Designing stiffening plates/elements on sheet metal walls 3

[Algirdas13]

Hi guys,

I have a question regarding the design of stiffening plates/elements on sheet metal walls. During all kind of analysis of designed equipment that uses stiffening plates/elements I came across different solutions and I would like to better understand why those solutions may be used. As an example I will use steel hoppers.

Some designers use only vertical, only horizontal, or both vertical and horizontal (combined) stiffening elements. I would like to better understand when to use vertical, horizontal or combined stiffeners. What rules/logics are behind this type of selection?
Horizontal stiffeners: Link
Vertical stiffeners: Link
Combined stiffeners: Link

Also, I'm interested in the selection of the stiffener type. Some designers use only sheet metal plates, some use channels, some use L-Profiles. On what criterions the shape/type of stiffener is selected?
Sheet metal plates as stiffeners: Link
Channels as stiffeners: Link
L-Profiles as stiffeners: Link

I will appreciate any insights, articles or any other form of information that could help to better understand the selection of stiffeners and their placement.

Thank you!

 

[Scuka]

It seems you're overthinking it a bit with the choice of stiffener type.

Most likely, that particular type was used because it was the simplest one to use for one reason or another (e.g. same profile was used someplace else on the structure, or company often uses that particular profile in multiple other products). Makes things simpler and minimizes the amount of different materials you need to manage.

I'm also willing to wager most of those weren't analyzed in too much detail. No one sat down trying to determine whether L profiles or rectangular tubes were a better choice. Most likely someone did a basic calculation at the most to determine whether the chosen setup was approximately strong enough, added a bit more so it would be obviously overbuilt, and called it a day. I doubt anyone bothered to optimize these things. The amount of steel you'd save by optimizing these things doesn't justify extra engineering hours.



That said, Blodgget's Design of Weldments has a chapter on this topic.

 

[human909]

It seems you're overthinking it a bit with the choice of stiffener type.

I disagree. And you follow up comments are mostly incorrect too. I personally have designed many such bins and hoppers. In a recent job I spent a decent amount of time reworking previous design to improve efficiency of fabrication and erection. When $400,000 is being spent it is pretty lucrative to reduce costs by 25% by doing a good design.

I have a question regarding the design of stiffening plates/elements on sheet metal walls. During all kind of analysis of designed equipment that uses stiffening plates/elements I came across different solutions and I would like to better understand why those solutions may be used. As an example I will use steel hoppers.

Good that you are asking these questions. Some answers are pretty obvious engineering, others are subtle and a few might just come down to lack of effort in design that Scuka was alluding to.

I've implemented designs that are pretty similar to all the examples given. Though I can see some inefficiencies in some, all approaches shown there and more I have used. In general due to labour costs I'll avoid 2-way spanning. But sometimes it is required by the needs of the job.

QUICK SUMMERARY In general you design to span the shortest distance and to points of restraint. Same basic rule of most beam design.

I'll elaborate by referencing you images and my opinions.

Image 1.
A hopper for free flowing product most likely. The stiffeners run circumferentially as that is both the shortest span and there is no clear point of support top and bottom. Angle is place in an inverted v shape most likely to avoid product build up.

Image 2.
A bin for quarrying or similar is my guess. The stiffeners run vertically in the top section because it is a short span and the stiffened rim and hopper compression ring are both necessary.

Image 3.
Again it looks like a quarry type setup, so loads are high. The walls have been designed as two way span.

Image 4.
Plate stiffeners are easy to handle and simple but not as efficient as angle stiffeners

Image 5.
Channels are strong and efficient and useful if your wall thickness isn't high.

Image 6.
Angles are generally quite efficient like this. But notice where the higher strength is required channels are used.


A quick summary:
Often for efficiency it is channels for the really high loads and angles for general use. Flats are a little simpler to deal with during fabrication and when used pointed down are often required to stop material build up. As the item gets smaller and less expensive it is just easier to use flats.

