AISC Seismic Provisions 2

[MichaelP1011]

Do the AISC Seismic Provisions allow an Ordinary Moment Frame of steel to be taller than one story?

The building in question is in Charleston, SC and is a two story commercial/office use building with every beam to column connection Fully Restrained as part of the Lateral (Seismic) Force Resisting System.

The Fabricator says it must be a Special Moment Frame, but cannot provide a rationale other than the AISC provisions, but we can't find any stipulations in the Provisions either.

 

[SlideRuleEra]

The second paragraph of C11.1, on page 170 of "AISC Seismic Provisions - 2005"

http://www.aisc.org/Content/Content...uctural_Steel_Buildings_incl__Supp__No__1.htm

(free .pdf download)

refers to "...siginicant height and other limitations..." placed on ordinary moment frames by ASCE 7-02.

I have not followed this lead to ASCE 7-02, but since Charleston has stringent seismic design criteria, the fabricator is probably correct.

Also, IBC 2003 may have language on this issue.

http://www.SlideRuleEra.net

 

[sundale]

Read IBC Table 1617.6.2. OMRF are not permitted in SDC D or worse, with two caveats (exceptions) in footnotes "h" and "i".

Two story would violate footnote "h". An office building would likely have floor dead loads and roof dead loads bigger than 15 psf and thus violate footnote "i".

You probably need SMRF's if you are SDC D is or worse.

Out of curiosity, does wind or seismic govern? Charlestown, SC is unique in that it has seismic and hurricane issues.

 

[SlideRuleEra]

sundale - Wind or seismic govern in Charleston? As usual, it depends on the size and geometry of the structure AND the precise location.

The worst seismic is about 25 miles inland - dropping as you approach the city (near the coastline).

The worst wind is on the coast, dropping as you go inland.

With so many variables, you have to put the ASCE 7 maps "under a magnifying glass" (almost literally) to find what conditions apply.

For waterfront structures you also have the possibility of major ship collisions. This whole combination was quite a consideration for the recently completed cable stay bridge

http://www.cooperriverbridge.org/

http://www.SlideRuleEra.net

 

[sundale]

Thanks SlideRuleEra. I was just curious. Things look fairly serious from a lateral perspective over there.

I agree that the maps they give you are typically far too small a scale to make any interpolation meaningful.

It would be nice if they gave you more printed maps at a smaller scale or a web site or something like that. I did find a website once that had Ss and S1 by zip code, but it said it was "not approved for design purposes"...

The contour lines are pretty close for the accelerations and wind speeds there.

 

[mrMikee]

Try these maps at USGS. They are fairly high resolution and make nice D-size prints. You should also try using the siesmic lookup by zipcode site which gives seismic data to a pretty small grid. The zipcode data should be compared to the map however because I've heard there are a few errors.

For IBC2000/2003:

Map for spectral response acceleration of 0.2 sec:

http://eqdesign.cr.usgs.gov/design/ibc/IBC1615-1us.pdf

For 1.0 sec:

http://eqdesign.cr.usgs.gov/design/ibc/IBC1615-2us.pdf

For seismic lookup by zipcode for IBC2000/2003:

http://eqint.cr.usgs.gov/eq/html/zipcode.html

For IBC2006:

http://eqint.cr.usgs.gov/eq/html/lookup-2002-interp.html

Regards,
-Mike

 

[sundale]

Thanks Mike. Those are nice. When I get some time, I'll goof around with trying to print the maps on the plotter.

Thanks for the warning about checking the zip code accelerations.

 

[SlideRuleEra]

Good links, mrMikee! I have the software versions of IBC 2003 and ASCE 7-02, and "blow-up" the included .pdf versions of these maps with Adobe Reader... BUT these same maps at this USGS website are much higher quality (easier to read with bifocals)

http://www.SlideRuleEra.net