overturning load case in Canada for Seismic loads

[SKKP]

Hi all,

with reference to the below mentioned thread, canwesteng says "the overturning load case in Canada is 0.9D+1.4W which works out to a FOS of just over 1.5". Could anyone please let me know what is it for Seismic loads per NBCC 2015 or CSA A23 or any canadian codes. It just gives 1.0DL + 1.0E. I want to know the Factor of safety for Overturning when we have seismic forces. FOS= Resisting moment (1.0DL)/Overturning moment (1.0EQ) = 1.0. please suggest


thread507-463767

Thanks & Regards,

 

[human909]

Equivalent static forces that we used to represent seismic forces are just as the name says. The are not the actual imposed load on the structure. The imposed load in the structure occurs at the base and are not fundamentally overturning forces.

Can overturning result for seismic action. Yes I suppose it can, but such behaviour is well beyond the scope of equivalent static analysis.

 

[driftLimiter]

FOS against overturning can be expressed as the Overturning moment / Resisting Moment. Resisting moment is typically taken for the lowest value of Dead Load. The Load Combinations themselves don't suggest any level of Factor of Saftey. If you say bring the Overturning moment = Resisting moment then your factor of saftey is 1.0. Excess capacity in the resisting moment would lead to a FOS greater than 1.

 

[ggcdn]

This is further complicated by the fact that foundations need to be capacity designed for the seismic overstrength of the SFRS. I suppose there is going to be a 'FOS' close to 1.0 when using the probable resistance of the SFRS as the demand, and that will result in a 'FOS' somewhere around 1.0 < FOS < Rd*Ro when using the seismic demands from analysis as the point of comparison.

-JA
try [link calcs.app]Calcs.app[/url] and let me know what you think

 

[KootK]

In many respects, it is difficult to pin down a global seismic factor of safety. That, because common code based approaches fundamentally have us design for displacement capacity rather than load even though it doesn't feel that way when you're running the numbers. To the extent that there is a global seismic FOS, one option might be:

[Actual displacement capacity of the structure (real life) / Planned displacement capacity of the structure (Rd, Ro choices etc)]

I'm sure that somebody knows what we've targeted that ratio to be but that somebody is not me.

For the design of an individual element of the structure that is not specifically designed to dissipate energy, one way to think of FOS would be:

[Capacity Provided / MIN( Nominal yield capacity of energy dissipating system, Overstrength level demand).

I know... it's rough. This article describes this a bit in the US context where foundations are, generally, not required to be designed for Overstrength demand:

https://www.skghoshassociates.com/blog/foundation-overstrength-factor/

 

[SKKP]

Thank you all for your replies and information.