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 timber bridge was designed to be aesthetically pleasing visually and extremely stiff structurally. After researching various design philosophies a combination of integral structural systems was selected. Vertically laminated plywood arches were used to support an integral box-beam deck. The arches are a classical structural form that are efficient and stiff while providing natural beauty. The box-beam deck provided for efficient use of materials by positioning most of the material in the compression and tension flanges away from the neutral axis of the deck system. Doing this also dramatically increases deck stiffness as determined by employing the parallel axis theorem. The bridge performed exceptionally well under load with the bridge deflecting 1.61 mm which was only 17% of the allowable deflection of 9.5 mm. The deck performed even better with the net deck deflection of 0.24 mm being only 4% of the allowable deflection of 5.4 mm for our bridge's deck span.

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 timber bridge was a three arch structure supporting an integral box beam deck that spanned between the arches. The arches were composed of vertically laminated 3/4" and 1/2" thick sections cut from 4x8 sheets of treated Douglas fir plywood. The middle arch was solid to provide increased strength and stiffness. The exterior arches were designed and constructed with holes to develop an aesthetically pleasing profile. The holes also reduced weight and provided increased efficiency in material use. The integral bridge deck was designed as a box beam that had shared components with the supporting arches. The deck was fabricated with 2"x6" treated Southern Pine lumber, 3/4" thick treated Douglas fir plywood, 5/4"x6" treated Southern Pine decking, and the integral arch tops. The 2"x6" lumber was used as webs for the box beams. The 3/4" plywood was used as top and bottom flanges for the box beams. The 5/4" decking was used as the wear surface for the bridge deck. The 5/4" decking was machined into a tongue and groove system to increase the load sharing performance of the deck. The bridge system ended up weighing a total of 952.5kg The bridge system was very stiff and performed superbly under loading. No creaking or popping was heard as load was applied with a hydraulic ram. The system deformed elastically through the loading cycle. The middle arch deflected 1.61mm under full loading. This was (1.61mm/9.5mm) 17% of the allowable 9.5mm deflection. The net deck deflection was 0.24mm under full loading which was 4% of the allowable net deck deflection for our bridges deck span of 540mm. After the full load was applied the bridge resisted excessive creep deformations. The creep deflection of the main arch was only 22% of the initial elastic deflection at full load. The creep deflection of the deck was only 20% of the initial elastic deflection at full load.

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:	Vertically laminated plywood arches
Deck:	Plywood and lumber box-beams integral with the supporting arches
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
	Dimension lumber and decking was pretreated with Copper Azole for ground contact. The plywood was pretreated for ground contact with an arsenic compound. These treatments were selected based on the materials readily available at local lumber yards. The treatments did not present any special problems other than using proper saftey equipment during handling and cutting.

7. Special Considerations
	The experience of designing and constructing a bridge was beneficial to the team. It taught us to work together to finish the project. It also taught us a lot about construction and the need for precision when designing dimensions and later constructing a full-scale structure. The importance of engineering "on-the-fly" during the construction process was also experienced when things couldn't be constructed as originally designed. During the design process a few conservative assumptions were made regarding material properties, composite behavior, and load-sharing between members. It was very beneficial to see the actual results under loading and comparing them to the originally calculated expectations. Many team members also did not have prior construction experience. As such, participating in the building process was found to be very beneficial for both personal and professional reasons. No special environmental impact factors were considered during the planning stage of design. The bridge will be employed for two uses in the future. First, the bridge will be used as a recruitment display for high school students interested in structural engineering. Second, the bridge will be put into service as a creek crossing at a local nature area.