Extra 2'' concrete on PT slab

[tedamiao]

Hello, friends,

We have finished calculation/drawings and passed plan check for a 10'' thick two way post-tensioned elevated floor slab. Everything is ready for construction. Right now, owner is planning to use 12'' thick and leave all PT cable and mild reinforcing at the same location/elevation as 10'' slab. This will bring us additional 2'' concrete dead load (25psf), which is not designed originally.
We originally designed by using ADAPT floor pro 1.5 based on slab self weight and 40 psf live load.

Anybody has better idea how to evaluate this situation rather than redo analysis?
Thanks.

 

[csd72]

I am sorry to say but you will probably have to reanalyse it.

 

[JAE]

I would re-analyze it myself. You are intuitively correct that the extra depth will be significant in reducing flexural stresses, etc. but with post-tensioning, the changes in axial stress, balanced loads, and other effects may not be as apparent.

Standard of care: what would another reasonable engineer do? I think most would at least re-check a number of typical spans to see the effect accurately.

 

[tedamiao]

Thanks, guys. I will run analysis to see........It will take a lot of time.......Plus there is no budget for this work. What happened is contractor does not want to change anything.....

 

[rday]

Agreed, re-analysis is required.

No budget?

The plans is being changed by the contractor after you were done, sounds like a change order.

 

[Taro]

Is seismic an issue for this project? The extra mass could have a significant effect on the lateral system.

 

[msquared48]

Any chance of using a lighter weight concrete of the same f'c? If you are using a 150 pcf mix now, one of 125 pcf would be the same dead weight.

Mike McCann
MMC Engineering

 

[nicam]

does it really take a long time to re-analyse? With the program I use, changing the slab thickness would only take about 10min to redesign if the bulk of the design has already been carried out.

Also like rday said it sound like you have a case to charge for a variation.

 

[csd72]

Do not do any work until someone agrees to pay you for it. Too many of us do additional work for no fee and it only serves to belittle our profession!

You also need to rationalise it a bit, can you check the extreme cases and then apply the assumed fix throughout?

You also need to check the load path to ensure that no other items will be effected by the additional weiight/bending moment.

 

[apsix]

Of course if the owner originally specified a 12" slab and you designed a 10" slab then its your problem.

If it's a late change by the owner then someone has to pay for redesign.
Will the Contractor also provide a 12" slab for the cost of a 10" because 'there is no budget'? - I doubt it.

 

[kslee1000]

Can you lower the strands by 2"?

 

[frv]

May I ask why on earth anyone would object to making a slab thinner in the first place?

Fire rating?

Sound proofing?

Seems to me that an owner would want the building system as efficient as possible.

 

[tedamiao]

Seismic is not a problem since it is at basement level and it can not go anywhere. Light weight concrete is not a choice due to many reasons such as anchor bolts capacity, budget..... Yes, it will take long time to re-analysis. If totally redo it, not only thickness need be changed, but also every cable elevation need be revised. Note, it is elevated slab, cable is parabolic, we have hundreds cables......A lot of work.

It is kind of coordination problem between us, senior engineer, architect and owner. From structural point, we made 10'' thick to bring client extra savings.....For some reason, client never claimed they need 12'' thick until plans approved and going to site construction....

Thanks, all your guys are great.....

Your guys are great, thanks.

 

[slickdeals]

Is it a basement slab or an elevated slab (as you say in your original post)? If it is a basement slab, then I am assuming you need the thickness for hydrostatic pressure?

 

[hokie66]

Adding 2" to the depth of the slab without adjusting the reinforcement goes against the grain for me. The reinforcement should be placed where it belongs. You didn't really say whether the extra 2" is added to the top or bottom or half each.

 

[tedamiao]

It is elevated slab at basement level. Extra 2'' concrete was on top as topping only.

 

[kslee1000]

Quick judgement: the positive moment should be ok (h & e both have increased resulting in higher moment capacity), may need to add some bars in the negativement region, if the slab is continuous over supports.

 

[hokie66]

Moment capacity will be greater in bottom, but not in top, as tedamiao said the tendons and rebar will be placed as for a 10" slab. Less effective prestress, more cracking is possible in top of slab. And if the drapes remain unchanged, the balancing changes, so deflections may increase.