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 fundamental idea behind our bridge was to design and construct a structure that did not incorporate the more standard or ubiquitous designs seen on most bridges, such as trusses and arches. Our idea was to build a bridge that resembles a tube, using plywood and fiberglass as the main components. The outer skin of the “tube” bridge had to be able to withstand the loading, as there were no conventional members, such as beams and girders. In order for the tube bridge to hold its shape, five interior frames were constructed in the shape of parallelograms, using treated Southern Pine 4”x4”s. The frames were held together with toothed metal connector plates. The frames were then laid out and evenly spaced, with a frame at each end of the bride, one in the middle, and one each at the quarter points. The next step was to attach the plywood to the trapezoidal frames. Sheets of treated plywood were attached to the frames giving the bridge its unique tubular, trapezoidal shape. The plywood was oriented on the frames so that a full sheet would be directly under the loading points. The plywood was attached to the frames using 3” wood screws. For the next layer on top of the plywood, we used a 50” wide E-glass fiber fabric mat impregnated with epoxy. The fiberglass mat was cut into correct lengths and covered with epoxy, then allowed to set for 3 hours allowing the epoxy to become tacky. In order to gain access to all four sides, the bridge was set up on a makeshift rotisserie, which consisted of a 20’ piece of pipe run longitudinally through the middle of the bridge. Doing this enabled the bridge to be rotated about its long axis. The fiberglass was then laid out on the bridge in two layers. The first layer consisted of lengths of fiberglass laid out longitudinally on all four sides of the bridge. The next layer was laid out transversally, using four 50” wide strips, which enabled a considerable overlap in the middle, further strengthening the bond of the fiberglass. The top wear surface was screwed into place on top of the fiberglass. Treated Southern Pine 1”x6” boards were used for the wear surface. A steel frame was erected for testing. The steel frame contained a hydraulic press to provide the 20 kN load required for deflection testing.

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 bridge is a composite tube with 5 trapezoidal wooden braces inside to hold the shape of the bridge during construction. As such there is no independent deck or main members. The trapezoids are composed of treated 4x4 Southern Pine members connected with toothed metal plate connectors. Treated 3/4" plywood was attached to the trapezoids with wood screws. Fiberglass was placed onto the exterior of the plywood in two layers. A longitudinal layer of fiberglass for flexural strength and stiffness was overlain onto the plywood. A transverse layer of fiberglass was overlain onto the longitudinal fiberglass to provide shear strength and material confinement. The 1x6 treated Southern Pine wear surface members were attached to the composite tube with wood screws. Finally, the 2x4 treated Southern Pine curbs were attached to the top surface of the bridge. The bridge was loaded using a test frame and a hydraulic ram with a calibrated load cell. The deflection of the bridge was made using four digital displacement gauges that measured to 0.01mm. Digital displacement gauges were placed directly under the left and right loading points as well as the center between the loading points. Since the bridge is a tube it can either be said that the bridge has no deck or that the entire bridge makes up the deck. As such we originally placed the fourth gauge at the center of the bridge. During the loading of the bridge occasional popping noises could be heard but after the hour had passed the displacement gauges showed that the bridge deflected slightly over half of the allowable deflection (5.05mm out of 10mm). A second loading was performed where a digital gauge was positioned inside the bridge transversely between the loading points to measure the net deflection of the top surface of the bridge. This deflection was reported as net deck deflection. The deck deflected less than half of the allowable deflection (5.94mm out of 14.4mm).

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:	The bridge uses plywood and fiberglass as a composite tube.
Deck:	The bridge is a tube. It has no specific deck.
Floor Beams:
Suspension:
Unique:	The bridge uses five wooden trapezoids to support the shell.

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
	We purchased pressure treated S. Pine lumber for the bridge from a local lumber yard. We selected the wood with this treatment because it was the only type of pressure treated wood locally and readily available.

7. Special Considerations
	Our team took no considerations over environmental impact factors when planning the bridge. The future plans for this bridge is to be used in a children’s playground.