Stress exerted on pipe column face by base plate stiffener???
Stress exerted on pipe column face by base plate stiffener???
(OP)
Hi
I was looking for a reference to determine the Stress exerted on column face by base plate stiffener, the base plate stiffener that I'm talking about is similar to this:
http://www.fhwa.dot.gov/bridge/images/sign36.jpg
I have only found an equation in "Design of Monopole bases" page 77. But the result is relatively small around 30 N/mm^2.
The problem is that when comparing the hand calculation with the Ansys FE model I'm getting larger stresses and ends up with the upper edge of the stiffener connected to column face is overstressed as well as the column surface at that point.
For the constrains in the Ansys I have tried both cases of making stiffener connected to the column as an assembly and then put "Bonded" contact constraint and tried to make the column and stiffeners as one part then connect them to the base plate, but in both cases I'm getting what is stated above.
So either the hand calculation is not conservative, or the Ansys analysis is not giving accurate values.
Any suggestions?
Thanks
I was looking for a reference to determine the Stress exerted on column face by base plate stiffener, the base plate stiffener that I'm talking about is similar to this:
http://www.fhwa.dot.gov/bridge/images/sign36.jpg
I have only found an equation in "Design of Monopole bases" page 77. But the result is relatively small around 30 N/mm^2.
The problem is that when comparing the hand calculation with the Ansys FE model I'm getting larger stresses and ends up with the upper edge of the stiffener connected to column face is overstressed as well as the column surface at that point.
For the constrains in the Ansys I have tried both cases of making stiffener connected to the column as an assembly and then put "Bonded" contact constraint and tried to make the column and stiffeners as one part then connect them to the base plate, but in both cases I'm getting what is stated above.
So either the hand calculation is not conservative, or the Ansys analysis is not giving accurate values.
Any suggestions?
Thanks






RE: Stress exerted on pipe column face by base plate stiffener???
Tara
RE: Stress exerted on pipe column face by base plate stiffener???
I have found small but important error in my hand calculation, and now I'm getting higher stress values than before.
Regarding to what you mentioned "and remove that peak stress component to give you the nominal stresses in that region."
I'm dealing with static load.
How should I remove these peaks?
There was a suggestion regarding the over stressing points - by a colleague in this forum - to use a stiffening ring at the place under consideration, it worked some times but didn't solve the problem in others.
The only solution was to use stiffening pipe extending from the base plate to few centimeters above the top of stiffeners. but that will end up with really thick section (45 mm) while the column it self should be only 27 mm to resist the applied loads.
Regards
RE: Stress exerted on pipe column face by base plate stiffener???
If the stress at the end of these stiffeners is still of concern then you could put an annular plate around the column, welded to the top of the stiffeners. like so
! !
_______! !
!_______! !
/ ! !
/ ! !
/ ! !
/__________!_!
This will take some of the peak stress out but you'll still have high stresses due to the sudden change in section. I'm not sure what another pipe over the stiffeners would help as you'd still have the peak stress from the edge of the stiffener plate acting on to the tube. It might reduce the nominal stress as you've increased the overall bending stiffness, but how do you access the holding down bolts with a pipe blocking them?
Tara
RE: Stress exerted on pipe column face by base plate stiffener???
RE: Stress exerted on pipe column face by base plate stiffener???
Just curious...in the photo, why the large bolt adjacent to the stiffener, then another smaller bolt which looks to be centred between the stiffeners?
RE: Stress exerted on pipe column face by base plate stiffener???
For an axial loaded base plate (Compression) I'm getting almost the same results in hand calculation and Ansys, but for the same base plate if the load is (Tension) the maximum principle stress will go up to 441 MPa (N/mm^2), however, the Von-Mises stress is almost exactly the same for the case of compression =340 MPa.
Here is a screen shot for the Maximum principle stress in MPa or N/mm^2
h
And I can't find any valid reason for this other than that the effect of deformation in Tension causes the top edges of the stiffeners to be overstressed!!!
If that is right, so I can't get these results for tension by hand?
@ hokie66
Maybe these are combination of holding down (anchor) bolts and ordinary bolts.
Thanks again for your valuable tips
regards
RE: Stress exerted on pipe column face by base plate stiffener???
The results from your loading may depend on the boundary conditions you've imposed. I would guess that in compression the whole of the base plate is restrained axially, whereas in tension only the holding down bolts provide restraint. This would give different results.
Tara
RE: Stress exerted on pipe column face by base plate stiffener???
I thought of adding fillet radius between them but I felt that it may not be correct to include the fillet weld material to the stiffeners.
I will try to add fillet radius to the stiffeners edges equivalent to the fillet weld dimension!!!
I have tried to add fillet weld but it only makes the situation more complex and more uncertainties comes up.
RE: Stress exerted on pipe column face by base plate stiffener???
Tara
RE: Stress exerted on pipe column face by base plate stiffener???
If it is just for axial force then blodgett design of tubular structures gives some furmula for the induced ring bending. They also give good formula on the annular ring mentioned above.
RE: Stress exerted on pipe column face by base plate stiffener???
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Stress exerted on pipe column face by base plate stiffener???
Dear csd72
These are results for the case of tension applied to the column, I did that for the sake of verifying the manual solution with the FE because it is easier than the one with moment.
Thanks for the reference, if you have a link where I can get it as a pdf I will appreciate it because I couldn't find any link.
Dear paddingtongreen
I don't have it but (I'm using fcu=40 N/mm^2), I have assigned the bottom surface of the plate to be compression only support (To allow for the upward movement which will only be resisted by the fixed washers) also I used the general support and made necessary settings to make it similar to compression only but with planar constraints.
