.46 | 12.7 | .98 |
| 10 | 262.62 | 1.94 | 12.0 | 1.30 |
II. Perpendicular or Across the Grain
SAMPLE 1
Length = 75.4 mm Width = 149.45 mm Breadth = 77.4 mm Area = 5835.96 mm2
| Trial No. | Load (KN) | Deformation (mm) | Stress (MPa) | Strain (%) |
| 1 | 5.23 | .15 | .89 | .20 |
| 2 | 10.64 | .34 | 1.82 | .45 |
| 3 | 15.81 | .51 | 2.71 | .68 |
| 4 | 21.12 | .62 | 3.62 | .82 |
| 5 | 26.20 | .81 | 4.49 | 1.07 |
| 6 | 31.78 | 1.12 | 5.45 | 1.49 |
| 7 | 36.85 | 1.78 | 6.31 | 2.36 |
| 8 | 38.71 | 2.85 | 6.63 | 3.78 |
| 9 | 40.74 | 4.23 | 6.98 | 5.61 |
| 10 | 42.09 | 5.25 | 7.21 | 6.96 |
SAMPLE 2
Length = 76.00 mm Width = 151.00 mm Breadth =79.1 mm Area = 6011.6 mm2
| Trial No. | Load (KN) | Deformation (mm) | Stress (MPa) | Strain (%) |
| 1 | 18.40 | .85 | 3.06 | 1.12 |
| 2 | 23.48 | 1.198 | 3.09 | 1.58 |
| 3 | 31.61 | 1.41 | 5.26 | 1.85 |
| 4 | 39.49 | 1.62 | 6.57 | 2.13 |
| 5 | 47.63 | 1.84 | 7.92 | 2.42 |
| 6 | 55.25 | 2.05 | 9.19 | 2.70 |
| 7 | 63.128 | 2.28 | 10.5 | 3.0 |
| 8 | 71.01 | 2.69 | 11.8 | 3.54 |
| 9 | 76.09 | 3.32 | 12.7 | 4.37 |
| 10 | 72.28 | 4.17 | 12.00 | 5.49 |
