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 design concept for the 2003 Timber Bridge Competition submitted by the Student Chapter of the Forest Products Society at Virginia Tech consisted of utilizing CCA (Chromated Copper Arsenate) treated Southern yellow Pine lumber which had been removed from service. The CCA treated Southern Yellow Pine (SYP) lumber had been removed from residential applications as part of a research project (Bailey 2003) that analyzed the feasibility of recycling CCA treated wood which had been used in exterior decks. Results of the research project concluded that 86% of the CCA treated wood removed is suitable for use in above ground applications based upon mechanical testing and chemical retention. Lumber which had not been tested was donated by Snavley Forest Products towards the building of this year’s timber bridge entry. The resorcinol used for the bridge was donated by Dynea. Using the recycled lumber lowered the overall cost of materials needed to build the bridge and also presented an alternative to disposing of the CCA treated SYP into a landfill. The main application for the above mentioned CCA treated SYP lumber was for the manufacture of two arches. The two arches were composed of three layers of 2"x 6"x 14"CCA treated SYP. The layers were held together with Resorcinol resin and screws. Bonding CCA treated wood is difficult and the bonds are notoriously non-durable. To correct this problem a coupling agent, hydroxymethylresorcinol was used as a pretreatment. HMR is an experimental coupling agent that creates remarkable improvements in the durability of typically troublesome and non-durable wood bonds. The HMR coupling agent fixes the durability issue and yields a strong bond. Virginia Tech is currently involved with the University of Maine and the Forest Products Lab to determine exactly how HMR works. 1/4 inch steel cables at 3 points connected the arch to 6 points along the deck. Overall the recycled CCA treated SYP lumber accounted for 50% of the materials in the bridge. The deck system is composed of carbon fiber sandwiched between 1/4?CCA treated plywood and ACQ treated 1?x 6?SYP. Resorcinol resin and screws were used to hold these three components together. The deck was supported on 5 I-joists which were pressure treated with ACQ. The i beams were cut into 3 equal sections to comply with the length requirement. The sections were then glued together with resorcinol with 2x8 blocks in the web for added stiffness. The deck structure was extended past the base of the arch to prevent the arch from spreading under the load. Throughout the bridge design resorcinol resin was used to maximize the stiffness of the structure. The engineered I-joists were used to help minimize the weight of the bridge but still minimize deflection. Bailey, D. 2003. The Feasibility of Recycling CCA Treated Wood from Spend Residential Applications. M.S. Thesis. Virginia Polytechnic Institute and State University, Blacksburg, VA. 206 p. Vick, Richter, River & Fried. Hydroxymethylated Resorcinol Coupling Agent for Enhanced Durability of Bisphenol-A Epoxy Bonds to Sitka Spruce, Wood and Fiber Sci. 27(1) 2, 1995.

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 performance requirements for the competition dictated that the bridge must be able to carry 20 kN, approximately 4500 pounds, for one hour and deflect no more than 9.5mm under this sustained load. The deck deflection was required to be less than 0.7mm (deck span/400) during the loading. In order to measure the bridge and deck deflections, 4 dial gages were placed at the center span of the I beams and on the deck under the foot of the load. . The load was distributed to the application points by attaching the loading blocks to a pallet and placing weight upon the pallet. 50 lbs lead blocks and 25lb bags of lead shot were placed into the box resting on the pallet in order to achieve the 20 kN load. The initial load of 5kN was achieved using the lead blocks and the weight of the wooden box. The bridge deflected more as each load increment was added according to the readings taken. The readings taken from the right side of the bridge were higher than the left side. This many have been due to the uneven tensioning of cables. The maximum deflection reached was 6.05mm and was within the stated guidelines. There was a discrepancy in our net deck deflection calculations.From the data one can see the right beam of the bridge deflected more than the left. When these values were averaged the gross deck deflection was greater than the average beam deflection. The gross deck deflection measurement was taken towards the left beam and using this data yields a positive number, although the posted data uses the average in order to comply with the guidelines.

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:	12" I-beams with osb webbing glued and screwed back together
Deck:	carbon fiber sandwiched between 1x6s and plywood with resorcinol
Floor Beams:
Suspension:	2x6 block arch with 1/4" coated steel cables
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
	All timber members were preservative treated and pressure impregnated with CCA in accordance with ASTM D1760 at Hoover Treated Wood Products, Inc., who donated the material to our team. After treatment, the lumber was kiln-dried to a 19% or less moisture content. This treatment minimized the moisture, thus reducing the weight of the bridge. The treatment occurred in the factory which had the proper equipment and safeguards.

7. Special Considerations
	The main goal of this years design was to incorperate the use of recyled CCA treated lumber. The lumber was recovered from old decks being torn down in the local area. The bridge will be put on display to further promote the Wood Science Department and the future of the wood industry.