How do I calculate the damage on a lightning pole from a car impact?

[Dalit]

PCB, contactor, RCD...
It's a smart pole.
The Pole is wired to a main voltage source.

 

[nvn]

Dalit: If you want to include older vehicles having bare steel bumpers connected directly to solid steel supports, then it is acceptable, due to your stated objective and scenario, to assume the vehicle is a rigid body. Also, the maximum strength of your beam that we could hope for is if we pretend your beam cross section does not experience local buckling near the base (even though local buckling near the base is likely, but has not been checked).

For an impact at a height of 500 mm, and pretending no local buckling of your beam cross section occurs near the base, your beam can absorb a total strain energy of U = 2540 N*m before rupturing. And the beam inelastic deflection (at a height of 500 mm) is 34.9 mm when your beam ruptures.

Therefore, for vehicle mass m = 2500 kg, and impact velocity v1 = 2.78 m/s, the vehicle velocity after your cantilever ruptures is v2 = 2.39 m/s. The impact duration is roughly 13.5 ms. If the vehicle mass is instead m = 1000 kg, and v1 = 2.78 m/s, then v2 = 1.15 m/s. Either way, this analysis indicates your beam is annihilated.

 

[xxChrisxx]

That's a really good response nvn.

It there any possibility of you fitting the main controls into the base Dalit?

 

[Dr.D]

nvn, you said, "The impact duration is roughly 13.5 ms." How did you come up with this value?

 

[nvn]

Dr.D: As a rough approximation, deflection divided by average velocity gives (0.0349 m)/[0.5(2.78 + 2.39) m/s] = 13.5 ms.

 

[Dalit]

nvn,
I don't follow your calculations.
you said that "strain energy of U = 2540 N*m".
you need the force at impact, Please advise how do you calculate it.

you also mentioned that "the beam inelastic deflection (at a height of 500 mm) is 34.9 mm when your beam ruptures." How did you calculate that?

In the third section, your calculations are based on velocity at impact 2.78 m/s (that's given), you assume that velocity after imact is v2 = 2.39 m/s. how come?

 

[nvn]

Dalit: I don't have time to try to explain nonlinear analysis here on the forum. You will need to study your favorite mechanics of materials or strength of materials textbooks, for a long time. You also need to be a good programmer. Even then, it will be a gross approximation. To get more accurate results, you could run a simulation of the impact using very advanced software. Perhaps LS-Dyna would do, but it is probably quite expensive and time-consuming. I only have time to give the results I obtained for your beam, but not further explanation.

The force versus deflection curve I obtained for your beam was P(y) = (y<1.26)(43083*y + 599.4) + (y>1.26)[37787(y + 3.589)^0.2261], for y = 0 to 35 mm, where P = load (in units of N) applied to cantilever by vehicle at a height of 500 mm, and y = cantilever deflection (at a height of 500 mm) in units of mm. Integrate the area under this curve to obtain the beam strain energy, U, to the point of extreme fiber rupture (y = 35 mm). This is based on mild steel. Once you obtain U in N*mm, convert it to N*m. Furthermore, although I didn't do this, you can probably divide the above U by 0.70 or 0.60, or maybe even 0.55, to try to make U also account for energy lost to heat and other forms. After you obtain U, write a conservation of energy equation for the impact, and solve it for v2 to obtain the vehicle final velocity.

Similarly, here is the force versus deflection curve I obtained if you increase your beam wall thickness to 6.0 mm. P(y) = (y<1.26)(99388*y + 362.4) + (y>1.26)[83513(y + 4.434)^0.2357], for y = 0 to 36 mm, where y = 36 mm is the point of extreme fiber rupture.