NAU
ASCE Steel Bridge Team
2007-2008
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Introduction to Team Members |
The Steel Bridge Team consisted of four Civil Engineering students.
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Introduction of Project |
Purpose: To design and build a steel bridge for competition in the annual ASCE/AISC Pacific Southwest Regional Conference. The NAU Steel
Bridge team was responsible for the design, fabrication, and construction of
a 1:10 scale model bridge that represents real environmental conditions. The bridge was to be 20 feet long and
fabricated strictly of steel. The team
participates in ASCE/AISC regional competition where there are standards for
strength, durability, constructability, usability, functionality, and
safety. These are the key regulations
that govern the design and construction of full-scale bridges. An initial design was chosen with the
following in mind: constructability, time, and weight. With the assistance of a technical advisor,
the team initially sized members with hand calculations then inputted them
into a structural analysis program.
This produced results to determine if the bridge would succeed in
competition. Changes were made until
the final design was selected after various calculations; the final design
chosen was a truss type bridge. The
team completed fabrication with the aid of welders. After much practice the team attended
competition and constructed the bridge onsite. |
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Design and Analysis |
Preliminary Designs The first bridge type that was researched was a single span beam bridge. It became apparent that a beam bridge is not adequate in deflection. Since during the loading aspect, the bridge must not sway horizontally more than 1 inch nor deflect vertically more than 2 inches. Therefore, a very stiff beam bridge would need to be designed, but to do so it would have to be extremely heavy to hold the maximum moment that the various load types apply. It was a priority to keep the weight of the bridge as minimal as possible. Since it was found a beam bridge is not adequate, different types of truss bridges are now considered. These types must fit certain design requirements given by ASCE/AISC rules. The bridge must 19-21, each member cannot exceed twenty pounds, dimensions of 3.5 x 6 x 6, and the bridge must be fully made out of steel with no variation. With these limitations it was determined that designing a truss bridge, a Pratt type would be a best fit design. This is due to the fact that all the diagonal members are in tension unlike the Warren and Howe. Yet the Pratt design has many connections between members which will raise our construction time and fabrication time, which are part of requirements to keep a low cost bridge. Final Bridge Members Design Since the load combinations from the rules showed that there were two applied vertical loads that may vary in locations from 47-82 and 122-157. As a result the maximum moment must be calculated by creating shear and bending moment diagrams for every load case by using hand calculations and EnerCalc, a structural engineering program. With the maximum moment calculated to be 5.37 k-Ft the design of each individual member could be completed. Summary of Member Sizes
The footings were designed based on surface area, not axial capacity. It was determined that a 1 x 1 twall = 0.083 is adequate but had little surface area, thus, a 3x 2, twall = 0.083 gave plenty of surface area for additional stability of the bridge. |
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Drawings |
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Fabrication |
Fabrication Process The fabrication process began even while the bridge was being designed. As member sizes were being designed, contact was being made with steel suppliers to see what was readily available. After researching what was available, it was decided that for our project the best option was to order the steel from a local supplier. Arizona Steel Fab. & Supply had most of the types of steel that we desired in stock and could easily supply the ones that they did not carry. A detailed list of materials to be used can be seen in Appendix B. This list aided us in the ordering of the correct amount steel. The team also decided that to save time some of the steel would be sent to Boyer Metal, a local steel shop, to perform some of the more difficult and repetitive cutting. However, this company took three weeks to complete this cutting, which ultimately set our fabrication process behind schedule. Some of the cutting which they completed was also completed incorrectly. Thus, we also had to spend time repairing or recutting the pieces that they cut. Nuts and bolts were purchased from Copper State Nut & Bolt Co. There were a total of 182 nuts and bolts, all connection pieces were Grade 8 and the bolts were 3/8 diameter. Three different sizes of bolts were used: 102 bolts of 5/8 length, 60 bolts of 1 ½ length, and 20 bolts of 2 length. The welding process began with the bottom chord members, a total of 6 members on each side. The most complex welding occurred while constructing these. The ½ x ½ and 1 x 1 hollow tubing as well as various different plates were welded together to form this member. Figure B.1 shows the chief welder, Randy Salcido, fabricating the bottom member. The lateral cross-bracing was then fabricated, consisting of built up members similar to the bottom chord members as well as 1 x 1 tubing. Plates were welded onto the end and holes were drilled in the proper locations. In total there were eight lateral braces along the bottom of the bridge. The entire bottom chords and cross-bracing were constructed before work continued on any other section of the bridge. Welding the Bottom Members Next the footings were fabricated, multiple plates were welded onto a 3 x 2 hollow tube to form holsters for the bottom and top chords to sit in. After the bottom chords were placed into the footing holster construction began of the top chords. The 2 x 1 hollow tubing and the end plates were welded together to form members which were assembled together to form the entire top chord. Two members end plates were mated together and holes were drilled. This mating together was done whenever two plates were to be connected. These two members had to then always be connected together. The top corner chord