g factor for vertical seismic of ASCE code

Hi All,

I know that CAESAR-II has function that it can automatically calculate the static seismic g's factor based on ASCE code. It is shown seismic g's factor for horizontal and vertical direction.
In my knowledge, g's factor for horizontal direction is calculated as per para. 13.3.1 of ASCE code.
But, I don't know calculation method for vertical g's factor. Could someone inform me the basis or document for calculation method for vertical seismic that it is using in CAESAR?

Thank you.

Taking the lead from ASCE 7, the vertical static seismic component would be 20 percent of the short term (0.2 sec) response to the ground motion. This value - 0.2*SDS - is used in the ASCE 7 load set considering response to seismic load. This is what we produce in the static seismic wizard.
Note that, currently, the wizard will also adjust the g loads by the 0.7 factor to account for the differences between the load resistance factor design method of ASCE 7 and the allowable stress design method of the piping codes.

Regarding the source of vertical is 2/3 horizontal. That approach was common in older seismic design criteria. I do not see that too much any more.

In NEHRP Provisions there is an interesting commentary that gives a reconciliation between 0.2 and 2/3 coefficients.

The title is NEHRP Recommended Seismic Provisions for New Buildings and Other Structures. The last edition is commonly referred as 2015 NEHRP Provisions/ or FEMA P-1050-1 and can be found on internet as fema_p-1050-1.pdf

"
Although no specific rationale or logic is provided in editions of the NEHRP provisions (FEMA 2009a) on how the value of 0.2SDS was determined, one possible way to rationalize the selection of the 0.2SDS value is to recognize that it is equivalent to (2/3)(0.3)SDS, where the 2/3 factor represents the often-assumed ratio between the vertical and horizontal components of motion, and the 0.3 factor represents the 30% in the 100% to 30% orthogonal load combination rule used for horizontal motions.

Although details regarding defining vertical ground motion spectra are currently well known, the committee elected not to define a vertical ground motion spectra in this standard because the approach provided by the equivalent static coefficient 0.2SDS is adequate. For situations where one wishes to explicitly include the vertical component of ground motion in design analysis, one may use the vertical ground motion spectra definition that is provided in the “New Chapter 23, Vertical Ground Motions for Seismic Design” in the 2009 edition of the NEHRP provisions (FEMA 2009a).
"

BR

Dear Dave Diehl and Mariog,

Thank you very much!!

The 100-30-30 rule (originally 100-40-40 rule) is considered as a reasonable method for spatial response combination replacing the- more accurate- SRSS method.

Giving credit to the explanations of "Provisions", it appears that a vertical seismic load of 0.2*SDS wouldn't be applied in a SRSS method, neither be further amended by coefficients as 0.3 or 0.4 in combinations.

As a suggestion, I think it is worth also to implement in Caesar the vertical ground motion spectra definition that is given in the Chapter 23, Vertical Ground Motions for Seismic Design” starting with 2009 edition of the NEHRP provisions. That one may be considered directly in SRSS or "equivalent" combinations, instead using the commonly ratio of V/H = 2/3 in engineering applications

Dave Diehl


According to what you mention if I carry out in cases of static load the combinations for earthquake in operation as follows:

P + T + W + U1
P + T + W-U1
P + T + W + U2
P + T + W-U2
P + T + W + U3
P + T + W-U3

Is the system already multiplied by the factor 0.7 to adjust to the ASD?

Or should I perform the following combinations?

P + T + W + 0.7U1
P + T + W-0.7U1
P + T + W + 0.7U2
P + T + W-0.7U2
P + T + W + 0.7U3
P + T + W-0.7U3

I thank you for your prompt reply.

In the later versions of c2 (Version 11 and later) you need to apply the multiplier for both wind and seismic loads. For seismic this would be 0.7*U and for wind it would be 0.6WIN.

For Versions prior to Version 11, consult the documentation.

Richard, thank you very much for the resolution, but based on what was commented previously in this thread, the factor for vertical load is mentioned, based on this, the load combinations should also consider the effect of the vertical load?
The combinations should be like this?

P + T + W + 0.7U1 + 0.46U2 + 0.21U3
P + T + W-0.7U1 + 0.46U2 + 0.21U3
P + T + W + 0.7U3 + 0.46U2 + 0.21U1
P + T + W-0.7U3 + 0.46U2 + 0.21U1

I ask you because in a project we need to compare the SAP loads of a cable-stayed bridge that supports our pipeline with the Caesar II loads, and in the Civil load combinations do they consider the 3 directions in the same state of load? or is it not advisable to carry out the seismic analysis in this way?

We also load the earthquake spectrum that we have for this project in the dynamic module of Caesar II but the loads differ twice and three times with respect to the static one.

I appreciate your comments on the subject.

For calculations under ASCE 7, CII User Guide mentions now
W+T1+P1+0.7U1 as operating case with seismic load for computing stress (which is as ASD method, Allowable Stress Design, that compares actual and allowable stress) and
W+T1+P1+U1 as operating case with seismic load for computing loads on supports and deflections (based on this case you can isolate seismic loads transmitted to structure for a LFRD calculation of your structural assembly (Loads and Resistance Factor Design, which provides the actual response of structural system to be compared with ultimate strength design).
So U or 0.7U (as horizontal seismic) and other combinations with vertical- it depends on your applicable Code (is ASCE 7 - which consider a MRI 2475 years as seismic definition/ or maybe other Code) and your scope (want to make an ASD analysis on your piping or want to collaborate with structural for their LFRD analysis).