Flat slabs-reducing hogging moments good practise?

[drile007]

Hi to all,

I’m have problem with large reinforcement area over columns in flat slab (30cm thick and 8.3m span). I’m in search for some guides how to model column supports to reduce peak hogging moments.
Can you help with ASCE provisions or any other guides, practices in USA.
I’m already found that i can smooth negative moments with modeling column as area support (elastic/rigit), cut momemnts at column edge, reduce peak momonet for degree etc… but I’m curius on which hogging moment you design long spans flat slabs.

Thank you for any comment/suggestion

 

[milkshakelake]

This is a bit outdated compared to the latest ACI 318 standards, but this is a good starting point:

https://www.concrete.org/store/productdetail.aspx?ItemID=421315&Language=English&Units=US_AND_METRIC

I've never had an issue with overly large reinforcement near columns. I don't know the magnitude of your issue, but a good amount of top rebar is common. I recommend doing a check of a few bays using the Direct Design Method in ACI 318M. In my experience, this is close to FEM results. If your FEM results are wildly different, there is something you need to fix in the model. If your rebar is still too high after doing this check, you might need to increase the thickness of slab, but the thickness you mentioned sounds reasonable to me.

 

[Tomfh]

Are you talking about peak moments in a finite element model? General practice is to average these over a wider strip, whilst ensuring you satisfy the code rules about minimum steel percentage over the column itself.

 

[ads0221]

As someone said earlier are you doing finite element analysis using plates? If so this modeling system has the tendency to accrue very high peak moments where you are placing concentrated loads.

You can try taking the average moment of each plate rather than the max corner or spreading out the applied loads to a more realistic area of the pedestals or slab area.

 

[HTURKAK]

My past experience using SAP 2000,
- Use the moments per unit length ( M11, M22 ) rather than point moments ,
- Compare the max. hogging moment with code specified approximate analysis ,

- You may prefer different FEM model .Refine the mesh at column area ( with 9 pts at column ) define rigid inclusion (thicker ) shell element at column area and define column line element at center point,
- There are approx. methods to round off . For example , pls look CIRIA -110.

...

He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48