Simple Lift Fixture - Need Advice to Relieve Stress Concentration
Simple Lift Fixture - Need Advice to Relieve Stress Concentration
(OP)
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!
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!





RE: Simple Lift Fixture - Need Advice to Relieve Stress Concentration
RE: Simple Lift Fixture - Need Advice to Relieve Stress Concentration
I have exausted efforts in changing the gusset geometry, weld termination, etc.... Let me know your thoughts, thanks!!!
RE: Simple Lift Fixture - Need Advice to Relieve Stress Concentration
i'd be interested to figure out why there's a low stress between the stress peaks ?
Quando Omni Flunkus Moritati
RE: Simple Lift Fixture - Need Advice to Relieve Stress Concentration
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.
RE: Simple Lift Fixture - Need Advice to Relieve Stress Concentration
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!
RE: Simple Lift Fixture - Need Advice to Relieve Stress Concentration
RE: Simple Lift Fixture - Need Advice to Relieve Stress Concentration
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.
RE: Simple Lift Fixture - Need Advice to Relieve Stress Concentration
RE: Simple Lift Fixture - Need Advice to Relieve Stress Concentration
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
RE: Simple Lift Fixture - Need Advice to Relieve Stress Concentration
RE: Simple Lift Fixture - Need Advice to Relieve Stress Concentration
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
RE: Simple Lift Fixture - Need Advice to Relieve Stress Concentration
there does look to be a funny mesh thing there as well.
RE: Simple Lift Fixture - Need Advice to Relieve Stress Concentration
RE: Simple Lift Fixture - Need Advice to Relieve Stress Concentration
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!
RE: Simple Lift Fixture - Need Advice to Relieve Stress Concentration
RE: Simple Lift Fixture - Need Advice to Relieve Stress Concentration
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 ?
RE: Simple Lift Fixture - Need Advice to Relieve Stress Concentration
RE: Simple Lift Fixture - Need Advice to Relieve Stress Concentration
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!
RE: Simple Lift Fixture - Need Advice to Relieve Stress Concentration
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