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 2000-2001 Clarkson University Timber Bridge Team considered numerous alternatives using both trusses and beams in the design before finally deciding upon a bridge composed of glued-laminated wood members with a superstructure featuring arched beams. Many hours were spent formulating this years' deck design. The result is a deck that is both cost-effective and lightweight. The design takes advantage of the consistent use of 38.10mm (1-1/2in) thick microllam blocks and deck "girders", and 9.53mm (3/8in) thick microllam planks. Together these members form a "building-block" system; once all members are cut to required dimension, assembly proceeds much like the way one might assemble a building block toy. The entire deck is fastened together using 9.53mm diameter steel rods (six total positioned at geometric centroids of every block section), glue, and finishing nails. Fiberglass reinforcement applied to the bottom of block sections insures stiffness of deck system in tension. This design is economically sound, because the repeated use of similar deck pieces suggests suitability for mass-production of components. Additionally, individual deck sections can be pre-fabricated before shipping to the bridge site. Sections can be combined on-site to form a deck of any reasonable length and width. The superstructure is best described as a "wishbone" design; four arched parallam beams meet at a center notch constructed of parallam. Each beam is 88.9mm (3.5in) thick and ranges from a maximum of 457.2mm (18in) deep at the abutments and a minimum of 152.4mm (6in) at midspan. The center notch is strengthened with a 6.4mm (0.25in) thick welded steel plate to provide adequate tensile strength. 6.4mm diameter bolts were used to fasten the beams into the notch. Then five 8.9x76.2mm (3.5x3in) grooves were notched into the superstructure to allow for parallam floor joists. The result is an aesthetically pleasing superstructure design that takes advantage of the strength of engineered lumber. The deck was attached to the superstructure using lag bolts. In years past, Clarkson has tended to construct timber bridges utilizing predominately practical methods. After some discussion it was decided to go ahead and be "daring" and try to design a more innovative bridge this year. Although the team was confident in the results obtained using the STAAD computer program, time became an important factor as the semester proceeded. As a result some of the critical fabrication steps were unfortunately rushed. The result was that the superstructure had a noticeable sag of almost 19mm (3/4in) at midspan before any loading and without the deck attached. Shimming was attempted to remedy this situation, but on testing day the negative nature of the results were undeniably clear. It should be noted that after "official" testing procedures were completed, an attempt was made to break the bridge to determine its capacity. It was not surprising to find that the bridge did not completely fail until a loading of 67kN (15.1kip-force), although deflection was significant. The team believes that, had more care been taken in fabrication, the design would have performed adequately in regards to deflection allowances

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)
	60-mm wide bases were cut from microllam members and placed atop concrete blocks to serve as the bridge abutments. The bridge was then placed in a reaction frame where the load was applied using a hydraulic actuator located at midspan of the frame. Three W6x9 sections distributed the applied load to the four 90x60mm loading blocks positioned as required by rules. Dial gauges were positioned under the bridge at all neccessary locations. Because the loading point selected for monitoring gross deck deflection was located along the extreme right edge of the deck along the curb, a single gauge was installed under the outside longitudinal beam closest to this point to record Beam L deflection. After the first 5kN load increment, the midspan dial gauge revealed a total bridge deflection of 1.78mm (0.07in). As the 10kN loading was reached, the gross deck deflection of 9.40mm (0.37in) was visually noticeable at midspan and bridge settling was audible. Maximum allowable bridge deflection of 10mm (0.39in) was exceeded just before the 15kN loading was reached. Despite an abundance of audible creaking as the 20kN load was reached, the deck and superstructure maintained overall structural integrity. Just before this load was reached, the gauge positioned to measure gross deck deflection reached its capacity of 25.4mm (1in) and was removed. For the remainder of the loading, the gross deck deflection was estimated as accurately as possible with a tape measure due to the removal of the dial gauge. During the one hour duration there was no audible settling nor any additional visible deflection of the bridge. The bridge deck showed no signs of punching failure, and total net deck deflection observed at the conclusion of the loading was approximately 9.6mm (0.38in) which exceeds the calculated value for maximum allowed net deck deflection of 3.05mm. Even with 19.43mm (0.76in) of total bridge deflection, the wishbone superstructure proved to maintain its strength during the hour-long load duration (there were no signs of bolt failure or beam fracture). Additionally, the parallam floor joists did not reveal any signs of failure.

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:	Wishbone design consisting of four arched parallam beams
Deck:	Building-block design consisting of repeated use of microllam components
Floor Beams:	Five 88.9x76.2x1371.6mm (3.5x3x54") parallam floor joists
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
	All parallam lumber used in the bridge was pre-treated with chromated copper arsenate (CCA) to a concentration of 0.6 pounds per cubic foot. Because Trus Joist MacMillan routinely has large batches of parallam treated they were able to have all parallam lumber requested by the team treated at no cost. However, the relatively small amount of microllam lumber requested could not be pre-treated by Trus Joist without significant cost and time investment. For this reason, all microllam lumber used in the bridge was treated ourselves with copper naphthenate to an approximate concentration of 0.4 pounds per cubic foot (wet). The problems this method introduced were (1) the appropriate precautions had to be taken to insure the safety of team members applying the treatment, and (2) the amount of time taken to insure adequate concentration. In practice this issue would be unlikely to pose such problems , as an outside contractor could insure adequate pre-treatment of all wood members used to construct the bridge. The treatment requirement presented an even greater problem, however, when we realized that the treatment was adversely affecting the stiffness of the bridge deck. The competition rules include recommendations that it is best to have all wood members treated before fabrication. As stated earlier, it just was not feasible for our team to have microllam members treated in this manner. Within days after applying the copper naphthenate treatment, the deck became noticeably more flexible, as evidenced by the visual deflection of microllam planks under minor loading. We believe that this phenomenon, coupled with the fabrication errors mentioned previously, are the reasons for excessive deflect

7. Special Considerations