Describe an alternate design method for bridge abutments with highly concentrated loads.
Existing bridge is a 3375 foot long plate girder bridge which was built in 1968. The bridge is considered fracture critical, due to having only two girders per bridge, no load path redundancy. Bridge carries US 52 over an industrial area, the Mississippi River and Railroad tracks. The Lafayette Bridge is one of the longest bridges in the Twin Cities. The existing and proposed consist of both a northbound and southbound bridge. Famous for bridge almost collapsed in 1975 due to a large crack that formed in one of the beams. Crack was repaired and bridge reopened.
Two designs and plan sets prepared, the contractor had the option of bidding either steel box girder or segmental concrete Proposed Bridge is 17 spans, longest span 362 feet, total length of 3210 Mn/DOT identified the need to replace the bridge based both on the condition of the bridge but also to improve the layout especially at the northern end of the bridge. Teamed with Figg Engineering on the segmental concrete option. Design was completed in the fall of 2010. Construction began in January, complete in 2014.
Abutments support the superstructure of the bridge and transmit load to the foundation. We used parapet type abutments with an expansion device due to the large amount of expansion and contraction of the bridge.
Two dimensional analysis works fine for multi beam bridges where load is evenly distributed along the length of the abutment. Moments are summed about the toe and pile loads determined. One pile load is determined for each row regardless of the location along the abutment. Typically designed using spreadsheets or MathCAD.
Typical loads were on the order of 2000 kips factored Beam Bridge may be 200-300 kips factored an order of magnitude more reaction
Wanted to know how the loads were distributed from the bearings to the piles We analyzed the abutments using standard 2 dimensional analysis without finite element and also with 3 dimensional analysis and compared results. Model was build using solid finite element blocks.
Finite element analysis (FEA) is a method for performing engineering analyses in which objects having complicated geometries are approximated with many small, simply-shaped elements. If mathematical solutions for the simply-shaped element are known, then a complex calculation is replaced with many simple calculations, which are performed on a computer. Common in aeronautical, biomechanical, fluids and automotive areas as well as structural design
Model of a wingwall for a different bridge, showing 2 dimensional finite elements or plate elecments. Common to use finite element for many different types of structures including bridge decks, piers, wingwalls. We use it quite a bit on environmental structures with odd geometry. Used where things such as beam theory don’t work well due to complex geometry. Even with plate elements assumptions still made, some of the issues with this include fixity with abutment wall. Generally fine for most bridges.
Model was build using solid finite element blocks. We used Staad Pro, but many other software programs available Solid finite element models require less assumptions, even looks more like the actual structure Blocks size used was approx 1 ft. The smaller the block the more accurate. Also the smaller the block the more time it takes to analyze. Each block has 8 nodes to distribute forces as opposed to 4 nodes with plate elements Drawing shows how stresses are distributed.
Adding the third dimension by using solids distributes the loads in both directions, along length and through the section Taller abutments tend to distribute the load more evenly so there is less advantage to using solid finite elements. Approximately 45 degree angle. Wingwalls are typically modeled separately but we were able to incorporate.
On taller abutments the results between 2-d and 3-d correlated somewhat closely. More beneficial with shorter abutments On the shorter abutments we used additional piles near the bearings to distribute the loads to each pile more efficiently. In the past technology was an obstacle, as computing power has risen going to see much more of this type of analysis.
Collaborative Effort Many more people involved especially from Mn/DOT Anti-Climactic, Steel was chosen, bids came in at $130M Don’t know how close concrete was, talking with contractors may have been fairly close.
Segmental Bridge Abutment Design Using Solid Finite Elements
Barritt Lovelace, PE WSB and Associates May 25 th , 2011 Segmental Bridge Abutment Design Using Solid Finite Elements
Lafayette Bridge <ul><li>Spans the Mississippi River east of Downtown St. Paul </li></ul>
Project Description <ul><li>Two options were designed </li></ul><ul><ul><li>Segmental Concrete </li></ul></ul><ul><ul><li>Steel Box Girder </li></ul></ul><ul><li>WSB Teamed with Figg Engineering Group to form the concrete design team </li></ul>
Abutments <ul><li>Resists vertical loads from Superstructure </li></ul><ul><li>Resists lateral loads from earth pressure </li></ul>
Two Dimensional Analysis <ul><li>Typical Design Method </li></ul><ul><ul><li>Loads from superstructure are assumed to be equally distributed along abutment </li></ul></ul><ul><ul><li>Pile loads are determined by calculating the neutral axis of the structure </li></ul></ul>
Segmental Bridge Loads <ul><li>Segmental concrete bridges typically have only two to three bearings </li></ul>
Three Dimensional Approach <ul><li>Produces large concentrated loads </li></ul><ul><li>Abutments need to be analyzed in three dimensions to determine distribution </li></ul>
Finite Element Analysis <ul><li>Definition </li></ul><ul><ul><li>The finite element method is a numerical technique for finding approximate solutions of partial differential equations as well as of integral equations. </li></ul></ul><ul><ul><li>In the FEM, the structural system is modeled by a set of appropriate finite elements interconnected at points called nodes. </li></ul></ul>
Advantages of 3-D Analysis <ul><li>Improved accuracy </li></ul><ul><li>Eliminates variability in pile loads </li></ul><ul><li>Reduces pile quantities </li></ul><ul><li>Capacity to more accurately place piles </li></ul><ul><li>Ability to integrate the wingwall geometry </li></ul>
Conclusions <ul><li>Very effective for shorter abutments </li></ul><ul><li>More accurate for all abutment heights </li></ul><ul><li>Potential to provide economical designs on all types of bridges and abutments </li></ul><ul><li>With better computer hardware these types of designs will be more common </li></ul>
Project Team Members <ul><li>MnDOT </li></ul><ul><li>Manjula Louis, Kevin Western, Paul Kivisto </li></ul><ul><li>FIGG </li></ul><ul><li>Russ Call, Mike Keller, Chris Burgess </li></ul><ul><li>WSB </li></ul><ul><li>Barritt Lovelace, Sabri Ayaz, Jim Archer, </li></ul><ul><li>Dave Vincent, Brad Robinson </li></ul>