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 feature in the design of our bridge is the built-up laminated arch section that span the required distance in the competition rules. An arch was used in our design to maximize the rigidity of the member while enabling us to minimize material usage. In using an arch design of this member, a component of the vertical load would be distributed to the horizontal tie rod connected to the base plates at either end of the arch. With the majority of the load being distributed to the horizontal tie member, the cross sectional area of our arched beam member would be minimized since the top region of the arch would mainly be in compression. The tie rod was incorporated in our design to prevent horizontal translation at the support points, therefore minimizing the bridge deflection under load. The radius of the arched section was determined using the required span and the maximum deck height requirements as constraints. It was chosen to design the member as a "one-piece" member to minimize the necessity for connections of vertical and horizontal members that would be fixed to the arch. This would minimize the potential for creep, or connection failure. The design of this member was accomplishedusing computer modeling (RISA) as well as hand calculations to determine the deflections and stresses that it would be subjected to under load. Since the member is structurally indeterminate, computer modeling proved to be the most efficient method of analysis with hand calculations being used to verify results.The laminated arched members were constructed with three layers of 3/4" Struc-1 exterior grade plywood, on edge, using staggered, spliced connections as a method of joinery. In order to optimize material usage, 4' panels were used giving 2'splices. Marine resin was used in the lamination of the member, with dowels being used at the spliced connections. As a method of fastening the 2"X6" deck beam system, notches were cut in the built-up member to accept the deck beams which were glued and doweled into place. No. 1 air dried Douglas Fir was used as a material for the deck beams, a low moisture content was desired to provide maximum strength and minimal shrinkage. The notches served as "blocking" for the deck beams providing efficient and strong connection. In order to reduce the deck beam span while maintaining structural stability, the deck beams were cantilevered 8" on either side of the longitudinal built-up sections. The decking consisted of 1-1/8" T&G exterior grade sub-floor, which was fastened to the deck beam system and the built up members by means of marine resin and builders screws, producing a diaphram. Since the point loads would be mid-span to any transverse members, this was a suitable material to minimize deck deflection. The curb for the structure was constructed of D. F. 2"x4" and was assumed to have no structuralintegrity

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)
	Our structure appeared to be very rigid under load. No creaking was heard during the load duration, which indicated sound joints and sufficiently sized members. Since the entire structure was assembled with dowels, and screws, and marine resin, it was essentially monolithic in nature and behaved so under load. Due to insufficient facilities at our campus, our bridge was loaded by means of a hydraulic jack with the use of a calibrated proving ring to monitor the load being imparted. The overall load was distributed to the point loads by means of a system of three I-beams of whose mass was accounted for, along with the jack and proving ring assembly, and was subtracted from the total required load. during the loading phase, the proving ring was monitored and adjusted accordingly to insure the load remained constant for all the loading phases. The dial indicators, which were located at mid-span under each arch, at the centroid of the point load at which maximum deflection was expected to occur and transverse from this location under each arched member, were zeroed before the apparatus was assembled on the deck of the structure. The decking and mid-span deflections showed to have linear relationship in the incremental loading with the maximum mid-span bridge deflection and deck deflection having values of 1.33 mm and 1.50 mm respectively at the 20 KN load increment.Our bridge and deck deflections showed to be nearly constant for the duration of one hour sustained loading period with the maximum deflection occurring at the later half of the loading period. This showed evidence of creep occurring which was assumed to be settlement at the base plate caps or possible horizontal translation of the base plates due to the thrust washers depressing into the arched member as a result of tensile loading. Our maximum bridge and deck deflection showed to be 1.40 mm and 1.59 mm respectively with these values remaining constant for 45 minute and 60 minute deflection readings. Overall, our bridge performed well under loading with the deflection values being close to that of our theoretical values being close to our design values. We were concerned with possible failure at the arch "springs", where the largest moment was observed to be occurring, and in particular at the spliced connections at these locations. The plywood used for the construction of the arched member had numerous voids in the transverse grain filler and the cross sectional area of the member was in question since the transverse grain filler was assumbed to have minimal strength properties compared to the longitudinal grain. This was also a consideration for the spliced connections on the longitudinal beam section of the arched member where the greatest force would be transferred to the arch from the deck beams adjacent to the locations of the point loads.

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:	None used
Deck:	Very strong and inflexible
Floor Beams:	Interacted well with arch and deck
Suspension:
Unique:	Arched beam with tie rod made exceptionally strong structure

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
	The treatment we chose was from the marine resin supplier. A copy is being sent. With the application of the outside finish and the resin glues we sealed and prevented dry rot. The system has been used for 40 years. The only problem we had was keeping the material warm enough long enough that it would dry. The two part epoxy requires a warm environment to cure. It penetrates the wood very well.

7. Special Considerations
	Actually we are talking to a number of people in our area. Especially the NPS, National Park Service, they always have a need for small bridges on their trails. In the city of Hercules we have many trails that span small water ways and I am working with them as well. Our basic consideration was designing a bridge which would have long endurance and withstand the elements. California is considered a desert state, however, in San Francisco this is not the case, we are right on the coast and we have quite an inflow of water from the delta. The choice of a marine preservative and marine glue was the natural step. It is environmentally friendly, and durable and non toxic.