Simple Lift Fixture - Need Advice to Relieve Stress Concentration

[AFish66]

All,

Hi, I'm new to the forums and am seeking advice on a simple, counterbalanced C-Shaped lifting fixture. All structural members are made of 4"x4"x1/2" A500 square tubing, and the gussets are A36 steel plate. The fixture is used to reach into an enclosure to rig a 2000lb load to to remove. I have completed multiple hand calcs using ASME BTH-1 showing my design is within margin, but am having trouble with my ANSYS model. Per our more strict internal design standards, we are using a factor of safety of 5 per the Ultimate Tensile Strength, which for A500 tubing is 11,600psi. Per my ANSYS model results, I am being directed to relieve a stress concentration in my design before I am able to move forward with my project (see attached powerpoint). . Any suggestions on how to support this area to redistribute the stress?

The ANSYS model is simple and does include simplified 7/16" fillet welds in the gusset to structural member joints. Any advice is greatly appreciated!

 

[AFish66]

Sorry... did not post the FEA result image properly the first time...

 

[AFish66]

Also, specifically I am speaking about the lower gusset to the vertical member. Here's another image to show the area zoomed in.

I have exausted efforts in changing the gusset geometry, weld termination, etc.... Let me know your thoughts, thanks!!!

 

[rb1957]

looks like something between modelling artifice (= fiction) and highly localised (and not structurally significant) yielding.

i'd be interested to figure out why there's a low stress between the stress peaks ?

Quando Omni Flunkus Moritati

 

[inline6]

is this a fatigue load case?

if not, then you are picking up effects (real and numerical) that no hand calculation will ever do. not knowing more info absolute stress levels predicted, kind of load case it is, codes being used it impossible to say it can be ignored at this end

to avoid this you can move gussets inplane with the RHS walls so shear forces in webs of RHS go through gusset more directly in a flare bevel butt weld rather than lap weld.

 

[AFish66]

Thanks very much for the replies, they are very helpful! This is not a fatigue load case, only static. I will try as you suggested inline and see how that iteration turns out.

Also rb, it is localized, but I am still being asked to minimize the stress concentration area before the chief engineer will allow me to move forward since this is a lifting device. The low stress between the peaks may be due to the way I have the vertical and lower beams connected on a 45 degree mate as opposed to a butted joint.

Thanks again for the help!

 

[AFish66]

I ran another iteration of moving the gussets in line with the square tubing wall section as suggested, but still am not having any luck. Any other suggestions? I am going to increase my gusset plate thickness and weld sizes next to see what happens. Thanks!

 

[Kwan]

What is the max stress concentration that the chief engineer will accept? You will have a stress concentration which can not be entirely removed.
Anyway, I would guess the next step is to reshape the gussets such that they taper into the tubing. So the free edge will have a curved shape.
The as you lower the angle of intercept to the tubing the stress should decrease.

Another suggestion is to butt weld the gussets and add a welded doubler at the stress concentrations.

 

[AFish66]

I think he is just looking to minimize the stress concentration area... I have tried more iterations throughout today and have still not had any luck. I also tried adding doublers without success... I may try to complete something with a curved gusset next. Thanks again for the help.

 

[desertfox]

Hi

Not sure if you need to worry about stress concentrations if fatigue is not an issue.
I had a similar problem years ago and it was solved by welding the gussets on the outside faces of the rectangular tube and also putting a curve in the gusset

 

[corus]

If there's room then make a circular hole inside the gusset plate and make a continuous weld inside the hole, attaching the gusset plate to the outer plate.

 

[rb1957]

is there a problem with your mesh ... looking carefully at the 45deg block (the weld) the outer nodes don't appear to line up with the nodes on the gusset plate.

maybe adding welds to the vertical edges will help ?

how about continuing the sloping edge of the gusset along the vertical tube face, so you have a sloping load transfer into the gusset. this'll probably move the stress concentration peak to the upper corner (ie the first load transfer out of the tube).

Quando Omni Flunkus Moritati

 

[inline6]

unless you have a radius there to lower the SCF that blends smoothly then you will always have a "singularity" at the sharp corner and the stress will just vary depending on the mesh density. if you have a radius you are likely to find a converged answer (if you did a convergence study) that is a real stress. if it is not fatigue loading then it doesn't matter anyway as long as the materials are sufficiency ductile and good fracture toughness which it sounds like it is

there does look to be a funny mesh thing there as well.

 

[AFish66]

Thanks for all of the replies guys. I ended up having success with an odd curvature shaped gusset and changing the 4x4 square members to 6x2 rectangular members with a thinner wall section. I will post some pictures once I clean up my weld models and re-run the analysis.

 

[AFish66]

All,

Here are some images of the updated design that utilizes curved gussets. I was not succesful connecting the weld beads together in NX, and this is where the stress concentrations showed up. In reality however, this will not be the case and I am thinking that the concentration will be dissipated through a full welded joint. Thanks again for all of the help!

 

[AFish66]

Sorry about all of the issues with attachments... here are some additional images that show the results.

 

[strainstress]

A safety factor of 5 over ultimate is only required on primary membrane and bending stresses. What you are looking at and worried about are local, secondary membrane and bending stresses. Small amount of local yielding will redistribute the stresses without affecting the overall integrity of the structure.

As far as local secondary bending stresses are concerned you can take it upto yield stress.

Look at stress categorization in ASME pressure vessel codes. I know it's bot the exact same thing as a lift structure, but a metal is metal and behaves in the same way.

BTW just curious to know did you manufacture the curved gusset plates shown in last pictures ? What did you chief engineer say after looking at the curved gusset ?

 

[corus]

Personally I don't see the benefit as you're terminating the welds on the outer face of each member where the stresses are the highest. If you look at welds on JCB machines, for example, you'll see that the welds terminate at the mid-section of the member so as to reduce the nominal stress near to the weld stop/start position. In other cases they'll use plug welds on the centre of the section, again to reduce the stress close to the weld ends. These plug welds also reduce the effects of stop /start locations where fatigue damage often starts.

 

[AFish66]

Hi Strainstress, thank you for the feedback. I have not manufactured anything as of yet - I did however have the updated/curved gusset design reviewed with positive feedback and am going to move forward with this design. I think overall that the chief was wanting me to exercise some due diligence to minimize the stress concetration area as much as possible since this is my first structural type project. It's always fun being the new guy....

Corus - thank you for the advice, and this makes a lot of sense.. I will take this into account as well.

Thanks again everyone.. this forum has been a huge help!

 

[rb1957]

for my money, this shows an undue focus on stress concentrations, other than as a learning exercise.

stress concentrations that we know about are not a problem (eg, Learjet doors have sharp corners; they've decided to live with the stress concentration and the weight penalty and have a simpler structure).

of course, in this case, the cost of reducing the stress concentration is probably minimal but if the original design was acceptable (from a stress perspective) then why re-design ?

and of course, consider the source of the "stress concentration" ... ie a linear FEM ... this stress would not, would never, appear in the real world.

Quando Omni Flunkus Moritati