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 design of the Cal Poly San Luis Obispo entry into the National Timber Bridge Competition is a hybrid of two structural systems that are known for being very efficient in carrying loads. Thus the main gravity force resisting system is an arched truss as demonstrated by the longitudinal members. While meeting the deflection criteria was the main concern, aesthetics and minimum weight were secondary criteria. Thorough study of past entries helped produce an optimum design. In order to minimize construction joints and weight, structural panels were laminated together to create longitudinal members that were constructed much like iLevel' Microllam' beams, only in an arched truss shape. These truss concepts were sketched on AutoCAD' and then modeled and analyzed using RISA-3D, a structural analysis program. The analysis results allowed material to be selectively removed from the trusses with minimal effect on the deflections. A test truss would later be built and tested to confirm these results. With the help of a computer numeric controlled router, dowel holes were added to the design to help optimized the construction process. Dowels helped align the truss pieces for laminating, but also helped stiffen the construction joints and kept the structural panels from delaminating. A ledger was added to each side of the trusses to provide points of attachment for the deck, and increased the stiffness of the trusses similar to the flanges of a TJI. The ledger effectively reduced the deck span to only 198 mm which helped increase the performance of the deck. A single truss was tested with a point load of 1500 lbs. (6.67 kN) at the mid span of the truss for an hour. The deflection after an hour of loading was 7 mm. These results were reassuring as no single truss is expected to carry more than 900 lbs. (4 kN) over two point loads/ To get the best performance from the deck, a 2"x12" deck was used to provide twice the stiffness of a typical 2"x6" deck. Each deck panel was screwed and glued to each truss. The glue between the deck panels and trusses helped transfer loads more effectively.

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)
	In order to meet the criteria for measuring bridge deflection and deck deflection, two separate tests had to be performed. The 20-kN load, applied in four intervals of 5-kN loads, and the total load was applied in approximately eleven minutes. Various concrete slabs and buckets of sand were used to achieve the 5-kN loads. The loads were applied with the help of a crane. Once each load interval was achieved, deflection readings were taken, and the next load was placed. Two gauges were used to calculate the maximum bridge deflection. These gauges were located at the mid-span of the two trusses receiving the most load. The two readings were averaged and submitted as the bridge deflection. After an hour of the bridge carrying the full 20-kN load, the average beam deflection was 1.45 mm. This deflection is much less than the maximum allowable deflection of 9.5 mm. The bridge had exceeded expectations. All deflection gauges used were accurate to a thousandth of an inch and then converted to metric units. In order to get the maximum deck deflection, the four point loads were moved transversely and longitudinally to get one of the loading blocks at the mid span of a deck member. Then deflection gauges were put at the center and both sides of the deck piece to get an accurate reading of the net deck deflection. The maximum net deck deflection was 0.58 mm, considerably less than the maximum allowable deck deflection of 1.98 mm.

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:	3/4 inch OSB Plywood laminated trusses with Titebond II Glue
Deck:	2 x 12 pressure treated lumber
Floor Beams:	none
Suspension:	none
Unique:	none

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
	Copper Green wood preservative was chosen as the wood preservative treatment for all untreated wood members. Copper Green is made up of copper naphthenate and hydrocarbon solvent. This treatment was chosen due to its ease of application and because it meets the AWPA ground contact standards. It was brush applied to all non-treated wood surfaces. All other members were pressure treated to ground contact standards.

7. Special Considerations
	Bridge will be placed in Poly Canyon to serve as a pedestrian bridge. Poly Canyon is a showcase for the College of Architectural and Environmental Design's senior projects.