STRUCTURE AS ART
Maura Chen / Stephanie Lloyd Sean Ostro / Baxter Smith Structures 150 Project 2 Alan DeMarche
DESIGN CONCEPTS INSPIRATION
precedent CaixaForum Herzog + de Meuron Madrid, Spain Completed 2008 The faceted structural core of la CaixaForum was our initial inspiration for the project, as we were intrigued to understand how a trussing system could allow for a structure to “blossom� as it moved vertically away from the ground plane. Our intent was to have a structure with a relatively small footprint and branch out as one moved vertically throughout the system. netting After having established an overall structural understanding for how to move forward with the project, we were curious to understand how some planes within the trussing system could be imagined as netted surfaces, as opposed to rigid diaphragms. Throughout further study of circulation patterns throughout the structure, we decided that these netted surfaces would conceptually allow children to move vertically throughout the structure, without compromising somewhat symmetrical truss patterns.
DESIGN CONCEPTS INITIAL CONCEPTION
Having agreed upon a scheme that allowed the structure to expand vertically, we spent some time trying to understand proportions between floors that would not only be structurally possible in terms of design constraints, but would also be aesthetically and functionally appropriate. After some design modifications, we reverted back to our original proportions,and made small modifications to the structural system, moving two members to allow for a front entrance. The sketches on the far right show the initial proportions for the project, with the study models on the right exploring possible changes in scale and proportion of the three floors. During this phase of design we also tried to understand how our members would come together at intricate joints. Using our initial design, we attempted to tie together a joint with the most members, 8 in total. The resulting detail can be seen to the right as well. We found that the simplest and most effective way to execute this detail was to simply drill 2 holes per member and tie as many connections together as possible. This would allow for a joint that would be able to entice member failure during the testing phase.
DESIGN DOCUMENTATION
PLAN ELEVATION
DESIGN DOCUMENTATION
AXONOMETRIC VIEW
EXPLODED AXONOMETRIC DIAGRAM
PERSPECTIVE VIEWS
DESIGN DOCUMENTATION
DESIGN DOCUMENTATION
DESIGN DOCUMENTATION
DESIGN DOCUMENTATION
ANALYSIS For the playhouse, we designed for a constant dead load of 15lb/sqft on the observation deck and a live load of an additional 40lb/sqft. We also designed for a lateral wind load of 40lb/sqft. Because of the way we chose to site our playhouse, we could determine the wind direction. To approximate a wind force over the entire projected area, the structure was loaded at the nodes colored red in the drawing to the right and pulled in the direction of the arrows.
SAP2000
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WIND LOADS. Undeformed shape
[NOTE: The large deflection on the left side of the structure seen from this view is due to the dead and live load, not the lateral load.]
After a number of revisions to the design, we settled on the truss structure shown to the right (all additions shown in red). We found that adding more pin connections to the ground improved the overall performance of the structure to withstand the applied lateral load. Also, including a complete ring connecting all of the foundational supports to each other and the ground gave the structure greater stability. Bracing was added to support the dead and live loads with and without the wind. A detailed comparison of an early iteration and the final iteration is shown on the next page.
The diagram to the right shows the forces applied for calculations in the final iteration. Forces were calculated by multiplying the applicable distibuted load (dead, live, and/or wind) to the tributary area that that joint covered.
SAP2000 v15.0.0 - File:project2_v3 - 3-D View - lb, in, F Units
SAP2000
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ADDED MEMBERS. Undeformed shape
SAP2000 v15.0.0 - File:project2_v3 - 3-D View - lb, in, F Units
FORCES APPLIED. Undeformed shape with lateral and vertical loads
ANALYSIS
The strength for wood was approximated to be 4000psi in compression and 400psi in tension. At 1:1 scale, 5277lbs was the maximum compressive force and 3152lbs was the maximum tensile force. Still at 1:1 scale, these forces required a crosssectional area of up to 9in2, which is a 3”x3” square cross section. At 1-1/2”=1’ scale, this cross section translated to 3/8”x3/8”. To account for members that required different amounts of strength (either compressive or tensile), two member sizes were used at 1-1/2”=1’ scale: 5/16”x5/16” and 1/2”x1/2” square dowels. These sizes err on the side of overestimating the amount of strength needed from the members.
SAP2000
Undeformed
SAP2000
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Deflected
SAP2000
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Axial Forces
DEAD + LIVE + WIND SAP2000 12/15/14 0:50:39
Deflected
SAP2000
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Axial Forces
12/14/1
EARLY VERSION
[NOTE: All moment forces at joints were released prior to analysis.]
