National Timber Bridge Design Entry

College & Team Information
College or University:		Student Chapter:
Address:
Phone:	Fax:	E-mail:
Website address:		Faculty Advisor:
Person In Charge of Project:

Team Member	Class	Team Member	Class










Hours spent on project:		Cost of Material ($ Amount)
Student:	Faculty:	Donated:	Purchased:

1. Abstract - (Max 500 word narrative)
	Our structural system was prioritized with the principal goal of resourcefully using wood as a structural material. The final design, a tied truss, was chosen due to its proven efficiency, ease of constructibility and aesthetic value. One of the advantages to using a truss over a girder bridge is the reduction in member sizes and therefore weight of the final bridge. We maintained a design that was simple from a structural analysis standpoint in order to keep our focus on minimizing wood member sections. We also maintained strict adherence to the dimensional requirements of the competition rules, which helped us limit the spans of members to the minimal required so that the bridge would not be over-designed Our team decided to use Douglas Fir-Larch for our bridge members due to its common availability and versatility in this part of the country. Douglas Fir is commonly used for structural purposes and is readily available as a treated lumber. We optimized the use of sawn lumber over glue laminated members to reduce the risk of fabrication errors due to our inexperience in manufacturing such products. The use of sawn lumber, although less efficient than wood I-Beams, added practicality by simplifying construction and theacquisition of lumber. With reduced probability for, error we did not have to be abnormally conservative in our design while retaining confidence that our design was adequate. The use of 2x members for our truss system was primarily based on the convenience of connection design, installation, and minimizing percentage of non-wood weight. Solid plywood decking panels were used because of their proven ability to distribute loads efficiently, thus minimizing the structural depth required. We also used a crosshatched deck topping made up of 1x6 treated dimension lumber to add aesthetic value and to act as a wearing surface. Considerable time was spent investigating alternative options for connections. For the superstructure truss connections we used 10 gauge powder-coated steel plates with custom sizing for each location. The plates used Simpson Ľ” SDS screws, which optimized stiffness and even helped improve the lateral stability of the truss. With these fasteners we were able to reduce the potential for additional deflections induced from connections. Without large holes, as would be used for a through-bolt, we could avoid loose connections. These smaller fasteners minimized the potential for wood splitting even without pre-drilling. SDS screws are manufactured for capacities above the more common #14 wood screw of similar size, allowing us to reduce the number of fasteners required. For the purlins and the deck joists we used manufactured metal hangers with nails which are proven solutions to meet these types of load paths.

2. Deflection Table
Deflection (millimeters - rounded to 2 decimal places)
Loading Inc.	Bridge	Beam L	Beam R	Average (L&R)	Gross Deck	Net Deck
5 kN
10 kN
15 kN
20 kN - 0 min.
20 kN - 15 min.
20 kN - 30 min.
20 kN - 45 min.
20 kN - 60 min.
1) Loading Increments.
2) Bridge - As measured at midspan of the longitudinal beam receiving greatest loading.
3) Beam L - As measured under the longitudinal beam to left of selected deck monitoring point.
4) Beam R - As measured under the longitudinal beam to right of selected deck monitoring point.
5) Average (L&R) - Average of 3 and 4.
6) Gross Deck - As measured under the loading point expected to experience maximum deflection.
7) Net Deck - Column 6 minus column 5.
Deck span (transverse distance between main longitudinal bridge support members measured from inside edge to inside edge) = mm / 100 = mm (max. allowable net deck deflection)

3. Materials List
Material Item	Weight (kg)




















Total Weight (Kg)
Weight Non-wood (Kg)
Percent Non-wood

4. Summary -Describe Bridge and behavior under load - (Max 500 words)
	The team set out to build a bowstring truss due to the structural benefit compared to the ease of construction. Since the team had little experience in carpentry, it seemed logical to have a final product that reflected the design and would not be subject to gross construction error. The final decision was made that the truss is the best design due to its proven efficiency, constructibility and aesthetics. The final product was honed to be practical, strong, and have extreme rigidity. We decided to use softwood for our bridge due to its availability and versatility, and in this part of the country, Douglas Fir is most commonly used. We also decided to use treated sawn lumber to increase the practicality of the bridge and to simplify the construction and acquisition of the lumber. Our team chose a solid panel deck because it distributes the loads more thoroughly. We also decided to apply some redundancy to the deck by using a panel and topping it with the same crosshatch tongue and groove system for a strictly aesthetic purpose. For the truss connections, we decided to make use of lag bolts and metal plates. The lag bolts preserve the truss’s two force member trait while the metal plates allow for more support and stability when needed. For the purlins and the deck, we used metal hangers and nails which allow the load to be efficiently transferred from the deck to the truss. trait while the metal plates allow for more support and stability when needed. For the purlins and the deck, we used metal hangers and nails which allow the load to be efficiently transferred from the deck to the truss. As we placed each load, we could hear some cracking of the wood and shifting of the members. The deck began to pull away from the supporting joists and purlins. Additionally, the decking out toward the supports began to bow up and separate from the supporting structural elements even more. One of the joists we were measuring for deflection developed a large horizontal crack through a knot located at the center of the joist. This crack extended significantly as the load increased, but did not spread much during the hour testing period. The loads were placed over a time of five minutes and 40 seconds. At the end of this loading, the average deflection at the gage locations was only 2.71 mm for the vertical bridge deflection measurement and 5.66 mm for the deck supporting joist deflection and only 4.24 mm for decking panel deflection. The increased deflection during the hour testing period averaged around .30 mm. Because the decking panel did not deflect as much as the supporting deck members, the net deck deflection was only 1.91 mm. The overall bridge deflection of 3.95 mm was only 42 percent of the allowable deflection. Overall, the bridge performed well under the loading. We learned that our connections were the cause of much of our deflection. Although the deck was over-designed, it experienced more deflection than we had anticipated. For future design projects, it will be most important to ensure that the connections are effectively designed to prevent unexpected deflection.

5. Project Drawings and Photos

Longitudinal Cross Section	Tranverse Cross Section	Trimetric View	Project Photo	Team Photo
Click on drawing or photo above for larger view.

6. Component Details
In ten (10) words or less per each component below, describe the bridge:
Stringers/Girders:	2x8 suspended from purlins with joist hangers
Deck:	3/4in plywood, transverse strong orientation with alternating 1x6 hatch
Floor Beams:	4x8 transferring joist loads into tied truss using hangers
Suspension:	2x8, 2x4 Tied Truss, attached custom plates and lag screws
Unique:

Describe preservative treatment for all wood members. Include type and concentrations. Also include a short statement of why this treatment was selected. Did the treatment requirement present any special problems? If yes, provide details
	Conrad Forest Products donated all of the wood members needed for the construction of our bridge. Conrad treated the lumber for the bridge with Copper Azole (CA-B). This treatment process provides protection against targeted wood destroying organisms and insects. It also allows the external surface to be painted and remains in the wood for leach resistance. The wood used in the bridge absorbed .21 pounds per cubic foot of Copper Azole. This meets AWPA standards for ground contact.

7. Special Considerations