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 main goal of our project was to design a structure which combined strength with beauty. Beauty and strength can both be found in one of the oldest design concepts known in nature, the arch. Our choice of an arch, as the principle structural member of our wood bridge, bred two difficult engineering hurdles: material selection and arch design and fabrication. In compliance with the rules of the competition we needed to choose a material that could be fabricated in the arch shape as well as meet the wood treated requirements. Our choice for this material was Clear Heart-Cut California Redwood in 4” x ˝” x 96” lengths. The material was approved by Bennie Hutchins, the Program Coordinator. Bent over an engineered plywood form, 12 layers of redwood lumber were placed and held together by PL Supreme Wood Adhesive and wood screws. The arch construction process began by constructing the first inner layer of the arch. This layer was then placed and mounted to the form. Every subsequent layer was then glued and screwed to the previous layer. The completed arches were allowed to cure for seven days, as set forth by the wood adhesive manufacture, so that the glue achieved its ultimate bonding strength. Once the arches had achieved there ultimate strength they were removed from the forms, the ends were trimmed, excess glue was removed, then the arches were planed and sanded to a smooth finish. The end trusses, decking, curbs, stringers, and floor beams were all constructed with regard AWPA standards. We chose to use Pressure Treated (ACQ) Northern California Douglas Fir, No. 1 & Better. The deck, end truss, curbs, and stringers were constructed from 2” x 4” members. The floor beams were constructed from a composite of 2” x 6” support members with 4” x 6” spacers. The bridge was design and constructed as follows: The two built-up end trusses members were used to in lateral support of the arch members. The floor beams were placed at quarter-span lengths underneath the 6 joists and were connected to each arch by a combination of 5/8” threaded steel rods and Ľ” steel cables and tensioners. The joists ran along the length of the bridge from end truss to end truss and were held together with flat metal tension plates. The deck consists of 43 individual DF members connected by two steel flat plates per deck member and was oriented perpendicularly to the joist members. The deck was connected to the joists by steel L-brackets. Once the bridge was constructed and assembled the testing set-up was placed on the bridge, calibrated, and the bridge was made ready for testing. Testing consisted of applying a 20 kN load centered at mid-span and set 4 1/8” from the right hand curb. The results of the testing were a total bridge deflection 1.124 mm measured under the right-hand, mid-span floor beam. Our gross deck deflection was 1.397 mm. Our net deck deflection was 0.1651 mm with allowable deflection 2.41 mm.

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)
	Four gages were placed under the bridge at the locations specified by the competition rules. The gage reading the deck deflection was located were the greatest estimated deck deflection was calculated to occur. This point was directly under the loading point and midway between the adjacent stringers and floor beams. The gage reading the maximum bridge deflection was located under the center floor beam. The competition requirements stated that a 20kN load must be applied on top of the deck for one hour. The maximum deflection requirement at the mid span of the longitudinal beam receiving the greatest amount of load is 9.5 mm and the maximum deck deflection requirement is 2.413 mm. Gages 2, 3 and 4 had an accuracy of 1/100th of an inch, whereas Gage 1 had an accuracy of 1/1000th of an inch. As shown in submitted drawings and photos, the force was created by wedging a hydraulic jack between a wood beam, a strain gage and a wooden post, which extended vertically up to a secure concrete beam. Using a calibration curve on the strain gage the hydraulic jack was jacked to the specified load amounts. Under the test load, the bridge performed extremely well with very little deflection. All of the deflections increased linearly, as the test load was applied. Throughout the one-hour period with the maximum load, the gages never read a deflection of more than 2 mm. After the one hour time period the maximum bridge deflection was 1.14 mm and the net deck deflection was 0.165 mm. These deflections are significantly less than the allowable deflections.

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:	6 Pressure Treated Northern Douglas Fir – 2” x 4” Stringers
Deck:	43 Pressure Treated Northern Douglas Fir – 2” x 4” Deck Pieces
Floor Beams:	3 Composite Pressure Treated Northern Douglas Fir – 2 2” x 6” Beams & 2 4”x 6” Spacers
Suspension:	6 5/8” Threaded A36 Steel Rods, Bolts, Washers, Ľ Steel Cable & Connectors
Unique:	2 Clear Heart California Redwood Laminated Arches – 3.5” x 6” Cross-Sectional Area

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
	In accordance to the parameters set forth by the competition rules, we purchased pressure treated Northern California Douglas Fir. The lumber was treated according to AWPA standards. Taking the environmentally friendly decision we decided on ACQ 0.4 pcf pressure treated Douglas Fir. Since Douglas Fir is abundant in Northern California, we had no trouble finding it in the lumberyards. We choose redwood for the arch because it is rot and insect resistant and does not need to be treated with wood preservatives. Furthermore, due to the requirements illustrated above, the ability of Redwood to be used in its natural state, untreated, was an environmental obvious choice, which combines both beauty and environmentally friendly engineering.

7. Special Considerations
	Coming from very diverse backgrounds, our group brought many different skills to the project ranging from carpentry and AutoCad to engineering design. In competing, we all broadend our knowledge and sharpened our engineering skills in what turned out to be a successful collaboration. We would also like to extend special thanks to John Richardson, Ernie Prien, and the SFSU School of Engineering for their generous donations to the project. Group Member Qoutes: Jim: “We hit it out of the ball park! Splash hit!” Rana: “Little successes breed confidence.” Faris: “People coming together from all different walks of life to establish a connection that will never be forgotten.” Bopha: “To take a concept and create it physically gives a great sense of accomplishment and inspiration.” Tom: “It was a challenging project which provided many engineering design and construction opportunities that brought us closer as a group.” Catherine: “I have never felt so connected and attached to a project as I have with this bridge. With only six minds, we created something that was not only beautiful, but very structurally sound.”