The vertical bending capacity of a twin UB laced strut can be calculated by $M_c = p_y \cdot 2Z_x$. Where $Z_x$ is the section modulus of a single UB section about its major axis.

For lateral bending capacity, can we calculate this by using $I_c = 2I_y + 2As^2/4$, where $s$ is the separation distance between centroid of the two UBs, and $Z_c = I_c/e$, where $e$ is the distance from centre of laced strut to extreme fibre?

Would the built-up member have a lower bending capacity due to the two beams being laced together at fixed intervals instead of being joined continuously together via welding?

In summary, can we design laced strut for significant lateral bending instead of mainly axial compression?


Yes, so long as $s$ is the distance between the centroid of one section and the centroid of the combined section, and $e$ is the distance from the centroid of the combined section to the extreme lateral fiber.

As to the comparison of laced vs. continuously-welded sections, you'll need to check the relevant code in your region. That being said, the codes I'm aware of have conditions which, if met, allow the beams to be considered as working perfectly in unison, in which case there is no loss of strength.

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  • $\begingroup$ Can you add in your justification? Thanks! $\endgroup$ – Jennifer Jun 20 '17 at 7:20
  • $\begingroup$ @Jennifer I don't know what you mean. My justification is the codes. So long as your laced beams are up to code, they can be considered as working perfectly in unison. I've edited my answer a tiny bit, but just to clarify a few points. $\endgroup$ – Wasabi Jun 20 '17 at 14:07
  • $\begingroup$ Can you quote the relevant section in the code that you used which says the above? $\endgroup$ – Jennifer Jun 21 '17 at 10:21

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