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)
	The goal of the 2006 OSU Wood bridge team was to make a bridge as simple and strong as possible. We knew we could control the deflections caused by the forces acting on the beam, but we also wanted to control the deflections caused by the imperfections of our connections. We thus set-out to make a bridge with as few mechanical connections as possible. Quickly into the design process, it became obvious that a glulam beam could be efficiently made to span the whole length with no mechanical joints. The individual pieces of the glulam could be cut to adhere to the rules of maximum member length, but the final product would span the full 3.8 meters. Many comparisons were conducted for the shape used in our beams. An I-shape gave us a very deep member that pushed the height limits of the competition, but was substantially lighter than a rectangular section. We selected our final shape by setting a target moment of inertia for minimal deflection and choosing the lightest section that fit the target. Because our school term is only 11 weeks long, we decided to ease the construction process and only make three beams. We found that it was easier to use a larger deck with fewer supporting beams rather than a smaller deck with more supporting beams. While this makes for a less intriguing deck design, it saved time and resources. A larger deck also contributed to our overall stiffness by providing substantial lateral support to the underlying beams. The only mechanical connections used in the bridge were 4” wood screws used to attach the deck to the beams. With only 174 screws total, the final connection weight not including the glue came to be a little over 3 pounds. The percent wood used totals over 97%. We feel that this is an admirable feature for our bridge’s weight. The distribution of loading caused the center beam to carry the largest amount of load. To simplify the construction documents and to provide a factor of safety, each beam was designed to be able to carry 50% of the load. After all three beams were constructed, the strongest and best made of the three was placed in the center and the other two placed on the sides. An additional feature was added to our beam near the end of the project which also helped our stiffness. Glass fiber reinforcing was attached to the tension faces of the beams using the very same glue we used for the wood. These strips of FRP helped increase our strength, lower creep and overall deflection, while adding only another three pounds of non-wood weight. In the end, our bridge was easy to construct, simple, stiff, and only moderately heavy. While the bridge weight was greater than we wanted, it was comprised almost entirely of wood and fit our goals.

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 bridge was expected to perform rather well, as it was essentially designed around the maximum allowable deflection, with a factor of safety of roughly 2.5 to 3. However, as we found out during the construction of the bridge, new problems can be created in constructed projects due to unforeseen issues. In spite of these unforeseen issues, the bridge seemed to deflect similarly to what was predicted. This bridge performed quite admirably under load. We did not hear creaking when the loads were applied to the bridge. Although cross bracing would have been needed if lateral loads were applied to the bridge, it did not deflect laterally upon application of test loads. The bridge was very stable both during and after the application of loads. Subsequently, it met all deflection requirements of the Timber Bridge Competition. Under the load for maximum vertical deflection, the total deflection of the bridge was roughly what was predicted from calculations. The maximum vertical deflection was approximately 3.3 millimeters, which is far less than the maximum allowable total deflection of 9.5 millimeters. Approximately 96 percent of the deflection occurred immediately under loading, and creep seemed to be of minimal importance with regards to our bridge performance. Under load, the maximum vertical deck deflection was also rather close to predicted values. The net vertical deck deflection in the bridge was approximately 2.5 millimeters, with a gross deck deflection of approximately 5.1 millimeters, and an average member deflection of approximately 2.6 millimeters. This was well within the allowable deck deflection of about 5 millimeters. Again, the vast majority of the deflections occurred under initial loading, with creep adding little to the total deflection after one hour of loading conditions. Overall, we believe our bridge performed very well under load. Although the design lacked lateral cross-bracing members, the deck was sufficient to distribute the load evenly to all members of the bridge. Also, issues within the constructed bridge, such as slight warping in two of the I-beam webs and a slight bend in the joint of the other I-beam web, did not seem to deter from the performance of the bridge under load. The utilization of fiber reinforcement in the extreme tension fiber may have assisted in terms of reducing creep in the bridge under sustained load. It is our opinion that our bridge would fare rather well in the Performance Awards section of the Timber Bridge competition, especially the Best Support Structure Award. Based off of results from the 2005 contest, we believe our bridge would fare very well in the Maximum Bridge Deflection category. The weight of our structure is not exceptionally low, but we have created a structure that is nearly all wood. Also, we believe the practicality of our bridge is an asset. Finally, even though an I-beam is not necessarily an innovative design, we believe the way in which our I-beams were constructed may be somewhat unique for the competition.

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:	Three Composite Glulam I-Beams
Deck:	3x6 Douglas Fir attached to top flange by screws
Floor Beams:
Suspension:
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
	Pretreated: Conrad Forest Products provided all of the treated lumber, which was chosen to be Douglas-fir for its local availability. They incised and pressure treated the wood with a chemical called Copper Azole - type B. The product name is "wolmanized residential outdoor wood". Post treated: After the construction process is completed we will put a surface treatment on any exposed areas since pretreatment primarily protects the outer surface of the wood. The chemical selected for this is called Copper Naphthenate.

7. Special Considerations
	An option, which may be beneficial to the OSU student chapter of the American Society of Civil Engineers, is to sell the bridge. All proceeds from the sale of the bridge would be donated to ASCE, which assisted with funding for this year’s bridge project. The money could be earmarked for use in future Timber Bridge Competition designs or for the general use of the ASCE student chapter. If the bridge were to be sold, it is possible that smaller sawn lumber members may replace the current deck. We believe that the 3x members used for the deck could be utilized in other ways, and could bring more money to ASCE if sold separately from the bridge structure.