IBC 2006 - Overturning question

[cessna98j]

It has come to our attention that the 2006 IBC now requires that you use only 2/3 of your dead load when calculating your overturning using ASD load combinations. Section 1605.3.2 states that "for load combinations that include the counteracting effects of dead and wind loads, only two thirds of the minimum dead load likely to be in place during a wind event shall be used". We do a lot of cement batch plants, with bins and silos on elevated legs - thus we can get some pretty high overturning forces. Since most of our stability is derived from the weight of the foundation, does this mean that our foundations will be 67% larger than they would have been if designed by the previous version of the IBC (2003)? What is everyone else's take on this, am I interpreting this correctly?

 

[JAE]

Take a look at this thread:

thread507-183966

 

[cessna98j]

JAE, thanks for the quick reply. I did read the thread that you mentioned, but I didn't find exactly the answer I was looking for. Typically we use the ASD alternative basic load combinations (1605.3.2) for design. In the 2003 IBC, you were allowed to use the full dead load of the structure (mostly the foundations in our case) for overturning. The 2006 IBC now states that you can only use two thirds of the dead load when checking wind loads, while using the exact same load cases. This just seems extreme to me, and will change our foundation design significantly if we are interpreting this correctly. Thanks again

 

[civilperson]

A ring footing with a continuous bottom,(like a bucket), will provide overturning resistance for large seismic and wind forces. Used for wind turbine foundations with the center filled with compacted soil.

 

[JAE]

In the 2000 and 2003 IBC this 2/3D was also present (see 1609.1.3)

 

[cessna98j]

Wow, I guess we never did catch that, but you're exactly right - thanks JAE