Modelling Complex / Stepped Diaphragms

[smb_PE]

This is my first post, so forgive me if I started this thread in the wrong location.

I'm looking for modeling tips for a fairly complex building structure. This building will have floors that ramp to accommodate levels for an existing building. In addition, there are certain areas where the diaphragms are vertically offset by 5' or so in the transition area. My main question is how can an analysis model be tweaked for this case?

In RAM, adding extra stories leads to multiple short columns, which make the design process difficult if sizes need to be tweaked in RAM Frame. I know in RAM Column, we can de-select 'floors brace columns', but there does not appear to be a good way to do this in RAM Frame.

It has been suggested that ETABS could handle this problem better than RAM, but I don't have enough ETABS experience under my belt to know if it would be worthwhile. I also have access to RISA Floor/3D but am doubtful of RISA floors composite design capabilities. Any guidance is much appreciated!

 

[SethGuthrie]

Here is a recent post on the Bentley forum about similar things.

https://communities.bentley.com/products/ram-staad/f/ram-staad-forum/148152/varying-deck-elevations

There are other online topics linked from there.
Ram can probably handle it, and I can point out any potential pitfalls if you include a picture.

 

[smb_PE]

Thanks for the link, Seth! We have already started a RAM model, so it would be ideal to continue on that path. I'll read up on the information you suggested.

I'm assuming one of the pitfalls would be auto-load generation. If you picture an elevation of the building, the right half of the building has additional levels. Unfortunately, the levels of the left and right side are at different elevations. I've attached a picture of a portion of a typical frame. The ramping portion of these floors is relatively small in comparison to the overall structure.

I've always wondered if there was a way to have separate level systems for each half of the building in RAM. Otherwise, when looking at the left half of my building, the frame columns are intersected by the levels of the right side of the building. This leaves roughly double the number of columns I need to override if sizes change from the frame analysis.

 

[SethGuthrie]

Right, automatic wind loads may not be quite right, but I usually find that seismic controls and those should be OK. It's the column design where the levels come together creating a short column segment often with a lot of shear that causes the most concern. When it comes to modeling, you may need an extra beam in the Ram model at the edge of one level that's really the beginning of another level. Duplicate columns can usually be avoided.