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)
	Wooden bridges are reliable, engineered structures because they use a material that is strong as a whole, but individually weak due to its variability. A wooden bridge was constructed as the final project for Wood Science 386 and as a preliminary qualification for the National Timber Bridge Competition. The final bridge design stemmed from many different initial conceptual designs. This bridge was designed to carry a 20 kN load with a maximum beam deflection of 8mm and deck deflection of 2mm. The main bearing supports of the bridge consist of four truss box beams. The truss box beams consist of a frame built from select grade kiln dried Douglas fir. It was found that Douglas fir was an ideal species because of its high density and strength. Douglas fir was determined to be the most economical species to withstand the shear stress and moment forces, which the bridge undergoes during loading. Kiln dried spruce-pine-fir (SPF) was used as bracing within the beam to withstand the compressive normal forces which are developed when the beams are loaded. The placement of the SPF members along the longitudinal axis was designed to make use of wood's natural resistance to longitudinal compression. The truss box beams were sheathed with oriented strand board to act as shear diaphragms and to help withstand as well as distribute the shear forces on the beam. PVAc glue and double-threaded screws were used to attach all joints in the beam to maximize the stiffness of the beam. Furthermore, all boards were planed before joining to increase the surface area contact between the joints. By using the truss box beams, weight was minimized because fewer wood members work together to transfer and distribute the load; therefore, the amount of material used in the beam's construction was minimized. Following beam construction, each beam was spaced appropriately using crate-like framing with finger-jointed SPF 2x2's. The ends of the crates were sheathed in OSB that functioned as a cross-brace to prevent the beams from buckling or "falling over". Next, the decking was placed atop of the evenly spaced beams. The decking consists of two layers of 1x4 SPF. The 1x4's were placed in a herring bone pattern to maximize overlap of the two layers which maximizes strength. Cross-linked PVAc glue and screws were once again used to inter-link the two deck layers to the beams. Although not the lightest decking material, budget constraints made 1x4 SPF the most economical choice.

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)
	After 720 people hours, the bridge had gone from conceptual design to a standing structure. The bridge was unique in design as it uses four truss box beams as the main supporting members. The truss box beams consisted of an internal truss frame and sheathed with oriented strand board. The truss box beams were laterally connected to each other with the use of cross members. The cross member was a wooden rectangular frame with oriented strand board covering the two ends. The oriented strand board was used to prevent the frame from shearing. Not only did the cross members strengthen and tie the beams together, but they also prevented the box beams from buckling from the load. The decking consisted of an alternating double layer of 1x4 placed in a herring bone pattern. By using the double threaded screws to fasten each joint, creep of the joints was prevented. After six days of construction, the bridge was completed and ready to be tested. Testing was performed by the University of British Columbia's Timber Mechanics Testing Department. The testing performed by placing steel plates onto the bridge according to the National Timber Bridge Design Competition rules and guidelines. The loads were placed, as stated by the guidelines, in the weakest points of the bridge. The Timber Mechanic Department staff determined the load positions. By using four box beams and the herringbone decking, the bridge transferred the load evenly throughout the structure. The average deflection of the right, center and left load positions for the bridge was 2.41 mm before the final load (20kN after 1 hour) was removed. The deck deflection was 0.05 mm. The similarity of the right and left deflection showed that the truss design distributed the load within the beam to minimize any area of concentrated stress. This also showed that the sheathing placed on the beams acted as a shear diaphragm, which was used to fight the shearforces experienced by the beam during loading. The bridge was designed to withstand the load and the bridge completed the task within the competition guidelines.

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:
Deck:	Doubled layered 1X4 herringbone pattern
Floor Beams:	Four Truss box beams evenly distributed within 1.4 meters
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

7. Special Considerations