Design rules of stiffening ribs

[eli28]

Hey,
I am wondering if anyone has a reference to a guide on metal (machined) ribs design in different parts for making it stiffer. I can think of some questions:
1. In which side I have to put the ribs? In the surface side that is in compression or tension? And why?
2. What should be the rib width and height relatively to the wall thickness?
3. What should be the distance between the ribs?
4. What is the right pattern for the ribs? Only Longitudinal or a combination of several directions?

I have a case of a vacuum cylindrical tank and I wonder in which side to add the ribs? On the outside surface that undergo compression or on the inner surface that undergo tension?

I am sure there are some good practice advices you can share with me

 

[dvd]

One can find good treatments on stiffening panels here -

Design of Weldments

Rectangular Industrial Duct Construction Standards

The Structural Design of Air and Gas Ducts for Power Stations and Industrial Boiler Applications

 

[GregLocock]

You could try a topographical optimiser in FEA to examine your options. I've used Altair Optistruct in the past. Curiously a colleague did some work on this for vibration reduction and found that making the main skin thicker rather than using ribs was the optimum use of material, much to his surprise.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[HTURKAK]

Hi eli28 ;

Pls find below my opinions for the thread,

1. In which side I have to put the ribs? In the surface side that is in compression or tension? And why?

Both are possible.. and depends on application. Should be decided acc. to future maintenance, which side shall be free from ribs.. etc.

2. What should be the rib width and height relatively to the wall thickness?

Should be decided acc. to thickness design of the plate , in general buckling of main plate and stiffeners control the design..

3. What should be the distance between the ribs?

This is an optimization problem also.. If the plate thick. selected, the distance between the ribs should be decided as per strenth and buckling resistance of plate.

4. What is the right pattern for the ribs? Only Longitudinal or a combination of several directions?

For straight plates ( not curved ) i prefered a combination rectangular pattern with 1:2 ratio..

I will suggest you to look :
Recommended Practice DNV–RP-C201 BUCKLING STRENGTH OF PLATED STRUCTURES

 

[eli28]

Thank you all for replying.
One thing that I am curious about is the preferred side the ribs should be located - the one that is in compression or tension? From reading different articles I saw that in many cases the preffered side is the one that is in compression, and I would like to know why. I guess that it helps to stiffen the side that is prone to buckling.
Another thing that is interesting is the location effect on the achieved stiffness - when considering this aspect, is there any preference?

thanks

 

[HTURKAK]

If i were free to choose, i would prefer ribs located at tension side due to stability..

 

[CWB1]

It depends largely on the material properties. Rolled steel can be subjected to conditions that cast aluminum or iron cannot.

 

[TheTick]

Most metals tolerate compression much better than tension.

 

[SnTMan]

Although then potential buckling of the ribs should likely be considered...

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[drawoh]

eli28,

Rules of thumb are for when you do not want to do calculations, and there is existing practise that works.

Fire up your FEA software and look for failure modes.

Talk to your machinist. There probably is a minimum rib thickness they can manage. Definitely, there is a relationship between the required diameter of the cutter, and the depth of your hole. The side your ribs/pockets do on affects the number of machining setups.

--
JHG

 

[SWComposites]

Manufacturing considerations probably drive the side to locate the ribs.

If ribs are on the compression side, then they have to be designed for local buckling.

 

[eli28]

I am sure that using FEA and optimization is the best way, but I wanted to get some rules of thumb that may guide me in the initial stages of preliminary design. In addition, maybe in this way I might develop some intuition instead of running numerous analyses for understanding the desired direction.

 

[drawoh]

eli28,

We do not know what your design does or how it works. Your part can fail in tension, or you can compress it and cause buckling, or it can be too springy resulting in some interesting vibration modes. We do not understand your cost model. Perhaps you are completely unconstrained, and you can machine your part from light, rigid beryllium!

--
JHG

 

[mfgenggear]

eli28

maybe this will be helpful maybe not.
look at this video by United Air Launch goto 3.56, 8.36 1.1-1.25, safety factor
I was at Convair/Space System in the 1990's and they were doing it then.
I highly recommend to watch the whole video.

edit

https://www.youtube.com/watch?v=o0fG_lnVhHw

 

[SWComposites]

Eli28,

What is this tank for?
Is weight critical?
Is cost critical?
How do you plan to make the tank?
Any other design requirements?