NCEES 2008 Structural PM problem #519

[PEin2010]

Where does the equation used for bearing pressure on the rectangular footing, p max = (2/3)*(Ptot / (y * footing width)) come from???? The answer using this is 2.7. y = resultant location.

This example is for eccentric loading on a rectangular foundation. I looked up the relevant chapter in CERM and there is an equation for p max, p max =(P/BL)*(1 + 6* eccentricity/B). If I apply this, the answer is close, 2.36, yet not quite the same.

Any guidance is appreciated!

Thanks.

 

[Ble_PE]

The equation you used to calculate 2.36 is only for footings where the resultant falls inside the middle third of the footing (i.e. e <= L/6). Since e>L/6 you have to use the equation qmax = (2/3)*(P/Be'). Here e'=(L/2-e)=y. Note that e is the eccentricity wrt the center of the footing.

This is included in Chap 2 of the SERM if you have that.

Good Luck!

 

[STEEL MAN]

agree, there are 3 cases for this eccentric load on footings and SERM clearly explains that, I wonder why CERM didnt show these types of cases.

 

[dastuff]

It's probably considered too advanced a topic to be included into the CERM (you can't have it all)

 

[PEin2010]

The equation you used to calculate 2.36 is only for footings where the resultant falls inside the middle third of the footing (i.e. e <= L/6). Since e>L/6 you have to use the equation qmax = (2/3)*(P/Be'). Here e'=(L/2-e)=y. Note that e is the eccentricity wrt the center of the footing.
This is included in Chap 2 of the SERM if you have that.

Good Luck!

YAY! I get an answer of 2.91 with that ...which is fairly close to the answer

The way they are calculating y in the book is, y = resultant = (Resisting moment - Overturning moment)/P total ---This is why the answer is different. However, I like your method way better! Why to calculate moments when you don't really need to?

Thanks!!!!

 

[playboyman007]

agree, there are 3 cases for this eccentric load on footings and SERM clearly explains that, I wonder why CERM didnt show these types of cases.

Yeah, CERM is better suited for the morning breadth.