 

[Scuka]

When $400,000 is being spent it is pretty lucrative to reduce costs by 25% by doing a good design.

You're saving $100.000 by using a couple of stiffeners less per wall?
Several of stiffeners comprise 25% of the cost of the entire project?

I don't understand, where did you pull that number from?

 

[human909]

You're saving $100.000 by using a couple of stiffeners less?

Yes. Though there was a little more nuance than "a couple of stiffeners less.

Stiffeners are 25% of the cost of the entire project?

No the project is 50x bigger. That design was just one part of it.

I don't understand, where did you pull that number from?

Directly from the job costings estimates based on prior inefficient designs and the actual cost of production from the newer more efficient design. I don't understand why you express doubt. Some engineers here work on small residential jobs, others work massive high rise or bridge construction. There are always efficiencies to be gained and the bigger the job the bigger the gains.

When you have large non circular surfaces subject to 100-200kPa (15-30psi) you end up with some pretty heavy and pricey steel bins. The 50kPa vacuum bin design was a doozy too. And my stuff is small compared to the mining stuff.

 

[Stick.]

Caveat: I work with large ducting and not hoppers, but the pictures you posted look pretty similar to what I generally work with.

Vertical vs horizontal vs both:

I do the same as human909 for picking a main stiffener direction, generally I pick the shortest span across the panel. As for when to pick both directions, that's generally driven by three factors for me. First is whether the sheet metal wall is over capacity. If it is, I may run stiffeners in both directions to break up the wall panel size. Second is whether stiffener buckling is a concern. Since I often design ducts subject to negative gauge pressures, the outer edge of the stiffener is in compression, so buckling may be an issue. If it is, I'll run stiffeners in both directions to shorten the unsupported length and take care of the buckling issue. Third is if there are any point loads on the panel (supports pads, lift points, etc). If that's the case, I'll often run stiffeners in two directions (only locally), just to transfer the load to both adjacent main stiffeners and not overstress the sheet metal wall at an unsupported corner of the support pad. As an example, in your first picture of horizontal stiffeners, I'd probably add a vertical stiffener on either side of the supports pads, just between the horizontal stiffener above and below the pads.

Stiffener type:

I don't have much to add since I almost exclusively use flat plate stiffeners, but one advantage of them is the design flexibility they give you. If you need a complex geometry (or just a very specific size) and have access to a laser cutter/waterjet/etc., there's very little restriction to what shape/size stiffener you design. Also, flat plate generally gives you a lot of flexibility with material choice compared to channels, angles, etc.

 

[dvd]

Another reason for stiffener orientation is the loading direction - horizontal hopper stiffeners may provide less stress variation along the weld and lower fatigue loading.

 

[Algirdas13]

Thank you everyone for the insights, I really appreciate that!
human909 and Stick., thank you for such deep analysis on different hopper designs, that helps a lot!

 

[Algirdas13]

Image 4.
Plate stiffeners are easy to handle and simple but not as efficient as angle stiffeners

human909, I would like to better understand why an angle is more efficient than a plate stiffener? Is it due that the plate stiffeners require laser cut/water jet, or for some other reasons?

 

[human909]

The stiffeners are behaving as a beam. A section that has more area at the extremity is more efficient than one that has less.

Thus for the same reasons that we extensively used I-sections and channels because they are more efficient. An angle section as a stiffener essentially behaves similarly to a channel with the stiffened plate behaving as the other flange.

 

[Algirdas13]

human909, thank you so much, that really makes a lot of sense!
For some reason when I was thinking about I-sections and channels resistance to bending, I was only taking into account that these sections has an advantage (comparing to tubes, etc.) only because of the ratio between element height and mass. And to be honest I didn't took into account also that the flanges (thickness of flange) of the I-sections and channels also creates an advantage in ration between mass and resistance of bending because of the tension and compression at the extremity.