NCEES 2011 Sample Problems Afternoon Problem 513?

[Lucky1]

This problem involves determining the thickness of the footing required to support the tower crane. The solution references a book by Shapiro, Cranes and Derricks, 3rd ed., p. 361. The solution contains equations to solve for the overturning moment and stabilizing moment. This appears to be a very complicated statics problem. But not having the referenced book wanted to know if the moment equations used are from the book and typical for a tower crane (e.g. can they be used to plug and chug for a tower crane problem). This is the only problem like this I have encountered in the reams of material I have studied. Can one of our veterans shed some light on this? Thanks!

 

[treyjay]

the tower crane problems are not as hard as they look....you need to be able to determine every moment generated by the main crane components, the load and the foundation....all you have to remember is overturning moments vs. resisting moments. since these moments will have to sum to zero and there will be at least one moment containing unknown the footing thickness (usually denoted "t") you can solve for the footing thickness....easy.

it helps to convert the weights given in these problems to kips to keep the number of numerals to a minimum because a misplaced decimal or dropped digit is the hardest thing to keep straight with these problems.

I would not try to memorize a "plug & chug" formula for this because the crane configurations and the applied loads can vary a lot. you just have to learn to recognize what is a overturning moment & what is a resisting moment.

 

[Jayman_PE]

To add to treyjay's comments above, he's correct - these problems are very easy once you recognize where to sum the moments about. In almost all similar problems that I've worked the tipping moment is summed about the edge of footing, toward the load side. The resisting moment from the foundation is the foundation weight (unit weight x volume) x 1/2 foundation width (level arm). Once you know that you just have to be very careful getting your summed moment directions correct and probably reduce weights from lbs (or N) to kips (or KN, if metric, etc.) to reduce potential for errors later. I know I've dropped a digit and had screwy numbers before, only to find I dropped a 0.... Just have to be careful, yet hurry because you only have 6 minutes ....

 

[Lucky1]

You guys have quickly summed this up and made the entire concept very clear. Thank you for your help!