Stites Tunnel is a buried two cell reinforced concrete arch structure that carries the Delaware Lackawanna Railroad over Paradise Creek and SR 191 in Monroe County, Pennsylvania. PennDOT Engineering District 5 determined that the tunnel needed rehabilitation and the SR 191 bridge structure that passes through one cell of the tunnel needed to be replaced. H&K Group, Inc. (H&K) was awarded the $8.3 million construction project for the tunnel rehabilitation and reconstruction of SR 191 with a 345-foot-long precast concrete culvert structure inside the tunnel and two new cast-in-place concrete approach spans. H&K retained Larson Design Group (LDG) to develop an alternate precast concrete design to provide further innovation for the approach structures and facilitate the timing and scheduling of the multiple site activities required for this complex project.
LDG worked closely with H&K as their engineer to brainstorm the alternate design and complete the final design and plans including securing approval from PennDOT following a rigorous review process. LDG also provided construction phase engineering including designing the temporary support structures and on-site verification of post-tensioning and grouting operations.
The project was separated into two different structure types and solutions to meet the complex geometry constraints. Inside the tunnel, a precast concrete culvert was designed to fit in the tunnel and provide for stream flow under the roadway. PennDOT’s design was a three-sided precast concrete rigid frame structures and a cast-in-place reinforced concrete slab to reduce the weight for erection inside the limited confines of the tunnel. LDG modified the design to use a four-sided precast concrete box culvert to eliminate the time-consuming construction of a cast in-place reinforced concrete bottom slab. LDG solved the constructability concerns by designing and pouring a thickened concrete mud slab capable of supporting two Taylor “Big Red” industrial lift trucks to work in tandem to move the 76 individual box culvert segments through the 345-foot tunnel and into their final position. Since the precast concrete box culverts were nonconventionally aligned with the top slab spanning perpendicular to the direction of traffic, standard PennDOT design software could not be used, so LDG engaged in a more rigorous analysis using finite element software to develop the live load forces for the design of the precast concrete box culverts.
Another non-conventional design was required for the approach span structures on each end of the tunnel. The existing bridge had the unique arrangement of the overflow channel of the creek taking two 90-degree bends under the approach spans. To accommodate this layout, LDG designed a precast concrete deck structure that was supported by new integral abutments under the outside shoulder and a new 94-foot-long post-tensioned concrete edge girder lengthwise on the creek side. To facilitate delivery and erection, LDG designed the combined precast concrete slab and edge girders to be fabricated in 8-foot segments and post-tensioned together after being erected on temporary supports (also designed by LDG). This alternate precast concrete segmental bridge design was an innovative technique for the structures and facilitated the timing and scheduling of the multiple site activities required for this complex project.
The unique project constraints and complex geometry necessitated using segmental bridge design and structural post-tensioning which are not normally permitted or used in Pennsylvania. However, PennDOT District 5 was very supportive of the LDG proposed design and recognized that this unique and innovative solution was required to provide the “side opening” using a precast concrete edge girder with post-tensioning. The integrated precast concrete edge girder with post-tensioning was used to solve the unique challenge of supporting one side of the bridge where an abutment could not be constructed. This bridge used the segmental bridge design features and details normally reserved for long-span structures on a medium-span structure to facilitate delivery and erection using readily available cranes and delivery trucks.
LDG demonstrated the feasibility and advantages of using post-tensioning on moderately sized projects to overcome difficult geometrical obstacles. As LDG embarked on designing this innovative structure, our engineers knew they had to create checks and balances in the design process to understand the design and develop a comfort level with the final product. Therefore, the design team developed an entirely new design process which started with using conventional simple span analysis with simplified end condition assumptions and proceeded to more sophisticated analysis using STAAD to develop two and three-dimensional models which could be checked against conventional analysis methods. Alternate designs are challenging due to short schedules and the need for the contractor’s engineer to quickly get up to speed with a design process that has progressed over several years between the owner and their engineer. LDG initiated early coordination with the owner and Engineer of Record at the preconstruction meeting. Through ongoing discussions and video conferences, LDG was brought up to speed on the design challenges that LDG needed to proactively address in the alternate design. Social and economic considerations were carefully weighed in the development of the alternate design. Reconstructing the bridge inside the tunnel using cast-in-place concrete as designed by PennDOT or steel beams would have extended the construction duration and the associated 7.1-mile car detour and 15.8-mile truck detour by an additional construction season. The alternate design was constructed at no additional cost to PennDOT.
This project was truly an innovative take on typical bridge work. The only resemblance of this project to a typical stream crossing was that it was designed by an experienced Pennsylvania-based engineer and constructed using multiple forms of precast concrete by a local contractor and fabricator using excavators, cranes and bituminous paving equipment. Even the precast concrete box culverts used inside the tunnel required a special finite element analysis and a nonconventional erection method using forklifts instead of cranes. Besides the complexity of designing a structure inside the tunnel, the approach structures were the most complex features since the design needed to accommodate the two 90-degree bends in the stream.