I would like a lab report done for Soil Mechanics - geotechnical Civil Engineering Lab

Wood is a material commonly used in the United States and understanding the property of the material is very significant for design work. The three most important design properties an engineer must satisfy when using wood are; shear, deflection and bending stress. Depending on the length of the beam, deferent behaviors are observed. For instance, a short beam is likely to experience shear, a medium size beam is likely to experience bending stress, and a long beam is more likely to deflect. Long beams are more likely to cause serviceability concerns than shorter beams, but short beams are much more likely to cause a physical failure. In this laboratory a long wooden Western Douglas fir beam was experimented on by using a four-point quasi-static loading and a universal testing machine (UTM). The four-point quas-static loading gives a constant maximum moment, while still allowing the moment to be accurate when calculating the Modules of Rupture. The purpose of this lab was to find the flexure strength known as the MOR and the modulus of elasticity (MOE)

Eq.1

Where: M is the moment

c is the centroid of the cross section of the wood specimen

I is the moment of inertia

= Eq. 2

Where: E is the modulus of elasticity for the wood specimen

P is the maxium pressure applied to wood specimen to cause fracture.

δ is the deflection of the wood specimen while it is loaded.

I is the moment of inertia

x and a are the distance from the pin to the point of loading.

l is the total length from pin to pin of the wood specimen.

Figure 1 contains a visual representation of the quasi static loading experimental set up for the lab.

Table 1 shows the values of ultimate load, modulus of rupture, and modulus of elasticity for Doug fir. The lab was conducted four separate times on a 2 by 2 in. by 36 in. long specimen of Doug fir. The units for the ultimate load are in lb, M.O.R. is in units of psi and the modulus of elasticity for Doug fir is in units of ksi.

The force applied in lbs. and displacement in in. were both measured in lab and were placed into excel to determine the Modulus of Rupture and the Modulus of Elasticity. The displacement had a precise measurement of 0.001 in. while the force applied was measured to the nearest lb. The Modulus of Rupture was calculated using the moment, centroid, and the moment of inertia. To calculate the moment, the applied force was divided by two and multiplied by the distance. The Modulus of Rupture was calculated in psi and the values had an average of 12,227 psi for the Douglas Fir specimen which is very similar to the accepted value of 12,000 psi. The Modulus of Elasticity was calculated using the equation in the lab. The four values of Modulus of Elasticity had an average of 1,154 ksi compared the accepted value of 1,900 ksi for Douglas Fir. The calculated Modulus of Elasticity had a slight similarity to the accepted value but was not as close as the calculated and accepted values for the Modulus of Rupture. All these values can be seen in Table 1.

During the laboratory tests four-point quasi static loading was used instead of the three- point quasi static loading. This was to create an area of constant moment over the length of the beam without interpolation. The points where the loads were applied can be seen in Figure 1.

In conclusion this lab has introduced how to determine several flexural properties of a wood specimen under four-point quasi static loading. The first properties that were determined from loading the wood specimen was the ultimate load. This was acquired by applying pressure until the wood specimen fractures. Then a reading from the machine would give the ultimate load in pounds. The second property attained from the loading was the modulus of rupture and was calculated using equation 1. The last property determined was the modulus of elasticity and the values for the modulus were acquired by inserting the deflection, lengths, ultimate load, and inertia values of the beam into equation 2. The average value for the ultimate load was 917 lb, the modulus of rupture had an average value of 12.226 ksi, and the modulus of elasticity had a value of 1,154 ksi.

During the experiment some of the wood specimens were drier than some of the other tested specimens. The drier specimens could have exhibited a much more brittle state giving it a higher loading rate compared to the mildly moist wood specimens that would have displayed a ductile state. The ductile pieces held less loading before rupture. The test could be done very accurately if all of the specimens had the same consistency involving the wet state of the wood.

Hibbeler, R. C. Engineering Mechanics. 12th ed. Upper Saddle River, NJ: Prentice Hall, 2010. Print.

Table 2: Deflection of Doug fir.

Sample Calculations for the Ultimate load of Doug Fir: