Analysis Model Simplification

[GalileoG]

I am designing a 150’x150’ expansion to a 1000’x1000’ industrial facility. The expansion and the existing building are steel moment-framed and the expansion will be structurally connected to the existing building columns at a higher level (the existing columns will be extended and reinforced as necessary).

I am creating an analysis model of the expansion. To what extent do I incorporate the existing building into the model? While I understand that modelling the existing building will capture the real behaviour of the final structure – I am sure there are simplifications I can undertake to keep the model as simple as possible, even if I need to go very conservative with these assumptions. I have the original construction drawings which show all of the original design forces on the existing columns where the new column extensions will be installed.

Any suggestions on how you guys typically approach this?

Have a great weekend everyone.

 

[rb1957]

your expansion is going on top of the existing building ?

I don't see how you can divorce the two. Sure having the design analysis of the existing building gives you some basis to superimpose loads. This can be done with simple conservative assumptions. But do you have a side wind loading ? seismic effects ??

your jurisdiction (design and approval) will probably affect the answer too.

another day in paradise, or is paradise one day closer ?

 

[TehMightyEngineer]

The simplest approach (and usually used from what I've seen) is to keep the two structures separate.

But, whenever I've had to add or modify a large portion of the LFRS of an existing building I modeled the entire structure (which isn't as bad as you think if you have all the drawings and the drawings are reasonably accurate). I remember modeling an entire pulp mill to determine approximate lateral forces in about a week or so.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[GalileoG]

Thanks for the response.

The original building drawings state that the columns and foundations were designed for loads from a future expansion, a preliminary check on my end corroborates this - so on that basis it does not seem reasonable to separate the structures if the reserve capacity is there. The cost of separating the structure (new columns, new foundations, etc.) is significantly more and would be a waste.

For the detailed design of the expansion, I want to make sure I am performing my due diligence without going overboard with the modelling. Surely there is a conservative simplification that practicing engineers use when altering or modifying existing structures?

 

[structSU10]

I think all you can do is model the pertinent components of the existing structure up to its (if any) expansion joints. You could try to simplify loading as much as possible in the areas far away from the new expansion for what its worth.

I think you are stuck though, as the stiffness of the moment frames will be very important to the behavior of any new moment frames you create, and how much load they carry / transfer back and forth between each other.