Frame Analysis

[LSPSCAT]

Please see attached sketch for frames on existing equipment. I am looking for some general comments with regards to the analysis of the frame. Specification requirements typically reference AISC 9th or use basic allowables like 4 to ultimate and 2 to yield. There are some questions here because part of the frame is typicaly cast iron and part is steel so a consistent factor of safety is questionable. The main girder is analyzed as a simply supported beam, usually fastened to the columns with a single row of fasteners. The vertical columns are usually analyzed..depending on length which can range from 3 feet to 40 feet...using P/A or appropriate column formula. The vertical columns are usually structural tubing or cast iron. The frame is typically braced back to another row of frames or back to wall. So out of plane bending is not really an issue. The bracing also prevents sidesway in the plane of the frame. Is it approriate to analyze as mentioned or should a frame analysis be performed and the additional moments in the columns looked at. It appears that most of the original analysis paperwork does not perform a frame analysis. Loads are typically around 50-100 KIP. Past performance has not shown any problems. The equipment is part of a retrofit/upgrade and there is talk about replacing some of the items and possible redesign issues.

 

[kslee1000]

To me, ASD and hand cal is the way to go. Watch out for additions that may change the load paths and alter the stress in the original structures.

Question yourself if any element is found to be under designed, you may need to check into older codes for such discrepancy.

 

[beton1]

Material strength confirmation would be a question; especially (if I read the thread right)with mixing of materials within one frame. Retrofit work with the older steel may be a challenge (weldability). I agree with kslee.
Bracing and connections would also be a concern.

 

[Ron]

If it were not for the different materials, I would do a simple frame analysis; however, with part of the frame being cast iron (assuming cross beam/brace?), then I would analyze as kslee1000 noted.

I would use a higher safety factor for the cast iron due to its potential non-homogeneity and brittleness.

 

[LSPSCAT]

The columns are cast iron...the beam is steel. The cast iron is designed with a safety factor of 5 to ultimate.

Best to assume frame analysis as a conservative case and then analyze cross beam as a simple span?

Some additional bracing is going to be provided for earthquake design as well. Main concern is in the lateral direction for sidesway.

 

[kslee1000]

Cast iron column obviously is not ideal material to resist lateral load. Before resort to frame analysis, try to locate and understand the existing lateral load resisting system, visually exam, and strengthen it if necessary. However, by doing so, it may consitute a major change in the eyes of building official (depends on the nature of changes). Then you may need to bring the entire structure up to current code. That would be fine if it is already spelled out in your contract, if not, talk to your client first, he might have something to say/ask.

Yes, if this is single span beam with ends bolted.

You opening statement implies that this building, on top of materials, has some geometry irregularities. If lateral load effect is the concern, a frame analysis is justified and useful.