Moment of Inertia of Weld Groups in 3-axes

[tc7]

Hello all-
We're designing a rather heavy test fixture to suspend and rotate a large mass. The fixture and mass will be rotated very slowly around the horizontal Z-axis and tilted fore and aft around the X-axis while we "operate" on our mass (all motions are slow enough and do not impart significant accelerations, i.e., static design is all that is required). The concept sketch of the fixture is attached. For the purpose of this question assume all plates are 1-inch thick (to be verified later).

THE PROBLEM: Need to determine the moments of inertia of the weld group which as you can see is configured along three orthogonal axes in order to design the fillet weld sizes. I'm not proficient in ASD or LRFD calculations so for our fixtures I'll use Blodgett's methods for designing welds. However every hand book on earth and every reference in the infinity of the internet only has charts of weld group properties for 2-dimensional single plane arrangements. Omar Blodgett's handbook includes a scant outline of a single computation for a 3-dimensional problem but seems to have made many simplifying assumptions that aren't explained and aren't obvious.

THE QUESTION: Can anyone advise on a source for weld group properties that includes 3-axis arrangements OR better yet, some other reference that shows details of handling this type of design. I can't be the only one to ever have to do this !!

Thank you for reading and all advice is welcome.

 

[JGard1985]

Hi TC7

Looking at your sketch, it should be two different problems to evaluate, not a single problem

1) Determine shear flow between vertical gusset and horizontal plate (36" long weld). Size weld as required to provide adequate shear flow and act as composite (if you need to) its possible gusset is strong enough without moment of inertia contribution of horizontal.

2) Determine moment of inertia of weld group at wall (right sketch). Determine bending moment at wall and check welds. Use 2 vertical fillets to start, Add horizontal if necessary. Remember if you use uneven weld equations to check the side in TENSION, not compression

Hope this helps



Jeff
Pipe Stress Analysis Engineer

http://www.xceed-eng.com

 

[tc7]

Thanks for you speedy reply Jeff, I'll start off my calcs with your suggestions.

Say by the way ! Since you are a piping engineer, let me ask you this: I happen to have an old piping design book by Grinnell that shows a piping run routed in three directions and they illustrate a method finding Ixx & Iyy in the X-Y plane, then find Ixx & Izz in the X-Z plane and then find the Iyy & Izz in the Y-Z plane. So after all that is done they sum up each pair of I's. The book does also list line properties in each orientation. The calculation process is very orderly and I understand them perfectly - are you familiar with the Grinnel method? and can I apply the same methodology to my design problem ?

 

[JGard1985]

No problem. I'm not familiar with that right now (I'm home). But if you add two moments of inertia that are orthagonal to eachother, you are calculating the torsional inertia of weld group "J". You don't have torsion so you shouldn't need to compute this. Can you upload Grinnell pages you described?

Jeff
Pipe Stress Analysis Engineer

http://www.xceed-eng.com

 

[tc7]

No, I'm not talking about the torsional inertia. In the Grinnell method, the line inertias are calculated for the projection of the 3-D configuration in each plane. For instance, Ixx is determined from the projection in the X-Y plane and then Ixx is determined from the projection in the X-Z plane, then each of these Ixx's are added together to provide a "Total Ix"; similarly, "Total Iy" and "Total Iz" are determined from their respective projection planes. Text pages are attached*.

What do think Jeff, is this a legitimate method to use when treating welds as lines and determining weld group inertias in a 3-axis system? It would seem to avoid need for simplified assumptions that appear to have been made in the Blodgett example.




* I don't think I am violating any web site rules, to give full credit, these pages are from the Grinnell text, " Piping Design and Engineering ", Third Edition, 1971.

 

[dik]

Can you treat the forces in the welds in a fashion similar to shear wall load distribution on a rigid plate?

Dik

 

[JGard1985]

TC7, I don't think this is the correct application of the the Grinnell Line Inertia. Thats for determining support reactions at end of piping:

1) The line body (piping) is only supported at two points, while your weld is continuously connected and transfers load along its entire length

2) I see how you are looking at the weld as a 3-Dimensional weld group. But take a step back and look at load flow. Any loading that flows from bottom plate through your vertical gusset still needs to be resolved at the wall. The 36" weld isn't a support reaction for your load. Your critical weld is the weld to the wall which is a 2D setup

Jeff
Pipe Stress Analysis Engineer

http://www.xceed-eng.com