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Sign Support Design Project

Team Hydra

Chris Byrne, Michiko Matsumoto Li, Kevin Stewart, Aoyong Tan, Yue You

MME 312 Section A. Spring Semester

Miami University: School of Engineering

Dr. Jeong-Hoi Koo

3 May 2013

Spring 2013 MME 312 Mechanics of Materials of 22

AbstractThe objective of this project was to create an alternative design for sign support structure

on a single lane parkway in Cincinnati, Ohio. Four separate designs were initially created and finally came to the final design which would be the basis for all the analysis (Appendix, Figure 8). Materials were researched to find what best coincided with the design. Three materials were selected: Aluminum 6061-T4, Stainless Steel 304, and Glass Fiber Reinforced Polymer Composite. Upon further investigating material properties, the material selection was narrowed to Aluminum 6061-T4 and Stainless Steel 304. The design was created in Abaqus and the material properties were applied to the model. After analysis was completed, the steel exhibited less stress and deflection as expected (Figures 1 and 2). When compared, although the steel displayed less stress and deflection, Aluminum 6061-T4 exhibited minimal stress and deflection and was much cheaper (Figures 3 and 4). Aluminum 6061-T4 was chosen as the final material because it met the desired standards and was a much cheaper alternative than Stainless Steel 304. The end product made with Aluminum 6061-T4 and the design created yielded the desired results.

Introduction The aim of this project was to design a new sign support structure for a single lane

parkway in Cincinnati. The sign in question is made of aluminum 2014-T6 and is 3 meters wide by 2 meters high and with a thickness of .001 meters. This sign is to be anchored to a single post with a circular cross section and can be either solid or tubular. Using the concepts learned from this course our goal was to determine a better design than the one currently being used. We would have to keep in mind the stresses and deflections from the wind, and also the structures own weight. Thermal degradation was also something we would keep in mind for the design.

Our main objective was to design a structure that minimizes cost, but still soundly support the sign. This required emphasis on which material we would use for the supports, as well as thicknesses and length of each support. If we minimized the amount of metal used, our total cost will do the same. Another objective was for the design to be easily assembled. This meant that we would try to use standard size pieces rather than having to heavily rely on cutting and welding to achieve the pieces we need. A positive for using standard size pieces means that production will be efficient as well, which leads to a faster construction time and lower labor costs.

After researching a few overhead highway designs used determined that there was some room for improvement. In most designs, there was an overuse of material used to support the sign. Our design would be able to do the same job while using less material while still conforming to highway regulations. These regulations required the sign to be high enough to avoid being struck by larger trucks, and also be able to withstand high winds. We expect to be able to design a stable structure to support the sign while keeping costs to a minimum. From our design and analyses using modeling software, we will show our design is a viable option to what is being used now.


Design and Analysis Results

Design SummaryAt our first meeting we brainstormed design concepts, keeping in mind our objectives

and goal. Our first design utilized two beams, one being parallel to the ground and the second anchoring the far end to the top of the support pole (Appendix, Figure 5). We determined that since we were utilizing a triangular shape, the sign would not be able to be secured in a way to prevent bending from the wind. The next design iteration we came up with was a u-shaped beam with a support in the middle (Appendix, Figure 6). Although it was unique, it would not work since we would be using too much material and it would be harder to manufacture. The third design we came up with featured two parallel beams attached to the support pole with a cross support in the middle (Appendix, Figure 7). Although this would be a sturdy design, we opted against it since we could achieve the stability with less material. The fourth design, which would be a basis of our selected design, featured two beams attached at one end to the pole and pinned in the middle (Appendix, Figure 8). This design looked good at first but it had a few problems. One of which was it would be harder to secure the beams where they cross since they were not keyed. We decided to cut a notch halfway through both beams where they cross so that lock together and allow for the sign to lay flat. The two beams could be welded to join them with minimal work. This design utilized the least amount of material, and could be produced and assembled fairly easily since it only requires two cuts and a weld. Based on these qualities, we chose this design. Before this design was modeled and analyzed in Abaqus, materials had to be chosen to test.

