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?

 

[Rabbit12]

If you look at the equation for Fp in Ch 13 0.4*ap with ap=2.5 is 1.0. So there is no difference in seismic force between your two scenarios correct? Chapter 15 doesn't utilize an ap in it's equation to calculate seismic force.

 

[Prestressed Guy]

Rabit12: but Eq 13.1-3 has the (1+2(z/h)which triples the load.
With Ch 13 I get F_p = 0.4*2.5*0.615-Wp*(1+2(z/h)/3 = 0.615W_p
With Ch 15 I get V = 0.615-Wp/3 = 0.205W_p
It just don't seem right!. My typical wall panels are designed for 0.197W_p, the ductile parts of the top connections are designed for 0.295W_p and the rest of those connections are designed for 0.923W_p
0.205 just doesn't feel right given the ramifications of what can happen if that thing 50' in the air decides to come crashing down.

 

[r13]

Here is an example on how to apply the 25% rule, and the weights to be considered. Per your description on weight of the supporting slab, it seems Ch13 is to be used.

 

[Prestressed Guy]

r13. I cannot get the link to work. After I down load it I get an error message. Can you upload again?

Thanks.

 

[r13]

This link works. The example starts on p.13-4.

Link

 

[human909]

human909 What types of system have you employed for this scenario. I see several options.

A frame similar to that posted by HTURKAK. 4 steel columns to the ground per bin. Braced in the direction of the truck movements and moment frame in the direction of perpendicular to truck travel direction. Heavy steel beams to support the bin legs as required. I've dealt with food grade systems regularly but not for this sort of application so I haven't faced the challenge where regular structural steel member can't be used.

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.

From what I can see the DT roof seems grossly understrength to perform this sort of task. (Though I haven't don't any calculations to support this judgement.) In fact any sort of 'roof' would generally be not suitable for these loads. You want these loads going straight into significant structural members.

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.

Trying to retrofit this load on an existing roof seems extremely challenging.

With the information available I'd consider looking at putting down steel columns external to the building on the 'left' side and picking up the vertical wall on the 'right' side. That said, how much capacity does this existing wall have?

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.

I would still favour this solution. Consider using stainless steel pipe or similar. I'd consider a braced steel superstructure with stainless steel beneath. Detailing the lateral restraint might be challenging likewise corrosion issues may need to be considered due to mixing stainless and mild steel. Alternatively consider a concrete frame.

 

[kipfoot]

regarding the r13 link: you can find the same file here

https://cdn.ymaws.com/

http://www.nibs.org/resource/resmgr/BSSC/p751_ch13.pdf

or google "soules nonbuilding structure design"

a good resource

 

[Prestressed Guy]

Here is a perspective on the system I am planning to develop. There are a couple of intermediate diaphragms that carry the bin legs and extend out to the 3rd stem beyond the legs. It should be noted that these double-T's are typically used for parking and office structures in the 60' - 65' range so this 24' span leaves a lot on the table.
Just for grins and giggles, I ran a calc for a single stem using the strand pattern that is used in a 70' span elsewhere in the building. It is loaded with the typical roof loads and a pair of 44k live loads and a pair of 95k reversible vertical seismic loads 5' each side of mid-span. In the moment diagram attached is shows that this single stem is only 60% overstressed in positive moment. As expected, negative moment is out the window. By spreading the seismic base for transverse EQ from 10' to 35' the loads on each stem will drop significantly. The walls are 8' and will not have any problem with these loads.

 

[Prestressed Guy]

Here is the moment diagram for one 36" stem with (2) 44k live loads and (2) 95k reversible EQ loads from a pair of legs. Once these reactions are spread over 4 stems it should work fine.

 

[HTURKAK]

I did not propose steel frame. Copy and paste from previous post ( 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;)
That is, i proposed solo 'stand alone ' reinforced concrete frame for the silo. Pls look the sketch again, the suggested sketch is RC frame with 4 columns, a thick slab with dropped beams under the ftgs of silo at elevation 35 and tie beams used for bracing at elevation 18.


Dear human909 , i did not propose frame with steel columns. Pls look to the above para.

Eng. Haydenwse, you are free to follow any of the advises in the previous posts . I just want to remind , several items based on having experience with living in high eq . zone ;

- It is not reasonable to compare the wt of silo (175 kips ) with the wt of total bldg ( around 10000 kips) since the SFRS for the silo will be some small portion of the total bldg. The subject bay is 24' x 125' , and DT plank with limited thick. can not behave as transfer plate .

- The DT planks connected to PC (8 in) walls ( I assume with some anchor plates ) with assumption of moment connection. I suppose the plastic hinge will develop at bottom and top of PC wall elements( NOT at DT plank drop beams). The assumption of plastic hinge development at 8 in PC plank really questionable. At least both top and bottom portion of the PC plank ( which the plastic hine would develop ) SHALL be confined with closed stirrups continuously and still the plastic hinge moment will be limited due to limited thickness. I invite you to look for the reason for providing separate PC frame .

- The behavior of bldg structures is misunderstood by a lot of young engineers. Pls consider that the EQ shakes the foundation , foundation shakes the vertical SFRS (columns and shear walls ), and vertical SFRS shakes the floors . In your case , 180 kips of elevated silo ( almost double of full loaded lorry ) 25 ft above the roof will be shaken by 8 in PC walls and DT planks.. Imagine the force flow..It is not reasonable to assume a ductile behavior will develop at PC walls around the silo and with the redistribution of forces to total bldg , the effect will be negligible for the total SFRS. I suspect, ( saying with observations) the DT planks will shear around the legs of silo and limited plastic hinge capacity at the top of PC walls will force the development of hinge at the bottom and will follow collapse of several PC walls together with DT planks. I suspect that may trigger the other portions of the total bldg.

