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)
	This years entry concentrated on making an efficient but easily constructible bridge. To satisfy this, a simple box beam was chosen for the support structure. These could be constructed in a relatively short amount of time and at a relatively low cost when compared to a truss system. The box beams were constructed using Micro-Lam LVL 2x8s (50x203mm) for the flanges, oriented on their weak axis. The webs were constructed out of 3/8in (9.5mm) plywood. A channel was cut into the flanges approximately 25mm from the edge and 19mm deep in which the plywood was fit into using an adhesive. Stiffeners were also used on the internal edges of the plywood to prevent buckling from occurring. The tension joints on the box beam were also reinforced with an e-glass/epoxy composite. This was utilized in an attempt to prevent joint failure from occurring and to increase the overall tensile strength of the beam under loading. The spacing of these beams was then determined using an in-house FORTRAN analysis model. This model was used to determine the optimal spacing such that the differentiated deck displacement at the cantilever tip and at mid-span were minimized depending on all possible load orientations. The transverse members were then attached using steel joist hangers, which rested against a block on the exterior of the plywood web. These blocks served not only to prevent the joists from trying to twist downward and possibly damaging the plywood, but also added additional stiffness to the web. A lightweight and stiff deck is essential for any bridge system to be efficient. To achieve this, a composite section similar to an orthotropic plate would be ideal. The design consisted of using continuous outer flanges separated by a 38mm composite web spaced at approximately 50mm to ensure that no direct shear was applied to the flanges. The deck was constructed using ripped down 2x6’s (50x155mm) dimensional lumber, planed into 13x140mm planks for the top and bottom flanges. A simple jig was constructed in order to space the webs all at once. This allowed the planks to be fastened quickly without the concern of spacing individual webs throughout the construction. The deck was then fastened to the box beams using an adhesive along with screws to ensure full surface contact.

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 spanned between CMU’s to the required span and was placed in a reaction frame. Load was applied to the bridge using a hydraulic actuator located at midspan of the reaction frame. W6x9 sections were used to distribute the applied load to the four specified points on the deck. Displacement transducers were attached at the prescribed locations and connected to a computer data acquisition system to monitor the bridges response. Testing was completed as specified in the 2000 National Timber Bridge Competition Rules, with a maximum load of 20kN applied for one hour. As the load was applied in 5kN increments, the deflection increased linearly. No creaking or visible changes were observed as the bridge approached an average defection of 2.865mm at the outset of the 20kN load. During the one-hour duration of the 20kN load, additional creep occurred resulting in a final average displacement of 4.23mm. The fiber-reinforced tension joints on the beam proved to maintain stiffness as was observed from data collected by strain gages located on their surface. The deck and floor beams performed exceptionally well under load and was determined from the loading that the floor beams could have been neglected from the design. The built up composite deck showed no signs of punching failure, and experienced a maximum deflection of 0.54mm at the conclusion of the one-hour test duration. This deflection of the deck is well below the calculated value of 1.8mm using the prescribed formula. The test setup was then repositioned to where the critical load would occur. The position of this occurred immediately to the left or right of the central loading position. Upon loading at this point, the bridge continued to perform as expected and no visible changes or audible creaking was observed. The maximum deflection recorded was approximately 5.0mm, which was located over the right side beam at midspan. This was well within the 8mm limit defined by the design criteria.

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:	Fiber reinforced box girders
Deck:	Micro-Lam webs sandwiched between 13mm thick planks
Floor Beams:	Micro-Lam floor beams hung with joist hangers
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
	Preservatives were not overlooked for the design of our bridge. All members used in the construction of our bridge are available pre-treated or can be treated after construction to meet specified requirements. For example, Micro-lam can be special ordered with the desired treatment, and our deck surface is sold as pressure treated, or can be treated effectively after construction. However, our facilities are not equipped or prepared to deal with such hazardous materials or chemicals, so therefore no treatment was used throughout the project. Proper ventilation is required to apply such preservative treatments, and the storage of the bridge is generally indoors to allow students to be able to work on it and to prevent vandalism. In addition, these chemicals are very toxic to the skin, eyes, respiratory system etc. and we felt it was unsafe to use them during any part of our construction. Students are in constant contact with pieces of lumber containing these chemicals throughout the construction as they size and alter members. It becomes a big hassle to have to get all dressed up in attire appropriate to handle such chemicals just to pick up a piece or make a quick saw cut. With this for an explanation, it is hoped that you understand why we did not use a chemical treatment, but kept them in consideration throughout our design.

7. Special Considerations