Material Alternatives and Selection

The material selection was one of the most imperative considerations affecting the sign support structure design. There were series of measurements and calculations conducted in the decision making. A research shows the U.S. Transportation Department has been using aluminum for transportation implementations since the 1950s. However, there were still several alternative materials considered on this design.

The objectives of the materials selection were based on the following four factors: (1) a series of physical and mechanical properties of the materials, strength, stiffness and ductility, (2) the load resistance of the wind [1], (3) chemical corrosion resistance, and (4) the cost and application. Decision making later maintained these four factors. There were four alternative materials researched for the sign structure design.

We started the research from aluminum which is frequently used in many applications. The material we picked was 6061-T4 (See from Appendix). Based on the specification that Matweb.com offered, aluminum has excellent joining characteristics, relatively high strength and workability. The application of 6061 was easily perceived from hardware. 6061-T4 has relatively high tensile strength. Its modulus elasticity is 68.9 GPa and a shear modulus of 26 GPa. Its


density is 2.7 ×103 kg/m3. The modulus of elasticity and shear modulus factor greatly into the decision whether the sign could resist the pressure from the wind. Its aging temperature is 160 °C, which means the material will maintain its strength under this temperature. Aluminum 6061-T4 also has good chemical corrosion resistance to bear aggressive environment. [3]

The second material we choose was Stainless Steel 304. The use of stainless steel is also relatively common in daily life due to its high tensile strength, high corrosion resistance. Based on the data that Matweb.com offered, the density of stainless steel is 8.00 ×103 kg/m3. It has a UTS of 500 GPa, yield strength of 215 GPa, range of modulus of elasticity is 193-200 GPa, and its shear modulus is 86 GPa. The research shows that stainless steel has high strength and good workability, which fulfills the requirements of fabrication. Its high ductility could help sign structure perform a stable dynamic behavior against the wind. Stainless steel 304 is also a low carbon steel, which means it possess a low susceptibility and high corrosion resistance.

The third material we considered were Glass Fiber Reinforced Polymer Composites (GFRPs). A research article shows that GFRP composites are feasible to use for crack connections of sign structure due to its decent mechanical properties [2]. Thus, we took it into consideration as one of the options in this circumstance. GFRP has a range from 19 GPa to 31 GPa of modulus of elasticity [2]. And its shear modulus is around 15 GPa [4]. It has low density compare to metals. The other advantages based on its mechanical property are good resistance on corrosion and under different environmental conditions. And research shows that GFRP composites have been adopted in various civil engineering projects such as composite beams and joints which could resist applied load. [2]

The Feasibility Comparison and CostThe very first two materials we considered for this design were steel and aluminum. The

comparisons were conducted after the research. The list below ranks the orders of different property. The final decision was generated from the multiple considerations below. Density: Stainless steel 304 > Aluminum 6061-T4 > GFRP composite Tensile/ Shear Strength: Stainless steel 304 > Aluminum 6061-T4 > GFRP composite Corrosion Resistance: GFRP composite > Aluminum 061-T4 > Stainless steel 304 General Cost: GFRP composite [5] > Stainless steel 304 > Aluminum 6061-T4

Due to the comparison above, we concluded Aluminum 6061-T4 would be the most feasible material to build the sign structure. It has enough tensile strength and shear strength which fulfilled the requirements. The density is relatively low compare to Stainless steel 304. Since, the demand of certain weight is not the factor we considered. However, lower weight of the material would be easier to transport and construct. The corrosion resistance is another non-negligible factor. It relates to the lifetime of any fabrication. Aluminum 6061-T4 has relatively stronger on corrosion resistance compare to Stainless steel 304. And GFRP perform as the most stronger on corrosion resistance among those three. The last option we need to consider is the cost of the materials. Aluminum 6061-T4 has lowest cost among those three which would reduce the total expense of building a sign structure.


