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)
	San Jose State University has entered the National Timber Bridge Contest and built and tested a truss bridge made of pressure treated wood. The main criterion of the contest is that the deck of the bridge is not to deflect more than 10mm at the end of one hour of testing. Therefore, this criteria was the main concern for the design of the bridge. Weight and aesthetics were a concern, but strength was the main priority. The net deflection was measured to be 6.8 mm at the end of one hour of testing. The bridge truss and support members consists of 2"x6", and 2"x4" pressure treated wood with a C-2 retention value. The deck consists of 2"x8" standard Douglas fir wood; the deck can be painted or stained to withstand the daily foot traffic and weather it would need to withstand. The bridge was tested on April 18th at the engineering building at San Jose State University. The bridge was positioned under a hydraulic press and supported by metal plates 60mm in width. Four 60 x 90 mm plates were placed on the deck according to the rules of the contest and aluminum beams were placed on top of them to form an H pattern. Five voltage meters were attached to the bridge via eyehooks inserted into the wood and wire string. Using the hydraulic press and pressing on the center of the H-pattern the test was conducted until a 20KN load was achieved. At each 5KN increment the deflection at the bridge at each measuring point was recorded. Then at each 15-minute interval following the deflection of the bridge was recorded until one hour had elapsed. The deflection of the bridge was calculated from the voltage meters. One change in voltage i.e. from 10volts to 9 volts means the deflection is equal to one inch. The testing process was video taped and photographed as evidence of the actual numbers. However, the quality of the video and an accidental erasure of some of the testing process damages the authenticity of the procedure. But, the entire process was conducted to the best of the team’s ability, without bias, and in the hopes of being considered eligible to participate in the contest.

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 model bridge has a span of 4-meters with a 1.2-meter wide walkway and a 0.9-meter vertical clearance. The support members of the bridge are all made of C-2 rated, commercially available, pressure treated wood. No individual piece of the bridge could be longer than 2.1 meters in length and a limit of 25% weight could be non-wood; therefore to achieve a span of 4-meters, pieces of wood had to be spliced together and the amount of non-wood material was minimized. The bottom chord of the two trusses are 4 – 2x6” members nailed together with slots in them to accept the 2x4 truss members. In other words, the bottom chord is acting as a gusset plate securely joining the bottom joints of the truss inside of it. The bottom chord has wood splices in order to abide by the 2.1-meter limitation, and as a result, Simpson 4X6 post base are used as mending plates. The top chord of the truss has 2x6 pieces not exceeding 2.1 meters nailed to the top of the truss on the exterior side of the bridge acting as a gusset plate. All truss joints are nailed with 16d galvanized nails and secured with a 3/8” galvanized carriage bolt. The deck joists connecting the two trusses are secured in place with Simpson 2x6 double shear joist hangers nailed with 16d-galvanized nails. The Douglas fir deck is nailed to the 2x6 deck joists with 16d-galvanized nails. The overall weight of the bridge with the deck is 345 kg; although this is a lot of weight, anything lighter (with a low non-wood percentage) in this design is unstable as our first bridge collapsed due to instability during testing. And, the total cost for this bridge is low due to the fact that we disregarded weight and purchased wet wood for a lower price. The bridge behaved well during testing, for the bridge did not make any creaking or settling noises as it was being loaded. The bridge did initially deflect as it was loaded, but as the test went on i.e. during the last half-hour of testing, the deflection was relatively zero. The net deflection at the end of the one-hour period was 6.8 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:	2"x6" pressure treated douglas fir 1.2m long
Deck:	2"x8" stud-grade, douglas fir, ready to be painted.
Floor Beams:	n/a
Suspension:	deck is supported by 2"x4" pressure-treated trusses.
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
	All wood except for deck is pressure-treated with a retention value of C-2. This wood was choosen becuase it is readily available at all wood supply stores and is reasonably inexpensive.

7. Special Considerations
	The bridge we built will become a permanent fixture in the backyard of the team leader's house. He plans to span a small coy pond on the side of his house.