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)
	Bridge Design Concept From the outset, our ultimate goal of the timber bridge project was to use our skills and knowledge, in structural engineering and wood sciences, to develop a design that minimizes deflection. After our initial evaluation of the required loading scenarios, we selected a Glue-laminated arch that supports the loads of the bridge deck, and all of its components. The load path for our bridge design is as follows: 20 kN load bears on the bridge deck in the prescribed locations, which transfers to the supporting girders, which then transfers to the purlins, which are connected to the arch via steel rods (see Load Path section for more details). The arch then takes the test load and the whole of the dead load down to the supports, thus completing the load path for the structure. With the use of the glulam arch, we are able to eliminate much of the bending that would otherwise be seen in the deck span. Although this is not the simplest of designs, it is really clear how the load transfers from one member to the next. The arch is a picturesque form, which makes the bridge a real eye-catcher to the common on-looker. So, in developing the design, we focused on minimizing the deflection and weight, while making the bridge aesthetically pleasing at the same time. Optimize stiffness/minimize weight In order to optimize the stiffness of the bridge, the arch provides the main support for the deck span. This approach gave us the opportunity to use fewer materials on the deck itself, thus minimizing the weight for the deck region. In order to have the same deflection without the addition of an arch, we would have had to use more substantial member sizes to support the same loadings. Thus, while maintaining an economical design, the bridge still meets the performance criteria. Another way in which stiffness was optimized was seen in the deck girders and purlins. The members were designed and placed to work about the strong axis, thus allowing us to maximize the amount of strength from each member, while minimizing the weight. Each section of the timber bridge design was developed to give the most stiffness with a minimum amount of material.

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)
	Prescriptive based designs require the engineer to interpret and apply applicable codes toward the goal of developing a suitable design. For the purpose of the NTBDC (National Timber Bridge Design Competition) a performance based design was implemented. Performance based designs require a better understanding of a structure�s behavior under loading, but allow for the maximization of building materials. Prior to design, several key goals were identified; the minimization of the structure�s weight, minimization of deflection under live load, and overall bridge aesthetics. The selection of a structural system that facilitates the efficient use of the structural materials was essential to meeting the performance based design criteria. Below is a description of the selected structural system and its response under loading. Load Path Live loads are applied directly to 2x12 No. 2 DF-L decking which is oriented transverse to the span. Tongue and grooving of the decking improved performance by distributing loads to adjacent decking members. The decking is supported by five girder lines, oriented parallel to the span and spaced at 1�- 3�� on center. Spanning 4�-1�� between supports, the girders are 2x8 No. 2 DF-L. Four purlins spaced at 4�-1�� provide support to the five girder lines. The purlins are made up of double 2x8�s and are supported at the ends by steel suspension rods. A glue laminated arch supports that rises above the deck provides the anchor for the other end of the steel suspension rods. The glue laminated arch transfers the vertical load directly into the abutments while longitudinal load is resisted by tying the adjacent abutments together via steel cable. The steel cable stressed in tension, creates a tied arch and resists lateral movement of the abutments. Limiting Constrains on Member Design: To ensure that the structure�s overall weight was kept to a minimum, all structural elements had to be used efficiently. Each member was evaluated as being deflection, bending, or shear controlled. Once the limiting factor of an element was determined, the design could undergo an iteration process which could improve the design. The iteration process involves changing the clear span distance, member size, or number of members.

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:	Five girder lines, oriented parallel to the span and spaced at 1�- 3�� on center. Spanning 4�-1�� between supports, the girders are 2x8 No. 2 DF-L.
Deck:	Live loads are applied directly to 2x12 No. 2 DF-L decking which is oriented transverse to the span
Floor Beams:	Four purlins spaced at 4�-1�� provide support to the five girder lines.
Suspension:	A glue laminated arch supports that rises above the deck provides the anchor for the other end of the steel suspension rods.
Unique:	The steel cable stressed in tension, creates a tied arch and resists lateral movement of the abutments.

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
	Alkaline Copper Quaternary: Type DAlkaline Copper Quaternary (ACQ) Type D has a lighter greenish brown color with a hardly noticeable odor. It will eventually weather to a brown color over time. Wood treated with ACQ will be protected from decay caused by fungi and insects. This treatment is especially good for wood exposed to the ground, which most bridges will be. The treatment is economical and readily available at most lumber yards. All of the decking members are pressure treated with ACQ-D. ACQ is more desirable for the environment, and with rising environmental concerns, it seems appropriate to consider environmentally sound options. The cost of this treatment is reasonable; therefore, offering an economical standpoint for choosing this treatment. Copper Naphthenate For the treatment of the Glulam arches, copper naphthenate was used for added durability and waterproofing. This treatment is semi-transparent, oily, non-corrosive to hardware, and extremely resistant to decay. It is listed as an approved preservative under AASHTO M-133, and was chosen based on the availability and the benefits. Since the glulam arches are planed down, we were required to provide a paint-on application of the copper naphthenate. This paint-on layer will protect the wood from decay due to weather exposure, as well as provide additional resistance to any other detrimental materials that the bridge may be exposed to throughout the duration of its useful life. The Treatment is applied prior to any testing procedures or use in the field. The goal in the application of this treatment is to retain the wood�s strength at the time of construction throughout the life-span of the bridge. This will ensure that design values remain accurate over time. Copper naphthenate has moderate toxicity to humans; therefore, care must be taken during the application process. As far as ecotoxicity, copper naphthenate is toxic to a wide variety of microorganisms, fungi, and plants. It is also toxic to fish. If the bridge is implemented over a stream, caution must be taken so that the copper naphthenate does not leach into the water system.

7. Special Considerations
	We planned to have the pressure treated wood that's touching the ground to be environmentally safe. The bridge will be donated and transported to Portland, OR, where it will be used as a crossing over a small creek. We are proud of our work and glad that it will serve a purpose after testing.