Support Condition in Structural Model

[xkybb]

I am modelling a RC frame structures founded on a thick mat foundation, and the mat foundation directly sit on rock as the rock level is shallow.
I model the RC frame structure and the foundation seperately in SAP2000 and SAFE, and I would want to know:

1. For the RC structure, shall I model the support as fixed or pinned? I understood that the modelling shall be related to my detailing and reinforcement, and does it mean that:
i. If modelled as fixed -> Then I will detail my column with adequate anchorage to the mat foundation. The mat foundation shall be more heavily reinforced and columns shall be less heavily reinforced when compared
to modelling as pinned, as both force and moment can be transferred to the mat foundation, vice versa.

ii. However, I also heard that the modelling as fixed or pinned also depend on the ground condition which my foundation sit on. As there will be moment applied to the mat foundation when modeled as fixed, there may
be the possibility of having the foundation partly bend upward ( and the soil, if not rock, underneath may not have tensile strength to resist it). Therefore, it is safer to model as pinned?

iii. In reality, The connection between the columns and foundation shall be bewteeen fixed and pinned. Therefore, let say If I have chosen pinned support and design for the foundation and columns, I may have
underestimated the loading on my foundation as in reality moment will transfer to the foundation and cause extra stress?

Thanks for your help in advance!

 

[xkybb]

Could anyone give me some advice?

 

[DaveAtkins]

If you design and detail the base connection in accordance with your modeling assumption, you should be OK. For example, if you model a pinned base, less reinforcing will be required, less reinforcing will be provided, and the reinforcing will yield to actually create a pinned base.

DaveAtkins

 

[manstrom]

It will behave how you detail it. I prefer to have pinned bases since it simplifies the foundation design.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[KootK]

I'd fix the column bases for the design of the raft foundation. Two reasons:

1) No matter how you detail the connection, it will behave as fixed until the flexural tensile stress in the columns exceeds the axial compressive stress. Pre-stressing effectively.

2) A raft foundation on rock won't have much rotational flexibility to it. If you ignore the column base moments, you'll probably underestimate raft punching shear demand. This would be especially true for any columns near the edges of the raft.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.