Our cost estimation was based on the market price. According to a material supplier in Ohio (McMaster.com), cost to build a 0.5 inch thickness, 18 meters (in total) fabrication sign structure would approximately cost 2,500 USD in total, not include workers wage. Stress/Deformation Analysis

To ensure the Aluminum was the best choice not only on paper, the model was first analyzed for stress and deflection using the mechanical properties for Stainless Steel 304.

Figure 1. Stress Analysis (Stainless Steel 304)

Figure 2. Deflection Analysis (Stainless Steel 304)


After Stainless Steel, the structure was then analyzed with the mechanical properties for Aluminum alloy 6061-T4. The analysis gave favorable results towards the Aluminum being the better option for its strength relative to its cost (Figures 3 and 4)

Figure 3. Stress Analysis (Aluminum 6061-T4).

Figure 4. Deflection Analysis (Aluminum 6061-T4)

From Figure 3 and Figure 4, we can know that the largest stress happened near the bottom of the pole which is 1.482*106Pa. Compare this to the yield stress of the Aluminum 6061-T4, which is 145MPa, the stress that acting on the pole is really a small number. Also, the largest deflection happened on the 2nd chord at the furthest point from the pole, which is 2.842*10-3m. Figures 8, 7, and 9 in the appendix show the deflection in the x, y and z directions. Note that the deflection is shown with signs to indicate their direction, therefore, the max deflection is the magnitude of these numbers: In x-direction is 2.795*10-3m; in y-direction is 4.872*10-4m; in z-direction is


3.488*10-4m. Overall, the volume of the whole structure is about 0.3441m3 and the mass is 929.082kg. When comparing the analysis of the stainless steel and aluminum or compared, the following conclusions were made:

1. There is almost no difference between the Max Stress. (1.471*106 Pa for steel vs. 1.482*106 Pa for aluminum)

2. There is a significant difference between the overall Max deflections. (9.311*10-4 m for steel vs. 2.842*10-3 m for aluminum)

3. There is a significant difference between the total masses. (2715.83kg for steel vs. 929.082kg for aluminum)

Hand Calculation ComparisonTo analyze the mechanical properties of the designed sign support structure, we basically

focus on the Stress, Deflection, and Angle of twist. We assume the only external force is produced by the wind on the area of the sign based on the weather condition of Cincinnati and neglect the wind force acting on the chord and pole since the area are very small compare to the sign.

By setting a 3D model of the sign support structure, the weight of the sign and the pole will create the normal stress to the section we chose (Appendix, Figures 15 through 19). And not only the wind force but also the moment of Z-axis will create the shear stress to the section .The moment acting on X-and Y-axis will create bending stress. Based on the parameters we set up, we can obtain the bending stresses are

σx=FwWC/2Iσx)max=183.86kPa

σY=PlC/2Iσy)max=747.19kPa

Then, it is easy to conclude that the maximum bending stress occur at the edge of the circular section, where the distance is the outer radius of the tube. The only one normal stress, which shows below will give the pole compression time

σz=ρ(l+a)g+Fw/Aσz)max=240.05kPa

So the maximum normal stress happens at the most bottom of the pole. The shear stresses produced by moment along Z-axis and shear force acting along negative X-axis are

τz=2.25PC/Jτz)max=220.63kPa

τx=PQ/Itτx)max=240.5kPa

It is obviously that the maximum shear stress also happens on the edge of the circular section.After the maximum values were obtained, we chose four points on the circular section to

analysis according to the direction and magnitude of each stress. Then by using Mohr circle analysis we got the maximum and the minimum normal stress, shear stress and twist angle will lead the extreme value.


Then based on the material property we calculated the maximum angle of twist, which is a very small value of 0.00024rad and within the range of twist angle to reach the maximum value. We can analyze all the points on the circular section by using Abaqus.

