Differences between calculation and reality 3

[m.piron]

Good morning.
I have the structure in picture. The material of the beam is steel S235.

The real F is 350N, but I continuosly put and remove the load, so I take into account 2*F (found by match the elastic and potential energy, 0.5*k*x^2 = m*g*x).
I found a stress of 453 MPa, so this beam couldn't carry the load.

My question is, I'm using this beam every day, and it's not broken, nor bend.
Why it resist?
What is wrong in my calculation?

Thanks

 

[BAretired]

Plastic Modulus Z = (d[sub]e[/sub][sup]3[/sup] - d[sub]i[/sub][sup]3[/sup])/6 = 650.7mm[sup]3[/sup]
Assumed Fy = 235MPa
Mu = Fy.Z = 235*650.7 = 152,907N-mm

If the impact factor is 2.0, then 2.0F*300 = 700*300 = 210,000N-mm or about 1.37Mu
For impact factor to be 2.0. the loads would need to be simultaneously released onto the beam. That would not be advisable.

It may be that the loads were placed individually and released slowly onto the beam. It is also possible that Fy > 235MPa.





BA

 

[kingnero]

Let me guess, you weigh (less than) 140 kg and you work out.
Your arms dampen the weight application. If this is the case, there certainly isn't 2x your weight applied.
If you weigh 140 kg, the distance between your shoulders is more than 400 mm. I'm guessing you spread your hands further apart, this also contributes to a lower bending moment.

 

[Tomfh]

Impact factor can be anything. 2x is just a code cookbook factor that’s generally conservative. You likely won’t yield the structure.

Regarding bodily impact forces - When you run you apply more than 2x body weight to your knees (don’t I know about this).

And if you fall from height onto a rigid surface you will experience peak impact forces vastly in excess of 2x your body weight.

Human body can handle brief impact of a few tens of g’s. But once you’re up near 100 g’s it is immediately fatal. This is why people die in car accidents. They simply come to a stop too quickly.