Equivalent Static Method vs. Simplified Method in NBCC 2015

[alannakate]

Hello,

I'm struggling to figure out the simplified method vs the equivalent static method in the seismic portion of the 2015 National Building Code of Canada.

The seismic hazard index is listed as IeFsSa(0.2) and IeFsSa(2.0) (Cl. 4.1.8.1 (2)). If these values are less than 0.13 and 0.03 then building is in the low seismic zone and the simplified method can be used.

Conversely, if IeFaSa(0.2) (NOT IeFsSa(0.2)) is less than 0.35, the equivalent static method can be used. I ran through a basic example (a 3 storey, braced steel frame, normal importance) using both methods and found the simplified method to give a significantly more conservative shear force than the equivalent static method. (The CISC does a similar example here:

https://www.cisc-icca.ca/nbc-2015-simplified-force-procedure-for-low-seismicity/).

My question is, why has the simplified method been added? If your building fits in the low seismic category then in many cases (at least where I live in Halifax) it also qualifies for the equivalent static method. Why would I choose to go with the simplified method over the equivalent static method if both are applicable and one leads to a significantly more conservative lateral bracing design and thus more steel, ect?

Thanks for any insight on this.

 

[KootK]

Try not to hate me for being trite but I suspect that they added the simplified method... because it's simpler? My understanding is that the method was intended to cover all of the structures that would have received no seismic design under previous NBCC versions. Given that those buildngs would have, in many cases, been designed by engineers less familiar seismic design, I think the intent was to give those folk a way to ease into seismic design gently. You know, less consideration of torsion, irregularities, etc.

But yeah, in some cases, there could be some money saved in construction costs by NOT using the simplified methods.

 

[alannakate]

That was the answer I got from some other engineers at my company as well. Glad to get another opinion that because it's easier is really the only reason to use one over the other.

 

[dik]

KootK: "But yeah, in some cases, there could be some money saved in construction costs by NOT using the simplified methods."

...but, a cost increase over not having to address the issue in the first place... in particular areas that have little or no seismic activity... like Manitoba.

Dik

 

[skeletron]

@dik: I believe the issue reason for the mandatory seismic check, for all areas, was a design philosophy shift that emphasized ductility rather than strength.