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)
	In designing the bridge, several choices had to be made. First, Southern Pine was selected as the wood product to be used for structural members due to its relatively light unit weight, as well as its relatively high modulus of elasticity. Also, it was decided that the bridge was to be made almost entirely of wood products, with a minimum amount of wood glue and steel fasteners incorporated in the design. It was also decided that the bridge be designed as a symmetric structure for ease of loading setup and construction. Once these choices were made, treatment options had to be considered. It was decided among the team members to use readily available pressure treated wood products treated for below ground use. Next was the decision to use an over slung truss design. It was the opinion of the team that this type of structure would provide the strength needed for preventing excessive deflections as well as good aesthetic value. The trusses themselves were assembled with 1X4 and 1X6 members spaced 3/4" apart to allow for greater strength while minimizing weight. Truss connections were made by attaching members to gusset plates. Plywood was the choice for the gusset plates due to its consistent engineering properties in different loadingconditions. The trusses were connected by transverse members, which were overlain by the decking. The deck surface was made from 1 x 6” planks placed perpendicular to the bridge’s longitudinal axis. They were connected to each other using glued tongue-and-grove joints and screwed to the twenty-six 1 x 3” stringers below. The 1 x 3” stringers were spliced in alternating locations in areas of low moment. Below the stringers, ľ” plywood sheets were used in the positive moment regions to stiffen the deck. Transverse beams were screwed into the bottom of the stringers at the negative moment regions. Basically, the deck was designed to act as multi-celled box beams at the positive and negative moment regions, and T-beams near the areas of contra-flexure. The 1 x 6” deck surface would act as the top flange, the stringers would act as the web, and the plywood (or top flange of the transverse beams) would act as the bottom flange. This would provide a very stiff deck, which would be equally stiff over the entire deck surface. The load from any bearing plate would be transferred from the deck surface to the stringers, from the stringers to the transverse beams, and finally from the transverse beams back into the trusses. The deck was designed to be built in two sections. This would make fabrication easier and if the bridge were in service, it would also allow one-half of the deck surfaces to be replaced, while keeping traffic open on the other half of the bridge.

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)
	Upon completion of bridge construction, the bridge appeared very strong in all directions with the exception of slightly wobbly trusses and wood that was still damp from treatment. It was decided to go ahead with the loading despite these problems. The bridge was placed upon blocks at each support. Between the block and the support a load cell was inserted to monitor the loading of the bridge, as well as to determine the dead weight of the bridge. Deflection gauges were set up at the appropriate points according to the given rules (directly beneath loading point, longitudinal member both left and right of loading point, and center of longitudinal member). The gauges were placed beneath the points that they were monitoring. The load was applied using a hydraulic jack. The jack would press against a steel beam extending freely through the truss and anchored into the floor. This action would load a steel beam under the jack, which in turn would load a solid wood structure constructed of plywood and 2X4’s, which would finally load the bridge deck through the steel loading plates on the deck surface. The first step was to obtain the dead weight of the bridge using the sum of all load cell readings. Next, the loading setup was placed on the bridge and again, the load cell conditioners were read to adjust for the weight of the loading setup on the bridge. Finally, the load was applied to the bridge as per the competition rules. The gauge readings at each loading point were then read as each 5kN load increment was applied. At full loading, the gauges were again read, as well as photographed with a digital camera. The bridge was subjected to the full loading for 1 hour, with deflection readings taken every 15 minutes. The bridge trusses performed very well during testing. Some cracks did develop in compression members experiencing heavy loading, but did not seem to result in any weakness of the structure. Maximum truss deflection at the center of the longitudinal beam was 1.68mm. The deck on the other hand, deflected much more than was anticipated from design calculations. The overall net deck deflection was 5.74mm, which was over our calculated 4mm max net deflection. It is the belief of the team members that the moisture remaining in the wide flange beams caused the modulus of the wood members to be much lower than the values used in 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:	Longitudinal beneath the deck surface, primarily for load transfer
Deck:	Transverse planks on longitudinal stringers with plywood support beneath stringers
Floor Beams:	Eight transverse beams, wide flange, 2X6 flange, 1X4 web
Suspension:	Overslung Truss, 1X6 compression members, 1X4 tension members
Unique:	Bridge built in four parts, 2 trusses, 2 deck halves (longitunal)

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 chose to go with commercially available pressure treated wood (from Lowe®s) that is treated for below ground use. This treatment was chosen for the practicality of buying the wood pre-treated, and to ensure a long post-contest life of the bridge. The treatment did however pose problems due to a lower modulus of elasticity of the "damp" treated wood. Also some shrinkage of the wood after drying caused splitting of the deck surface.

7. Special Considerations
	The bridge will be displayed at UMR Campus events for viewing by students and faculty, as well as campus visitors.