hopper bin on roof structure 2

[Prestressed Guy]

I am looking at a project that will have a 14'ø by 40' tall hopper bottomed bin mounted with (4) legs to the roof of the structure. The CG is about 25' above the roof level with at max weight of 175k.
For seismic loads, would this fall under ASCE 7 chapter 15.4 with R = 2 Omaga-0 = 2
Or would it be under table 13.6.1 for roof mounted stacks, cooling and electrical tower laterally braced below the CG. with a_p = 2.5 R_p - 3?

 

[r13]

For roof top structure that tall, be conservative, unless you are 100% certain which case it belongs.

 

[Prestressed Guy]

That was where I was looking. This is a elevated storage vessel / bin which leads me to 15.4 but on the other hand it is roof mounted and is extremely heavy and top heavy to boot. It doesn't really fit the discription in 13.6.1 (stacks, cooling and electrical towers)but it is roof mounted and braces a long way below the CG.
13.6 will give about 2x the lateral load of what I get from 15.4.

 

[r13]

I think the linked article may help you to determine which provision is applicable to your case. [link file:///F:/Engineering/Seismic%20Design%20of%20Nonbuilding%20Nonstructural%20Components%207-16.pdf]Link[/url]

 

[HTURKAK]

In your case, the elevated hopper with four legs supported by other structure and is not part of the primary seismic force-resisting system.

You should look 15.3 ( NONBUILDING STRUCTURES SUPPORTED BY OTHER STRUCTURES..) 15.3.1 or 15.3.2 is applicable as per the seismic mass ratio.

If you provide more info. regarding the supporting structure, ( a sketch etc ) you may get better responds..

 

[Prestressed Guy]

The S_DS=0.615
here is a view of the bin

 

[Prestressed Guy]

r13.: I was unable to get the link in your post to work.

 

[r13]

Here is the article.

 

[r13]

Not mean to comment on a preliminary design/sketch, but the support building seems way too slim. Watch out.

 

[HTURKAK]

That sketch implies an industrial i single storey PC building.. is the opening 14'X 16' for truck access ? and the structure is for truck loading?

please provide more info . for the supporting structure to get better responds..

 

[Prestressed Guy]

R3: Thanks for the link to the article. This is one 24' bay of a 340'x125' total precast structure. This bay is 24' x 125' with double-T roof.

Given that the span-to-depth ratio is greater than (3) it cannot be idealized as rigid so I will be modeling it as semi-rigid and the walls to the adjoining higher bay will be transferring much of the lateral load to the large upper diaphragm.

Given the lateral loads and very high CG I anticipate some significant net uplift loads which DT's do not handle well so will need to revise the typical configuration to increase load distribution by means of intermediate vertical diaphragms and/or thick composite topping/house-keeping slabs.

 

[Prestressed Guy]

HTURKAK: All of your assumptions are correct. See previous post for more info. I investigated the possibility of taking the vertical loads through the roof to an "X" frame that would allow columns to grade at 12'x12' but that is not an option. Prelim design shows that I can get the vertical downward capacity from 36" DT's but not the needed net uplift due to narrow stem width.

 

[r13]

This is a big challenge. I envision you will need a very strong floor, and sturdy columns/walls to resist and pass the resulting forces to the foundation, if you can't bring the legs of the hopper frame to the ground level. Good luck.

 

[HTURKAK]

Dear Haydense, it is not reasonable to support the silo having 175 kips weight and 40' tall on the PC bldg with deck PC DT planks . It needs to be supported properly with a separate supporting structure. Sds=0.65 means moderate seismic zone.. I will suggest a separate RC supporting frame structure with four columns to grade at 14' X14' and with a thick slab supporting the legs of the silo. Similar set up like this one;

 

[human909]

For what it is worth I've been designing about half a dozen of these sorts of structures for the last couple of years. I presume it is an outloading structure of some kind for loading product into trucks. I've been typically dealing with 100 metric ton bins. Naturally seismic loads, particularly in moderate to high seismic areas are very significant.

I might be able to answer specific questions though as I don't work with ASCE code I cannot respond to code related questions.

One thing I will say I've seen some of these things MASSIVELY under designed also confirmed by others. Be sensible about things.

This is a big challenge. I envision you will need a very strong floor, and sturdy columns/walls to resist and pass the resulting forces to the foundation, if you can't bring the legs of the hopper frame to the ground level. Good luck.

