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 goal was to build an innovative and simple bridge with a good compromise between light weight and small deflections. We felt that having a design utilizing two triangles with steel hangers was simple and it allowed us to have a deep bridge, which maximized the effectiveness of a minimal amount of steel. We chose pressure treated southern pine dimensioned lumber and Grade B7 4140 Alloy Steel due to their availability and strength. The main diagonal structural members (the ones that make up the triangles) are constructed using two 2"x6" pieces laminated with wood glue and screws. This increased the stiffness of the structural members and allowed us to make them strong enough to cross the 4 meter span using only 2.44 meter (8 ft) long pieces of wood. The deck is affixed to the horizontal beams of the two triangle supports. These horizontal beams are fixed to the diagonal structrual members of the triangle using 1/2" diameter 4" long bolts at the bottom ends and using hangers with steel plate-topped wood caps at equal points along the diagonal beams. We conducted significant testing on the connections in the diagonal structural members with the steel-topped wood caps. These tests showed that the caps would successfully transfer the vertical load evenly to the diagonal structural member. For the deck we created a wood grid with 2"x4" pieces of wood and covered with 1"x6" pieces of wood over the top. The design was chosen to reduce the deck weight and to disperse the load throughout the deck. We modeled the effects of the load on a deck supported by a grid using equations for plate deflections and determined that a grid made up of 4"x4" squares was optimal for reducing deflections and reducing weight. The edges of the grid were connected to the horizontal beams using screws and top flange deck hangers. The intent was the grid would transfer the load to all parts of the horizontal beams. In order to prevent global buckling bridge, connections at the bottom of the triangle and the top of the triangle were fixed as much as possible. At the top we used 4"x4" and 2"x6" pieces of wood, connected between the triangles, to achieve this.

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 required a 20-kN load be applied through 60-mm by 90-mm load blocks placed at four specified locations on the bridge deck. The loading was increased in 5 kN increments up to 20-kN. Steel angle sections and two steel beams were used for the 20-kN load and divided into four 5-kN groups. The steel beams were used to distribute the load to the four 60-mm by 90-mm wooden load blocks. Three of the four 5-kN loads consisted of entirely steel angles while the first 5-kN load consisted of the two distribution steel beams and enough steel angles to total 5-kN. Each 5-kN load was hand loaded until the 20-kN load was achieved. Deflections were measured and recorded after each 5-kN load was applied, and at 15-minute intervals for one hour after the full 20-kN load was applied. Deflection gauges were placed at the mid-span of the bridge under one of the girders to measure the maximum vertical bridge deflection in accordance with the competition requirements. The gauges used each had an accuracy of 1/100th of a millimeter. After one hour under the 20-kN applied load, the maximum vertical bridge deflection was 6.53mm which was less than the allowable 10mm. When discussing our design with Mr. Hutchins, the point of contact for the competition, we determined that our grid decking "squares" were so small we should consider the deck to be one unit. Therefore, when we measured the deck deflection, we moved one of our point loads to the center of the deck, and adjusted the others accordingly. To find the net deck deflection, we averaged the deflections under the girders and subtracted the deflection under our center loading block. The deck span was 1479.6 mm which resulted in an allowable deflection of 14.8mm. After the 20 kN load was applied for 60 minutes, the net deck deflection was 3.86 mm, which was less than the allowable. Under the test load, the bridge performed very well and showed exceptional resilience to the 20-kN weight. Most of the deflections took place immediately after putting the load on due to gaps between members. After the completion of the tests we realized that we should have designed stiffer supports because there was 1 to 2 mm of deflection in the supports alone.

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:	Triangle Shape, two 2x6s act as 1 member
Deck:	Grid shape, made of 2x4s
Floor Beams:	Incorporated in grid deck, made of 2x4s
Suspension:	7 3/8" threaded steel rods on each side
Unique:	2x6 drilled caps to distribute weight on top

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
	In accordance with the rules of the competition, we used pressure treated Southern Pine. The timber was treated to AWPA standards as per competition rules. The treatment was ACQ 0.25 pcf. We chose Southern Pine because it was commonly available from the nearest lumberyard. The treatment of the wood posed several problems related to construction. We had to wait for the wood to dry before working with it and had to prevent it from warping as it dried. We spread the wood out and loaded it with several thousand pounds to prevent it from warping as it dried.

7. Special Considerations
	During the planning stage we considered being able to donate this bridge to the cummunity as a stream-crossing in a park or some similar application. Finding a place to put it on a military reservation is tough, though, and transportation of the bridge might be a problem as well.