Silo bracing 1

[MikeG7]

Hi All, see attached photograph
What is the logic of bracing back to the cone on this silo?
We have had some discussions and it seems that it is to limit the unbraced length of the leg in the weak direction?
Any thoughts

 

[Ron]

That's part of it, but also to create a shear plane/diaphragm to resist lateral loads. Prevents wracking.

 

[MikeG7]

Thanks Ron. Thought the X-bracing was there to resist lateral loads? 'wracking' isn't a term I'm familiar with. Care to expand?

 

[Ron]

Making a square into a parallelogram

 

[JStephen]

That would be "racking", I think.
Reducing unbraced length would be one motivation. Allowing a moment connection to the silo may be another motivation.
It's kind of hard to tell what the design philosophy is on something like this- you can make different assumptions about what is fixed and what isn't.
I don't care for the little tabs that the cross-bracing is attached to, looks like they should be in line with the bracing.

 

[MikeG7]

Yes, agreed the tabs should be in-line. A Client has made up a basic prototype without a great deal of engineering attention to details.
The leg columns are bolted in a few rows of bolts so that there is a fair amount of engagement with the silo walls. I think it would be reasonable to say this is a moment connection. Comments?

 

[JLNJ]

The whole way it's framed looks a bit goofy to me.

I'm not sure the person who designed it could articulate his/her own design intent.

 

[MikeG7]

I did model this bracing up in an analysis software to see what the radial bracing does when an overturning moment is applied to the structure. It definitely has a small influence on reducing the tension in the X bracing (I have to consider/assume that the 'ring' which the radial bracing attaches to is stiff).

 

[MikeG7]

My last post refers to the influence of the radial bracing (reading it now it seemed a bit out of context)

 

[JStephen]

In general, the design of tanks/silos on legs is a mess. Bednar's pressure vessel handbook gives one approach. Megyesy's pressure vessel handbook gives a different approach. Gaylord and Gaylord's Structural Engineering Handbook gives a different approach. AWWA D100 includes some requirements on the topic, but doesn't have a full design procedure, either. Each of these approaches is pretty limited and there seem to be some major shortcomings or oversights in each one. Looking at actual tanks and silos shows all kinds of detailing and arrangements used.
You can assume the legs are fixed to the cylindrical shell. Or that they are fixed at the foundation. Or some of each. In this case, it looks like the legs are cast into the foundation? If you assume either end if fixed, you can wind up trying to support a large moment at that point. If you assume either end is pinned, you have to figure out how to ensure flexibility at that point.
Assuming the cross-bracing on this one isn't pre-stressed in some way, presumably, you'd get compression loading from gravity loads in the cross-bracing?

 

[SAIL3]

I have never seen this type of bracing before.....unless it is to reduce the unbraced length in the weak direction, as mentioned in the post....I would have expected a ring on the vessel @ the horiz bracing for any substantial load or to provide enough stifeness for bracing...assuming there is no thermal growth in the vessel...can not figure out the conn @ base of cols which show bolts in the fla of the col...does not look like it was designed by a structural engr...

 

[Ron]

JStephen....now you've racked my brain

 

[MikeG7]

Hi fellas, thanks for your insights and questions/comments.
This kind of radial bracing is very typical from what I've seen in the silo world. Almost every commercial silo I've come across has it. Hence my questions, because it's just not well supported with documented reasoning why it's there, but I can tell you from the way these silos are constructed, if it's not needed you can be sure it's not provided!
JStephen, those references are helpful, thanks. I've been using the Moss approach, which actually doesn't consider the bending moment on the leg in the case of X-braced legs. Bending stress in leg is only considered (in that method) for unbraced legs. I take it Moss kind of assumes that the bracing eliminates the bending. There is interestingly a small note also about "lateral bracing". I assume this to mean the type of bracing as in the first post.
I've included another photo here of the base attachment, it's not a great photo but you can see the implementation.

 

[MikeG7]

I just had another thought...the other possible reason for the radial bracing could be likely to "accidental" loads from things bashing into the silo columns . Because the X-bracing does not provide much resistance to a force directed inward, which could easily buckle one leg and send the entire silo over.?

 

[dik]

I've gone through my 4 metal silo design books/manuals and can find no reference to the bracing shown. It may have been added to provide added resistance to the outlet.

Dik

 

[StrP88]

dik (Structural,
I am new into Silos and heard you said 4 books, would you be able to share the name of these books so I can read them

Thank you