Prefabricated Steel Bridge Systems: Final Report
7. Conclusions And Recommendations
In this study two steel bridge concepts are developed using innovative technologies and techniques that will accelerate the construction of bridges. Both concepts are based on modular units made of steel girders and concrete deck and are presented in discussed in this report.. The first concept is made of all-prefabricated system including the deck slab, whereas the second is made of prefabricated steel girder system including a cold-formed steel form that will receive cast in place concrete deck. Parameters such as normal weight concrete (NWC) versus light weight (LWC), normal strength concrete (NSC) versus high strength concrete (HSC), and 8 inch versus 6 inch slab are studied and compared for performance and cost. Advanced optimization techniques are utilized to optimize the two concepts and to compare between the resulting different systems and assess the influence of the studied parameters on cost.
The following conclusions can be drawn from the study:
Maximum span lengths that can be achieved
- For 200 kips maximum weight, the maximum span lengths that can be achieved are approximately as follows: (i) for CIP (NWC and LWC): 225 feet and 250 feet for NSC and HSC, respectively; (ii) for NWC Precast slab: 130 feet and 155 feet for 8in. and 6in. slab, respectively; for LWC precast slab: 146 feet and 167 feet for 8in. and 6in. slab, respectively.
Effect of slab thickness
- For PrecastNWC, as slab thickness is reduced from 8 inches to 6 inches, the maximum length achieved for 200 kips total weight is increased from 130 ft to approximately 153 ft, i.e., an increase of 17.70 %.
- Similarly, for PrecastLWC, as the slab thickness is reduced from 8 inches to 6 inches, the maximum length achieved for 200 kips total weight is increased from approximately 146 ft to approximately 166 ft, i.e., an increase of 13.70 %.
- It is concluded that from the cost point of view the use of 6 inch slab is more beneficial in NWC than in LWC.
Effect of using HSC
- For precast NWC concrete, the use of HSC results in no gain in span length and given the span length, it translates into a more costly bridge, compared to NSC.
- For CIP concrete, the use of HSC can provide a gain in performance for span lengths above 160 feet for both LWC and NWC. This means that for CIP concrete bridges below 160 feet span, it is clearly advantageous to use NSC.
Effect of using LWC
- The gain in length due to the use of normal strength LWC in CIP concrete bridge is marginal (less than 1%), regardless of the span length. Therefore the cost of both LWC and NWC CIP bridges is the same.
- For Precast NSC, the use of LWC results in a maximum gain in bridge length of around 12% for 8 inch slab and 8% for 6 inch slab, compared to NWC, for any given span length. However, the cost analysis for 120 ft span showed that the cost of both LWC and NWC Precast bridges is similar.
The following tasks can be recommended for future research:
- Half scale bridge units for the two concepts should be tested for static as well as fatigue. In particular the effect of using LWC and the 6 inch slab on fatigue should be carefully studied.
- Constructability test should be carried out to assess the applicability of the details of the concepts.
- Extending optimization work to include various unit width and girder spacing as well as exterior girders.