Saddle Analysis By PV Elite / Dennis Moss

[TheMehanic08]

Need some help guys,

Was performing an saddle analysis and came across one thing that caught my mind.

The equipment weight is 6300 kg
The equipment shear in seismic is 4725 kg.
The same was matched up in the pvelite file


and the saddle calculation (for one saddle) was as follows:

Saddle Reaction Force due to Wind Ft [Fwt]:
= Ftr * ( Ft/Num of Saddles + Z Force Load ) * B / E
= 3.00 * ( 701.4/2 + 0 ) * 405.0000/380.0000
= 1121.4 Kgf

Saddle Reaction Force due to Wind Fl or Friction [Fwl]:
= Max( Fl, Friction Load, Sum of X Forces) * B / Ls
= Max( 47.10 , 1679.27 , 0 ) * 405.0000/3000.0000
= 226.7 Kgf

Saddle Reaction Force due to Earthquake Fl or Friction [Fsl]:
= Max( Fl, Friction Force, Sum of X Forces ) * B / Ls
= Max( 4725.55 , 1679.27 , 0 ) * 405.0000/3000.0000
= 637.9 Kgf

Saddle Reaction Force due to Earthquake Ft [Fst]:
= Ftr * ( Ft/Num of Saddles + Z Force Load ) * B / E
= 3.00 * ( 4725/2 + 0 ) * 405.0000/380.0000
= 7554.7 Kgf

Load Combination Results for Q + Wind or Seismic [Q]:
= Saddle Load + Max( Fwl, Fwt, Fsl, Fst )
= 3358 + Max( 226 , 1121 , 637 , 7554 )
= 10913.2 Kgf

Now we see here the overall reaction on one saddle is around 11 tons. Is this justified ?
As the operating weight is only 6300 kg and max seismic shear is only 4725 kg for the entire equipment and now the saddle reaction only for one saddle is nearly 11 tons

 

[SnTMan]

TheMehanic08, hard to say what all the numbers mean, but at a guess:

Your software is calculating a vertical load due to seismic (Fst) and adding to dead load (3358).

Be aware that PVE is making a number of assumptions about the geometry of your vessel. You need to confirm them.

Regards,

Mike

 

[JStephen]

I don't know what all the numbers mean either, but yes, it could be like that. Specifically, if you had a short vessel (or at least with saddles close together) relative to the height, then the longitudinal seismic would be adding a vertical load to one saddle and uplifting on the other.

 

[SnTMan]

I have also seen cases where seismic loads were considered to act vertically as well as laterally. Could be either, only the programmers know

Regards,

Mike

 

[TheMehanic08]

Thanks Guys for your input, sorry if i was not very clear with my question.
Well i actually wanted to know how is this ftr = 3 arrived at in both the below formulas

Saddle Reaction Force due to Earthquake Ft [Fst]:
= Ftr * ( Ft/Num of Saddles + Z Force Load ) * B / E
= 3.00 * ( 4725/2 + 0 ) * 405.0000/380.0000
= 7554.7 Kgf


= Ftr * ( Ft/Num of Saddles + Z Force Load ) * B / E
= 3.00 * ( 701.4/2 + 0 ) * 405.0000/380.0000
= 1121.4 Kgf

Anyone can explain that???

 

[doct9960]

TheMechanic08,

The explanation is in page 169 of the Dennis Moss book (3rd edition). If you don't have a copy of the book, google "Pressure Vessel Design Manual".

You are using PVElite? What does it say about Ftr? Usually there are manuals for design software like PVElite. Or go to the Help menu. I would like to know the term they called "3".

 

[gr2vessels]

As doc9960 said, check in the help menu the recommendation for the Ftr value. Click on the Ftr input data box and press F1 for help. From the menu on left hand side, select the Ftr and 'voila', the explanation is there (also I copied for you the PV Elite help on the topic)
'Moment Factor, Ftr
Enter the moment factor for calculating the saddle reaction force due to the wind or earthquake transverse load. The recommended value is 3.
The value of 6 is conservative in that it assumes that the maximum edge load is uniform across the entire base, when realistically it occurs only at the edge. A more accurate method converts this triangular loading into a more realistic uniform load, leading to the value of 3. The following figure shows an end view of a horizontal vessel with a transverse load, simulating wind/seismic loading:...'
Cheers,
gr2vessels