NCEES 2008 Problem #3

[xsciencex]

I think I might need a crash coarse in basic statics, but I simply can't wrap my mind around how the the pull-out force (T) was determined by dividing the Moment exerted by the wind force on the top of the frame by 30 feet (which I assume is the width of the frame). Can someone explain this to me: if given a moment (in this case 3000 kip-ft) which I assume is exerted on the pedestal bases closest to the the wind force, how would I convert this to a pull-out force experienced by the same pedestals. Since the base is 30 feet square, I am unable to determine which direction was chosen to obtain the moment arm. There is something very basic here that I am not grasping...

Thanks for you help

 

[bootlegend]

I don't have the problem in front of me, but in general all four pedestals will have vertical load (plus or minus) due to the moment. They form a force couple for the wind load and are added into the gravity load reactions.

Sum moments about the pedestal farthest from the wind and you'll be able to solve directly for the pullout force.

 

[pmblair]

The Overturning moment generates a Resistance couple in the pedestals.

Tension equals compression times the couple arm which is the spacing of the pedestals. Two pedestals are in compression and two are in tension.