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 structural concept for the 2002 Virginia Tech Timber Bridge was 2, 2-dimensional, moment resisting trusses with ledger beams attached supporting the bridge deck. The objectives for fabrication of the bridge were to utilize easily obtainable components that could be put together quickly with a minimum of expertise, while keeping the weight and amount of non-wood components as low as possible. All wood utilized was CCA pressure treated Southern Pine. Trusses were compri- sed of 2x4 web members and 2x6 upper and lower chord members. To max-imize the stiffness of the trusses, pressure treated 3/4 inch thick plywood gusset plates were used to connect truss members, resulting in a series of moment resisting connections. Truss connections were held together with resorcinol phenolformaldehyde (PRF) resin and screws. Screws were used mainly to apply clamping pressure for the resin. Cen-ter portions of the ledgers for the deck were 2x10’s and the outer portions were 2x8s. Both were adhered using PRF resin and clamped using 3x1/4 inch Simpson screws. The 5/4 deck boards were screwed on top of the ledgers, creating the bridge deck. The decking was add-itionally supported along its center with a 2x8 stringer, spliced in the center, that was stiffened with 5 sets of angled 2x4 struts. Transverse 2x4 braces were placed at 4 locations near the bottom of the trusses to avoid splaying of the lower truss chords. While resorcinolic adhesives are known as the best and most durable thermosetting wood adhesives, it is well known that CCA treated Southern Pine surfaces are extremely difficult to bond. This was a significant issue for the bridge because adhesion was a critical design element for desired stiffness. Researchers at the Forest Products Laboratory developed a coupling agent called HMR, for hydroxymethyl resorcinol. The HMR coupling agent represents a big advance in wood adhesion because it dramatically improves durability of troublesome wood adhesive bonds (1). HMR even improves the durability of adhesive bonds to CCA treated Southern Pine (2). Unfortunately, the HMR coupling agent is still experimental and is not commercially available. We followed procedures described in references 1 and 2 by making a dilute aqueous solution of resorcinol, formaldehyde, and sodium hydroxide. We allowed this mixture to react at room temperature for 4 hours prior to application to the bonding surfaces, in order to reach optimum HMR molecular weight (1). We allowed bonding surfaces 24 hours to dry and then applied the PRF adhesive, which was formulated as a gap filling resin, and was appropriate since our bonding surfaces were not knife planed. According to the literature, the HMR coupling agent should provide delamination resistance sufficient to pass ASTM D2559, which is the standard for exterior grade structural thermosets. While improved durability from HMR will not improve the performance of the bridge, since the competition is a short-term load application, we thought it was important to optimize the adhesive durability since the competition is intended for practical, exterior, long-term applications. References available upon request.

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 9mm under this sustained load. 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, deflection gauges were placed on the floor and monitored the absolute displacements at the locations indicated on the plan drawing submitted. Deck deflection was measured beneath one of the four points where the load was applied, as well as in the longitudinal members adjacent to the maximum load point. The point of maximum deck deflection was located directly between the left truss and the center stringer, 320 mm from the truss. Deflections were obtained at center-span for each of the two trusses. The load was distributed to the application points by attaching loading blocks to a pallet and placing weight upon the pallet. Concrete masonry units, buckets of drywall joint compound 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 either of the trusses 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 only deflection limit that was exceeded was that of net deck deflection. The net deck deflection, after subtracting out the deflections of the adjacent longitudinal members was greater than 3.5 mm, also shown on page 4 of the online application.

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:	Two-dimensional moment resisting trusses with spliced stringer down the center
Deck:	5/4 decking boards running transversly resting on ledgers on trusses and stringer
Floor Beams:	Center stringer supported by angled struts which transfered load equally to the bottom chords of each truss
Suspension:
Unique:	Truss members connected utilizing plywood gussets adhered with resorcinol phenolformaldehyde

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
	CCA pressure preservative treatment eas used on all wood members. The plywood gussets were cut out of 3/4 inch thick marine grade CCA pressure treated plywood. All solid wood members were donated untreated and the CCA treatment was donated separately. All metal fasteners used were treated for outdoor use. HMR coupling agent was used for maximum durability of resorcinol phenolformaldehyde glue bonds.

7. Special Considerations
	All materials used in the fabrication of the bridge were selected for their known durability and resistance to natural deterioration (such as insects, rot, and corrosion). Additionally, a state-of-the-art coupling agent was applied to all adhesive bonding surfaces in order to increase the bondline longevity. This year, as in previous years, we had the privilege to work with several companies that donated materials and services for the bridge. We were also able to allow students working on the bridge to participate in various processes related to use of wood products. There was involvement with seeking material donations, treating and drying the wood after pressure treatment and working with the HMR coupling agent and FPR adhesive. Cooperation such as this gives the students direct interaction with manufacturing processes while also reflecting positively on Virginia Tech and the sponsor companies.