Mars Hill/Lowell Observatory Truss Bridge

Design Process

The design process for this Town Lattice Truss Bridge included research, a review of applicable codes, demand calculations, design, and confirmation of the results using SAP 2000.

Before any calculations or design could be accomplished, preliminary research had to be conducted. The design team had to become familiar with the engineering properties of wood and wood processing because the client requested the use of Ponderosa Pine from his own property for this project.  The design team also had to become familiar with Town Lattice Trusses and general bridge components necessary for a combined vehicular and pedestrian bridge.

Communication with the City of Flagstaff and the project advisor resulted in the recommendation of design manuals and documents to use as references.  These included AASHTO Standard Specifications for Highway Bridges 2002 (17th Edition), ASCE/SEI 7-05: Minimum Design Loads for Buildings and Other Structures, 2010 ADA Standards for Accessible Design, and the City of Flagstaff Building Codes and Engineering Standards.  The codes were reviewed to find design information.

With the use of applicable codes, demand calculations were performed for the bridge and thus finding the resulting forces.  AASHTO was used to determine the allowable stress of the timber.  Based on the demand and available capacities, bridge components were designed and sized.

The Town Lattice Truss Bridge was drawn in SAP 2000 to the dimensions determined by the design team.  The bridge was then loaded to determine the forces acting in each member, and these values were used to confirm the allowable stress in each member was not exceeded.  SAP 2000 was also used to determine the shear force each connection joint must sustain.

 

Significant Milestone

·         30% Design - 2/21/11

·         Scale Model - 4/28/11

·         60% Design - 4/28/11

·         Presentation - 4/29/11

 

Project Obstacle

Some of the obstacles that Ponderosa Engineering has overcome were due to the steep learning curve for the project.  The team was unfamiliar with wood and bridge design; therefore more hours were allotted to research to further our understanding of these concepts, which caused a delay in our schedule.  The team also encountered another delay due to unfamiliarity of detailed design for a timber bridge and time demand associated with the coming up to speed on these aspects. Ponderosa Engineering got back on schedule by holding extended meetings to complete the 60% of bridge design, website, UGRAD poster, and scale model.

Ponderosa Engineering and BGK Engineering had the unique challenge of concurrently developing designs that were dependent on one another.  As a result, each group had to maintain effective communication with the other to minimalize delays.