Moss method to determine horizontal load distribution Vn for vessel supported by legs.

[jrcan]

I'm trying to use MOSS (3rd Ed) Procedure 3-4 "Seismic Design - Vessel on Unbraced Legs" as I think I found a problem with what Compress is doing.

In my case, I have 8 legs that are W12X120 shape. The horizontal load distribution is said to be calculated as: "The horizontal load on any one given leg, Vn, is proportional to the stiffness of that one leg perpendicular to teh applied force relative to the stiffness of the other legs. ... Thus the general equation:" Vn = V*I/Sum(V1).

I think this is very straight forward, so I calculate the different I's depending on the load angle for each leg (Iz from figure 3-11). Then I just calculate V*I/sum(I) for all leg positions...easy. But I must be doing something wrong as my resultant Vn's don't match the output calculated by Compress. I should be able to match this exactly for a given V and I can't figure out what I'm doing wrong. Anyone have any guidance?

For V = 14176.9 lbs
Ix = 1070 in^4 (matches Compress)
Iy = 345 in^4 (matches Compress)

Leg Position	I (what I calculate)	Vn (what I calculate)	Vn (Compress)
0	345	861.1	519
45	707.5	1772.1	1772.1
90	1070.0	2680.1	3025.2
135	707.5	1772.1	1772.1
180	345	864.1	519
225	707.5	1772.1	1772.1
270	1070	2680.1	3025.2
315	707.5	1772.1	1772.1

As you can see, the Vn values match for 45, 135, 225, 315. I have to making some simple error, but I can't figure it out.

Thanks,

 

[GregLocock]

I get same as you. Perhaps the orientation of the legs is wrong in your Compress files?

 

[Trestala]

If you go to https://support.codeware.com/s/article/235, their formula for shear:

Vx = F(Iy/Ix) / 2(1+ Iy/Ix)
Vy = (Vx)Ix/Iy