Why are stiffeners chamfered? 1

[LearningENG-]

Hi folks,

This might be an obvious question but it has been in my mind for a while. Why are some of the stiffeners on a flat face of a pressure chamber chamfered? See pictures attached. I couldn't justify why they are made that way...

TIA

 

[LittleInch]

~Mainly personnel protection.

You're not going to be as damaged if you hit a chamfered end compared to a sharp point, equally won't rip you clothes / skin if caught accidentally and they don't really change the effectiveness of the stiffener.

Also stop people standing on the ends....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JStephen]

If they're being designed as a simply-supported beam, maximum moment is at the center, so it doesn't much matter how the ends are shaped, which leads to LittleInch's response above.

 

[Compositepro]

Safety and appearance are important factor, but it does also reduce the shear stress concentration at the tip of the stiffener by making the stiffener less stiff at the tip. Stresses concentrate where there are sudden changes in stiffness. It does make the structure stronger. It may not be necessary to make the structure work in any given situation, but it does improve the strength of the structure, and it looks better. No calculations are needed to know that, so the design feature is often a default choice, even though it may add a little cost.

 

[r6155]

45 degree angle vs 90 degree angle to avoid acid traps and other design flaws.
Sharp corners could cause acid traps, where some of the acids continue to corrode at sharp corners.
See “rain cap” in PIP VEFV1112 legs support

Regards

 

[LearningENG-]

Thanks to everyone! Another question on this: Why does the stiffeners (like the ones on the chamber in the picture) stop about 1 or 2 inches away from the edges? Wouldn't it be stronger to make it all the way to the end?

Regards

 

[Compositepro]

Again that would create a greater stress concentration. Each wall of the chamber is stiffened at the edges where it is attached to adjacent walls. The stiffening ribs are there to keep the flat plates from bending. They are stiffeners and not strengtheners. Imagine that the stiffeners were infinitely stiff. Then all the forces on the walls would be applied at the point the stiffening ribs and adjacent walls meet. That being a relatively small area, means the stresses would be very high.

 

[DriveMeNuts]

The stresses are greatest at the middle length.
There are negligible to no stresses that need to be stiffened at the ends of the stiffener.
Most of the material at the ends of a stiffener is pointless excess weight.
An optimum stiffener would be a triangle or segment of a circle, where the height is greatest at the mid-point.

 

[Compositepro]

There are lots of stress at the tip of a stiffener, and not just in the stiffener. If not, why not remove the tip and make the stiffener even shorter. But then there is still a tip where there are still stresses. Every geometry has to be analyzed individually for the full range of applied loads. Loads will be different for vacuum versus pressure in this chamber.

 

[3DDave]

A failure mode I've seen in non-boiler structures is what I call "can-opener," where a stiffener ends abruptly and puts a stress concentration right at the end of it. It tends to punch a fracture just like a can-opener does, so it's a detail to carefully consider anywhere there is a sudden change in section/stiffness.

 

[r6155]

Speculations are not a good engineering practice. Ask (OP) for more information before replying.

Regards

 

[LearningENG-]

Hi All,

Thanks a lot for the replies. I'm mainly working with vacuum chambers and in my case, the stresses are the highest at the adjacent walls. I did some quick FEA study and they seem to reflect what Compositepro says (Sorry I'm having troubling uploading images so I attached the file in the link below). But I'm having some trouble understanding it in a theoretical way. Should I be checking indeterminate beams for an answer?

https://files.engineering.com/getfi...82-057e-4136-a61a-30481f071bcb&file=image.jpg

 

[DriveMeNuts]

The purpose of a stiffener is to reduce the primary bending stresses and the associated deflections at the mid-point of a flat plate. In all three of your stress plots, that is achieved. And you can see from the low stresses at the ends of the stiffener (in all three plots), that the stiffener ends are largely made up from excess material and could actually be a triangle or circle segment to stiffen the flat plate. Anyone who understands the basics of simple beam theory will understand this.

The bending stresses at the corner junction of the two flat plates are secondary and have nothing to do with 'stiffening'. The corner joint needs 'reinforcing' (not stiffening). I'm guessing that Compositemro is getting confused between the concepts of stiffening and reinforcing.

You can see from the plots that with the stiffener/reinforcement plate extended all the way to the edge of the flat plate, that the additional material reduces the corner joint stresses, hence provides improved 'reinforcement', however as you can see there is still elevated stresses. You can make the reinforcing plate as long as you want and even allow it to extend beyond the edge of the flat plate and those elevated stresses will remain. To reduce the corner stresses the reinforcing element needs to wrap around the corner joint onto the other flat plate.

You may learn a lot by doing hand calculations from fundamentals using a spreadsheet. Roarks and ASME VIII Div 1 Appendix 13 are good resources. Appendix 13 has separate equations for stiffening the mid-span and reinforcing the corner junction.

 

[LearningENG-]

Hi DriveMeNuts,

Thanks for you reply.

I think I was confused by the concept of a stiffener and reinforcement: I call these extra bars "stiffener" but in fact I think I should not simply call them that. Like you said, they don't only stiffens the plate but also reinforces it. Yes if it's just for stiffening purpose a trianguler or circle segment would be enough.

You can see from the plots that with the stiffener/reinforcement plate extended all the way to the edge of the flat plate, that the additional material reduces the corner joint stresses, hence provides improved 'reinforcement', however as you can see there is still elevated stresses.

In terms of this, from the plots, the closer the ends of the bars are to the edge, the higher the stresses values are at the junction. It seems like you see it otherwise? But I guess this is the question that I really intended to ask.

 

[DriveMeNuts]

I've just read your graphs again. It looks like when you extend the stiffening plate all the way to the end of the flat plate, at the corner joint, it causes the stresses to shift from the horizontal flat plate and concentrate in the vertical flat plate.
In other words, while the stiffener is short, both the horizontal and vertical flat plate are able to split the Secondary bending stresses at the corner joint 50% each.
When the stiffener extends all the way to the end, the top flat plate becomes more constrained, therefore its bending stress is shifted to the vertical flat plate, meaning that the vertical flat plate takes 100% of the corner joint bending stress, hence a higher stress.
For this specific geometry, shortening the stiffener or using a different profile (i.e. Triangle, 45° chamfer) will allow the stresses at the corner junction to distribute more evenly across both sides of the corner joint. But watch out for the can-opener issue described by 3DDave.
If after trying these ideas, this still results in excessive bending stress on both sides of the corner joint, a reinforcement element that wraps around the corner joint may be needed (or increase in flat plate thickness). Appendix 13 is good for making this assessment.