4. Materials Testing
4.1 Discussion of Important Properties
To enable accurate computer modeling, specific material strength properties need to be determined for the individual components of the physical bridge model. The most relevant material property for the wood members is the modulus of elasticity (MOE). The MOE relates the linear deformation of wood to applied flexural loading. This property is specific to wood because of the very low shear stiffness that occurs in wood, roughly one-sixth of the axial stiffness [Criswell, 1982]. The low shearing modulus of wood can result in significant elastic deformation caused by shear stresses. In practical usage the MOE combines the deflection caused by flexural strains and the deflection caused by the shear deformation. Also it is important to evaluate the Young's Modulus (E) of the wood piles used. E relates the axial strains of a member to applied axial stresses. The difference between E and the MOE is that the former does not include shear deformation and the latter does.
The MOE value for each component was evaluated by conducting non-destructive beam tests. The stringers, pile caps, and cross ties were tested as simply supported beams, and the tie rails were tested as cantilever beams. The details of the beam tests are described in subsequent sections. Segments of the wood pile material were tested for their E values by conducting standard destructive compression tests. The specific procedure for the compression tests is discussed in the next section.
Testing for the E of the wood pile material provides the information needed to include the individual components of the foundation system in the computer model. Analytically modeling the pile interacting with the surrounding soil will enable future considerations of different soil material, not just the sand used in this research.
Eight samples of the pile material obtained from the five original wood poles were tested in compression. An ATS1660 Universal Testing Machine was used to conduct the pile material testing. The load and deformation values were recorded at 445 N (100 lb) increments until the sample failed. Failure was defined when severe cracking and a significant decrease in the sample's load carrying ability occurred. A stress vs. strain plot for each sample was created from the load vs. deformation data that was collected during the tests.
For the linear portion of the stress vs. strain data linear regression was used to create a "trend line." Each E value was determined as the slope of the linear regression "trend line." Table 4.1 displays the E and ultimate strength (sult) values obtained for each sample. The average E value for the pile material is approximately 630 ksi for compression parallel to grain.
|Pile Sample Name|
The MOE and E values for each member of the bridge model were evaluated using non-destructive flexural tests. The stringers, pile cap source beam, and fifteen 2.4 m (8 ft) long nominal 51 mm x 51 mm (2 in. x 2 in.) dimensioned lumber boards, with actual dimensions of 38 mm x 38 mm (1.5 in. x 1.5 in.), were tested as simply supported beams. MOE values for the tie rails were evaluated using a cantilever beam test because of the very small cross section to length ratio. The tie rail members each have a 16 mm x 16 mm (0.625 in. x 0.625 in.) cross-section and a 3.7 m (12 ft) length.
The simply supported beam test was used to evaluate the MOE and E of the stringers, pile caps and crossties. Figure 4.1 depicts the general test setup. This setup consists of applying equal point loads to a member at equal distances from the supports then measuring the deflection of each beam under the load. Actual load levels and span dimensions vary for each component type and are discussed subsequently.
The stringers were tested in weak axis bending to eliminate lateral torsional buckling effects. Nearly identical MOE values result from both weak axis and strong axis bending of wood beams [Criswell, 1982]. The other components have square cross sections. As illustrated in Figure 4.1, three string potentiometers were used to measure the deflection of each member along the span: one at mid span, and two equidistant from mid span, but between the applied loads. The calculation of MOE used the standard deflection equation for a simply supported beam under the two-point load configuration used in the test. The standard deflection equation was algebraically re-arranged to solve for the MOE. The modified equation is:
Δcl is the measured deflection at mid span relative to the supported ends. The first method calculates an MOE value where the shear deformation is included in the deflection measurement because the load configuration creates a shear diagram with constant shear between the supports and the point loads and no shear between the loads.
The second method estimated the E value for each component and was done by calculating the relative deflection of the beam between the intermediate deflection measurements and the mid-span deflection. Half of the middle segment of each beam was then treated as a cantilever beam with an applied point moment at the free end.
The application of the equal point loads at equal distances from the supports created a stress condition where the beam experienced zero shear stresses and constant flexural stresses between the applied loads. As before a standard beam bending equation was rearranged and used to calculate the E value. The modified equation is:
The MOE calculation method included shear deformation. The E calculation excluded shear effects due to the absence of shear stresses in the simple beam segment.
Each member type required specific load levels, and deflection measurement lactations. Figures 4.2, 4.3, 4.4 and 4.5 illustrate the specific setup dimensions for each of the 4 ft stringers, 8 ft stringers, crossties and pile caps respectively. Tables 4.2, 4.3 4.4 and 4.5 list the calculated values for each property and component type.
|Average 'MOE'||Average 'E'|
|Average 'MOE'||Average 'E'|
|Average 'MOE'||Average 'E'|
|1||13925||2020||14545||2110||T1, T18, T35 & T52|
|2||13591||1971||13883||2014||T9, T22, T36 & T48|
|3||13004||1886||15192||2203||T23, T37, & T50|
|4||7154||1038||7346||1065||Not Used (Too Weak)|
|5||14409||2090||16465||2388||T10, T24, T38 & T51|
|6||13491||1957||14703||2133||T11, T25 & T39|
|7||14741||2138||14753||2140||T12, T26 & T40|
|8||14653||2125||15382||2231||T13, T27 & T41|
|9||16324||2368||18161||2634||T2, T14, T28 & T42|
|10||14353||2082||14580||2115||T3, T15, T29 & T43|
|11||18324||2658||17965||2606||T4, T16, T30 & T44|
|12||12842||1863||12952||1879||T5, T17, T31, T45|
|13||11284||1637||10908||1582||T6, T19, T32 & T46|
|14||14737||2137||13375||1940||T7, T20, T33 & T47|
|15||14350||2081||15085||2188||T8, T21, T34 & T48|
Figure 4.6 illustrates the resulting MOE values for each stringer of the three-span semicontinuous specimen.
The tie rail members were tested for the MOE using a cantilever beam test. A standard bending equation was algebraically rearranged to solve for the MOE instead of the deflection. The rearranged equation is:
Using linear regression, the slope of each load vs. deflection plot was estimated with a "trend line." By definition the slope of these plots approximates the term 'dP/δΔ' in equation 4.3. Substituting the slope values of the "trend line" into equation 4.3 resulted in the approximate MOE value for each tie rail component. Table 4.6 displays the calculated MOE value for each tie rail,
All of the beam tests resulted in similar values for the MOE and E value of each member. Having these two methods of measuring each member's stiffness result in similar values corresponds to very little shear deformation being experienced within these members under load. This was expected because all the members have a very large span-to-depth ratio. Large span-to-depth ratios indicate that a smaller portion of the applied load is resisted by shear stresses.