I have been trying to find out the significance of the usual streetlight design - Why is there curvature of the arm holding the bulb of the light? I have been unable to any reason as to why it may be. Here is an picture of the "usual" of which I am referring to: enter image description here

  • $\begingroup$ to provide light? what are you asking? $\endgroup$
    – agentp
    Commented Mar 7, 2017 at 3:38
  • $\begingroup$ Sorry for being unclear. I mean why the curvature of the design from the stick holding the bulb of the light. $\endgroup$ Commented Mar 7, 2017 at 3:43
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    $\begingroup$ I don't know that is a specific reason, but one advantage of having the light bulb at the end of a curved arm is to increase its height above the traffic to give greater head clearance. Another possibility is visual appeal & esthetics - making things pretty $\endgroup$
    – Fred
    Commented Mar 7, 2017 at 13:07
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    $\begingroup$ There are many street lights that do not have a curved arm. In looking at the wide variation in street lights, it should be clear that most of the differences between designs is visual. $\endgroup$
    – hazzey
    Commented Mar 7, 2017 at 14:04

2 Answers 2


One possible reason is that it reduces the radius of the moment arm a bit. So, the downward force is the same, but the moment arm is smaller. The fixed joint will be under less stress.


remember, the weight acts vertically

so to find the moment arm we should draw a line perpendicular from the vertical pole and another line from the weight that goes straight up and down (incidentally parallel to the main pole)

where the two intersect, that's how we can find the length of the moment arm

Another clarification, my explanation assumes a shorter distance between the two

enter image description here

  • $\begingroup$ Could you clarify your answer? Reduces the moment arm compared to what? If you used a straight arm that had the same horizontal distance from the main pole, the moment arm from the center of mass to the joint would be the same. Based on what I think you're saying, I disagree with this answer. $\endgroup$ Commented Mar 7, 2017 at 14:49
  • $\begingroup$ the weight of the lamp doesn't operate perpendicular to the moment arm, it always operates down. $\endgroup$ Commented Mar 7, 2017 at 14:50
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    $\begingroup$ This still doesn't explain how the curved arm offers any advantage over a straight arm with the same horizontal distance (i.e. same moment arm) $\endgroup$ Commented Mar 7, 2017 at 14:59
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    $\begingroup$ Adding a curve does not decrease the required length "L". The horizontal distance between the pole and the light is specified for where the lightbulb is desired relative to the fixing point; adding curvature to the arm doesn't change this. $\endgroup$
    – AndyT
    Commented Mar 7, 2017 at 17:06
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    $\begingroup$ This answer is just plain wrong. The moment arm is the horizontal distance out from the lamp post, not the distance along the support structure. In other words, it's where the lamp ends up, not the path of the support to get it there. $\endgroup$ Commented Mar 7, 2017 at 19:03

okay heres a thought.

I have had similar thoughts about sign boards and their life. We know what happens when there is a cantilever beam. There is more stress at the ends (SFD) So think of the deflection to be maximum at the ends with respect to the original reference or starting point of the pipe.

If we keep a curved pipe, the deflection with respect to the starting point will be lesser than the first case.

The life and the amount of stress the pipe can handle would be more by using the curved pipe

Think of this example of a camber in rollers, we change the shape a bit to accommodate the pressure the rollers face.

Roll Bending


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