Correct. For what it is worth I haven't found the design of such structures to be too much of a challenge. One you accept the loads involved and the need for heavy members and heavy connections it actually works out quite elegantly. It is not uncommon bins to have numerous legs arranged around the cylinder. So the design often involves numerous transfer beams to bring the load to the main supporting columns.

I've used heavy steel columns with rigid connections to the foundation to keep deflection to something reasonable. A thick heavy foundation with ground beams to resist moment and overturning.

The other option that is sometimes used is square bins. The structural designs of the square bins is less efficient than that of cylindrical bins but the it makes the sub structure easier to build and you can store more product in the same footprint.

 

[Prestressed Guy]

I would like to try to bring this back to the question at hand. Chapter 13 or chapter 15? I read the STRUCTURE article and it raised all of the questions that I had already been going over but did not provide much in the way of conclusions that apply. I am inclined to use 13 with a_p = 2½

HTURKAK: I aggrege that it is totally out of the realm of possibility that this bin can be put on a typical DT roof structure as I indicated in my earlier response. Given the geometry, each pair of legs would land on a single stem with significant uplift which will never work. DT is the roof structure but it will need to be highly modified from typical 36" DT's on 10' centers. Your steel frame down grade with only lateral loads going to the roof structure was my first suggestion but it was rejected due to conflicts with use in the load-out building and need for cleanout. The facility is food processing using cleanup with a fire hose, The owner does not allow any steel inside that is not stainless. All structure must be concrete.

human909 What types of system have you employed for this scenario. I see several options.
1. locate bin legs mid-way between two DT stems and create intermediate load distribution diaphragms like you see on prestressed bride girders to spread the load to several stems each side of the bin. These would be created by holes cast through the top of the stems for top reinforcement with the bottom reinforcement passing below the stems. The anchor bolts would be cast into these transverse diaphragms and the load would be spread over as many stems as needed to control the loading. This would be statically indeterminant so I would perform a dynamic modal analysis to find the needed strength and stiffness.
2. If this does not work for whatever reason, I can look at increasing the thickness of the composite topping slab to make it a joist supported elevated footing. These DT's have a very short span for the section so there is a great deal of excess strength available.
3. The last option is to replace the DT's under the bin legs with prestressed girders.
In all cases, I would still use the intermediate diaphragm to distribute the lateral loading from wind and seismic perpendicular to span out to a wider base.

 

[bones206]

I would like to try to bring this back to the question at hand. Chapter 13 or chapter 15?

Answer:

In your case, the elevated hopper with four legs supported by other structure and is not part of the primary seismic force-resisting system.

You should look 15.3 ( NONBUILDING STRUCTURES SUPPORTED BY OTHER STRUCTURES..) 15.3.1 or 15.3.2 is applicable as per the seismic mass ratio.

 

[Rabbit12]

Haydenwse whether you use Chapter 13 or 15 depends on what your SFRS is. If the seismic load of this will be resisted by the building SFRS then you'll use Ch 13 unless the weight of this is greater than or equal to 25% of the effective seismic weight of the structure. In the latter case you'd use Ch 15.

If it's a standalone SFRS you'd use Ch 15.

 

[Prestressed Guy]

In section 15.3 it discusses a 25% limitation. "15.3.2(2) The nonbuilding structure weight is greater than or equal to 25 percent of the combined weight of the nonbuilding structure and the supporting structure"
When they say "supporting structure" is it talking about the structure as a whole or just the elements of the structure that support the bin?
In this case, the bin, even at 173k is just a small fraction of the total weight of the total structure. At this point the total roof framing weighs about 3,500k and the walls add another 6,400k so this bin doesn't amount to much.
Each of the DT's that are under the bin on the other hand only weight about 18k and the walls are 100psf so if a 20' slice of this bay is supporting the bin, the 20' long "supporting structure" will weigh 158k so the percentage would be 173/(173+158)= 52%.

 

[Prestressed Guy]

Rabbit12: Per ch 15 a_p is not included in the lateral load but when I look at ch 13 all of the stuff that is supported below the CG have an a_p of 2.5

For the life of me cannot see any logic of why a roof top electrical tower weighing a couple of kips would have a_p = 2.5 and R = 3 and a 173k 40' tall roof mounted, top heavy bin, would only need R = 3 and no a_p.
Logic tells me that anything roof mounted with the CG well above the supporting structure should have a_p = 2.5 but so far I have not found a code justification for that. Am I missing anything?