Structure Height hn

[SteelPE]

ASCE 7-10 has a definition for structure height hn of "The vertical distance from the base to the highest level of seismic force resisting system of the structure. For pitched or sloped roofs, the structural height is from the base to the average height of the roof."

I have always taken this value from grade to the roof. On a structure that is on a sloping site with the following floor and grade elevations:

Below grade basement elevation = 0'-0"
Floor 1 back grade = 12'-0"
Floor 2 front grade = 24'-0"

Where would you start this elevation from, 24'-0"?

 

[Agent666]

I'd typically say its wherever your seismic load is coming out of the structure and resisted by the ground. Or alternatively where is the loads imparted form the ground coming into the structure.

This article may help from structuremag

 

[ggcdn]

The commentary of ASCE 7 has a lengthy discussion of the seismic base, including sloping site scenarios. It's well worth a read.

 

[KootK]

What's the nature of your superstructure? Often, if it's a system where the above grade structure would be much more flexible than the below grade structure, I use the two stage procedure. It tends to make for friendlier superstructure loads and is a much better reflection of reality in my opinion.

 

[SteelPE]

KootK,

This doesn't have anything to do with a structure I am responsible for. I was asked to look at a building for a client. The building was a 8 story (above grade) building with 1 story on a sloping grade and 1 story below grade (10 overall). Overall building height was 140' from basement to penthouse.

We both found out soon after that the building had already been designed by another engineer (he had bought the project and documents from a developer). The project was outside of what I was comfortable with so I rejected the project before it even ended up in my email (Client = "Can you design an 8 story building for me" Steelpe= "nope"). However, when it did show up I spent 30 min going through the drawings. One item that stuck out to me was the use of R=3 for the LFRS. The local jurisdiction places limits on when you can use this type of system (I think they cap it at 100' for SDC=B and 65' for SDC=C). So this structure appears to be in violation of this code requirement.

I was just wondering if they were cheating the elevation of the ground level to make hn work (I still don't think this would work as the penthouse is 110' above upper ground level). The EOR is from out of state and might not have even looked at the IBC overwrites from the jurisdiction.

I always wished I cold do projects like this, but unfortunately my background has always been 60' and less in height.

I have never done a two stage analysis, didn't even know that existed. However, it appears to be used for system with mixed building materials (wood over concrete or wood over steel). This project was all steel but the foundations.