Vertical Seismic Load Effect (ASCE Ev) vs. Vertical Response Spectral Analysis 1

[Howlyn2]

Good Evening All. ASCE clearly outlines that a structure shall include both the effects of horizontal and vertical Seismic Load Effects. Depending on the Seismic Design Category and other variables such as frame construction, structural irregularities, height limits, etc. several different Permitted Analytical Procedures are allowed. ASCE, depending on the Seismic Design Category, also states that the horizontal loads shall be applied in each of the two orthogonal directions. That leaves one to wonder though how is the vertical seismic load effects to be considered when performing a Modal Response Spectrum Analysis? Should you just move forward with the vertical seismic load effect as per the code using the site's Sds values, or would it be more prudent to include the scaled spectral acceleration based on the structure's specific period and mass (found via a dynamic analysis)?

Code doesn't clearly outline whether the vertical seismic effect applies to one analytical procedure or another. Fema 451b did shed some light on the fact that the 0.2 second spectral acceleration is approximately 2.5 times the peak ground acceleration (PGA). Perhaps this is something the ASCE committee is comfortable with and allows as a simplification? Instead one could have been using the various spectral acceleration factors to create a spectral response spectrum to find a MCEr - Risk Targeted Maximum Considered Earthquake Ground Motion Acceleration. This would lead me to believe a scaling factor applied to the vertical seismic force would be appropriate. Could that be allowed?

Thank You for Your Input.

 

[JoshPlumSE]

Typically for building structures, you'll do an Response Spectrum Analysis (RSA) for both horizontal directions. But, you will merely use the 0.2Sds value on the dead load (positive or negative depending on the load factor for the DL).

However, I have seen people do an RSA analysis for all three directions before. Though, I think I've only seen this for nuclear work.

 

[HTURKAK]

Will you pls be more specific?? ..

The vertical seismic effects is considered for horizontal cantilevers for buildings SDC S,,E,F and for tanks and vessels ( Section 15.1.4 )..
Pls look again the 2.3.6 Basic Combinations with Seismic Load Effects (Comb. 6 and 7)

 

[Howlyn2]

Good Morning Team. Thank you for the responses. JoshPlumSE accurately summed up my main question. HTURKAK also provided some good insight regarding when the calculation of Ev can be overruled. C15.1.4 pointed me to 11.9 and ultimately Eqn (12.4-4b). This new insight prompts me back to my original and main question. When should we choose to use the response spectrum analysis instead of the 0.2Sds value for loads in the vertical direction? Reviewing C11.9 & Figure C11.9-1 it seems like depending on the structure's period would let us know what type of forces to expect. Specifically, I am interested in tower like structures with Vertical Structural Irregularities.

 

[HTURKAK]

I will suggest you to look again ;

- C11.9 VERTICAL GROUND MOTIONS FOR SEISMIC
DESIGN
- 15.1.4 Nonbuilding Structures Sensitive to Vertical Ground
Motions

As far as i understand ; the use of response spectrum analysis instead of the 0.2Sds value for loads in the vertical direction procedure is only required for three classes of nonbuilding structures..
according to Section 15.1.4 of the standard..( Tanks, vessels, hanging structures, and nonbuilding
structures incorporating horizontal cantilevers....)

My English is not great .. if you post ( if it is a real project ) some details , you may get better responds..

 

[KootK]

Most conventional buildings and tower structures will be very rigid vertically relative to their lateral stiffnesses. For that reason, I don't feel that a response spectrum analysis for vertical loads is prudent. Building vertical movement under excitation is more rigid body-ish than "whippy".