Questions and discussion for this lecture live here. Fire away by hitting Reply below 
Hello Sean,
In the following equation:
basePushPull = -baseMomentOverturning / buildingWidth
Why do we add the “-” (negative) sign on the right-hand side of the equation?
Additionally, in these calculations:
F_down = round(max((0.5selfWeight + basePushPull)/1000), 2)
F_up = round(min((0.5selfWeight + basePushPull)/1000), 2)
How do we determine that a positive value of basePushPull corresponds to a compressive force? What is the reasoning behind this sign convention?
Thank you!
Hey @mrgurer - the negative sign just indicates that the moment generated by the push-pull force times the lever arm is opposing the overturning moment. In truth, of more interest in this calculation of basePushPull is the absolute magbitude - the inclusion of the minus sign is not strictly necessary.
For your second question - you’ve raised a really good point. The only logic at play here is that the compression force F_down is the result of adding the max positive push-pull force to the self-weight and F_up is the difference - obtained here by adding the max negative value of basePushPull to the self-weight.
An alternative (and I think more robust) approach would be to simply determine the maximum absolute value of basePushPull…
maxAbsPushPull = round(max(abs(basePushPull)/1000),2)
…and combine it with the self-weight as outlined above…
F_down = round(maxAbsPushPull + (0.5*selfWeight)/1000 , 2)
F_up = round(maxAbsPushPull - (0.5*selfWeight)/1000 , 2)
I hope that helps - thanks for making me think a little more on this!
Seán