For a waterway crossing, the designer must consider the backwater elevation and flow velocity for both the proposed and existing structures. It is recommended that the same hydraulic model be utilized for both the existing and proposed structure. Any increase in backwater elevation or stream velocity must be thoroughly analyzed and the upstream and downstream effects considered. For a grade separation structure the designer must consider both horizontal and vertical clearances. The shape of the replacement structure must be considered when determining the minimum clearances.
It is imperative that an accurate and complete survey of the existing structure be conducted. This will aid the designer in determining the maximum prefabricated structure size that can be installed at a particular site.
In certain situations it may be possible to reuse portions of the existing structure in the design of the replacement structure. The most obvious example is reuse of the existing foundation. If the foundation type is known (i. e., concrete spread footer, concrete on piling, etc.) standard geotechnical engineering calculations for assessing the suitability of the foundation must be completed. The designer is cautioned against using existing unknown foundation types.
One of the primary benefits of utilizing a prefabricated culvert as a bridge replacement is that much of the existing structure can remain in place. This reduces construction time and reduces the work limits required for the structure installation. For single-span structures with vertical wall-type abutments, it is typical to leave the existing abutments in place. It may also be possible to leave the deck in place. For multiple-span structures, existing abutments, piers, foundations, and deck may all be left in place depending on site constraints. The required size of the replacement structure, along with site access will typically control how much of the existing structure can be left in place.
Another consideration for the designer is the void space between the existing and proposed structure. If there is insufficient void space to properly place, compact, and test soil backfill, the use of flowable fill is common. Where flowable fill is utilized, it is recommended that the proposed structure size be maximized. This is because the cost of the additional structure size is typically far less expensive then the cost of the flowable fill.
Lastly the designer must determine the structural capacity of the replacement structure and the existing structure. If the two structures are very close or if the existing deck is left in place, then the composite strength of the two may be considered. The finite element method is well suited for this complex analysis. In the absence of sophisticated computer methods, the designer can conservatively ignore the contribution of the existing structure. However, typical design assumptions regarding surrounding soil support must be verified prior to the use of the closed form design methodologies presented in Art. 5.8. The designer must also consider external grouting pressures when flowable fill is used as the backfill material.
