A 1400 mm long galv steel beam, size 40x20x2. 80 mm of the beam on one end is welded in flat position to a platform. How much weight can I put safely on the other end? What if instead of being flat it will be on a short side/rib? The platform is meant to spin, will this effect the weight the beam can hold at the other end?

I am not an engineer and do not understand the formulas I found in other posts. This is for a home project.

  • $\begingroup$ Welcome to Engineering! I have rolled back your most recent edit, since it fundamentally changed the question and made the existing answer invalid. If you have a subsequent question on this topic, feel free to ask it as a new question and to add a link to this question if you believe it will be a relevant reference. $\endgroup$ – Wasabi May 22 '20 at 0:18

Assuming your platform is solid enough to fully support the moment created by the beam. we can consider your mechanism as a cantilever beam with a free length of $L=1400-80=1320\text{ mm}$.

For this beam $c = 40 / 2 = 20$.


$$\begin{align} F_y &=250\text{ MPa} \\ I &= I_{ext}-I_{int}=\dfrac{20\cdot40^3}{12}- \dfrac{18\cdot36^3}{12} = 44,458.6\text{ mm}^4 \\ M &= PL \\ \sigma &= \frac{Mc}{ I } =\frac{20M}{44,458.6} = 250\text{ MPa} \\ 20M &= 250\cdot44,458 \\ \therefore M &= 555,733.3\text{ Nm} \\ P &= M/1320=421N=421/9.8=42.9\text{ kg} \\ \end{align}$$

This is an un-factored stationary load. if we allow a factor of safety of 60% $$P=25.8\text{ kg}$$ If you need to turn the plate then you need to know the angular acceleration and calculate the angular moment of inertia and the lateral strength of the beam for that.

  • 1
    $\begingroup$ +1 Though I like being overly conservative, so I'd've used the full span ($L = 1400$), since we're dealing with DIY welding, we can't be sure the beam can be assumed fixed at the very start of the weld. $\endgroup$ – Wasabi May 21 '20 at 15:08
  • $\begingroup$ "we can't be sure the beam can be assumed fixed at the very start of the weld" what if I weld it to the platform + add a plate on top, thus joining everything together on the bottom and top. Is the danger that the weld joint will break off or the beam will bent? $\endgroup$ – anm767 May 21 '20 at 20:48
  • $\begingroup$ Weld material is designed to be stronger than the sections they weld. My answer is meant to give a way of starting estimate of strength. Otherwise one can test the weld and the base to come up with a rigorous answer. $\endgroup$ – kamran May 21 '20 at 21:53
  • $\begingroup$ Just looking this over - Is Fy a pressure, or a force? 250N, but MPa? If you are building playground equipment, you might look into some design guides: cpsc.gov/s3fs-public/325.pdf ; there's an official one ASTM F1487-17, that might talk about build details too. $\endgroup$ – asylumax May 22 '20 at 0:02

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.