ASCE 113 Vertical Seismic Forces

[martintstruct]

Can anyone explain to me how vertical forces due to seismic loading are accounted for in the load combination in this publication? I am familiar with the IBC provisions of 0.2*Sds*D and applying this term with the dead load factor. However, since the load combinations in this ASCE publications are different, and there is no mention of how to apply vertical seismic loads, I am not sure the correct way to apply the seismic load when modeling substation structures use this publication.

 

[JAE]

I don't have the 113 publication - can you provide a sample of their load combinations so we can see how they are formatted?

 

[martintstruct]

Case 1: 1.1D + 1.2W*Ifw + 0.75SC + 1.1Tw
Case 2: 1.1D + 1.2IwIfi + 1.2WiIfiw + 0.75SC + 1.1Tw
Case 3: 1.1D + 1.0SC + 1.1Tw
Case 4: 1.1D + 1.25E*Ife + 0.75SC + 1.1Tw

D = structure and wire dead load
W = extreme wind load
Wi = wind load in combination with ice
Iw = ice load in combination with wind
E = earthquake load
Tw = horizontal wire tensions for the appropriate wind and temperature condition
SC = short-circuit load
If = importance factors (Ifw, Ifi, Ifwi, and Ife)

E is defined as being calculated by equation 3-10:

Fe = (Sa/R)W(Ife)(Imv)

Fe = seismic design force, lateral force applied at the center of gravity of the structure or component
R = structure response modification factor
Ife = importance factor for earthquake loads
W = dead load (including all rigidly attached equipment and 50% of the weight of the attached wire)
Sa = design spectral response acceleration
Imv = 1.0 for dominant single mode behavior or 1.5 when multiple vibration modes are consider by the designer

W is defined as being calculated by equation 3-1:

F = Q * kz * V^2 * Grf * Cf * A

F = wind force in direction of wind (lb,N)
Q = are density factor, default values = 0.00256
kz = terrain exposure coefficient
V = basic wind speed, 3-s gust wind speed (mph)
Grf = gust response factor (for structure and wire)
Cf = force coefficient
A = projected wind surface area normal to the direction of wind (ft^2)

Most of the factors for the equations are given in the various sections and tables of the publication. However, there is no mention of how the vertical seismic forces are factored into the load cases. There is also a quote that says:

"The vertical ground acceleration used in combination with the horizontal base shear should be 80% of the design horizontal ground acceleration. Friction forces due to the gravity loads shall not be considered to provide resistance to seismic forces."

To my understanding, this is not useful since its talking about ground accelerations and not spectral accelerations.

 

[JAE]

So what you are showing above is that there is no reference to a 0.2SDS(D) type of vertical seismic correct?

If so, it may be that this particular code doesn't have a vertical seismic component.

 

[martintstruct]

Does that mean that I shouldn't use this publication for a high seismic region?

 

[JAE]

Well, it depends on whether the publication is applicable I would guess. The vertical seismic component was a "new" thing added I think with the 1997 UBC. So if your 113 document is older than that it just may not have been a part of seismic design procedures.

If it is more recent, I'm surprised it isn't included in the language of the code.

 

[martintstruct]

After doing some test, depending on the geometry of some substation structures and the equipment mounted to them, the publications doesn't always give me higher base reactions when compared to following the vertical seismic design of ASCE-7. Another factor that makes ASCE-7 combinations give higher loading sometimes is the overstrength factor that is used with the lateral seismic load. I have seen other calculations using ASCE-7 using table 15.4.2 taking R=2 and omega=2 under "All steel and reinforced concrete distributed mass cantilever structures not otherwise covered herein including stacks, chimneys, silos, skirt-supported vertical vessels and single pedestal or skirt suppored."

For a simple fixed-free column, it is evident that using an overstrength factor of 2.0 and a coefficient of 1.0 for seismic in the load combinations given in ASCE-7 Section 12.4.3.2 will produce higher base moments than using no overstrength factor and a coefficient of 1.25 for seismic in the load combinations given in ASCE #113.

This issue mostly bothers me for base plate and anchor design.