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 goal for this year’s bridge was to maintain practicality in the design. Complicated designs were not pursued due to the difficulty in calculations and construction. Instead a bridge that incorporated a feasible design, which allowed for simpler construction and reduction in weight, was chosen. This design also allowed wood members to be cut and shaped in a more replicable fashion. The bridge consisted of two I-beam girders with stringers integrated into the top flange topped with a single layer of decking. I-beams were used because of their reduced weight, high moment of inertia, and ease of construction. The fourteen foot long I-beam webs were constructed of four layers of 0.5in plywood all glued together. Screws were added to make sure there would be sufficient contact between plywood layers. A 4x6 pressure treated board was used as the top and bottom flange. A 1.5in deep by 2in wide longitudinal strip was removed from both the top and bottom flange allowing a groove/slot to insert the web. The total length of the flange and web were cut to comply with the maximum member length requirement as stated in the rules. Lap joints were fabricated to reconnect the flanges. Premium wood glue and screws were used to fasten pieces ofthe flanges together and also the web to the flanges. Truss plates were added to strengthen the lap joints in order to provide a more continuous acting beam. Caps were placed on both ends of each beam. The caps consisted of extra lengths of unused flange material. These end blocks provided lateral stability of the I-beam as well as reinforcement against shear. A 1in x 1.5in transverse slot was cut into the top flanges every 6.5in on center. The 5 foot long 2x4 stringers were placed into the slots and secured to the girders with screws. Locations with flange lap joints did not have slots made. A cut out was instead made on the stringer. The stringers with the cut outs were fastened to the girders with nails and hurricane ties. By having the stringers sit into the I-beams, it offered more lateral support and allowed for easier connections. Fewer metal hurricane ties were used, thus reducing the non-wood weight of the bridge. With the stringers integrated into the I-beams the bridge acted more as a single unit, and allowed the applied load to be distributed more uniformly throughout the structure. The stringers were set on their shorter side so the load would be applied along their strong axis. This provided a strong support system for the deck to be placed on. A single layer deck of 5/4in x 6in pressure treated southern yellow pine was laid longitudinally. The deck was fastened to each stringer with two screws to give a secure fit. This bridge accomplished the goals of a practical, elegant, lightweight, and structurally sufficient design.

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 first load setup, measured vertical bridge deflection, consisted of the four point load system placed in the center of the bridge according to the schematic given on the rules page. Each end of the girders were placed on a roller, therefore the supporting bases were well below the maximum 60 mm requirement. Rollers were prevented from moving horizontally by clamped pieces of steel to the base. The second load setup which measured vertical net deck deflection used the same four point load but in a different location on the bridge. For this load setup one of the point loads was placed at the centroid of the largest deck panel. In order to do this the four point load system straddled one of the girders. For both load tests the load was applied by a computer controlled hydraulic actuator. The loading was placed in 5kN increments every two minutes, until a maximum of 20kN was achieved. The 20kN load was maintained for one hour. Linear variable differential transformers (LVDT) were used in place of dial gages to measure deflections. Data was acquired by the LVDTs, using a computer program in LabView. The bridge did not make any noise during either load tests.Due to the close spacing of stringers the deck did not appear to show any noticeable deflection.For the first load test (total bridge deflection) an LVDT was placed under the center of one of the girders since each girder received equal amount of loading. For the second load test (deck deflection) an LVDT was placed at the centroid of the largest deck panel. Two more LVDTs were placed, one on each side of that particular deck panel, on the stringers. The deflection of the bridge at full loading was 5.8563mm, after one hour at the same load, deflection was 7.0696mm. The difference was 1.2133mm after one hour, which indicates that wood does experience noticeable creep. However the creep was small and the rate of creep decreased as the duration of the test increased. At full loading the gross and net deck deflection were 5.99033mm and 0.48221mm, respectively. After one hour the gross and net deck deflections had crept to 6.9014mm and 0.4269mm, respectively. At no time did the girder delaminate or the stringers separate from the girder flanges. Bending was noticed in the girders during both load tests. During the test, the bridge appeared to handle the load well and remained rigid. All joints of the bridge remained intact during the full test.

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:	(26) 5ft 2x4 6.5in on center/(2) 14ft I-beams plywood web, 4x6 notched flange
Deck:	(1) layer 5/4in boards placed longitudinally over stringers
Floor Beams:
Suspension:
Unique:	stringers recessed 1in. into I-beam flanges

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
	In accordance to AWPA standards and competition rules approved pressure treated southern yellow pine was used in all locations where the bridge would be exposed to the environment and in contact with the ground. Deck boards and stringers had alkaline copper quat (ACQ) pressure treatment with concentrations of 0.25. Flange wood had chromate copper arsenate (CCA) pressure treatment with a concentration of 0.4. Commercially available pressure treated wood was selected due to reduced maintenance and up keep over the structures service life. All exposed wood will be treated with a water repellant to help further protect the wood against weathering.

7. Special Considerations
	The project was very beneficial to the team. The project provided real life engineering applications and was a good learning experience. It introduced us to various components such as cost, construction practices, design, structural strength, safety, and time management that must be considered and balanced in an engineering project. During the design phase the team decided to use a combination of pressure treated wood and commercial wood protector as opposed to only a wood protector. The team could thereby reduce its exposure to the wood protector/sealers fumes. The bridge will be donated to a local ATV club where it will be used on a trail to allow easier crossing of a ditch.