Non Uniform I Beam Calculation help

[paulpatrick]

Hi
I have to do a number of non uniform I beam Calcs I need some one to take a gander at my calcs. I get a different value for the second moment of area of the beam. The results are both of a similar magnitude but not equal they are supposed to be. I have just done the calcs for mm to the power of 4....
The first method is using a formula from efunda.com,

http://www.efunda.com/designstandards/beams/UnevenIBeam.cfm.

the other method is using the axis parallel to the nuetral axis. I have about 10 of these to do I need tojust get one right in my head an I willl be sort .. please take a look ....

 

[rb1957]

your shift terms look a little off ...
16.97-5.5/2 = 14.22 (not 13.93)
(5.5+94.3+5.7/2)-16.97 = 85.68 (not 85.97)

IMHO ... don't enter hard numbers for dimensions that you've listed (like b) ... use the cell address, label the cell if you want

also i guess the POWER() function is clear to you, i prefer "^", like b^3 instead of POWER(b,3)

haven't bothered to follow through the efunda calc, i notice the efunda NA is slightly different to 16.97.

 

[rb1957]

you need a Ay2 term for the web

i get 3666510

i like to set up the calc as ...
b d A=bd y Ay Ay^2 Io=bd^3/12
where y is the distance from the datum axis to the element CG. then yNA = sum(Ay)/sum(A) and I = sum(Ay2) + sum(Io) - sum(A)*yNA^2

but there are many ways to skin cats

 

[paulpatrick]

Hi I did my calcs and I got a number of 3666795 its slightly different from yours...Asked my little Bro the structual engineer of the family and He got something like 4000000....Seems funny that no-one gets the same...anyhow just to confirm some theory .. the bd^3/12 formulae is about the Nuetral axis.. and the BD^3/12 one is for about an edge of the section? Thanks for your number it was confirmed by AutoCAD...My Boss showed me this neat way of sort to get it.

 

[BAretired]

This is unbelievable!!!!

BA

 

[dhengr]

BA:
What’s so unbelievable? He probably sold his Mechanics of Materials book at the end of the semester, for ten cents on the dollar, assuming he would never need it again as an engineering reference book. What with Eng-Tips, efunda calcs and the like anyone can now be, or call himself, an engineer. I’m no longer surprised at this kind of stuff given what I’ve seen here, but it does scare the hell out of me. People might buy that product or walk under it when it is under load. And, I suspect his boss doesn’t really know the extent of what he doesn’t know.

 

[BAretired]

dhengr,

Wow! Yours is a sad commentary on engineers. Fortunately, it does not apply to all of them.

BA

 

[paulpatrick]

Look Its been like 6 years since I covered this material

....Give me a break... I have always wanted to do the maths and science part of engineering but up till now I have never hand the chance.

All the jobs I worked in till now required zip in terms of calculations or maths.....
I understood all the principles and could design stuff using it no problem as I was doing it everyday 7 days a week for five years.
One of design classes we had to design a scale bridge ..I fulfilled the brief and got good marks for it.it had to weigh less than 5 kg span 1m by 0.5 an apply only a downward force to the piers. It had to hold a 2 ton and not be over engineered either. My Design failed after 2.2. due to the fact that the metal thickness joined were not similar or chamfered where they met. It was the next semester we had weilding science. It was suppsed to fail at 3.0ton I learned from it. To research more and or ask more experienced people.
It was my favourite and best class.

But where anyones life is envolved I always get someone to check... especially when it beeen a while since I did something like it or it my first time doing something.
and just because your boss has been in the business for years does not always mean he knows everything Or he is right ..I have caught my old bosses out numerous times. They just got good at covering their mistakes.

Last job was in passenger vehicles light rail to be precise and while I understand that things are done to a budget there is no excuse for producing crud( shit).. and people wonder why China and elsewhere get the manufacturing jobs.. why buy expensive crud made in your own country when it comes from overseas and is cheaper.. and probably works better too...makes poor business sense in the ong run.
For the sake of a few dollars or euros extra the goods manufactured would benifit emensilly... Take as an example...a curved door into a toilet for able and challenged people...one would think it should be reliable i.e. open and close on demand endlessly... "company" contracted its design to someother company in the know and They supposedly did "testing". The door traps people inside continuously or jams etc.... I had to make it work as our company had wasted loads of money needlessly elsewhere.. for a little as possible ..all I could do was ad extra wheels on a track to prevent it skipping.. did not stop it but reduced its occurances Significantly

Between HVAC Tool and DIE and electronics all my work was basically fixing other engineers mistakes and making their failed designs that got funding and approval work ....

And these are probaly all your age BAretired and dhengr.

I have met many engineers of different ages and experience some good some bad .. and age is irrelevent here.. competant and incompetant...in college and industry. Book smart and life smart...qualified and unqualified.

As for me I spent the last 8-12 months out of work chasing anything that paid peanuts + in a foriegn country where I had no industry experience what so ever to pay the bills when the GFC hit and at home it hit even worse.. heck they are trying to dig themselves out of it too still.

I design things to last and I like to make sure before I build stuff that it works and I have fullfilled my role ..I take pride in my design objects ..i.e. I have design something that will fullfill its purpose and last and be safe.. .. not like all the current stuff thats out there by The older throw away generation. Bring back where it lasts and it can be repaired or reused.. more jobs too..friendly to the environment ..less landfill and less emissions.

As for the book comment I am in another country from home and I just recieved my books from home today .. I could have tossed note books etc but I kept them as I know they are more important than gold ..just wish I had my notes as they were ten times better than the books.

Sorry mateys but you didnt know what my situation was.... And I appologise to rb957...and thank him for his words.

 

[rb1957]

bd^3/12 is I for a rectangle about it's CG (=NA)
bd^3/3 is I for a rectangle about it's base.

sorry, but i echo some of the other posters comments ... this is a very basic structural question. sure you may have known it in school and your experience since has been more on the mechanical side, but none of us responders know this from your post. If this is your first structural assignment in years, there should be support for you at work (your boss knew something) but we shouldn't (IMHO) need tools like AutoCad to solve this simple section.

 

[dcarr82775]

At least paul isn't a structural. A young engineer (about to sit for PE so not too young) in our office came to me with a similar problem a couple weeks ago. After showing him what he did wrong he argued with me that it was I who was in fact wrong and not him.....

now that is unbelievable

 

[BAretired]

paulpatrick,

Sorry...no offense intended.

The efunda formula is based on the thickness of top and bottom flanges being the same, namely S. You have S1 and S2 which are slightly different, so the efunda formula is not exact.

In your calculation of centroid, Lines 13, 14 and 15 should be 2.85, 52.85 and 102.75 respectively. This will account for minor differences in your results.

I'm not sure why you are asking about the moment of inertia of a rectangle about its base because you don't need it for this exercise. The moment of inertia for the beam is [∑]bd[sup]3[/sup]/12 + [∑]bdy[sub]bar[/sub][sup]2[/sup] where y[sub]bar[/sub] is the distance from the centroid of each rectangle to the centroid of the beam. Whether it is positive or negative, the square is positive.

BA

 

[rb1957]

the efunda is exact IF you apply it carefully ... s1 goes with b, s2 goes with a.

on your sheet "locating centroid", d (cell I21) should refer to s2 (b4) not s1 (b3). with this correction, efunda produces the right result, 3666510.

their calc shows you the correct application of bd^3/3 ... they calc the inertia of a rectangle b*(d-y) about its' base (being the NA of the section) and subtract the rectangle (b-t)*(d-y-s1).

many ways to skin cats