Acceptable column deflection?

[jlcochran1]

I am investigating the design of a replacement column. Based on current wind codes the theoretical deflection of the existing column that has been in service over 20 years would be well over 10”/100’. Wind design is driving column thicknesses. To reduce the column deflection to 6”/100’ would require substantial thickness increase and cost. When is it acceptable to exceed the guidelines of 6”/100’ deflection? Is this strictly a function of tray performance? One consulting engineer stated he had also seen 8”/100’ as a common guideline. If the existing column has been operating satisfactorily, is there any reason not to exceed the 6”/100’ guideline with customer approval? Is this typically a tray manufacturer requirment for performance guarantee?

 

[kstaylor]

Both ASCE 7 and the IBC (not the root beer) get modified every couple of years based on better understanding of structural response to external loadings, primarily wind and seismic. We have replaced several vertical vessels that are over 20 years old that were properly designed to the standards in effect at the time of original fabrication. The replacement vessels were designed using the more recent standards and have always had increased thicknesses in the lower portions (compared to the original vessel) due to the governing wind or seismic loads. The increased thicknesses were always due to the use of these newer structural standards. For these replacements we have not deviated from our internal corporate engineering standards of 6" deflection/100 ft length for vertical vessels.

One could argue that the original vessel design served well for its service life but when you replace it with a new vessel, good engineering practice (not to mention vessel regulations in most jurisdictions) would require using the most current Codes and standards. Try buying a new vessel built to the 1980 ASME Code. No fab shop that I know will agree to do this for you.

The 6"/100 ft. criteria is fairly universal. I have not seen any maximum deflection criteria from any major engineering firm or owner/user that exceeded 6"/100 ft for vertical vessels. If you have ever been on top of a 150-200 ft. vertical vessel in a stiff wind I think that you might think that 6" deflection/100 ft. is too much! One can get motion sickness or at least have a strong desire to decend fairly quickly to get off that swaying pole.

Your client may or may not want to waive their maximum deflection criteria.

KST

 

[warrenw]

jlcochran1;
ASME STS-1-2003 "Steel Stacks" section 4.5 Lateral Deflections - "There is no practical limit placed on the maximum deflection a stack can experience"
6"/100' may be a nice rule of thumb but is not required by code.

 

[jlcochran1]

Thanks for the feedback. In this case the customer opted for 8"/100' deflection. This provided less deflection than the existing column at about a $15,000.00 savings over the 6"/100' option. The tray manufacturer was not concerned from a process standpoint of deflections in the 6-10"/100' range.

 

[DSB123]

warrenw,
ASME STS-1-2003 is for Steel Stacks and not Pressurised Process Columns.