- I is not reasonable to assume this case a simple meach. attachement or nonstructural component at the roof ( say roof type HVAC etc) and apply the rules of Chp. 13 . You are expected to look 15.3 ( NONBUILDING STRUCTURES SUPPORTED BY OTHER STRUCTURES..) 15.3.1 or 15.3.2 is applicable as per the seismic mass ratio.

- English is not my first language moreover, one of the languages i am familiar and hoping you understand my concerns and advise. If you post the SFRS of the whole str. Just curious for the SFRS of the whole str. in X direction ..

- Still my advise will be, provide a separate self standing RC frame similar to the sketch i provided . If your only option is supporting the elevated silo on PC DT roof planks and with PC 8 in thck walls vertical SFRS, I hope never occur , in case of an EQ, remember this thread and my post.

PS. This forum is international and the members , sometimes without having past experience and knowledge , are making brain storming ( ofcourse they are free to do). The OP should have discretion to choose the valuable responds. IMO, it is not reasonable shooting in the dark with machine gun with the assumption of one of the bullet will hit the target.

 

[human909]

Dear human909 , i did not propose frame with steel columns. Pls look to the above para.

Sorry for misrepresenting you.

For what it is worth, I concur with you previous post.

 

[Rabbit12]

I agree with HTURKAK. You really need to pay attention to your load path and make sure can get the forces from this silo back to whatever SFRS you use.

 

[Prestressed Guy]

First off, the short answer to the primary comments of HTURKAK

Thanks for the comments and I do not think we are as far apart in our thinking as you may assume. For background, I am an SE in CA and WA and have been designing primarily SDS D precast/prestressed since 1998. Many of my buildings have had big stuff on the roof just not 175k stuff with a 10' x 10' base with the lateral load 25' above the structure.
I fully agree with your assessment of EQ being bottom up and wind being top down. That is why I keep going back to the A_p because we have a "crack the wipe" geometry with a 173k cracker.

First off, this is a new design/build project and all construction is new and currently in the schematic phase of design so all options are on the table (within the sanitation / water proofing / corrosive environment limitations set by the owner) It will be a continually damp area and will be cleaned with high-pressure fire hoses. Per RFP all structural elements are precast concrete except for foundation and slabs.

As for SFRS, the building is per ASCE 7-10 table 12.2-1 Type A.5. Bearing Wall System - Intermediate Precast Shear Walls.

Wall to DT / beam connections are all pinned. Stems sit unrestrained on bearing pads which take only vertical force. All in-plane and out-of-plane loads are taken out at the top of the member. This is done in several ways but at this location, there will be a significantly thick topping slab that will be tied to the walls with rebar dowels. I will probably detail the dowels with 7bd debonding in the slab at the slab wall joint to increase ductility of this connection.

Given that the bay is 125' long it will be too flexible for X loads so the wall on the right side will need to redistribute the lateral loads up to the roof and and down to the slab. This wall line will be designed for this OOP bending.

None of these elements will be designed as moment frame structures. All connections are pin.

Overall, the structure is very stiff. Lots of concrete wall in both directions with very few openings of any size. All roof diaphragms are DT and with the exception of this bay and another at the opposite end of the building meet the requirement to be idealized as rigid.

At this time, the primary direction that we are investigating is to place prestressed girders under each pair of legs at the 10’ spacing. These girders will span 24’ and be supported by pilasters in the walls. This narrow spacing with no distribution to joists on either side will result in large net uplift so the beam to column and column to foundation connections will be designed for net uplift. The gap between the girders will be infilled with a DT narrowed to fill the gap.
The idea of the intermediate diaphragm is not a first choice due to the precast erection crew not being excited about forming these beams after erection.

Z axis lateral loads will be taken though diaphragm to shear walls on each end of girder / DT span. X axis lateral loads will be redistributed through wall on right up to high roof and down to slab.

All connections will take into account the overstrength provisions.

 

[r13]

Is the 8' thick wall a typo, or as is? If so, wow!

 

[Prestressed Guy]

r13: Nope, not a typo.

8" prestressed wall using slender wall design in LecWall software. I have not run any numbers yet so do not know if it will work but that is the first try. all DT's are loaded concentrically into the walls on pockets rather than on corbels which reduces the gravity induced moments. When you start with 225 psi minimum prestress, you can do some fun things with concrete walls. The exterior walls are 4"+4"+4" sandwich insulated composite walls.

 

[r13]

Yeah, the project could be very interesting and enjoyable. Good luck.

 

[dik]

any issues with snow accumulation?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Prestressed Guy]

No snow load. Just 20psf roof plus a bunch of mechanical roof top units. Other than the Bin, it is a pretty straight forward precast project.

 

[Rabbit12]

So Haydenwse, back to your original question. Following the code you'd determine seismic force per Ch. 13.....do you agree? As you pointed out this increases your force by 3 or so.

 

[Prestressed Guy]

Rabbit12. That is where I ended up. The lateral force is S_DSx W. Further discussion with the project owner found that the 173k is contents weight and the self-weight of the bin is 27k so total is 200k. ap = 2½: Rp = 3: Ω0 = 2½
This gives a horizontal seismic load of 123k at 23.5' above the roof plane

Using a beam under each leg pairs, the beam reactions are
DL = -13.95k
LL = -43.25k
EQx = leg 1 = +60.6k: leg 2 -60.2k (note: loads on all beams acting in the same direction)
EQz = leg 1 = ±145.15k: leg 2 ±145.15k (note: loads individual beam act in same direction, alternating beams acting in the opposite direction, i.e. beam 1 up, beam 2 down)
Will end up with a net uplift of about 60k at the foundation for loads in the Z direction.