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 Washington University design team built a post truss bridge with the goal of minimizing weight and deflection. We also wanted connections which were easy to construct. There are four trusses total, made up of two inner trusses attached together and located at the center of the width of the bridge, and two outer trusses spaced 581.0 mm (1'-10 7/8") from the center. Each truss is made up of a 2x4 top chord, two 2x4 posts at a distance of 1812.9 mm (5'-11 3/8") from each end of the bridge, and tensioned with steel cable. The splice for the top chord of each truss is staggered so that weakness is minimized. There are ½” plywood gussets attached to the posts for strength. The deck is made up of 2x6, 2x8, and 2x12 members laid flat longitudinally, also spliced at different locations to minimize weakness. The deck is wider than the truss structure, so triangular blocks are attached between the top chord of the outer trusses and the deck to minimize the deflection of the edge of the deck. Purlins (2x4) are spaced typically 609.6 mm (2') o.c. to help distribute the load on the deck to the trusses. The connection between the tensioned cable and the top chord of the truss consists of eye bolts bearing on a steel plate. The plates are attached to both the top chords and the deck, positioned at an angle so that they are normal to the cables. The steel cable was tensioned using the same eye bolts that connect the cable to the top chord. Wood glue and “Gorilla Glue” are used for the majority of attachments, however, clamps and wood screws were used in the construction of the bridge to hold the members together until the glue set. Some screws remain in the structure. The bridge is supported on eight “feet” made of short 2x4 blocks and reinforced with ½” plywood gusset plates.

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 load was applied in 1/4 increments to the total load of 20 kN using a hydraulic press supported by a rigid steel frame. The deflection behavior was as expected for proportional loading increments. The load was transferred to the bridge via 3 I-beams in an H-formation. The point selected for deflection measurements was midspan, centered between two of the trusses. This loaction was critical for all the required measurements. We did not experience a great deal of creep duirng the one hour loading because the cables were taut and did not experience much additional strain.

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:	2x6,2x8,2x12 members laid flatwise longitudinally
Floor Beams:	2x4 purlins
Suspension:
Unique:	(4) post trusses. Tensioned w/ lug-bolt, 3/16" galvanized steel cable, steel bearing plate.

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
	Alkaline Copper Quarternary (ACQ) 0.25 pcf. The wood was allowed to dry for several weeks before construction, so the preservative posed no problem.

7. Special Considerations
	The bridge is going to be used over a small creek in Wisconsin to transport cattle from one pasture to another.