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)
	Timber Bridge Abstract Many elements were considered during the design of this year's entry. The trusses are simple Pratt Deck Trusses, optimized for stiffness, aesthetics and efficiency. The top chord was constructed by splicing members under the specified length of 1.4 m using finger joints. These splices were located at the intersection of the verticals to limit the number of steel plates required. Vertical members were fastened into the truss system using steel shear plates. All diagonal bracing, and the bottom chord members were fabricated from 3/8" A36 round stock. The bottom chords were constructed from straight members bent to the appropriate angles required at the end panels. Ends of the steel members were individually threaded to allow for pre-tensioning. The spacing between the two trusses was designed based on an in-house FORTRAN analysis model. This model was used to determine the optimal spacing such that the differential deck displacement at the cantilever tip and at midspan were minimized depending on all possible load orientations. The selected truss spacing also permitted the centroid of the load to be located between the two trusses at all times, permitting the use of frictional connections between the floor beams and the trusses. The floor beams and top chord were notched in order to interlock the deck with the trusses. These joints were designed as frictional connections rather than mechanical connections to decrease the use of non-wood materials in the structure. Floor beams were designed in accordance with the determined truss spacing to resist shear and bending, and to ensure the deck deflection was below the allowable 2 mm. The floor beams were cut to a height of 76.2 mm from standard 2x4 sections to minimize weight. A lightweight and stiff deck is essential for any bridge system to be efficient. In order to achieve a lightweight deck, various deck designs were built and tested isolated from the main structure. It was determined that a composite section similar to an orthotropic plate would be ideal for the decking. The final design consisted of continuous outer flanges separated by a 38 mm composite web. The deck elements were designed assuming an effective width of 400 mm, and webs were spaced at 60 mm to ensure no direct shear was applied to the flanges. The deck was constructed from Micro-lam webs sandwiched between 6 mm plywood. All wooden truss members were constructed from Micro-lam engineered lumber. These boards were readily available, and were chosen over lighter alternatives. Micro-lam timbers were ripped to size and spliced making sure to preserve the strong axis orientation of the wood grain.

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)
	Summary of Bridge under Load The bridge was assembled on steel abutments and placed in a reaction frame. Load was applied to the bridge with a hydraulic actuator located at midspan of the reaction frame. Steel channel sections were used to distribute the applied load to four specified points on the deck. Displacement transducers were attached at the prescribed locations and connected to a computer data acquisition system to monitor the bridge response. Pre-tensioning of the steel members was completed 48 hours prior to load application. Testing was completed as prescribed in The 1999 National Timber Bridge Competition Rules with a maximum load of 20kN applied for one hour. As the load was applied incrementally, the deflection increased linearly. While the load was increased to the 20kN design load, creaking was audible and the bridge experienced an average deflection of 6.10 mm at the outset of the 20 kN load. During the one-hour duration of the 20kN load, minimal additional creep occurred to a final average displacement of 6.42 mm. The deck and floor beam system performed well under the applied load. The built up composite decking showed no signs of punching failure, and experienced a maximum deflection of 1.7 mm During loading, cracking of the North vertical at midspan occurred between laminate plates of the Micro-lam. This was due to seating of the steel diagonals at this location. This cracking resulted in additional displacement of the North truss. In general, the deck and trusses performed well. However, the overall performance of the structure could be enhanced in the future with improved detailing of the steel timber interfaces.

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:	None
Deck:	Micro-Lam webs sandwiched between 6mm plywood
Floor Beams:	Notched Micro-Lam to interlock with the truss system
Suspension:	none
Unique:	Steel tension members and diagonal bracing

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

7. Special Considerations