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Re: 97 UBC 2213.8.5 - mechanical penthouses=stories? braces have str ength f...

[Subject Prev][Subject Next][Thread Prev][Thread Next] Mark,

Here are some of my thoughts.

The seismic force is OMEGAo*Eh and not divided by 1.4 even when designing for ASD.   Page 89 of the SEAOC Seismic Design Manual, Volume 1 talks about checking for "Design Strength" since it is believed to be more realistic than the arbitrary ASD force levels.  You can use the appropriate adjustment factors when checking the adequacy of the member (1.7 increase for ASD and resistance factor Phi = 1.0).

I would try and use and satisfy the AISC Seismic Provisions for Buildings where possible in addition to the 1997 UBC since the 1997 UBC does not consider the resistance factors (phi) which are included in the AISC Seismic Provisions (for both LRFD and ASD).  I believe this was an oversight in the 1997 UBC.

Whether a mechanical penthouse is a story or not I think depends upon the size of the penthouse and materials used in construction of the penthouse.  If the footprint of the mechanical penthouse is equal to 75% of the building base footprint, then it probably should be considered a story.  If the footprint of the mechanical penthouse is less than 25% of the building footprint, than it probably doesn't need to be considered as a story (but make sure to include the mechanical penthouse mass with the roof mass for seismic analysis).  Between 25% and 75%, then it is probably left up to engineering judgment. 

Types of construction can come into play.  Example: If the main building is cast-in-place concrete (floors and roof), and the construction of the mechanical penthouse roof is steel beams and bare steel deck (with no topping slab), the penthouse is rather lightweight compared to the rest of the building.  Is it reasonable when doing the vertical distribution of seismic forces acting on the building to consider the penthouse as a story level with the maximum story force occurring at this level? or does the penthouse go along for the ride with the main roof structure during an Earthquake. 

In past design for taller buildings, I believe it was common design practice to just lump-sum the penthouse roof mass with the main building mass and disregard the penthouse as a story level if there were significant differences in mass (bare steel deck roof versus cast-place concrete floor).  In making the engineering decision, if the mass of the penthouse is comparable to the mass of the floor/roof below, then the penthouse should probably be considered as a story.  If the mass is small, then it can probably be neglected as a story level (still include the penthouse mass with the roof level).

If the penthouse is neglected as a story level, what force should be used to design the penthouse level?  If the penthouse is small in area or of little mass, then should probably use  (Fp = 4.0 Ca Ip Wp) or the more refined version; equation 32-2.  If the
mechanical penthouse foot print covers more than 50% of the building footprint, then maybe you could use the building base shear equations and treat it like a one story building.  Again, an area for engineering judgment.  If I was using Fp, I don't think I would be amplifying the design force by Omega (I would use Fp and not (Omega)Fp).

I believe the brace frame exception for mechanical penthouses design is intended for when you don't consider the mechanical penthouse roof as the uppermost story level of the building. Therefore the exception can apply to small mechanical penthouses on the roofs of both lowrise and highrise buildings.  If you did consider the mechanical penthouse roof as a story level, then the two story exception would still be acceptable only if the building was two story in height including the mechanical penthouse level.

Hope this helps,

Michael Cochran, S.E.

In a message dated 2/12/2002 5:04:43 PM Pacific Standard Time, HaanSM(--nospam--at) writes:


For an ordinary braced frame building where the braces do not meet the b/t
or KL/r or <70% tension or compression,  97 UBC 2213.8.5 says "Braced frames
not meeting the requirements of Sections 2213.8.2 and 2213.8.4 may be used
in buildings not over two stories in height and in roof structures as
defined in Chapter 15 if the braces have the strength to resist OMEGAo times
the design seismic forces."

These are ASD provisions,  does this section mean the braces need to have
the strength for OMEGAo*Eh/1.4 or OMEGAo*Eh?    
Are mechanical penthouses considered stories for the purposes of this
section?  The section says and roof structures defined in Chapter 15 - what
is this saying - anyone know a code section?


Scott M Haan P.E.
Plan Review Engineer
Building Safety Division
Development Services Department
Municipality of Anchorage