10.2 Steel Composite "Banana" Farm Access Bridge
Subjects Covered
- Composite beam structures
- FE webs; 3D structures
- Virtual members
- Member eccentricities
- Joint editing
Outline
This access bridge is constructed with two steel plate girders supported on “H” piles acting compositely with a concrete slab. The top flange of the beam has an arched profile and it is deeper in the centre than at the ends.
The slab, diaphragm and upstand are created with grade C32/40 concrete and the girder with grade 355 structural steel.
The structure is modelled using a 3D shell finite element slab (curved in elevation) with a steel composite beam inserted as an FE web beam to explicitly model the web as finite elements and the flanges as beam elements. Upstands are added as edge beam members with the appropriate vertical offset and the diaphragm is represented with finite elements.
The beam, a 30m span, is assumed to be continuous at its ends as there will be some hogging at this location and this will affect the effective breadth of concrete flange. The adjacent spans (the piles) are assumed to be 4m long and fixed into rock at the remote end.
The girder has uniform thickness of web and flanges throughout (28mm & 50mm respectively) and the flanges are 500mm wide. The overall depth is set to 1000mm deep but with a sagging profile such that the overall depth at the ends is 500mm. This is done with 20 straight segments, as the curved profile is limited to hogging shapes. The profile points are not exactly on a circular curve but are close to it.
The slab in the beam representation is 2m wide and 0.2m thick, but is offset by 0.5m.
An edge upstand 200mm wide and 250mm deep is added above each edge of the slab and is assumed to be structural and cast with the slab.
Mid Span Section
| Profile of Top Of Beam | |
|---|---|
| 0.00 | 0.00 |
| 1.50 | 0.19 |
| 3.00 | 0.36 |
| 4.50 | 0.51 |
| 6.00 | 0.64 |
| 7.50 | 0.75 |
| 9.00 | 0.84 |
| 10.50 | 0.91 |
| 12.00 | 0.96 |
| 13.50 | 1.00 |
| 15.00 | 1.00 |
| 16.50 | 1.00 |
| 18.00 | 0.96 |
| 19.50 | 0.91 |
| 21.00 | 0.84 |
| 22.50 | 0.75 |
| 24.00 | 0.64 |
| 25.50 | 0.51 |
| 27.00 | 0.36 |
| 28.50 | 0.19 |
| 30.00 | 0.00 |
The carriageway on the bridge is a single lane of 3.8m, centred on the deck, with no footway or verges.
Procedure
The general procedure is as follows:
- Create a Beam for a composite girder and add tiles etc.
- Define the materials needed
- Define a composite design beams representing the north girder
- Save the beam for subsequent use in a structural project.
- Create a refined model and add title etc.
- Define a design beam in the project using the save file and embed the data (which will add material properties to the project).
- Create a 2D sub-model consisting of a flat 2d finite element slab and the edge upstand beams.
- Change the 2D sub-model to 3D and adjust vertical coordinates to give a vertically curved slab.
- Create design section in the project to represent the upstand and the piles.
- Assign the composite girders to the structure as “FE Web” members and the edge section to the edge beam elements.
- Define two 2D sub-models containing the piles and diaphragms and assign properties and supports.
- Define dead load and SDL load cases and compilations.
- Live load optimisation for one point on the south main girder.
- Analyse the structure/load cases.
- Transfer the results to the design beam load effect tables.
- Perform one or two design checks.