consisted of two 2 x 1 tubing with angled cuts welded together. Plates were then welded over this weld joint to allow for the inner members to connect to this chord, this is seen in the Figure B.2. Once all of the top chord members were fabricated, connections were made to the footings. Following this work began on the lateral bracing between the two top chords. There were a total of two lateral braces placed along the top chord. This bracing consisted of a 1 x 1 hollow tube with a rectangular plates welded onto each end. The member was placed in the desired location and holes were drilled to allow for a nut and bolt connection. Corner Chord Connection At this time the rods as well as the inner member 1 x 1 tube were fabricated. Plates were welded parallel to the rods and holes were drilled at the correct location in order to have a perfect connection from top to bottom chord, the welding of the rods can be seen in the Figure B.3. The inner chord consisted of two separate members, due to the length restriction of each member. Plates were welded onto one end to make the connection between these two members. A hole was drilled at one end, and then a mark was made on the other end where that hole needed to be. We ran into a little difficulty with one section of the bridge with these members. Two of the rods and one of the 1 x 1 members had to be lengthened from the original design in order to have sufficient shear out distance from the hole. Welding of the Plates to the Rebar The final members to be fabricated were the top chords knee bracing. These consisted of 1 x 1 hollow tubing with angled cuts. Plates were welded onto the end and holes were drilled for connections to the top chord and lateral bracing members. After the entire bridge was fabricated and constructed one more step was completed. This was to weld labeling onto each member. This permitted us to be able to construct the bridge repeatedly in the exact same manner. Lastly, the bridge was spray painted and labeled as Northern Arizona University as required by the guidelines. The primary location for all of the welding occurred at Randy Salcidos personal machine shop. The NAU Machine Shop was also used for some welding, all of the remaining cutting, and the construction of the bridge. Look here for pictures taken during the fabrication process of the bridge. |
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Competition |
Competition was held in Northridge, CA this year. A total of 16 schools participated in the Steel Bridge competition. The competition is judged in a number of ways. The factors that determine the placing of the bridges is weight, timed construction, penalties during timed construction, and deflections due to horizontal and vertical load testing. During the timed construction of the bridge, the team suffered several penalties. Two tools dropped, one fastener dropped, one wet water violation, which is when a builder steps onto the tape that is said to be the river, and a decking height penalty of approximately 1/8th inch too high. The team believes that this was due to the local conditions of the site because the team measured the decking height before leaving for competition and it was a lot less than 1/8th inch. The construction time was approximately 22 minutes and 12 seconds. After the bridge was constructed, the team carried the bridge over to the loading area. An initial horizontal load of 50 pounds was placed laterally on the bridge. The bridge deflected less than the one inch maximum on this test. Then the vertical load was placed on the bridge. The load was placed in two different places, 61 inches and 122 inches from the edge of the bridge. This was determined by a role of dice the night before competition. The bridge held all 2500 pounds that was placed on it. The vertical deflection was approximately less than half an inch, less than the maximum deflection allowed of two inches. But with this vertical load on the bridge, the deflection laterally was more than the inch allowed; it was a total of one and a half inch. The bridge was disqualified due to the lateral deflection during the vertical load test. After the team discussed in great detail everything that could have allowed for the lateral deflection, the team decided to reload the bridge once back at campus. The team was convinced that the slope of the ground where the bridge was loaded had a significant impact on the lateral deflection of the bridge. The team reloaded the bridge, using the same load conditions that were used at conference. The following table, Table 1.1, shows the deflections at both conference and on campus. Load Deflection
With this information, it was determined that the slope of the ground was the main influence on the lateral deflection. Look here for pictures taken at conference during construction time as well as load testing. |
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Conclusion |
The steel bridge team worked attentively to
design and fabricate a competitive entry forthis years PSWRC. All together,
from design, fabrication, practice construction time, and aesthetics, a total
of 740 man hours were spent on the steel bridge this year. This years bridgeevolved
into one of the best bridges Northern Arizona University has ever seen. This
was because it was very light weight, the connections used, as well as getting
the construction time down by practicing many times before conference. The steel bridge team has come to conclusion
that very few design options could be improved. One improvement would be to
have lateral diagonal cross bracing on the bottom chord of the bridge. This
could have helped prevent the lateral deflection the bridge had during loading
caused by later torsional buckling. Other obstacles the team faced this year
were communication and various amounts of interest amongst the team members.
All the team members had very different schedules preventing a set schedule
for the semester. Due to this unbalanced schedule meetings were held at night
to work on the bridge. This kept the team on schedule for finishing before
conference. What did set the team behind was trying to find if a steel shop
would be able to fabricate the bridge for us. As well as steel being sent to
Boyer Metal to have the plates cut. Boyer Metal took three weeks to finish
and the cuts were not all perfect, therefore, the team spent more time fixing
their mistakes. |