Bending Strength of a deformed section

[ajc2014]

I work in an industrial plant and I get a lot of small questions to support other projects, usually their pretty easy. This one made me think twice.

A blind flange on a davit port to a tank is suspended from a 1 1/2" sch40 pipe bent to a 90 deg angle. I know a bent shape should keep a strength of Mp until failure, but does that apply at a bend of 90 deg? wont their be a significant change in area?

In this situation the loading is small (250#) but this could fall on a worker. The dims of the arm are 24" high reaching out 14" with a bend radius of 6".

Thanks in advance for any help.

Adam

 

[AELLC]

What is the "theoretical", service level, actual bending stress based on that load and eccentricity?

 

[prex]

Yes, the strength of the bent pipe is (possibly much) smaller than the straight one. A piping code (e.g. B31.1, B31.3) has formulae for the so called 'stress intensification factor' (that here has nothing to do with the fatigue strength). It will depend on the thickness to diameter ratio and on the bend radius to diameter ratio.

prex
[URL unfurl="true"]http://www.xcalcs.com[/url] : Online engineering calculations
[URL unfurl="true"]http://www.megamag.it[/url] : Magnetic brakes and launchers for fun rides
[URL unfurl="true"]http://www.levitans.com[/url] : Air bearing pads

 

[ajc2014]

I started calcs using Mu = (lrfd load comb factor)*P*L=1.6*.25*14= 5.6 K"
aisc has an Mp = 1.11 K' = 13.32 K" for an undeformed 1 1/2" std pipe in tbl 3-15 e13
-but I wasn't sure if this applies since it's been plastically deformed.

 

[AELLC]

I don't know the allowable stress for bent pipe. I just wanted to see the relative magnitude of stress.

Apparently bent pipe has reduced strength properties due to the severe bend, and the failure mode may be vastly different compared to a straight pipe loaded to failure. Buckling mode etc.

 

[ajc2014]

Thanks for your help!

I found an eq on pg 40 of B31.1 - this checks out. But it says that this is for
"elbows, miter bends, and full size outlet branch connections"
which are (or made of) hot rolled sections. Correct me if I'm wrong.

 

[AELLC]

I have a gut feeling this will deflect significantly before buckling, so without any legal calculation of allowable stress, I think it is nevertheless OK for 250 lbs.

The kids over in the FEA corner of the industry would perhaps have fun with this.

 

[ajc2014]

I should also add that, when it it checks- the moment strength is approximately the same for the straight section. Mu = 0.292K' (same calc as above but ASD) vs Mu= 0.256 K' ASME method. 12% change.

Adam

 

[dhengr]

Ajc2014:
You say... “A blind flange on a davit port to a tank is suspended from a 1 1/2" sch40 pipe bent to a 90 deg angle. I know a bent shape should keep a strength of Mp until failure, but does that apply at a bend of 90 deg? wont their be a significant change in area?” Whatever that means? I wonder how many hundreds of free body diagrams could be drawn which fit that verbal description. Each one, of course, causing slightly different stress configurations and considerations. Generally speaking, you should take into account and changes in thickness and shape which may have occurred due to the pipe bending process, they change the mechanical properties of the section. Also, the bending process causes yielding in some areas of the pipe shape. Instead of thinking in terms of Mp whatever that really means to different people, look up Bauschinger Effect. You have strained the material in the bending process, or you are now working the material further up the stress/strain curve. With loading or reloading the steel will still follow essentially the same “E” slope on reloading, and if you increase the loading it will move further up the stress/strain curve, still following the same “E” slope, until it reaches failure at Fu. Today, Mp is kinda a nebulous thing with all the multiplying and reducing factors which have been introduced to loads, strengths, probabilities, etc. Today, with Mp we are taking about the section being able to tolerate some plastic deformation without failing drastically, but that is probably not your situation.

 

[MikeHalloran]

Most field bending methods will leave a 4D bend pretty distorted, with the outer wall thinned and probably flattened, and the diameter at tne neutral surface expanded.
You might want to measure the bend and estimate the actual section properties there.




Mike Halloran
Pembroke Pines, FL, USA