Crush strength calc for short steel member
Crush strength calc for short steel member
(OP)
Our customer has asked for an axial crush/column strength calc for a short steel tube we are developing for them. The tubing is 1026 DOM with an OD of 0.945" and ID of 0.708" and is 2.028" long. They want to know the load at which 3% yield occurs. Here's what I've used:
total elong. = 3% x 2.028" = 0.0608"
F = unknown
l = original length = 2.028"
E = 30,000,000 psi
A = 0.308sq in
total elong. = Fl/AE
Solve for F I get 277,000 lbs. That seems high to me, but I checked that the length is short enough to use a purely compression calculation. I just need a simple calc model for this. Have I totally missed something or am I on the right track?
total elong. = 3% x 2.028" = 0.0608"
F = unknown
l = original length = 2.028"
E = 30,000,000 psi
A = 0.308sq in
total elong. = Fl/AE
Solve for F I get 277,000 lbs. That seems high to me, but I checked that the length is short enough to use a purely compression calculation. I just need a simple calc model for this. Have I totally missed something or am I on the right track?





RE: Crush strength calc for short steel member
You question is difficult to understand. I believe you are not on the right track.
1) FL/AE will calculate an elastic deflection, assuming the material remains elastic. This is unrelated to yielding.
2) 3% yield is a term I do not understand. Does your client require the load at which the total strain (elastic + plastic) is 3%? Or the load at which the plastic strain alone is 3%?
3) Usually, a crush strength involves the member shortening, so I'd stay away from terms like "elongation". I know it sounds picky, but we engineers are a picky bunch...
tg
RE: Crush strength calc for short steel member
RE: Crush strength calc for short steel member
RE: Crush strength calc for short steel member
RE: Crush strength calc for short steel member
CoryPad, why is this not valid for steel in the elastic range?
I don't believe this is a buckling problem since the slenderness ratio is so small.
If they want to know the load at which the plastic strain is 3%. How do I calculate that?
RE: Crush strength calc for short steel member
To determine plasticity, you need a method to measure strain independent of the load train. This is done with strain gages, extensometers, or video methods, among others.
RE: Crush strength calc for short steel member
Being a short member, forget bending.
RE: Crush strength calc for short steel member
Why doe the customer require this calculation would it not be easier to supply a material certificate with guarenteed minimum tensile strength an yield on it.
desertfox
RE: Crush strength calc for short steel member
This is a buckling issue...
RE: Crush strength calc for short steel member
RE: Crush strength calc for short steel member
kl/r is less than 15 so buckling not an issue.
AMMOYER...I approached from different direction and got your same answer...277k. Does seem extraordinarily high. I can't see how you could mobilize that kind of load on such a small tube....except in a testing machine.
I think the magnitude is so far off because we are assuming the stress-strain relationship is the same for tension and compression in the steel...Not so. I'm not sure what the compression modulus is for your material, but it might behave more like a secant modulus rather than a tangent modulus. That could change the answer by a factor of maybe 10?
You might pursue the variation in tension-compression moduli to solve this.
RE: Crush strength calc for short steel member
The numbers sound high but, assuming a 50ksi yield strength, your tensile yield load would be about 162 kip and, if it doesn't buckle locally, it should hold more in compression.
RE: Crush strength calc for short steel member
This request is vague.
1. Load at 3% of yield strength of the metal?
2. Load at 3% of yield strain of the metal?
3. Load at the metal when it reaches 3% strain?
(1) does not make sense (a straight cal).
(2) what type/grade is the metal?
(3) agree with others, the strain is beyond elastic range. Unless the grade (fy) is greater than 870 ksi (29000*3%).
RE: Crush strength calc for short steel member
Is there a simple way to estimate the load at the 3% strain in compression? I understand that steel behaves roughly the same in tension and compression below the yield point, but what about above there? The actual failure mode or failure point is not critical unless it will help me determine the load at 3%.
I appreciate all the input.
RE: Crush strength calc for short steel member
RE: Crush strength calc for short steel member
Why does the customer want to know a load beyond the elastic limit?
desertfox
RE: Crush strength calc for short steel member
RE: Crush strength calc for short steel member
RE: Crush strength calc for short steel member
AMMoyer made an error with the units in his last post: the load (force) is ~ 22,000 pounds for compressing this tube/sleeve to a plastic strain of 3% using 1026 steel. This is very close to the value obtained using the approach described by kslee of just calculating the load based on a stress at yield (perfectly plastic behavior): 18,500 pounds for a yield strength of 60ksi; 21,500 for 70ksi yield; etc.
AMMoyer,
Yes, there are methods for estimating the performance in compression. I will have to look through my files to find some. Usually this type of behavior is confirmed through part testing, so even if you have to start by machining samples, you could relatively quickly get some data.