|
Request for Preliminary Bridge Girder Comparisons
Preliminary Bridge Girder Comparisons
In order to achieve economy and efficiency in steel bridge design, engineers must be familiar with the most advanced design techniques and construction materials available. Many factors are involved. From a materials standpoint, the engineer must determine the most economical steel grade level for a given design. What is the cost savings for unpainted weathering steel versus painted high strength steel? What is the most cost effective web depth? What is the most economical span arrangement and girder spacing?
Conducting thorough studies of this nature can be both time consuming and tedious for design engineers. Furthermore, many of the cost factors required for these studies are not readily available to the designer.
Bethlehem Steel provides a service to aid engineers in the economic evaluation of parallel-flange plate girder bridges. Bethlehem has a unique computer program that optimizes preliminary girder designs according to cost, not least weight. This has proven to be a helpful tool in determining the most efficient and economical use of steel for bridges. It is based on the current AASHTO Standard Specifications for Highway Bridges, with price/cost parameters being updated regularly.
The preliminary comparison will be most valuable if it is conducted at the time of initial planning, immediately after basic alignment for the structure has been established. Perhaps best of all, the service is free of charge and carries no obligation.
Information Required:
In order to assure an effective turnaround time, the following information is needed to proceed with a bridge study:
- Name and location of structure
- Cross-section of the deck
- Proposed girder spacing
- Span lengths
- Design loads
- Restrictions or unusual design criteria
Preliminary Cost Optimization:
For a given set of design criteria, (span length, material grade, and girder spacing), the program is given a range of girder depths to investigate. For each specified depth increment within this range, optimum designs are developed and corresponding costs computed. Each design is optimized by considering the trade-offs between fabrication and material costs.
For example, a designer has the option of selecting various web thicknesses for a particular web depth, each of which requires a corresponding number of stiffeners. The scheme weighing the least utilizes the thinnest web permissible, but requires the greatest number of stiffeners. This scheme may not be the most economical since more fabrication is required.
In the program, web designs are generated for each depth by first selecting the minimum thickness permitted by the AASHTO Specs. Web thickness is then increased by 1/16" increments with corresponding decreases in the number of stiffeners required. A new design with a corresponding cost is produced in every case. This procedure continues until a girder web is developed which needs a minimum number of transverse stiffeners. The program then selects the most economical web from all of the costs generated for that depth. It then moves on to the next web depth.
A similar process is used to determine the most economical flange plates. At predetermined points along the length of the flange, the program evaluates the trade-off between continuing a heavier flange plate and the cost of a butt splice.
The program then compares the optimum designs (web and flanges) at each depth and selects the low cost combination as the overall optimum design. Relative cost indices are reported for several depths each side of the optimum (Figure 1). This information is often valuable in cases where depth of the superstructure is a major concern.
FIG 1.
| Depth(inches) |
Web Thickness(1/16") |
Web |
Flange |
No. of Stiffeners |
Weight |
Cost |
| 51 |
Variable |
A572 |
A572 |
34 |
1.01 |
1.04 |
| 54 |
Variable |
A572 |
A572 |
36 |
1.01 |
1.04 |
| 57 |
Variable |
A572 |
A572 |
34 |
1.02 |
1.03 |
| 60 |
Variable |
A572 |
A572 |
38 |
1.00 |
1.00 |
| 63 |
Variable |
A572 |
A572 |
38 |
1.00 |
1.00 |
| 66 |
Variable |
A572 |
A572 |
46 |
0.98 |
1.00 |
| 69 |
Variable |
A572 |
A572 |
40 |
0.98 |
1.00 |
The program considers not only material cost, but also the fabrication costs of stiffeners (transverse, longitudinal, diaphragm, and bearing), flange splices, horizontal and vertical web splices, web-to-flange welds, shear connectors, and blast cleaning of the steel. Painting costs are also included and are based on one shop coat and two field coats. For unpainted A588, only the cost of blast cleaning is used. The costs associated with notch toughness requirements for both FCM and non-FCM plates are also included for main load carrying members.
No consideration has been given to other details whose cost would be common to all designs; i.e. field splices, detail material, bearings, etc. Thus the program addresses factors affecting about 75% of the delivered steel cost. Substructure items such as foundations, piers, and abutments are not included in the study and have to be considered by the designer in order to optimize the entire structure.
|
