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 Rose-Hulman 2001 design team's goal is to construct a sturdy, practical bridge utilizing a truss frame for support. We are building this bridge for the Timber Design Competition, but we hope that we could donate the bridge for public use somewhere. The goal of our design was to use readily available material to construct a bridge that was both strong and economical. Using Visual Analysis 3.1 we have analyzed our truss system and have found that our maximum deflection was estimated at 5 mm. Furthermore, we feel that the designs for the decking, joists, truss members, and connections will gain favor with the economical and constructability criteria. Decking Our decking design provides both stability and aesthetic characteristics to our design. We gave special attention to the decking design, because the deck deflection category is of great importance in the competition. The decking will be constructed of 2 layers of tongue and groove 1x4's with a modulus of elasticity of 1300 ksi. The layers will be placed at a 45° degree angle to the deck and the top layer of decking will be placed perpendicular to the bottom layer. The tongue and groove members will be both glued and screwed. Joists The deck joists will be constructed with 2x6 Southern Pine #1 members with a modulus of elasticity greater than or equal to 1700 ksi. The bridge will be constructed with 9 joists. The two end joists will be 250 mm from the end of the deck and the seven other joists will be spaced at 500 mm. Truss Members We feel that the truss is the strongest component of our design. We were able to design the truss using all 2x4 Southern Pine #2 members. For extra support we added a steel tie rod that will provide a great deal of tension capacity to the bottom of the truss. We plan to use ½ " diameter steel rod to tie both ends of each truss together. We also plan to use a steel bearing plate to prevent crushing of the truss members from the force of the steel rod. Connections There are three categories of connections in our design. The first connection is between the truss members. For this connection we plan to use both plywood gusset plates and half-lap joints. The plywood will be ½" pressure treated CCA sheets that we will rip to form the plates and then nail and glue to the truss members. We will refer to the PDS manual for design specifications for the gusset plates. The half-lap joints will be nailed together as well. The second connection will be a steel bearing plate used to connect the steel chord to the truss. The last connection detail will be that of the deck to the truss. This connection will consist of 2x6 members attached to the top chord of the truss, which will then be screwed to the deck joists to provide lateral stability of the joists.

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 two main trusses are braced against racking by x-bracing at either end of the bridge. Also, they are braced at the bottom chord with knee braces connected to the underside of the joists. We were concerned at first about the possibility of the trusses folding over due to the eccentric nature of the load. However, during loading little or no bending or leaning of the trusses was observed. Therfore, we conclude that the x-bracing was more than adequate. Under load, the bridge behaved acceptably. As can be seen by the deflection readings, the creep occuring in the bridge nearly disappeared after about 45 minutes. Overall, the bridge behaved as expected except for the fact that we had not anticipated quite the amount of deck and overall deflection.

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:	Four triangular 2X4 trusses act as main girders.
Deck:	Two layers of 1X4 T+G oriented at 90 degrees to one another.
Floor Beams:	2X6 transverse beams span the two sets of trusses.
Suspension:
Unique:	Tension rod at truss bottoms to resist spreading forces.

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 material is Southern Yellow Pine CCA pressure treated lumber of at least 0.4 retention as this is mostly all that is available in our locality.

7. Special Considerations
	As with all treated lumber, leaching of the preservative into the environment may be a concern if acids are present. We plan to donate this bridge to a local boy scout camp or other youth camp for use as a footbridge. We will recommend that the bridge be periodically sealed so as to prevent leaching and extend the bridge life.