Structural International

Structural Engineering 103

Final Report: Luxury Vacation Home

December 7, 2015

Team 32

Thunyathorn Chutinuntanakul

(Eve)

Khristian Decastro Tak Lun Law Pete Phathayakorn Samuel Trejo (Sam)

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Table of Contents

Background and Purpose 2

Conceptual Design 2

Engineering Principles 3

Structural Performance 4

Aesthetics 7

Project Challenges 8

Team Responsibilities 8

Conclusion 8

Appendix 9

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Background and Purpose The purpose of the project is to apply engineering concepts from class to design an elevated house in a

certain location. The goal of the project was to build a lightweight substructure that can hold up the lux, withstand a minimum lateral load of 35 lbs, and be economically viable. For the house, Structural International’s goal was to build a structure that was both environmentally friendly and aesthetically pleasing. The roles of the team members were Tak Lun Law as the Design Lead, Samuel Trejo as the Construction Lead, Pete Phathayakorn as the Design Analyst, Eve Chutinuntanakul as the Lead Drafter, and Khristian Decastro as the Architect. The location was chosen to satisfy TRITONS Real Estate Investment Association (TREIA) in obtaining clients who love off­roading and dirt biking during their vacation time. The location also gives the client the option of more relaxing activities such as golfing and walking in the state park. Moreover, the design of the structure allows the client to view the precious mountains for future adventures, the sunrise in the mornings as well as sunsets, and allow future clients to explore the area’s casinos and restaurants. Figure 1 shows the site plan for the project while Figure 2 shows the zoomed in view with dimensions.

Conceptual Design

The basis of the structure’s design consists of a wooden frame with diagonal truss members for rigidity and fixed restraints at the bottom of the substructure. The entire structure was built using square balsa wood members that were cut into appropriate lengths. The team’s design went through a two­stage process due to unforeseen circumstances that occurred during construction.

Initially, the team decided on a square base that was 5x5 inches with 4 tapering columns that would span the height of the tower. The top of the columns were fixed to a smaller square that was designed to be 2x2 inches. This braced frame structure was built using 1/4x1/4 balsa wood sticks. The tapering of the tower was designed to resist lateral forces applied to the top. For example, when a horizontal force is applied to the top of a vertical member that is fixed at the bottom, the member becomes an axially­zero force member and does not contribute to the rigidity of the structure. This causes the majority of the load to be resisted purely by a reactionary moment at the fixed joint. This redistributes some of the load and causes less stress concentration at the joint. Tapering the columns of the substructure allows them to act as if they were truss members themselves. This increases rigidity in cases where a lateral load is applied to the top end of the substructure.

The substructure was sectioned into 3 levels due to requirements dictating that no member be more than 6 inches long. Between each level were horizontal beams that braced the columns to each other. These horizontal beams were made with 1/4x1/4 balsa wood sticks. Within each level, diagonal trusses of 1/8x1/8 were used to form cross braces that acted across each face of the structure.

During construction of the structure, however, it was deemed difficult to properly construct a frame that tapered all the way to the top given the initial planned dimensions. As everything was hand made, accuracy in obtaining the proper angles in 3 dimensions for the joints proved to be a struggle. After constructing the first level

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which had a 5x5 square base that tapered to a 3.75x3.75 top, the team decided to continue on with the second and third levels with vertical columns instead of continuing to taper the columns upward. This modified design would not have the theorized benefits of the original design, however the team decided that the benefits from a tapered tower did not outweigh the risk of having improperly set joints and connections. As an extra precaution against joint failure, exposed surfaces of the joints were covered with thin slices of balsa wood, acting as gusset plates.

