section modulus brain fart

hey guys and girls!
I'm running into a serious mental block right now! maybe its the fact ive been at this project all day or maybe im just way over thinking something but here it goes... I have a section modulus "adder upper" set up in excel with the final SM units as cm2 m. Pageing thru the ABS rules i have several SM requiermets written with and end unit cm3. what am i missing?! can i just make a simple conversion or should i write a new sheet for the cm3 units? If this makes no sense to anyone please let me know and i'll try to explain better! thanks advance for all your help! ~dan

 

Hmmm are you sure that lemonade stand wasn’t selling hash cookies too!
Who told excel it was in cm2,how does it know, it could be cm3,it could be out to get you.Gee I could do with some of those cookies right now

 

Section modulus in all rules I know are in cm3. Working with cm2 m seems quite strange and unpractical.

 

SM, S+M, and AY hell

Section Modulus is always expressed in 'units' cubed. What I mean by 'units' is inches or centimeters. I use inches for my units because I can visualize an inch better than I can a centimeter (approx. 3/8ths an inch)

It been almost 16 years since I did an 'AY^2' table. And I had forgotten how to do it. Lucky for me, I saved one example from one problem on a test (who's answer I got wrong) and from there was able to discover a few steps I left out.

BTW Inertia is expressed in units^4. Why that is, I used to know. But now I just say: "It is, so it is."

No one will ever cut themselves to death on MY brilliance .

Bob

 

Section modulus is, as just mentioned, in "units" cubed. Typically, these are inches^3, mm^3, or cm^3. As you know, you have to calculate moment of inertia first, which is in units^4, and then you divide moment of inertia by the distance from the neutral axis to the extreme side, above and below, which is in plain units, to get section modulus. In some classification society rules, the area part of moment of inertia may be in inches^2, cm^2, or mm^s, and the moment arm may be in larger units, such as feet or meters, thereby giving moment of inertia units in inces^2-Feet^2, or as you are probably doing, cm^2-Meters^2. Since the distance measurement is usually in the larger units, that is why you may end up with section modulus units of inches^2-feet, or cm^2-Meters.

More specifically, the units are units of length, and any mix of length units can be used, just so long as they are properly labeled.

In OceanBoy's case, therefore, for units of cm^2-Meters for section modulus, you don't have to recreate the spreadsheet. Simply use a conversion factor for the meters part of the number by multiplying 100 cm/M.

Eric

 

Eric. But was the original problem, actual section modulus or required section modulus?

 

bhnautika,

They way I read it, Oceanboy's Excel spreadsheet tabulated the results in cm^2M, but the ABS rules give requirements in cm^3. So Oceanboy has to convert his units to match those of ABS. The ABS value is the minimum required section modulus, and Oceanboy has to have a value of actual section modulus of at least that much, or preferably something greater. The bigger the section modulus, the stronger the part is, and the lower the stress in the member.

Eric

 

The sectional modulus of a brain fart is difficult to measure, but rest assured I have a few that are right off the scale (always cubed though).

I think I have been working too hard.