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 bridge was designed as three separate subsystems, each optimized to serve a specific purpose. Due to the relatively light loads and small allowable deflections stiffness was the leading design criteria. The deck: Composite action between deck members was obtained through two glued biscuit joints, spaced every 10 inches along the members. Allowable deck deflection was maximized by running the boards parallel to the length of the bridge. The flexure formula and LRFD load tables were utilized with varying end condition fixities to determine the degree of composite action and required orientation of the members with an excel worksheet. This composite deck transferred load to five joists. Engineered I-Beam Joists: Designed to transfer loads to box trusses. Simply supported I-beams provided an efficient use of material and high moment of inertia. Excel was again utilized to optimize weight, moment of inertia, and deflection. A dado blade and standard 2x4 members, and the same plywood utilized in the box trusses made the members practical to construct. Box Trusses (supertruss): Our approach to the bridge contest began with an investigation of what forces the load would develop in any standard truss design. A force analysis immediately indicated that strength was not the critical design factor. An innovative idea to load the trusses in their "middle" through bearing on the joints allowed deep trusses and the possibility to utilize two trusses with joists perpendicular and decking parallel to the bridge span. Struggling with how to best design the truss joints, the solution of plywood gusset plates quickly developed into a box truss concept with the truss being laminated on both sides with plywood sheathing (glued and screwed). A test truss with 2x3s was constructed and loaded on an instron machine first with one side laminated and then with both sides laminated. The plywood and second sheathing dramatically increased stiffness. We adopted 2x4's for our final design at the expense of weight due to the unavailability of pressure treated 2x3's and the poor grades available. By designing each subsystem individually and then mating them together we developed a very stiff structure while keeping weight within a reasonable range. We also made our design to be modular. The three subsystems easily separate with the removal of a few screws. The bridge can be assembled, from its three subsystems, in 15 minutes by a team of 4 soldiers.

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 United States Military Academy Timber Bridge for 2005 was designed around two very stiff box trusses. These 1.2 m deep trusses, stiffed with plywood diaphragms, were expected to carry the load with minimal deflection. The I-beams used as transverse members where also very stiff compared to the expected loads. By biscuit jointing the deck we gained almost complete composite action allowing us to mobilize a much greater moment of inertia. Under the 20 kN load we saw very load deflections in both areas. Our maximum bridge deflection of 1.55 mm was within our expected range given the very effective design. The deck was such an efficient load path to the I-beams that our overall deck deflection was less then 1 mm. The biscuit joints and longitudinally oriented members performed even better then expected. Overall we are very pleased with the behavior of this design.

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:	Box Truss (Truss double sheathed with plywood)
Deck:	2x4 on weak axis with biscuit joints at 10" intervals
Floor Beams:	9" deep I-beams
Suspension:
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
	Commercially completed pressure treating. This was selected for ease of constructability and its ready availability.

7. Special Considerations
	While there are no specific applications planned for this bridge it was designed with light military units in mind. With the modular design the whole bridge can easily be transported with the HMMWV and assembled in 15 minutes by a 4 soldier crew.