Grillage Analysis of a Bridge Deck

[ZeroStress]

Hello

I am currently assessing a bridge deck by grillage analysis. The deck consists of T4 prestressed beams closely spaced together (500 crs) and fully infilled with insitu concrete with 75mm topping.

I have modelled the longitudinal grillage member representing the main prestressed beams and assigned the relevant section properties.

However, I am confused in what should be the section properties for the transverse members. In the case of a prestressed beam with just a flange insitu concrete (not full depth infill), the transverse section is represented as the flange depth x spacing of transverse members. Is the same principle applied to the fully infilled deck i.e. total depth of the T4 beam and topping x spacing of transverse members?

If I assign the section properties based on full depth of the section, I get a heavily stiff deck in transverse direction which is eventually transferring most of the load towards the edge longitudinal member eventually over stressing it.

Could anyone please advise?

Thanks

 

[MotorCity]

I am a building guy, not a bridge guy, but this seems like a very unusual arrangement that would be highly susceptible to cracking of the infill and topping concrete.

But in any case, it looks like you would gain strength in the transverse direction primarily from the lateral (weak axis) strength of the beams with the topping acting as a diaphragm of sorts. The infill concrete seems like it is just dead weight that would not contribute to the strength.

 

[Agent666]

Just a thought, it would be quite easy I would think to put some ducts cast into the base of the prestressed beams webs that some transverse reinforcement or post tensioning could be passed through to tie it all together at the base, then you have a bit more certainty over the transverse stiffness.

 

[ZeroStress]

@MotorCity
This is fairly standard method of PS Beam bridges. The infilled concrete adds valuable strength to the capacity with minimal addition to the dead weight.

@Agent666
You're right there are holes in web's of the beam and transverse reinforcement is passing through the webs connecting the beams transversly.

 

[BridgeSmith]

The differential stresses between the essentially inert infill concrete and the prestressed beams could become substantial when the beam concrete creeps.

I'm struggling to understand why you would fill between the webs anyway; it likely adds more weight than it does strength. This could likely be accomplished much more efficiently using precast box beams





Rod Smith, P.E., The artist formerly known as HotRod10

 

[Settingsun]

For reference, Hambly's book Bridge Deck Behaviour has an example where transverse I is 5% of longitudinal I. His calculations aren't explicitly shown but the factors include cracked properties for transverse, reduced effective depth as the transverse reinforcement is above the bottom flange, and lower strength insitu concrete compared with the precast beams.

I think overload of the edge beam is common. One Australian state road agency bases its edge beam longitudinal stiffness on the ratio of capacity compared with the interior beams.

https://www.google.com/url?sa=t&sou...FjAAegQIAhAB&usg=AOvVaw3P630_z1DrdiHRG-Wfye2l