Support C.A. ?

[jlcochran1]

Currently Compress has inputs for corrosion allowance for skirts only. Suggest adding c.a. for legs, saddles, lugs, and base rings.

I have customer requirement for 1/16" c.a. on legs, any suggestions on best way to approach with Compress?

 

[dgrande]

I run across this situation quite often. Currently, I simply calculate my supports 1/16" thinner then what I intend to draw/fabricate them. To make the matter clear to my drafters/customers, I put a note on my calculation coversheet stating what I did.

 

[jlcochran1]

Hopefully Compress will eventually add this feature as this becomes more difficult with I-Beam sections. ie by the time you add c.a. to both the web and flange it may double the section weight, where a less expensive section (#/ft) would work. It becomes an iterative process more suited for the computer. Simple angle sections or plate supports are easier to deal with.

 

[TomBarsh]

An interesting question. I can see this leading to problems for sections with properties tabulated in AISC steel design manual. In the past I have heard of people not being able to verify the tabulated values "precisely".

To specify a corrosion allowance for these types of sections (AISC) would require that COMPRESS have an algorithm to calculate the section properties. People would try to verify the calculation by entering a minuscule corrosion allowance and comparing to the AISC published values (well, I would try to do so myself). Problems and complaints would result when they were not able to do so.

Mathematically it should be relatively easy to determine the section properties of a wide flange beam, etc., only the bookkeeping is tedious (but computers are good at repetitive, tedious tasks). It's an almost trivial problem except for the small spandrels where the web meets the flanges.

However, it may not be possible to exactly duplicate the AISC tabulated values based on the assumed mathematical model of flange, web, and spandrel geometry.

In my free time I may write a spreadsheet to perform this calculation and see how close I can get to the AISC values once the spandrels are accounted for.



For support lugs, support rings, saddles, base rings, anchor bolt chairs, etc., I suggest adding the corrosion allowance to the detailed part. In most cases there will be little difference in weight (weight changes can affect the part design). Of course, the larger surface areas of saddles and support rings will result in larger weight change. But I can't see this materially affecting the design of the components. My opinion only.

Tom Barsh
Codeware Technical Support

 

[TomBarsh]

Clarifying this issue with a numerical example, consider a W8x13, probably appropriate only for very small vessels. Applying a 1/16" corrosion allowance to it results in approximately 50% loss in all the relevant section properties: area, and moment of inertia and section modulus for both weak and strong axes.

For expediency in calculating values in the corroded condition I ignored the effect of the spandrel; it can be shown that this results in only a small, ~1% or 2%, difference from the tabulated values in AISC steel manual.

Thus for "small/thin" sections it can be expected that there will be a large loss of strength due to relatively small amounts of specified corrosion allowance.

For a stockier section, consider a W8x40. The 1/16" corrosion loss results with about a 25% reduction in the section properties.

A really stocky member, a W12x190, loses only about 8%-9% of its strength based on the 1/16" corrosion (I am surprised that there was this much effect, I would've expected less).

Based on these results I am not surprised that the weight per foot of a typical vessel "leg" may double based on a small corrosion allowance.

Note that COMPRESS provides the complete database of sections listed in the AISC steel manual. Like the ASME materials, the sections listed in the default installation include only a few sections. You can add additional sections from the AISC database (Materials menu) to refine the automatic selection of leg sizes.

Tom Barsh
Codeware Technical Support