Tunnel Top Slab Flexural Analysis

[EngSD]

In the past I have found this forum extremely helpful as I have researched questions but this is the first time that I am posting my own question. The question pertains to an existing tunnel that will now be seeing additional load due to changes on a project(attached is sketch of the tunnel top slab and all the reinforcement). I have ran the analysis and the slab works for shear but the trouble I am having is with the flexure. The factored moment demand on the slab will be 69k-ft but analyzing the top slab as a 9' simply supported span and accounting only for the #8 bottom bars I only get a capacity of 53.5 k-ft (f'c=4,000psi and fy=60,000psi). My questions is whether or not the shear bars can be used to account for additional flexural reinforcement or if there is any other things I am not taking into account that would provide additional flexural support?

Thanks to everyone for helping.

 

[Teguci]

1. Consider the top bars for compression reinforcement. It probably won't, but may increase your moment arm.
2. Consider some restraint from the walls. Slab will not behave entirely as a simple span.
3. Test the concrete compressive strength. Concrete may behave like 5000 psi or so.
4. Roughen the surface and add a 2" topping slab.

Shear bars, because they are not continuous, will not add to your moment capacity.

 

[EngSD]

Thanks, I had been thinking about your option #2 of the slab not compeletley acting as a simple support.

I had also thought about your option #4, but that was quickly shot down by the contractor.

 

[JAE]

Those shear bars cannot be used for flexure as they are not continuous.

The top bars (#4 @ 12) might be some help if you go to the trouble of calculating the section as a doubly reinforced section. Your 53.5 ft-kips/ft. is based on singly reinforced capacity neglecting the top bars.

The other thing you could do is analyze with some level of fixity from the walls. The walls look much thinner than the top slab so there probably won't be much negative moment at the ends. And you'd have to check whether the walls can even take the negative moment.

 

[JAE]

Oops - I was late on the trigger.

 

[EngSD]

Determining partial fixity at the wall to top slab connection is not something I am very familiar with. Do you have a good source I can use to brush up or can you give a quick explanation?

Thanks again for all your help.

 

[phuduhudu]

It looks to me like the top slab is only dowelled into the tops of the walls with a single line of bars so I don't think you can assume any moment capacity for that connection. It looks like it was just added as a simple lid. Also as others have said you would then need to check the wall section too. I think your option 4 might be the one to look at. More complicated would be some additional external bonded reinforcement under the tunnel lid slab.

 

[yakpol]

You will have to check serviceability (crack control) for AASHTO. For #8@12" reinforcing the unfactored allowable moment will be around 25 k-ft.

 

[JAE]

...only if AASHTO applies of course.

 

[hokie66]

Those "shear bars" are probably just supports for the top steel. No benefit in bending. Adding a topping slab would be my suggestion, if you can get at it.