Besides, to consider the elongation based on the difference of the temperature, we found out the highest and the lowest normal temperature in Cincinnati, which is 103F and -22F, the thermal strain we calculated are very small, which can be neglected. Also there will be a deflection on the portion where the sign attached on the pole, the total deflection is 1.3mm, which can cause the pole to deflect and cause the sign lower. But based on such a small deflection compared to the whole structure we can neglect it.

ConclusionsFrom all of the research done on materials the Aluminum 6061-T4 appeared to be the

best choice for our structure design when considering strength and overall cost. After further hand calculations and loading analysis in Abaqus, this was proved true. Although Stainless Steel 304 had lower values of stress and deflection, the aluminum alloy still had minimal stresses and deflection and was much more cost efficient. Concepts from Mechanics of Materials were integral for every part of this project. Stress analysis had to be performed in each beam, bolt, and point of attachment. This stress analysis included all different analysis such as bending stress, normal stress, and shear stress. Concepts for finding deflection also had to be used in order to make sure the sign structure was not bending too much and possibly lower and hit a semi-truck passing under. Overall this project was a very good test of all skills learned in Mechanics and was very interesting relating those skills into a real life application.


References

[1] Kaczinski MR, Dexter RJ, Van Dien JP. Fatigue-resistant design of cantilevered signal, sign and light supports, NCHRP Report 412, Transportation Research Board, National Research Council: Washington, DC; 1998.

[2] Pantelides, Chris P.. "Repair of Cracked Aluminum Overhead Sign Structures with Glass Fiber Reinforced Polymer Composites." (2003): 9. Print

[3]Metals Handbook, Vol.2 - Properties and Selection: Nonferrous Alloys and Special-Purpose Materials, ASM International 10th Ed. 1990.

[4] Federal Highway Administration. "Behavior of Fiber-Reinforced Polymer (FRP) Composite Piles under Vertical Loads." [Online] April 2011. <http://www.fhwa.dot.gov/engineering/geotech/pubs/04107/chapt2.cfm>.

[5] Chakrapan, Tuakta. Use of fiber reinforced polymer composite in bridge structure. (2004): 68. Print


http://www.fhwa.dot.gov/engineering/geotech/pubs/04107/chapt2.cfm

Appendix

Table 1. Properties of Aluminum 6061-T4.

Density 2.7×103kg/m3

Tensile Strength, Ultimate 241 MPa

Tensile Strength, Yield 145 MPa

Modulus of Elasticity 68.9 GPa

Poisson Ratio 0.33

Shear Modulus 26.0GPa

Aging Temperature 160 °C Rolled or drawn products; hold at

temperature for 18 hr

177 °C Extrusions or forgings; hold at temperature for 8 hr

Table 2. Properties of Stainless Steel 304.

Density 8×103 kg/m3

Tensile Strength, Ultimate 505 MPa

Tensile Strength, Yield 215 MPa

Modulus of Elasticity 193-200 GPa

Poisson Ratio 0.29

Shear Modulus 86.0GPa


Table 3. Properties of Glass Fiber Reinforced Polymer Composites.

Figure 5. Deflection Analysis x-direction (Stainless Steel 304)


Figure 6. Deflection Analysis y-direction (Stainless Steel 304)

Figure 7. Deflection analysis in z-direction (Stainless Steel 304)


Figure 8. Deflection analysis in x-direction (Aluminum 6061-T4)

Figure 9. Deflection analysis in y-direction (Aluminum 6061-T4)


Figure 10. Deflection analysis in z-direction (Aluminum 6061-T4)

Figure 11. Design 1


Figure 12. Design 2.

Figure 13. Design 3.


Figure 14. Final Design


Figure 15. Structure Parameters and Calculation Assumptions.


Figure 16. Stress Calculations.


Figure 17. Stress Calculations Continued.


Figure 18. Mohr's Circle Analysis.


Figure 19. Stress and Deflection Calculations of Structure and Bolt.


Table 4. Dates and Time Spent in Meetings.

Dates Minutes10-Apr 6017-Apr 6025-Apr 1201-May 902-May 240


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