## ASCE 113 Vertical Seismic Forces

## ASCE 113 Vertical Seismic Forces

(OP)

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.

## RE: ASCE 113 Vertical Seismic Forces

## RE: ASCE 113 Vertical Seismic Forces

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.

## RE: ASCE 113 Vertical Seismic Forces

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

## RE: ASCE 113 Vertical Seismic Forces

## RE: ASCE 113 Vertical Seismic Forces

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

## RE: ASCE 113 Vertical Seismic Forces

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.