The tower was fixed with aluminum plates on all four bottom sides. These were glued on to the middle of the bottom horizontal beams with JB Kwik epoxy on each face. Each plate then had 2 small holes, approximately .10938 inches in diameter, drilled on opposite ends of the plate’s length, both holes being .4 inches above the bottom of the plate and their center. The holes are .5 inches away from the length ends. The top of the structure featured 2 aluminum plates placed opposite to each other and secured with JB Kwik. Each of the top plates had 2 holes drilled into them, .25 inches in diameter, on opposite ends of the plate’s length with each hole 1.5 inches away from the ends and in the center of the plate’s width. Figure 3 shows the final design of the substructure

Engineering Principle The team decided to apply slope­deflection concepts for the hand

calculations. The structure is a frame with trusses, yet the team had to simply analyze at first in order to ensure SAP2000’s model match theoretical values. Columns, beams, and diagonals were virtually separated for analysis; given lateral, shear, and moment at each end. The team decided to analyze a simple frame first in order to compare the accuracy of SAP2000. Hand Calculations are in the Appendix. Figure 4 shows the simple frame with each column and diagonal having fixed­ends. Table 1 shows the hand calculation and SAP2000’s result. Figures 5 and 6 show SAP’s frame and deflection.

Table 1 : Comparison of Hand Calculation and SAP2000 Calculation

Hand Calculation (in) SAP2000 calculation (in)

9.75 9.68

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Figure 5: Sample of Fixed

Frame Figure 6: Sample Deflection of Frame

Afterward, horizontal beams were attached and the same concept was applied. Later, the team created diagonals and connections to make up lateral systems. The purpose of this lateral system is to transfer the lateral load, from where force was applied, to the ground without damaging the substructure. Using fixed­end moments for each member, the team determined the deflection for the test frame.

To determine the ultimate failure mode of the structure, the stress and critical buckling for diagonal members were determined since they were constructed with a thinner cross sectional area. Since the intersection between 2 diagonals were glued

together, the team assumed pinned­ends for the worst case scenario of the substructure. Each analyzed diagonal had half its length applied to the buckling equation since the intersection was considered a joint. This process was applied to members facing the most stress on SAP2000 and were compared to their critical buckling values that were determined by hand.

Structural Performance By looking at the load combinations per ASCE 7­10, the team found that case 3, 1.2D+1.0E+L+0.2S,

governs the design. The values that the team used in finding the governed design are in Table 2. Figures 7 through 9 show the pattern of deflection for every case and Table 3 shows the max deflection for each case. While comparing the deflections in Table 3 to the deflection the team obtained for the testing scenario with 35 lbs of lateral force and the weight of the plate, the team found that the max deflection for the substructure is 0.15 in.

Table 2: Load Type and Value

Wind Load (lb) Live Load (lb) Dead Load (lb) Roof Live Load (lb) Earthquake Load (lb)

3 15.3 15.2 0.42 8

Figure 7: Case 1 Combination

Figure 8: Case 2 Combination

Figure 9: Case 3 Combination

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Table 3: Deflection for Different Load Combinations

Case 1 Deflection (in) Case 2 Deflection (in) Case 3 Deflection (in)

.0115 .0266 .0625

The loads that were applied to the structure during the testing are the dead load of the top plate and the

wind load applied by the actuator. From SAP2000 analysis, the team believed that the critical members would be the sloped columns in the side farther away from the applied load, one of the top crossed braces, and the bottom crossed braces from the two sides parallel to the direction of the wind load. These members would fail due to buckling when applying the wind loads. The top diagonal member would receive the most compressive force because it would resist the applied load. From the equation, , the critical load of this diagonal member is 15.56 lb. Due to crP = π EI2

(kL)2

the fact that the bottom diagonal members are longer than the rest, it would fail first. Their critical load is 9.7 lb. This judgement was based by their length and cross sectional area. Hand calculations were performed to determine the critical buckling force and only partial length was taken into account since the intersection of the diagonals was glued. Hand calculations are in the Appendix. After determining the buckling value, different loading cases were applied onto the structure and the team looked at the max applied stress to determine its force. Theoretically, the substructure was able to handle over 35lbs of lateral load. Hence the team increased the load until reaching 40 lbs. The team decided to lower the max load to 37.5 lbs since it was believed that manufacturing error would occur.

The load paths start at the top plate screwed into the structure. The wind load and the weight of the plate went to the beams connecting to the two aluminum sheets. This load went to the connections and then the diagonal members and the four columns. From the diagonal members, the load went through the bottom beams then onto the columns. This process continued until the load reached the ground. However, because the substructure was being pushed on one side, the load path of the right side would go up, while the bottom reaction force is pulling that side down. Two of the diagonals in each floor would receive the upward load path as well because the columns would pull the diagonals. The team assumed that all the connections are fixed connections because the connections transferred the moment to the members. The connections transferred both the axial force and bending force. The load path is shown in Figure 10 .

