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 main objectives in determining the overall design concept for this bridge are: to create a bridge that has a high strength to weight ratio, is economical and easy to build, and yet still retains the aesthetic appeal of timber structures. In order to do this, the design process entailed breaking the bridge into two main sub-systems – the deck and the understructure that supports it. Several options for each system were designed and tested, including two beams for the understructure and two deck options. The design for the understructure was chosen because it very easy to build and is therefore practical for construction in the field, is relatively lightweight, and it is also an economical, low-cost design. The design consists of four evenly spaced, built up I-beams, each consisting of ½ inch pressure treated plywood for the web, and two 2”x4” SYP pressure treated boards fastened with 3” deck screws on either side of the plywood and flush with the top and bottom of the plywood for each respective flange. In order to stay in compliance with the competition rules, the web and flanges of each beam are divided into three lengths so as not to go over the length limit of any individual member. To increase the stiffness of each individualbeam, five, (evenly spaced along the length of the beam) 10” long 2”x4” SYP web stiffeners were added to both sides of the web between each flange. These web stiffeners also act to prevent cracking and buckling of web during loading. The design for the deck system was chosen because of its practicality and ease of construction, as well as the excellent stiffness that it has. The design consists of 2”x4” stringers running perpendicular to the understructure with 1’x6” boards on top of them running longitudinally. The stringers span the entire required 63 inches of the bridge’s width, and are connected to each girder in the understructure with 3 inch deck screws. In order to increase the stiffness of the deck system, the stingers are oriented along their strong axis and are spaced at 7.5 inches on center. The combination of the small spacing as well as orientation of the stringers is intended to decrease the tributary width of each member and therefore the amount of load each will have to endure during testing. The top layer of the deck system (1”x6” boards) was nailed longitudinally onto the stringers as was chosen because they are lightweight, easy to install, and larger boards were not really needed given the strength of the stringers support.

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 competition requires a 20kN load to be applied to the bridge through four 60x90mm loading blocks. The 20kN was loaded in 5kN increments in a period of just under twenty minutes. Four pallets were loaded with steel angle sections and concrete masonry units to achieve the required 5kN and then were placed on the bridge using a forklift. We fabricated the 60x90mm loading blocks out of wood and used small W shaped steel sections to span between the blocks and distribute the load from the pallets to the blocks. We positioned dial indicators at our four specified gauge points to measure bridge deflection, left and right beam deflection, and gross deck deflection. The gauges were positioned under the center of an interior beam and under a loading point in the center of a deck panel. We took readings after each incremental 5kN load was applied and then every 15 minutes once the full 20kN was on the bridge. The dial gauges that we used had an accuracy of 1/100th of an inch. After one hour with the 20kN load applied the bridge deflected 6.35mm which is below the allowable 9.5mm. The deck performed very well during the loading. The net deck deflection was measured by positioning dial gauges in the center and at the edges of a deck panel experiencing maximum deflection. The allowable net deck deflection is the span/100 and in our case that is 1.54mm. The deck only deflected 0.0889 mm or 5.8% of what it is allowed. This is a very small deflection and is a result of the added stiffness of transverse stringers and small tributary areas that they each supported. The stringers take advantage of the strong axis of the 2x4”s and help to resist the bending moment caused by the load. As the bridge was loaded there was no audible creaking or settling of the bridge. We preloaded the bridge prior to the competition in order to work out all of the initial deflections that would occur as the bridge settled beneath the load. Under the 20kN load the bridge deflected at a decreasing rate throughout the hour period of loading. This was due to further settling of the bridge and a declining creep rate. The bridge is an extremely light and efficient design considering it weighed just 311 kg and keeping that in mind the bridge performed exceptionally well in terms of bridge and net deck deflections.

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:	2x4 6.75 in OC / (4) 3.8m I-Beams plywood web, (2) 2x4 flange
Deck:	1x6 boards placed longitudinally on stringers
Floor Beams:
Suspension:
Unique:	(10) 2x4 web stiffiners per girder

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 lumber and plywood is pressure treated. This treatmeant was chosen because of competition requirements and so the bridge can be used outdoors after the competition.

7. Special Considerations
	This individual study project was a great experience for all team members. We were presented with a real world problem and given free reign to come up with any solution we wanted. The project really allowed us to apply many of the design principles learned in the classroom. One particularly beneficial aspect was that we were able to discover how design calculations are only good until you actually construct the individual systems of the structure, and that complicated designs, while good on the drawing board, are difficult to bring to fruition given our level of building experience. After the competition, our team is looking into various venues for the bridge to be placed in around the West Point community, mainly focusing on either the West Point Golf Course or Middle School.