Questions and discussion for this lecture live here. Fire away by hitting Reply below 
Hi Sean,
Bit confused with the SFD for vertical members BE, BF, CG and CH, your drawing all of the loads on the same side of the neutral axis yet the reaction forces of each pair of columns are acting in opposite directions from each other, I’d expect to see the load data for CG and CH to be drawn on the opposite side of the NA, can you please clarify?
Regards,
Andy
Hey Andy,
I’m not sure I fully understand your question…do feel free to follow up.
But I’ll take a stab at trying to clarify anyway…
For the vertical members, if we consider each of the column segments individually, it’s easier to see how the Shear forces map onto the SFD. So, take column segment FB for example; if we consider an origin of our SFD axis for this member located at F with a vertical axis (along the member) that that is positive upwards to B and a perpendicular axis that is positive to the left, we can see that the shear force H_f has a magnitude of 3.8 kN, pointing to the right. So, it would appear as a negative 3.8, extending to the right of the column at F.
If you follow this convention for all vertical members, you can ‘follow the loads’ along each column and construct the shear force diagram quite easily - just like we do for horizontal members.
Use the same logic and look at column HC. At H, the shear H_c is 5.32 acting to the left, which on our previously defined axis system is positive. Then, we see that on the SFD, the SF extends to the left of the member (positive axis direction for this member).
Apply the same logic to the other column segments and you’ll be able to see how shear forces calculated for individual column segments map onto the overall shear force diagram.
As I said above, if you’re still not sure, let me know and I’ll help you get to the bottom of it.
S
Hi Sean,
Apologies, my original query isn’t the clearest.
It’s the fact that in lecture 10 the SFD for HC and CG is drawn on the RHS of the axis and called off as -5.32kN and -7.47kN, and as you say above they’re acting to the left so they should be drawn on the positive side of the axis system. Just need to make sure I’m understanding it correctly.
Andy
Ah yes - I see exactly what you mean! I was somewhat careless in drawing the SFD for H-C-G…it contradicts what I explained in my previous reply, so I can see why you were confused.
To clarify, what I explained above is correct and drawing the SFD the way I have in the video lecture is not consistent with that.
Now, you’ll note that I didn’t say it was incorrect! When it comes to shear force (unlike bending moment) the direction of the shear is not significant. Rather it’s only the magnitude we’re interested in since when we come to designing for shear, we don’t really care if the shear points from left-to-right or right-to-left! This is obviously not true when it comes to bending moment - here direction really matters because we need to know which side of the section is in tension.
All that said - we should still be consistent with our shear force diagrams and apply the same convention for shear sign/direction throughout the structure and not have one member be inconsistently signed as I have in the video!
Well spotted!
Seán
Cool, thought it best to query.
Bit of a side note, just coming to the end of the 1st reasonably consistent year studying your courses, it’s been a really enjoyable year, feel like I’ve developed a pretty solid understanding of structural analysis, (albeit the basics) and looking forward to making even more progress in 2025.
Thank you very much for creating this fantastic learning environment, hope you have great Christmas and all the best for 2025!
Andy
Thanks Andy! Great to hear that your finding my courses so helpful!
Keep it up.
Seán