During testing, the substructure failed in the contact between the testing apparatus and balsa wood­aluminum sheet. This is because the team used JB Kwik instead of JB Weld glue; JB Kwik has significantly less adhesive strength. . Hence the member failed due to not sustaining the shear force traveling along the member.

The applied load made half of the substructure receive compression while the other half obtained tension. The side with the applied force was in tension, as well as the blue diagonals in Figure 12. The side farther away from the applied force was in compression because the substructure deflected, as well as the red diagonals in Figure 12. The critical connections of this substructure were the connections between the slanted columns and vertical columns. These connections would receive the most stress due to the tension and compression of the columns. These connections were solely held together by glue, which could break under heavy stress.

Figure 10: Load Path of the Substructure

The team’s estimated PI value is 9852. Table 4 shows all predicted and actual values. The possible failure modes were the buckling of the diagonal members and the breaking of the connections where the slanted columns meet the vertical ones. To minimize the effects of these failure modes, the team glued the diagonal members

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together in order to reduce the length of the members, which in turn increased its critical load. To help reinforce the connections, the team added small gusset plates to the side of the columns. During experimentation, the substructure withstood 7.205 lbs. The experiment caused the bottom beam, which was parallel to the lateral force, to crack. The team believes cracking occurred due to the glue used to fix the aluminum plate onto the bottom beam. To minimize construction time, JB Kwik was used to contact the bottom beams with the aluminum plates. JB Kwik was fast­drying, but this quick curing time compromised the strength mixture of the epoxy. The analysis did not incorporate the aluminum plates in SAP2000 since the team believed no significant difference would occur. Moreover, assuming no aluminum plates would provide the worst case scenario for the substructure.

Table 4: Comparison between Predicted and Actual PI

Cost ($) Weight (g)

Predicted Displ. (in)

Predicted Force (lbs)

Displ. (in) @ 20 lbs

Ultimate Displ. (in)

Ultimate Force (lbs)

Predicted PI

Actual PI

12,380 97.6 .150 37.5 1.5 .124 7.205 9852 170.63

Figure 11: Failure Location of

Substructure

The result was fairly different from the predicted value since the connections by JB Kwik did not perform as well as the team predicted. When the JB Kwik epoxy gave in, the support reaction moved inwards from the side of the structure. This caused a bending moment in the the bottom horizontal member. Figure 11 shows the failure of the substructure under testing.

Figures 12 and 13 show the expected performance of the structure with the predicted connection, while Figures 14 and 15 show what happened in the experiment. The errors in the prediction were caused by the wrong assumption of the strength of the epoxy. If the team were to use JB Weld, the structure would have a higher ultimate force than 20 lbs, giving a higher stiffness factor.

Figure 12: Predicted Force Diagram of the

Substructure

Figure 13: Predicted

Deflection

Figure 14: Estimated Force Diagram of the Substructure after

Testing

Figure 15: Estimated Testing Deflection

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Aesthetics The structure was intended to blend into the desert environment to minimize disturbances to the natural

beauty of the surrounding landscapes. To do so, Structural International designed the truss system beneath the structure to allow visitors to see the surrounding landscapes. Additionally, the team used sandstone on several walls to help further the home’s blending into the environment. Different earth tones were used in the structure as well. To keep up with the vastness of the desert environment, the architect designed vast open windows facing the east and west, as well as an open deck on the structure’s west side to allow the client to feel one with the environment as the sun rises and sets. To add on to this, each story has very high ceilings at a height of fifteen feet, and includes a grand living and dining area that is two stories tall, as shown in Figure 16.

The significant volume of natural light entering the structure helps decrease the amount of electricity needed to produce light throughout the day. To compensate for the expected very cool evenings and nights, the two­story wall, facing east in the grand living room was painted black in order to absorb heat reflected off of the inner surfaces; the same was done for the roof of the structure. To prevent the interior of the structure from heating up too much, the team utilized tinted windows to prevent an uncomfortable amount of radiated heat from entering. The sandstone exteriors are also meant to reflect light and be a strong insulator so as to strengthen prevention against heat. The solar panels on top of the structure are curved in such a way that they follow the path of the sun’s movement throughout the day, as the sun in the northern hemisphere typically sits toward the south for most of the year. Figure 17 showcases the colors, textures, and environmental considerations in 3D from the Southwest.

