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21. 4. 19 오후 7:58

This webinar shows a case study about the steel composite ladder deck girder bridge. The webinar will highlight major factors to consider in the design of this type of bridge, such as shear lag and buckling effects. It will provide recommendations on suitability and use of different model types. Since many of ladder deck bridges are constructed using the push launch method, special considerations for the construction sequence will be discussed.  


In this session, Martin Bosak from Barry Transportation shares the presentation regarding modeling approaches including the geometry, model properties, loads, and construction stage analysis with the steel ladder deck bridge.

 


This case study highlights:

 

1. Purpose of Different Model Types for Steel 

More than one type of structural model may be necessary for the steel bridge design. The speaker will talk about the purpose of different modelling methods and how those models can be created. 

 

 

2. Deck Slab Considerations

The presentation will discuss whether and how to include effects of shear lag in concrete deck as well as cracked concrete over bridge supports.

 

 

3. Special Considerations for Push Launch Construction

Stability of the bare steel structure during construction may be a deciding factor in the design. The webinar will provide guidance in this important aspect of the bridge design.

 


 

1. Purpose of Different Model Types for Steel Bridge Design

 

The model for case study is a steel composite ladder deck girder bridge. The bridge has twin steel girders and the concrete deck. And it is constructed by Push Launch Method. Here are information of the bridge.

 

Fig02

Fig03
Fig. Details of the steel composite ladder deck girder bridge

 

Now, let's look at various modeling approaches  of this bridge. When we create and analyze a model, we have a number of options as a designer. Here are approaches for modeling.

 


 

Approach no.1. 3D Frame(Grillage) Model

 

Purpose

- Construction stage analysis(Launch, deck construction)

- Global static analysis

- Support reactions(Bearing design), Deformations, Member design and verification(Main girder)

Fig04
Fig. Grillage Model View and Bending Moment Diagram on Main Girder

 


 

Approach no.2: Mixed Type Model

(Deck and main girder web as plates, crossbeams and/or girder flanges as line beams)

 

Purpose

- Stability verification during construction

- Transverse deck analysis and design

Fig05
Fig. Transverse Bending of Deck Slab

 

 

 


 

Approach no.3: Full Plate Model

(All elements modelled as plates, including crossbeams and stiffeners)

 

Purpose

- Detailed analysis of crossbeams

- Local verification(Stiffener buckling, local web buckling, stress concentrations)

Fig06
Fig. Stress Concentration at Transverse Stiffener

 

Fig07

 


 

2. Deck Slab Considerations

 

If you decide to use the grillage model for your model, you have some considerations to model the bridge. In midas Civil, you can model each part using Composite section type and Steel section type. And you can use dummy elements to complete creating the grillage model.

 

Fig08

 

 

It is necessary to take into account effects of shear lag in concrete deck when you have the grillage model. There are provisions in Eurocode 4 about how to calculate participating width of concrete slab. The steel composite ladder deck bridge has just 2 main girders. The distance of 2 girders is not short so, we need to calculate participating width called the effective width.

 

 

Fig09

 

 

Next, we need to check the effect of cracked concrete. We know that the tensile stress or large hogging moment at the internal support makes the crack at the concrete deck. It makes the stiffness of concrete to be decreased. In order to consider the effect, midas Civil provides Section Manger to adjust the stiffness as the scale.

 

 

Fig10
Fig. Adjustments of cracked concrete section with Section Manager

 

 

If you decide to use plate elements for your model, you don't nee to deal with the effective width of the slab - shear lag effect is included implicitly within FEM model. The effect of cracked concrete slab over supports may be modelled by material properties of concrete(Reduced the elasticity modulus value, using the orthotropic material). And it is necessary to pay attention to proper mesh size and element shapes.

 

 

Fig11
Fig. Details of girder made of plate elements

 

 


 

3. Special Considerations for Push Launch Construction

 

The steel ladder deck girder was constructed by using the push launch method. Here are considerations for the push launch method.

 

Fig12

We have 2 options for modeling approach of the push launch method. Depending on designer's decision, automatic generation method or manual definition of several critical positions. 

 

Fig13

 

After the push launch work, we have the deck construction. We have 2 options for the deck construction to consider this work in SW. 

 

Fig14

Fig15

 

We had looked into considerations for the general design and modeling. In the next blog content, we will have a time to check detailed modeling works with midas Civil.

 

 


 

Watch the full webinar video

 

 

 

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Martin Bosak | Senior Engineer | Barry Transportation

Martin Bosak has over 15 years of experience in structure design such as concrete and steel bridges, road, rail, and foot bridge design.

Bridge designer in major infrastructure projects across Europe:
- Czech Republic (D3 Motorway, R48 Expressway, I/11)
- Slovakia (D3 Motorway)
- UK (A5 Northern Ireland, A737 Scotland)
- Ireland (M3 Motorway, N9/N10, M17/M18) 

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