Load Path Around Opening of Round HSS-to-HSS T Connection.

[VT17]

I have question regarding load path around an opening of a t-section made out of round hollow sections.

So I’m doing the structural design a sculpture that is essentially cantilevered from the ground (like a pole). The connection that makes the transition from the rest of the sculpture into the vertical support is a T shape moment connection. Before this was accomplished by welding a round HSS vertical post to a horizontal round HSS member and making the adequate checks under table K3.1 of AISC 360-10 and using an adequate weld. Furthermore, the horizontal HSS was going to have a drilled hole concentric with the vertical HSS to run electrical conduit inside the HSS sections. However, now we will be casting pieces instead of using HSS sections so this T shape will be one uniform piece similar to this in shape ([URL unfurl="true"]https://www.oilybits.com/316-stainless-steel-reducing-tee-150lb-bsp.html[/url]). My question is whether it poses a problem to have that opening in the horizontal section the size of the inside diameter of the round vertical “stem”? The horizontal section is experiencing shear in both orthogonal local axis, moment in all three axis, and axial load simultaneously that need to transfer to the vertical stem. Since this is all one piece, the load should travel from the horizontal section to the vertical section around the opening correct? In other words, for all those loads I mentioned, I should be able to use the section properties of the full round section for the horizontal member correct? Also is there any stress concentration that occurs at this location?

Thank you in advance if you read this far and for any input you provide.

 

[BAretired]

I think it would depend on the ratio of diameters.

BA

 

[VT17]

BA,

The ratio of the diameters of the horizontal member to the vertical member is about 1 to 1 if that's what you meant?

In terms of stress concentration I was recalling how there are factors but as I reviewed my mechanics of materials, they only stipulate factors for a flat bar with a hole under axial loading. For pure bending, there are the cases of a flat bar with a sudden change in width or a flat bar with grooves. These are not exactly what I have so I'm not sure if a stress concentration would occur in the first place, and if it did, to how to even approximate it (although maybe the aforementioned factors could help).

 

[BAretired]

A ratio of 1:1 means that the vertical and horizontal RHS have the same diameter. You cannot expect to cut a hole the size of the horizontal RHS and use its full section properties when calculating stresses. It will be seriously weakened at the hole location. I don't know how to advise about stress concentrations other than to suggest a finite element analysis of the tee shape. There may be approximate methods but I'm not aware of them.

BA

 

[hokie66]

What is the proposed diameter of the members? What is the material of the casting? How will the verticals and horizontals be connected to the tee?

 

[SAIL3]

AS BA noted the horiz member is seriously weakened with such a large hole....being that it is a cast member it would be difficult to add any reinforcement by welding...there may be something in ASME that may be similar and worth a check....I would go back to the original design which may be also be possible to cast....

 

[VT17]

Thank you all for your inputs!

Hokie: The diameter of the vertical post is 4.5" and the horizontal (varies) about 5" at the joint. The material is stainless steel and the rest of the cast pieces are attached to the tee with CJP welds.

BA: My first thought was that I could not use the full section properties because of the hole. When I checked a "half" section with an equivalent S and Z it obviously failed. But then I started thinking this would be the case if I simply had a horizontal member subject to a whole bunch of loads and then simply cut a large hole in it. By having the opening right where the vertical post occurs I started thinking the loads would resolve themselves around the opening and transfer to the vertical post. However, I'm struggling to confidently determine how the loads resolve themselves with this opening at the joint.

My problem with an FEA analysis is thinking how to properly do it. In other words how to create this shape. With Ram Elements or Risa this connection is currently a round HSS framing into another HSS where all degrees of freedom at the intersecting node are fixed. This is obviously not the case anymore in reality. I'll try to think of something (maybe Inventor?) but I welcome any suggestions you all may have in terms of modeling or software to use.

SAIL3: I will look into ASME to see if they have anything helpful.

 

[dhengr]

VT17:
My understanding of the shape of your structure is like this “T”, with the vert. limb (pole/post/column) being a 4.5” dia. pipe and the top, horiz. member being a 5” dia. pipe. I would make the horiz. member continuous through the joint, and cut/shape the vert. member to fit the bottom outer shape of the horiz. pipe at the joint, with appropriate attention to welding prep. I would drill the hole (1 or 1.5” dia.) for the electrical conduit in in the bottom of the horiz. member at the center of the post. This is akin to a cont. stl. beam running over the top of a stl. column, which beam cantilevers a few feet either side of the stl. column. This allows the loads and stresses in the canti. beams to tend to stay in the beam element and pass over the col. rather than having to pass through the col. Then the only forces and stresses which do go down into the col. are those which are not balanced btwn. the two canti. beams. Alternatively, the two canti. beams could have moment connections at the col., but then the balancing loads and stresses must pass through the col., and that and the detailing in general is just a mess, and the connections must be designed for the max. loads and stresses. A well proportioned sketch with dimensions, sizes, thicknesses, loads, etc., all the pertinent engineering design info. needed for the design and a meaningful discussion would really be helpful in defining your problem. I would want to do a FEA of the joint area for study of the stresses, and I would model it with 3D solid elements, and a fairly fine mesh in the immediate region of the joint. You will see pretty high stresses right in the region of the fillet weld btwn. the two pipes, specifically in the regions of 5-7 o’clock on the horiz. pipe.

Since this joint is now going to be cast, you can vary the thickness and dia. in the immediate region of the joint to accommodate the higher stresses there. You can also provide generous radii at the transitions btwn. the two pipes since these are high stress regions. Finally, at the ends of each of the legs you can cast a transition to a smaller dia. round pipe shape, with a bevel btwn. the two dias., with the smaller dia. fitting snuggly inside the pipe extension beyond and also acting as a back-up bar for your welding at these joints.

 

[BAretired]

dhengr's suggestion seems workable. The small hole for electrical conduit would have only a small effect on the strength of the horizontal member.

BA