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 Merrimack College entry into the National Timber Bridge Competition was one of the projects for the Timber Design class. The students undertook the responsibility of designing, constructing and testing their entry as partial fulfillment of their class work. The first design concept considered was construction of a wood truss. Because wood is weak in tension, the truss was designed to have as many members in compression as possible. Members acting in tension were constructed in steel. The bottom chord of the truss was constructed as a composite beam. The beam consisted of spliced 2x6 dimensioned wood with plywood sheathing. This construction enabled the students to maintain the minimum member length requirements as specified in the rules and still span the specified distance. (The beam composition is given in the description of the “stringers/girders” on page 8 of the on-line entry form as well.) The upper truss members were constructed with wooden dowels because they were inexpensive and when tested in compression gave good results. All dowels were clear of knots and other discontinuities making them excellent wood members. The truss members were connected with sections of steel pipe welded together at fixed positions along the bottom chord. Because these welded joints did not act as pin connections, the support structure of the bridge, although looking like a truss actually acted as a built up beam.

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 was loaded at the civil engineering test facility at the University of New Hampshire. It was loaded at four points on the bridge deck equidistant from the center, at the position in the longitudinal direction, as designated in the rules. These points were considered the weakest point on the deck because of the support system under the deck. During loading, the beams deflected much greater than anticipated. Although it was expected that the deck could experience some deflection because there were no cross members directly under the load points, the structural analysis (computer modeling) performed prior to finalizing the bridge design indicated that the beams would be stiff enough to met the established criteria and would not deflect more than 8 mm. Some sections of the deck experienced a slight rippling effect when under load. No visible crushing was evident. The dowel truss members showed some signs of bending. During the testing, there were sounds of the glue cracking. Besides the sounds of cracking glue, the test was quiet. During loading, the beams deflected much greater than anticipated. Although it was expected that the deck could experience some deflection because there were no cross members directly under the load points, the structural analysis (computer modeling) performed prior to finalizing the bridge design indicated that the beams would be stiff enough to met the established criteria and would not deflect more than 8 mm. Some sections of the deck experienced a slight rippling effect when under load. No visible crushing was evident. The dowel truss members showed some signs of bending. During the testing, there were sounds of the glue cracking. Besides the sounds of cracking glue, the test was quiet. The beam deflections for the right side and left side were similar. At no time when defections were recorded did one beam defect more than 4mm greater than the other beam.

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:	Two 2x6s glued and screwed between 5/8in plywood strips.
Deck:	Primed pine srpuce fir 1x6 members
Floor Beams:	Pine Spruce Fir 2x4s
Suspension:
Unique:	Truss Members, 7/8in dowels surrounded by 1/2in dowels glued together.

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
	BEHR Wood Preservative. Unfortunetly, because of lack of funds, this was all that was availible for use. The Concentrations were unknown.

7. Special Considerations
	Because of the planned use of the bridge, the design of the truss had to be above the bridge. It's future home is at the home of the Timber Design Profesor's sister's house where it will be a means of crossing a small creek in their back yard. The truss had to be above to preserve the stream bed profile.