Displacing Longitudinal Reinforcing in Kerbs 1

[asixth]

Hi guys,

Referring to my rough freehand sketch schematic. I have a prestressed deck unit bridge with a composite in-situ concrete deck bridge structure which has a slight cantilever overhang (2' / 600mm). I need to provide drains through the kerb units and as a result, some of the longitudinal reinforcement in the kerb needs to be displaced to avoid any clashing with the drains. This bar serves as a longitudinal bar for the torsional reinforcement for any traffic barrier loads which are imposed on the kerb.

What is the best solution / what are my options for this problem?

 

[ishvaaag]

Outright displacement may be directly OK on the proportions you are showing. The amount of rebar and the ear size look as able to redistribute any proportionate local effect present there. If in doubt, it may be analyzed. Imagine then substructures taking all present actions.

 

[hokie66]

asixth,

I can't see that the kerb is in torsion. I would just move the bar inside the drain. You need one at the top of the slab also. If you are concerned about impact loads cracking the kerb, you may want to add some diagonal bars across that corner, with 45 degree bends each end.

I'm sure this is not the first time this has been done. How does Main Roads normally detail it?

 

[rowingengineer]

asixth,
Hope you do not mind but I have provided a sketch with a bit more info for those that do not get to play with crash barriers very often.

I believe placement of the drain needs to be between the crash barrier posts, if possible. I would not like to see the displacement of the reo for the length of the kerb, because you do have large forces at the post location. I would prefer to splice at at drain locations only. However, the slab will help with the tensional strength, however I never found out how to calculate the slab effect.

Note: the crash rail from the main roads standard is not designed for all of the 250kn (for a medium) to be transferred in bending to the kerb at one post location, the design is to take this loading over a few posts. If you do an analysis of the crash rail, you can reduce the amount of loading, thus the torsion in the beam.


When in doubt, just take the next small step.

 

[ishvaaag]

On the slab contribution...

FEM solid model always helps.

Anyway, as in any RC in bending action we have two situations: cracked and un cracked. One has to examine whether the tensile stresses standing on the analyzed plate will be causing cracking, upon which situation one needs to reinforce not only by mandatory clause, but technical requirement. The usual approach of using some tributary tensile force may or may not be safely applied depending on how such force is derived; for it is obvious it is between other things a flexural situation and sometimes is not thought this way.

 

[asixth]

hokie,

the kerb goes into torsion when it is transferring the collosion load from the top of the kerb to the slab which goes into negative moment bending before the load can be taken out in the deck units back to the abutments.

rowingengineer,

I like the ideas that you present in your post and I will dig through AS5100 tomorrow to see what provisions I can make to reduce the distribution of barrier loading. I also like the idea of redirecting the drainage and I will put this idea forward to the appropriate people.

Thanks

 

[rowingengineer]

Just a quick note to save some time, the barrier you are using is probably at tested barrier. I have been informed Main roads Queensland have a tested all there barriers in accordance with AS5100. For my bridge, we justified the reduction in post loading to 190kN (could have gone further less but this wasn’t up to me). We did this by analysis and testing. The post was assessed using plastic limit state analysis of the bolts, weld, plates and post. The weld for the crash rail we had were 8mm fillet, which by calculation ignoring all safety factors failed at about 180kN. We also tested the post and again failure in the weld at 180kn. hence we used 190kn as the loading. I also did load sharing model of the crash barrier, because the 250kN (for medium) is distributed along 1.1m.

When in doubt, just take the next small step.

 

[hokie66]

asixth,

Yes, the kerb is in torsion. I just don't think of it that way. The wheel load is like any point load in a bending element, where the problem is deciding over what length the bending occurs. The moment and shear is greatest at the point of load, and dies out over some length each side. The top kerb bars would be the most important thing in distributing the load.

 

[rowingengineer]

Maybe I'm confused, are we talkinga about barrier on a kerb or a kerb barrier? ie are we talking about a 2000 kN point load at the top of the kerb or a 250kN load applied to a carsh barrier at 800mm above deck?

I am talking about the 250KN load at 800mm above deck.


When in doubt, just take the next small step.

 

[crossframe]

Why can't the drain go through the deck?

 

[hokie66]

Good question. If that doesn't work, why not continue it out through the kerb, missing the reinforcement, instead of turning it down?