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)
	Our bridge is a composite I-beam structure built of traditional treated wood. We approached the competition searching for the most practical and simplest way to fulfill the requirements. We chose treated wood over engineered wood products on account of its availability and well understood structural properties. The design was chosen from several other design-concepts based upon Washington University’s prior experience and ease of construction. All designs were modeled to estimate behavior under loading and discussed as a group. This I-beam design offered a simple construction with a high level of performance and minimal room for error. All materials for this bridge were traditional available wood construction products. All of the wood was purchased pretreated with 0.40 pcf of Chromated Copper Arsenate. Plywood was used as in the web of the beam and as connecting scabs. All remaining lumber was Southern Pine No. 2. Wood glue was the primary fastener, but steel screws were used where glueing was not practical. The composite I-beams carried the wooden plank deck over the 4.06m distance. These I-beams were designed for a high moment of inertia with low weight.The top inside flange was shortened below the top of the beam. The bridge deck was then placed inside of the beams and resting on them. This increased the composite action between the beams and the deck. The web of the I-beam was composed of two layers of ½ inch thick plywood for a total thickness of 1 inch. The flanges were constructed of sawn dimension lumber. These beams ran the length of the bridge with plywood scabs connecting the staggered splices. Outside the region of maximum moment, flange material was trimmed to minimize weight without compromising stiffness. The construction drawings indicate the sizes and locations of the flange pieces. As in all connections, glue was applied to all contact areas to achieve as much composite action. The deck consists of 2” x 6” planks spanning 2 x 6 transverse stiffeners. The deck was built in place with cross clamping during glueing to increase composite action. Cross bracing provided lateral support during the loading. A curb was constructed of a 2"x4" and was assumed to have no structural integrity.

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 bridge was very rigid and stable under loading. No visible deflections were observed and no creaking was heard during the loading. The load was applied using a hydraulic pump anchored from a rigid steel frame. The force from the hydraulic pump was transferred to four load points by using steel I-beams in an H-pattern. The bottom two transverse I-beams rested on top of bearing plates. These bearing plates were the point of applied load to the bridge. The hydraulic pump pressed against a third I-beam, which spanned the two transverse I-beams, forming the middle of the “H”. Care was taken during test-setup to insure an even distribution of the load. The mass of the testing materials was accounted for and subtracted from the total required load. The deflection dial gages were zeroed before this this 227 lb load was applied. Our deck deflection was required to be less than 2.86 mm according to the L/400 requirement. The total bridge deflection was required by competition rules to be less than 10mm. The load points were positioned to be transversely even from the sides of the deck, providing even load to the weakest portions of the deck. After applying the first 5kN load, the bridge deflection and deckdeflection were .56mm and .54mm respectively. The deflections increased linearly with each increment to a maximum of 4.42mm bridge deflection and 2.48mm deck deflection at 20 kN of load. Our bridge and deck deflections increased 45% and 10% respectively during the hour of constant load. This small creep may have been due to flexible individual members due to the high moisture content of the wood. The deck deflection reported after 15 minutes of the full load was in disagreement with the other data points. We expected a small amount of creep or increase in deflection during each time interval. At the 15 minute mark, deflection jumped 0.2mm from time zero before decreasing by 0.1mm to the 30 minute value. This discrepancy may be attributed to a variation in the hydraulic pump. Measurement error may also be a factor, a product of testing in an uncontrollable environment. We assumed the deck deflection should linearly increase from time 0 to 30 minutes of loading. These deflections were within the competition requirements and acceptable in a real-life 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:	Stiffened plywood webs; 2x6 bottom, 2x8 top flange; 2x4 ledge
Deck:	2x6 interlocking, 2x6 transverses, grain parallel longtudinally for composite action
Floor Beams:
Suspension:
Unique:	cross-braced for stability, 2x8 outer flange area reduced for weight

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
	.40pcf Chromated Copper Arsenate was the preservative treatment used on all wood members.

7. Special Considerations
	After testing, one of the team members removed the bridge and installed it on his rural property. The bridge now spans a small creek just outside of St. Louis.