Despite all the effort put into blending the structure into the environment, the team still wanted to maintain a sense of uniqueness and did so by designing with hard lines and strong corners, thus providing the structure with a hierarchy of shape against the smooth curves of the surrounding mountains. The mere scale of the building and its elevation, also help stand out from the environment despite its desert­inspired colors and textures. Altogether, the structure aims to be simultaneously harmonious with the surrounding landscape as well as eye­catching enough to catch the attention of passersby.

Figure 16: The Dimensions of the Structure from the South

Figure 17: 3D Model of the Structure from the Southwest

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Project Challenges One main challenge was trying to decrease the moment that the structure would experience from the lateral

force. Slanting the column toward the wind force helps reduce the moment on the substructure since the force perpendicular to the cross sectional area of the column would decrease. The team realized that the more slanted the columns are, the better the structure can reduce the moment at the bottom of the structure. However, in the original design, the team could not decrease the angle anymore as doing so would risk decreasing the area at the top of the structure to hold the vacation house on top, hence they were placed at 83 degrees.

Structural International faced a major setback during construction. Due to the complex geometry of the original design, it was very difficult to construct the substructure with accuracy. After building the bottom section, the Construction Leader realized that the complex shape could jeopardize the given criteria. The team was also not well­equipped to accurately fabricate the beams needed to build the structure. In order to minimize costs, the Lead Analyst kept the bottom section the same and redesigned the rest of the structure. Both leaders realized that one of the most important elements of a structure under lateral is the connections. Having to conserve the cost and weight, the team used balsa wood with a small surface area, creating challenges to make strong connections. To ensure that the connections would not be a problem under loading, the team used TITEBOND III wood glue as well as adding reinforcements on the joints.

The overall team dynamics on this project were outstanding. One challenge in team dynamics was that there were strong opinions between team members on the design of the substructure. Samuel wanted a design that would perform better, while Pete wanted another design that is more cost efficient. The entire team met to find a design that satisfied each member’s preferences. The result was a design that can withstand sufficient force and was economically efficient.

Team Responsibilities Tak and Sam were responsible for the original design of the substructure, while Pete and Eve redesigned it

in order to reach a compromise between the team members’ interest. Sam was responsible for the SolidWorks model of the substructure with the assistance of Tak. Eve’s task was producing the site layout and drawings through AUTOCAD. Pete and Sam were responsible for the structural analysis and predictions. Khristian created the original design of the house on SolidWorks, with revisions done by Pete and Tak to reach the final aesthetic design, which Khristian in turn again modeled within SolidWorks. Pete and Khristian went out to buy necessary materials for constructing the entire model. All the team members worked together throughout the construction of the entire structure. The total hours the Eve, Pete, Sam, Tak, and Khristian put into this project are 48, 48, 49, 43,and 13 hours respectively.

Conclusion The team learned a huge difference between the strength of JB Weld and JB Kwik epoxy. Moreover, they

learned the importance of strong connections in the lateral system. Concepts such as Slope­Deflection were applied for analysis to determine deflection and compare the accuracy of SAP2000. During the construction period, they learned that the pre­construction planning is one of the most important processes for successfully constructing without having to waste any time or material. Unfortunately the team was not able to test their structure to the full limit due the poor connection by JB Kwik. The actual PI was 170.63 while Structural International expected 9852. However the predicted PI assumes the entire structure was constructed properly. During the quarter, the team became aware of how to accurately model in SAP2000 in order to maximize the efficiency of the program, how to draw site plans using AUTOCAD, and create solid parts and assemblies in SolidWorks. They also knew the limitations of computer programs and have taken that into account for their predictions, thus emphasizing the importance of the engineer possessing knowledge and skills in the field in addition to computer analysis.

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Appendix

Figure 18: Substructure with Applied Fixed Frames

Figure 19: Substructure During Experiment

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Figure 20: Hand Calculations

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Figure 21: Hand Calculations Cont.

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Figure 22: Critical buckling for Predicted Failure Member

Figure 23: West View of the Structure

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Figure 24: East View of Structure

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Figure 25: North View of Structure