How would you model this? And thoughts on lack of diagonal braces?
How would you model this? And thoughts on lack of diagonal braces?
(OP)
Hi everyone,
I just want to get your thoughts on how you would model this structure (having some differing opinions within office). Basically, the tank on top is being increased to hold more water and I am tasked with making sure the stand can support it. It is made of 2x2x1/4" mild steel square tubing. It has angle iron bracing at mid-height roughly welded all around. The top (triangle portion) of the stand is also made of the same square tubing welded all around. The bottoms are welded all around to a square plate and bolted with 4 bolts at each corner. See attached pic. Two of the legs are nearly vertical and the third is angled due to space constraints.
(PIC REMOVED)
I will be modelling in STAAD (not a fancy Finite Element) and have limited budget to complete this analysis. Thoughts on setting up model? What supports to use, etc? Also, what are you thoughts on the lack of a diagonal brace connecting the top to mid point to bottom. Looks like a bit of torsion would twist the stand pretty easily?
Thanks
I just want to get your thoughts on how you would model this structure (having some differing opinions within office). Basically, the tank on top is being increased to hold more water and I am tasked with making sure the stand can support it. It is made of 2x2x1/4" mild steel square tubing. It has angle iron bracing at mid-height roughly welded all around. The top (triangle portion) of the stand is also made of the same square tubing welded all around. The bottoms are welded all around to a square plate and bolted with 4 bolts at each corner. See attached pic. Two of the legs are nearly vertical and the third is angled due to space constraints.
(PIC REMOVED)
I will be modelling in STAAD (not a fancy Finite Element) and have limited budget to complete this analysis. Thoughts on setting up model? What supports to use, etc? Also, what are you thoughts on the lack of a diagonal brace connecting the top to mid point to bottom. Looks like a bit of torsion would twist the stand pretty easily?
Thanks






RE: How would you model this? And thoughts on lack of diagonal braces?
This looks like an industrial setting, my guess would be it was made to fit, not engineered. I also think it will fail under current conditions and structural models. It is unstable with all pin connections, but you don't have moment connections because connecting elements were just assembled not designed. Adding some bracing will stabilize it, but can those 2" columns handle the axial load, and when at an angle? My guess is no.
Can you brace it to the catwalk behind? and not overload the catwalk?
It sounds like your budget isn't enough to stick your neck out. I'd say the owner will be better off paying for a proper engineered new support for their new tank.
RE: How would you model this? And thoughts on lack of diagonal braces?
RE: How would you model this? And thoughts on lack of diagonal braces?
how much does it deflect if you shake it ?
is anyone (else, like the plant manager) concerned about it ?
why do analysis now ? (after the horse has bolted ?)
another day in paradise, or is paradise one day closer ?
RE: How would you model this? And thoughts on lack of diagonal braces?
Seemed pretty sturdy when I tried shaking it... but I'm a small guy :P
Stand is brand new. When they went to commission it, they were told the tank needed to accommodate a 4' water column. Thus the concern to quickly retrofit the tank so that it can be commissioned and be up and running. Plant manager is concerned about the extra weight.
RE: How would you model this? And thoughts on lack of diagonal braces?
that's a lot of water, a lot of weight !
the plant manager is right to be concerned.
add diagonal bracing, explain if you have to, that the load has tripled.
another day in paradise, or is paradise one day closer ?
RE: How would you model this? And thoughts on lack of diagonal braces?
Probably pins everywhere but continuous columns. Maybe some fixity in the frame at the top depending on what you see up there. I agree with the sensitivity to torsional buckling. I imagine that you'll want to lace it up with bracing. With the right comfiguration though, I suspect the columns could be made to work. A rebuild may indeed be less work though.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: How would you model this? And thoughts on lack of diagonal braces?
RE: How would you model this? And thoughts on lack of diagonal braces?
as a model, simple rods ? could use beams if you're confident about the moment connection between members.
another day in paradise, or is paradise one day closer ?
RE: How would you model this? And thoughts on lack of diagonal braces?
RE: How would you model this? And thoughts on lack of diagonal braces?
I've gone through this routine with a few Winnipeg manufacturing clients and they always end up seeing the light once I give them a basic hand sketch showing how much bracing would be required.
Approximate the weight required to be supported by a single column, determine the allowable unbraced length of the column (accounting for the fact that it's not overly concentric loading), then provide bracing to meet those requirements.
RE: How would you model this? And thoughts on lack of diagonal braces?
Too often we assume e=0" and it will be stable under gravity fine (look it is standing now!) :) I respect the why model answer, i do the same too often :)
RE: How would you model this? And thoughts on lack of diagonal braces?
-Total factored axial load in legs = 6 kN (1350 lbf)
-Bending moments in legs found to be negligible
-Longest piece is 105" long (between mid-point bracing and top)
-K = 1.0 for pin-pin
-KL/r = 146 < 200 so OK
-Checking axial resistance assuming an Fy of 160 to be conservative (who knows where they got their material), I get a resistance of 71 kN which is well beyond the applied
-Given the dimensions, this is a class 1 section.
So far, it looks pretty good. EngineeringEric, that is definitely my next step. I will apply some eccentric loading to the top and see what sort of numbers I get. Nonetheless, I think I'd like to add additional bracing though...
RE: How would you model this? And thoughts on lack of diagonal braces?
RE: How would you model this? And thoughts on lack of diagonal braces?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: How would you model this? And thoughts on lack of diagonal braces?
I would treat the triple column as unbraced in torsion except at the base where each column should be pinned to a solid foundation. That means adding diagonal braces on three sides above and below the middle brace.
BA
RE: How would you model this? And thoughts on lack of diagonal braces?
I'd take a look at sloshing with something this flexible. (Definitely a P-Delta run.) It's one of those where the pipe (going into it) might stabilize it.
RE: How would you model this? And thoughts on lack of diagonal braces?
WARose, not too concerned about sloshing. It is a distributor tank meant to eliminate sloshing the way the inside compartments are designed. I have played around with calling the bracing pin or fixed and added the notional loads to the top (0.005*gravity loads). I've been super conservative and still seem to have decent capacity... I'm going to suggest more bracing though for peace of mind
RE: How would you model this? And thoughts on lack of diagonal braces?
RE: How would you model this? And thoughts on lack of diagonal braces?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: How would you model this? And thoughts on lack of diagonal braces?
BA
RE: How would you model this? And thoughts on lack of diagonal braces?
RE: How would you model this? And thoughts on lack of diagonal braces?
RE: How would you model this? And thoughts on lack of diagonal braces?
Dik
RE: How would you model this? And thoughts on lack of diagonal braces?
Yeah, except that the more complicated you make it [for example, using hss rather than angles as KootK suggested] the more work and cost and not to mention complexity per connection. As KootK also said, the more exaggerated the brace angle the less effective it will be, i would rather add lattice to make sure that the individual tubes are stiffened correctly.
RE: How would you model this? And thoughts on lack of diagonal braces?
Yeah, that's a thing. I'm sure the verticals were not oriented to suit easy trussing. And they're four sided things in a three sided system. One option could be to cope the outstanding angle legs so that the remaining leg forms a tab that can be bent to suit. Another could be to run horizontals inside the legs and then diagonals outside the horizontals. This is a small load thing so I wouldn't sweat the implied connection eccentricities.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.