Understanding Silo Support Condition During Analysis. 1

[mooncrackers2]

Hello,

I en-route in designing a Raft foundation with 6 tall circular hopper bottom Silos i.e., Silos with steel column legs carrying the silo structure.

The vendor data and drawing indicates the columns connection at the interface column – raft interface is using only 2 bolts, placed on each side of the web of the I – section column.

The use only 2 bolts means, the connection is considered as pinned. Below is my question.

1) Should I consider during analysis, that the support is pinned structure.?
2) If I consider pinned support, how can I generate the overturning moment on the raft for dead, live and lateral loads like wind and seismic.
3) Should I be designing the raft for only tension and compression forces obtained from Pinned Support condition or Should I consider a fixed support in order to generate the moments at each supporting column.

Kindly note, from the vendor technical inputs, he is supplying horizontal and vertical forces for Dead, Live, Wind and Seismic load cases (without moment) and then has another 2 rows with a huge moment of 5700 kn-m for wind load case and 59000 kn-m for seismic load case.

So I want to understand the procedure in generating these forces before I carryon with the analysis of Raft Foundation. Kindly share your valuable experience and thoughts which will help me.

 

[JStephen]

Lacking more detailed information, I would assume that the base plates were being treated as pinned, and that the moment being transferred to the slab was due to vertical forces applied at the column locations- typically, assumed that force due to moment is proportional to distance from the neutral axis.
If in doubt, check with the vendor in question.

 

[12345abc6ttyui67]

I am used to seeing Vendors provide a global overturning moment, which they then decouple into tension / compression over the columns. Is that what they are trying to show? Best bet is ask the Vendor to provide more info.

 

[mooncrackers2]

Yes the vendor is supplying a global overturning moment on the raft, however my confusion is how can one analyze a tall cantilever structure with pinned / hinged support condition. Isn't is wrong. Its like having a shear wall in a tower being analyzed using a pinned support condition.

If the Silo analysis is analyzed using a pinned support condition, why does the supplier provide a global overturning moment ?

Please share your valuable thoughts.

Thank you.

 

[12345abc6ttyui67]

I assume there are multiple columns around the perimeter of the silo, hence the overturning moment from the silo is resisted by tension / compression force couples shared between the columns. As JStephen said, these forces are based on the distance of each column from the neutral axis of the entire column group. The tension / compression loads are then resisted by the pinned connections.

Note, this assumes the 'top' end of the columns (where they join the silo) are moment connections.

That is what I would expect at least, you will need to clarify with the Vendor.

 

[mooncrackers2]

Ok. I understand that I need to check with the supplier for more information. Thank you for your support.