Skirt With 'Seismic Clips' 1

[Cobra17]

I'm looking for a valid way to verify 'Seismic Clip'. The clips are pieces of angle welded to a skirt and bolted to the ground. The skirt on it's own is enough for seismic conditions and I'm thinking that if I do procedure 4-15 from the 4th edition of the Pressure Vessel Design Manual (Moss), which is based on the centered neutral axis method, that I can prove the bolting, base plate thickness and skirt, then a weld shear check for the skirt to lug. I would use the shear and bending moment that I get from running seismic calcs (Compress) on the skirt.

 

[r6155]

And the question is ???
Regards

 

[Cobra17]

do you think the method is valid?, or do you have any suggestions?

 

[HTURKAK]

This procedure is acceptable if the vessel relatively small.. ( pls look to the notes )

Apparently the skirt plt thk. is the bearing area with conc. base .. You would need a base plate having width (w) and the bearing stress should be LT 0.25 fc.

 

[Cobra17]

HTURKAK:

they are small vessels, ranging in size from 10" to 30" diameter, and thin walled.

In the Chemical Engineering Design - Volume 6, 4th edition, I did find the same equations as in Moss's book, but they also had a base plate width calculation, which is the 'L' dimension from my first post (see attached for calc reference)

Lb = width of base plate
Fb = compressive load on the base ring, which isn't in Moss's.
fc = actual bearing pressure on the foundation, same as Moss's.

Where Lb = Fb/fc

Is that the width you mentioned? or the 'W' dimension from my first post? and how would you determine the width (circ direction) of the clips?

 

[HTURKAK]

Yes... similar calculations . But Lb = width of the base ring (13.94 in your second attachment ). However in your first post, there is no bearing ring but some individual angle clips ( could be four pcs)..

If the vessels small , you may use angle clips instead of bearing ring but the calculation will differ..

 

[Cobra17]

This is the part that I need help with. I'm not an Engineering, and don't deal with structural very much. How would you alter the formula to not account for only the clip width?

 

[HTURKAK]

Can you post the necessary info for one the vessels ? pls look to the your second attachment and thk. of the skirt ..

 

[Cobra17]

Here is the calculation sheet I'm working on. it has the Moss formula's so it's not like it's a big secret. let me know if you need more.

 

[HTURKAK]

Mr. Cobra17 ,

Pls find (attached ) seismic force calculation of the support clips of the subject vessel.We are at very different time zones so i responded today ..

I assumed conservatively , the vessel is erected on a prefinished smooth surface concrete plinth with post installed anchors and the vessel skirt does not touch to the concrete surface and supported on angle clips..

With this set up ,( assuming your seismic shear and moment is correct ) , tension force does not develop at anchor bolts moreover, base shear is resisted by friction and no shear force develop at anchors either..

In this case, you may use nominal size anchor bolts .. say 3/4 in. is fine..

 

[r6155]

@ Cobra17
For the diameters in your case the skirt is not a good idea. Legs is the option. All surfaces must be accessible for inspections.
Calculations are useless when the design is wrong.

Regards

 

[Cobra17]

HTURKAK: Thanks for the calcs. The vessel does rest on the skirt as shown in my first posts attachment, and the skirt is sufficient for seismic forces. I'm not sure how, or if what you have provided should, or needs to be modified, but I do have a few questions:

1. For the Area of 3 pts = 3 in your calcs, is the 3 because there are 3 clips? If there were 4 clips, would 'h' be 29*COS(45) = 20.5061?.
2. Why calculate Pmin?
3. fc = 353 psi is ok because it's less than my legs allowable stress 'S' of 16,600 psi correct, not my skirt (which isn't shown)?
4. where you have fc as 'check conc. stress', 'conc.' is concentric correct?

I attached an update calc sheet where I added your calcs on lines 9,10,11,16,17,18

 

[HTURKAK]

1. For the Area of 3 pts = 3 in your calcs, is the 3 because there are 3 clips? If there were 4 clips, would 'h' be 29*COS(45) = 20.5061?.

A= There are 3 clips and i assumed each one has 1.0 unit area.. If there were 4 clips, 'h' would be 29*COS(45) = 20.5061. so , true..

2. Why calculate Pmin?

A= In order to see if tension develops at one of the legs.. and Pmin = 2320 and in your calculation , fs = -2062.3759 lp ( meaning compression)... Abr= -2062.376/7000 = -0.2946 is not true.. but you may provide a nominal size bolt.. ( If compression, the force will be resisted by angle clip or base ring unless a counter nut is not provided under the base plate or angle clip )..


3. fc = 353 psi is ok because it's less than my legs allowable stress 'S' of 16,600 psi correct, not my skirt (which isn't shown)?

A=fc is bearing stress developing under compression leg..allowable bearing pressure, 2000 lb concrete can be assumed 500 psi

4. where you have fc as 'check conc. stress', 'conc.' is concentric correct?][/quote]

A='check concrete stress'

 

[Cobra17]

Thanks for the feedback. I have a few follow up questions if you have some time.

2. Why calculate Pmin?

A= In order to see if tension develops at one of the legs.. and Pmin = 2320 and in your calculation , fs = -2062.3759 lp ( meaning compression)... Abr= -2062.376/7000 = -0.2946 is not true.. but you may provide a nominal size bolt.. ( If compression, the force will be resisted by angle clip or base ring unless a counter nut is not provided under the base plate or angle clip )..

" the force will be resisted by angle clip or base ring.." how do I know the angle clip is wide/long enough? tb (line 14) gives thickness (from Moss), Can I then use formula 13.93 from the chemical engineering design book to get Fb (total compressive load on the base ring), and use the fc that is the actual stress on the concrete to calculate Lb (13.94)? should fc = MAX(Pmax,Pmin)/f'c?

"Abr= -2062.376/7000 = -0.2946 is not true" Would ABS(fs)/Sb be more accurate? or MAX(fs/Sb,0) ?