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 concept for Virginia Tech's Timber Bridge 2000 entry was a bridge without any metal fasteners, only wood, Resorcinol/Phenol Formaldehyde adhesive and carbon fiber reinforcement. The use of adhesives results in rigid, lightweight structure and carbon fiber is known to be a very strong yet light tensile reinforcement. Additionally, a bridge without metal fasteners was thought to require less maintenance. The structural concept for the bridge was a pair of longitudinal beams supporting the deck, which were in turn supported by arches. Spokes transferred loads from the beam to the arches. Each arch consisted of 3 inch wide, 1/16th inch thick strips of yellow poplar veneer, laminated into a smooth curve, 4 inches thick at the base and 2 inches thick at the apex. The arches were tapered because more load needed to be carried at the bases of the arches than at the top of the arches. The laminating process lent itself extremely well to tapering, as well as to placing strips of carbon fiber between the laminations in regions of the highest tension forces. Therefore, the fiber was placed along the full length within the glue lines on two of the outer layers and part of the way up the arches for two additional layers near the bases of the arches. The beams were built of laminated Southern Yellow Pine (SYP), No. 1 2x4's ripped down to 3 inches wide. Carbon fiber strips were placed in the glue lines for the two bottom layers because these were areas of highest tension. The last two layers were shorter because more wood was needed at the midspans. A ledger of SYP No. 2, 2x8, was ripped down to 5 inches wide and used for the third layer down from the top to support the deck joists. Beam and arch laminations were staggered to avoid failure planes in consecutive layers. The arches and beams were connected using lapped dovetail joints. Pressure treated SYP No. 1 joists made from 2x4's ripped down to 3 inches wide were glued onto the ledgers, spanning the beams at 5.25 inches on center. Pressure treated SYP No. 1 decking boards planed down to 0.75 inches thick and 5.25 inches wide were glued on top of the joists at 5.25 inches on center. Pressure treated, 0.5 inch diameter pegs were driven through the joists into the ledger and through the curb and deck boards into the beams to provde additional connectivity among the beams, joists and deck. The center struts were 1 inch thick and 2.5 inches wide. The flanking struts were 1 inch thick and 3.5 inches wide. All of the struts were glued and pegged to the side of the beams, then glued to the side of the arches and pinned to blocks that were glued to the outer surface of the arches. By using laminations and reinforcement, more material was placed where stresses were higher and less material was placed in area of lower stress, resulting in a more efficient timber 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)
	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 8mm under this sustained load. The deck deflection was required to be less than 3.5mm (deck span/400) during the loading. In order to measure the bridge and deck deflections, the method described in ASTM Standard D-198 was used. A 1x6 yoke was pinned at the end of each beam and a deflection gauge was placed at the center of the span on the beams to obtain the beam displacements. The deck deflection was measured beneath the centroid of the four points where the load was applied. A deflection gauge that utilized a 1x6 yoke that was pinned to the inside of each beam to obtain these measurements. The load was distributed to the application points by attaching the loading blocks to a pallet and placing weight upon the pallet. Concrete masonry units and cat litter were used as weights to load the bridge in four increments of 5 kN, approximately 1125 pounds, up to the required 20 kN. The bridge deflected more as each load increment was added according to the readings taken. The bridge deflected minimally throughout the sustained loading. At no point during the testing did the deck or either of the beams deflect more than 2 mm, as shown on page 4 of the online application. Therefore, the bridge was clearly within the necessary deflection limits. The bridge easily withstood the load that was applied to it and deflected minimally under the weight. We believe that the deflections measured on gauge 2, the right longitudinal beam, were actually less than those reported for this submittal. Between the readings for the 15 kN and 20 kN deflection measurements the yoke was accidentally bumped. We have included in the mail-in submittal a graph of the deflections. This graph shows the sudden jump on gauge 2 which corresponds to the accidental bump. Deflection measurements taken during the unloading of the bridge, in the same increments as the intitial loading, indicated that the measured deflection for that increment was exaggerated. We mention this not in the hopes of leniency during the judging, but rather to ensure proper scientific methodology.

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:	glue laminated southern yellow pine with carbon fiber reinforcement
Deck:	3/4" decking and 2x4 joists spanning between the two stringers
Floor Beams:
Suspension:	tension struts pegged to arches and stringers to transfer loads
Unique:	tapering glue laminated poplar arches with carbon fiber reinforcement

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 deck, curb, spoke and joist members were CCA pressure treated. The arches were saturated with adhesive and treated with two coats of 9.08% copper naphthenate. The adhesive saturation provided a matrix similar to that found in wood/plastic composites which have a high resistance to decay. It was thought that this matrix, in conjunction with the copper naphthenate treatment would be an effective insect and decay deterrent. CCA pressure treatment of the arches was impossible due to their size and could have compromised their structural integrity. The beams were twice coated with 9.08% copper naphthenate. Additionally, a mixture of borax and boric acid was placed into 0.5 inch diameter holes, 10 inches on center and 7.5 inches deep to serve as borate rods. The holes were plugged with pressure treated dowels. Borate rods are an accepted after-market treatment. Our club chose to produce our own rods using borax and boric acid mixture (1.2:1 by weight). This action saved our organization $125.00. We felt this was an innovative and cost effective treatment solution. Because we chose an innovative bridge design, it required an innovative treatment solution. CCA pressure treatment of either the glue laminated longitudinal beams could have compromised their structural integrity.

7. Special Considerations
	This year, we had the privilege to work with 5 companies that donated materials and services for the bridge. Cooperation such as this gives the students direct interaction with industry. It also reflects positively on Virginia Tech and the sponsor companies. The bridge competition is an excellent community relations tool. In addition, it promotes the use of wood products, one of our few renewable natural resources. Last year our participation in the bridge competition generated a great deal of interest and positive publicity. We hope this year's competition will generate the same interest.