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Weld stress in joint

Weld stress in joint

Weld stress in joint

(OP)
Suppose I have a weldment with a tri-pod design (see attachment). The load is 36,000 lbs, so each leg is supporting 1/3 of that weight. What would the stress be in the weld that secures the leg to the center tube? My attempt: the reaction (Rby) at each foot at the base is 12,000 lbs, the opposite reaction in the vertical direction on the tube-to-leg weld group (Rcy) would also be 12,000 lbs. The tube, cut at an angle at the point of attachment, has dimensions of 1 x 4.73 and is welded all the way around. Following the information in Shigley's, this grouping has a total throat area of 1.414h(b + d). In this case, b = 1" and d = 4.73". With a 1/8 fillet weld, the total throat area is calculated to be 1.414*(.125)*(1+4.73) = 1.012" Shear = F/A, so 12,0000/1.012 = 11,857 psi. According to this I can get away with an 1/8 weld, but that just seems small given 18 tons being applied? Is this methodology correct or am I missing something?

RE: Weld stress in joint

I think your calculations are about right. Personally, I would disregard the 1" dimension and consider the length of weld to be 2*4.73" = 9.5", but I would still weld all the way around. And I would use a minimum of 3/16" fillet weld.

Check the wall thickness of the circular vertical tube for concentrated loads from the legs. If the extended centerline of each leg does not meet precisely at the midpoint of the circular tube, a torsional moment will exist which must be resisted by the three legs, but also by the wall of the circular tube. Thus the legs must be positioned rather accurately.

The stability of the assembly is dependent on the thickness of the wall of the vertical tube. If the attachment of each leg to the central tube allows free rotation about a vertical axis, the assembly is unstable.

BA

RE: Weld stress in joint

I agree with BA...I never use a !/8" fillet weld on any significant structural conn...3/16" min

RE: Weld stress in joint

Are the feet just bearing on the foundation?

What is value of the estimated maximum transverse force at the top of the tripod (Rax?).

How Are the tripod legs oriented in the top/plan view?
120 degrees?
http://g-ecx.images-amazon.com/images/G/01/aplusau...

or inline?

Regardless, if the interfaces to the "ground" and I-beam are not very secure, and slip free, A few ties lower on the legs might provide some nice redundancy.
http://g-ecx.images-amazon.com/images/G/01/aplusau...

RE: Weld stress in joint

(OP)
The legs are bearing on the foundation, equally spaced at 120 degrees. The thing that confuses me is that since this is more of a rigid frame, and not a pinned structure, wouldn't there be a moment reaction at the fixed connection to the tube in addition to shear? I believe Rax would be about 5547, because the compression in each leg is 36,000 = 3R_legSin(65), where R_leg = 13,240 lbs. Then you can solve for Rax using Pythagorean theorem. Also the reason for the 1/8 weld is the tube wall is .120. Is 3/16 possible with that thin of a wall?

RE: Weld stress in joint

Wall thickness is 0.120" for all tubes? Or just the legs? What is the size of the center tube?

BA

RE: Weld stress in joint

Issues to consider: can the feet slide, will the stand deflect under load, is the load 100% purely axial, is there any dynamic loading, is there enough friction to keep the feet from sliding, is the load applied quickly, is failure likely to cause injury, will the heat of welding compromise the hardness or temper of the parts, will the leg cuts be curved to fit the pipe. I would tie the feet together with small chain or wire rope.

RE: Weld stress in joint

(OP)
wall thickness is .120 for all legs and crossmembers, center tube wall is .226. The FEA simulation indicated stresses higher than just pure axial, there is some bending present I just don't know how to verify that with hand calculation. Probably over thinking it though.

RE: Weld stress in joint

I don't see any crossmembers on your sketch and the diameter of the central tube is not shown. What is the boundary condition at the bottom of the central tube? In your FEA simulation, did you consider the legs hinged at the base?

With only 0.12" wall thickness, you are correct when you say that by code, you cannot exceed a 1/8" fillet weld.

I don't believe you are overthinking it at all. You should be satisfied that the computer results pass a sanity check.

BA

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