Also regarding the Elastic support, I have noticed that it restraint the upward movement!! Should it?
regards
RE: Stress exerted on pipe column face by base plate stiffener???
Even though it is related to geometry, but it gives the feeling that the structure is overstressed :]
RE: Stress exerted on pipe column face by base plate stiffener???
It would be interesting to see your hand calcs. and exactly how you treated this connection in that analysis. Why do you suppose the stiffeners are wide at their base and taper to almost nothing up at their top? Might you reduce your FEA stress concentration by shaping the stiffeners differently that a straight slope to 1" wide at the top? Should you weld up the sides of the stiffener, and then across the top of the stiffener (its thickness) or just up the sides of the stiffener? What sort of stress raisers and stress concentrations have you introduced by you welding details and weld starts and stops? These are probably more important, as failure mechanisms than the stress you think you are seeing, however you model it. If you could model them, every weld start and stop or imperfection will cause the same kind of stress concentration you think you are seeing, and many of these will be at a point where the steel has been hardened by the welding process. Of course, you will see very high stresses where ever you have stresses flowing around a reentrant corner, like at the top of the stiffener, as Corus suggests @ 14FEB, 8:38.
RE: "How should I remove these peaks?" 14FEB11, 4:15: Maybe what you should do is, once the structure is all welded up, take a 1" core drill and centered on that high stress point, drill right through the stiffener and pipe wall, leaving a 1" hole with zero stress. But, be careful not to leave that piece of high stress inside the pipe.
RE: Stress exerted on pipe column face by base plate stiffener???
BA
RE: Stress exerted on pipe column face by base plate stiffener???
RE: Stress exerted on pipe column face by base plate stiffener???
That's just a fit-up night mare without much real improvement or advantage, as I see it. With more than four stiffeners, their fit-up on the interior is a mess, and what is their function on the interior other than dead weight, the base plate loads are on the exterior, from the A.B's. Then their good tight fit-up in the slots in the pipe wall becomes a real tolerancing problem for a good weld at the root. You've lost the continuous weld btwn. the pipe and the base plate and traded that for a discontinuous weld with starts and stops at the highest stress location. Finally, you don't really improve the weld termination up at the top of the stiffener. There's still a 50/50 chance that the welder will weld around the top of the stiffener and leave notches on the edges of the stiffener or else leave a water catching opening.
It seems to me you have to make a decision about where you want the continuity, and I would prefer it btwn. the pipe and the base plate. The stiffeners then distribute the concentrated A.B. loads sufficiently up into the pipe. One would hope that the stiffeners and the A.B's. are symmetrical w.r.t. each other. And, a bending moment and the potential of fatigue certainly complicate our design worries, but the stresses don't change much one way or the other.
RE: Stress exerted on pipe column face by base plate stiffener???
Don't be so humble, you do too know how to interpret FEA results, and you just proved it. You know enough to ignore the pretty little red spots (you've probably already had chicken pox too, but have you had your 'shingles shot'?) and wonder how the hell a stress field can just jump from one magnitude to another without any apparent inducement. Having said that... Usually you can fix that dumb green stress band (a lower stress field) within a higher stress field (the yellow) by trading one indeterminate boundary condition or restraint for another indeterminate boundary condition or restraint. We're never quite sure which ones are the right ones anyway. The scary thing is that when I increase the size of his link so I can read it, the size of the high stressed area gets larger too, and I'm wondering if the whole darn thing will become over stressed at some point. Alternatively, you can just photoshop the picture to change the colors to your liking.
RE: Stress exerted on pipe column face by base plate stiffener???
My suggestion of stiffeners through the column was based on the original photo, which only showed four stiffeners, I think. That's not a difficult fabrication.
RE: Stress exerted on pipe column face by base plate stiffener???
I was thinking the same thing, but we were not raised on these programs. I had difficulty in the nuclear power business making the models run without these nonsensical results. I knew the structure was good when I created it, but I had to make the model results agree or my design was condemned.
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Stress exerted on pipe column face by base plate stiffener???
Continuing the stiffeners through the HSS reduces bending stress on the wall to nearly zero. No need for ring thickening or other expensive methods.
BA
RE: Stress exerted on pipe column face by base plate stiffener???
Dear dhengr
I enjoyed reading your comment, it looks like you do like FE :) (Just kidding).
Well, I used Ansys for such non-complicated design considering the values provided by the software to be something close to what would happen in real experimental test (which is not always possible to be done). Also I wanted to verify the accuracy of the results because I have other cases with moments.
I'm concerned about the stresses although there are several factors of safety in both materials and loads because having overstressed structure could cause the structure to collapse as stress is just another way to interpret the applied loads. What is the use of the structure if the base goes in failure!
Regarding the shape of the stiffener, there is a valid point to have such shape, as if you use for example a rectangular stiffener, the extra part other than shown in the picture will have almost 0 stress and I have tried several sizes and arrangements.
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Dear BAretired and hokie66
I have tried to do such arrangement and it does reduce the stresses for the continuous stiffeners but others won't have much reduction.
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Dear hokie66
If there were no stiffeners there would be green then yellow at the top, but due to stress concentration at the top end of the stiffeners a yellow zone will appear as shown then it will become green when the influence of the stiffeners reduces then it will be yellow again as it is approaching to the end of the column section in the model.
Thanks every body, I have an idea maybe it will solve the concentration problem, will be back soon...
Regards