DEAD + LIVE ONLY
Max. Deflection: -1.24x1018, 0, 2.07x1017
SAP2000 SAP2000 v15.0.0 - File:project2_v2 - 3-D View - lb, in, F Units
SAP2000
12/14/14 23:36:40 SAP2000 v15.0.0 - File:project2_v2 - Deformed Shape (DEAD+LIVE) - Kip, in, F Units
SAP2000 v15.0.0 - File:project2_v2 - Axial Force Diagram (DEAD+LIVE) - Kip, in, F Units
SAP2000
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[NOTE: deflection was extremely high because there was not sufficient tringulation. This was corrected in later iterations.]
SAP2000 12/15/14 0:50:53 SAP2000 v15.0.0 - File:project2_v2 - Deformed Shape (WIND) - lb, in, F Units
Max. Tension: 6189 lbs
Max. Compr.: 3932 lbs
SAP2000
12/14/14 23:37:24 SAP2000 v15.0.0 - File:project2_v2 - Axial Force Diagram (WIND) - lb, in, F Units
FINAL VERSION
This page shows a series of results from SAP, demonstrating the performance of an early iteration and the final iteration of the design. Deflection and axial forces are shown for under two conditions: just dead and live loads applied and all loads applied (dead, live, and wind).
Max. Deflection: -0.0088, 2.84x10-5, -0.003 SAP2000 v15.0.0 - File:project2_v3 - Axial Force Diagram (DEAD+LIVE) - Kip, in, F Units
SAP2000 v15.0.0 - File:project2_v3 - 3-D View - lb, in, F Units
SAP2000 v15.0.0 - File:project2_v3 - Deformed Shape (DEAD+LIVE) - Kip, in, F Units
SAP2000 v15.0.0 - File:project2_v3 - Deformed Shape (DL+WIND) - lb, in, F Units
Max. Tension: 3152 lbs
Max. Compr.: 5277 lbs
[NOTE: Although max. compression increased from earlier iterations, wood is significantly weaker in tension, so the goal was to decrease that value.]
SAP2000 v15.0.0 - File:project2_v3 - Axial Force Diagram (DL+WIND) - lb, in, F Units
12/14/1
TESTING
Ready to load We were able to test the structural model using a tie off ring from the joints to a sail winch. In order to meet the minimum load requirements the structure had to be able to take 40 lbs of force. We achieved 70 Height: 1’ 7� lbs of force before two Weight: 31.1 oz (~2 lbs) Maximum load: 140 lbs initial break joints failed.
220 lbs max Strength/weight: 70 initial break 110 max
Max loading
Loading with scale
Scale
Sail Winch
Joint failure at base 1
After these two initial failures we were able to load the structure up to 220 lbs with several other joints failing but maintaining overall structural integrity. Finally, we removed the scale in order to load the model to total failure.
220 lbs max scale
Initial Failures
Load at 70lbs
Loading without scale
Broken string
Tie-off ring
Joint failure at base 2
Broken string
Total failure
CONCLUSIONS SAP and Sizing An initial challenge we found with the project was how to withstand a lateral load with a pin connection. In order to do this we came up with a joint that would allow us to meet these demands using bass wood and string. Using SAP we were able to make iterations of the project. In our final iteration we removed several members in the structure, leaving open spaces where netting could be hung for circulation. This kept the structure stable enough for live and dead loads while sticking to its nature as a play house. With this final design change and our chosen joint, we plugged in the structure to SAP and addressed member sizing. We determined that two sizes would be necessary to over come the anticipated loads, 1/2” and 5/16” at model scale.
Test joint
SAP Analysis
Joint and Member Failures Another challenge, which lead to the initial failures and continued integrity of the structure, were the connections to the base. We built the structure sideways, so the applied lateral load was facing down. This helped us to tighten the joints as they would be loaded. We were able to wrap the joints to the base, but not securely enough. When loaded the structure shifted into its built position and took on 70lbs of force, at which point the shear force was able to over come this connection, leading to its failure. After loading the structure to the initial failure we decided to keep testing it to try and get a member to fail. We eventually did, but were unable to determine at what force. The member failed where a hole was drilled for the string, which is a weak point in each member. None of the members failed in the middle, which would have expressed a buckling force, which is what we were hoping to achieve through our joint technique and member sizing.
1/2” member
5/16” member
SAP2000 v15.0.0 - File:project2_v3 - Axial Force Diagram (DL+WIND) - lb, ft, F Units
Initial Failure 1
Initial Failure 2
Member Failure
Maintained Integrity After these failures we continued to load the structure, and it was able to maintain its form while the rest of the base connections broke away. This lead to the main structure touching down on new base points created by the floor above.
Final testing
Base connections
Maintained form