Method of Analysis for Stud welded baseplate for Nonstructural Component Stand

[CaliEng]

Seeking a bit of guidance on how to analyze the shear capacity of a threaded stud used as a baseplate connection for leg of equipment stand. Concern is that seismic lateral forces may shear off at the shreaded stud section (example in 3rd photo). I have the entire structure modeled in SkyCiv (Structural analysis software) and the anchor bolts modeled in Hilit Profis. But I want to look at one specific foot, and the shear going into the 2" threaded stud. You can see a couple reference photos for context.
PROFIS Shows sufficient anchorage.

The Skyciv shows an exceedence in material yield for this member section (circular 6" HSS) to solid 2" section).
However would a simple shear calc be applicable? (i.e. T = F/A (40kN/2026 mm2)) shows only 20 MPa

FYI due to the exceedence from SkyCiv i was proposing a reinforcing clamshell (2nd photo) but if this is not necessary, then save money/time.

Thanks for the guidance!

 

[TLHS]

I don't understand the failure mechanism in that picture. It looks like there's dishing or something? How was the stud fastened down to the plate? Was the plate bolted down? What does the rest of the structure look like?

You're not shearing a two inch steel threaded shaft with actual certified material properties under 40kN of load. If you get a little bit on the long side, you could get a bending failure. Its not a great angle to tell, but I don't really see signs of a bending failure. It looks like whatever the fastening was just let go.

 

[CaliEng]

The 2" stud coming from the HSS is welded to the base plate. In this case the baseplate was not anchored to the slab, it typically is.


Upon reviewing the results of the SkyCiv analysis more closely, it appears that that it is a top bending stress within that stud section. Will such a small stud, I would assume this is not a concern

 

[CaliEng]

Update / Context:

Within on of the rear reduced sections there is a top bending shear just above the material yield (215 > 205 MPa). together with Axial, the Top Combined gets to 280 MPa
However when AISC member design checks are completed (AISC 360-16 LRFD) all members OK.
Design Check NG for AISC 360-16 ASD for these reduced sections.
As TLHS stated shear values are not much of a concern; however is the bending stress? Or due to its small size, no?
Appreciate the input & guidance.
Thanks,

 

[phamENG]

1 - what is your total base shear?

2 - what does SkyCiv say your maximum base shear is (max lateral reaction at a support), and at which support does it occur? (Point to it in your picture).

3 - Have you broken it out and chased the load path by hand?

SkyCiv is a great program for what it is. I use it, and enjoy it. But like all programs, it's not perfect - and neither are most of the people using it. Just looking at your renderings, I'm not sure you've modeled the transition from the HSS to your threaded rod properly. But, then again, I don't think you should model it. If you're going to run a model at all, I think you should model the HSS to a pinned joint, and then apply the joint loading to a hand calc of your connection.

Welding a threaded rod can be tricky. Be careful about material spec - not all threaded rod is weldable on a chemical level. You may consider having a partially threaded rod with enough blank material at the bottom to achieve a good fillet weld. Then run the threaded portion up into the leg.

 

[CaliEng]

PhamENG thanks for the reply

1- Total Base shear calculated to be 99 kN
2- at the support that is seeing the that highest bending, the Reaction is 17kN shear, 134kN vertical (point highlighted, along with location where point loads are applied for shear force)
3- I have not. I may need a bit of guidance on that as well to be honest. This may be a whole thread in itsself..

Re Skyciv, Yeah I like it. I can see that it is relatively rudimentary, but I am somewhat new to the structural world so it works great for me. Super intuitive and quick results.
Right, I just have an abrupt transition between the 6" HSS to 2" solid stud (actually modeled as a HSS with near full thickness, so that it can go through the AISC member design check).
Model the HSS all the way to the ground, or have the pinned joint at its actual location. Thats another thing I am looking for, is the actual method of analysis for this specific (unique?) connection.


From what I have seen this is fairly common, stainless steel through and through. But it may be that the portion nearest the baseplate is blank.

Thanks

 

[phamENG]

99/6=16.5, so 17 checks out. I'm with you there. Not sure what an engineer in California is doing using that darned metric system, though. (22.3kips and 3.82kips for the kids at home.)

This is actually a whole lot easier to chase by hand than trying to use a 3D model.

Start by looking at the base shear at the base of the worst case leg and design the weld of the plate to the leg for that shear load. Then look at the hole in the middle. I'd design that as a bearing condition for a pin in a hole and not a bolt. Bolt design calcs assume you have confinement of surrounding material from a tight bolt head/nut and that doesn't apply here.

Then you need to put the base shear load on the stud and base plate. Here you need to assume the base is fixed and apply the load from your leg to the cantilevered threaded rod. If you assume both are pins, your structure is unstable. This will mean moment in the weld from threaded rod to plate, bending in the plate, bearing on concrete, and tension in anchors. Take a look at AISC Design Guide 1 for assistance here.

I noticed the legs were awfully bright in that picture. When you say AISC, are you talking about the steel construction manual? If so, you're in the wrong place. That is not applicable stainless steels - at least not directly. AISC has another design guide (DG 27) that addresses the structural design of stainless steel. There are a lot of similarities but several fundamental differences, too.

 

[Blackstar123]

Concern is that seismic lateral forces may shear off at the shreaded stud section (example in 3rd photo).

To prevent this, you need to make sure the shear capacity of weld/connection is greater than the design shear force.

The 2" stud coming from the HSS is welded to the base plate. In this case the baseplate was not anchored to the slab, it typically is.

Base plate is not anchored to the slab? Then how was the base connection resisting the lateral force? You said that the welded connection between stud and base plate failed due to lateral loads right?

However when AISC member design checks are completed (AISC 360-16 LRFD) all members OK.
Design Check NG for AISC 360-16 ASD for these reduced sections.

Probably because flexure capacity as per ASD is limited to elastic limit state.

Right, I just have an abrupt transition between the 6" HSS to 2" solid stud (actually modeled as a HSS with near full thickness, so that it can go through the AISC member design check).

Your software is probably checking the design capacity of the stud as a HSS section with very thick walls as per AISC LRFD which I believe isn't the right way to design it.
However, just for the sake of true modeling and analysis of your actual condition, you should probably defined a solid circular section with right material properties and correct support conditions. Have you defined a moment release at the base of your actual HSS section?
Once you are sure of your analysis results, you could check the design of stud manually.

Having said this all, I'll too, stress the following point.

I think you should model the HSS to a pinned joint, and then apply the joint loading to a hand calc of your connection.

Seeking a bit of guidance on how to analyze the shear capacity of a threaded stud used as a baseplate connection for leg of equipment stand.

PhamEng has nailed this. I'll advice you to follow his guidance verbatim.