huangyhg

ibc 2000 - seismic-force-resisting system
has anyone used option #7 for a seismic-force-resisting system (structural steel sytems not specificaly detailed for seismic resistance)? i am designing a single story office building and i don't particularly want to mess with seismic detailing.
this item (7) is usually used in very low seismic regions. what area is your project in?
the project is in oklahoma city with ss=0.342g and s1=0.085. soil site class = c, which results in seismic design category = b.
well, you'd have to weigh the relative costs of the detailing in seismic, using higher r values, verses the sledgehammer approach with an r=3 (lots higher seismic forces)..
simply compare the r=3 seismic forces with your wind loads to see if there is a significant difference. if the r=3 loads are comparable to wind - that would probably be the way to go.
go with r=3!
using under ibc chapter 22, using r=3 allows you to by-pass the aisc seismic provisions. using any greater value for r would mandate that you conform to the aisc seismic provisions even though your project is sdc = b.
we have done the comparison for braced frames using r=3 and no seismic detailing or r=5 for ocbf's (if allowed) and r=6 for scbf.
mrstohler - i read through the referenced thread and agree that for special moment frames the more complex pushover or inelastic analyses are probably required to determine the "max. force delivered by the system". however, for more simple structures, such as an ordinary braced frame, with a metal deck diaphragm, usually the max. force delivered to the system is either the overturning force on the footings or the failure of the metal deck in shear.
these make for a more simple determination of the max. force delivered vs. a moment frame.
also, the decision to go with r=3 or a higher r isn't always just a slam dunk decision - it really needs to be compared to the wind forces that otherwise might control. in a 90 mph vs. a 120 mph wind zone, the r=3 vs. r=higher could be affected.
thanks for the thread reference.
jae - your suggestions are what i initially thought as well until i asked aisc. true the foundations might control the force that can be delivered to the system if a spread footing is used. this does not apply to other foundation systems.
the more complicated analysis not only also applies to moment frames but braced frames as well. consider the aisc definitions of ocbf and scbf both require the structure to withstand some inelastic deformations, in a brace this means full yield.
please refer to the following:
aisc seismic provisions 2002 sections 13 and 14 (including commentary)
鈥淒esign of special concentrically braced frames鈥? structural steel education council steel tips may 2004
鈥淪eismic behavior and design of special concentrically braced frames鈥? aisc journal 2001 3rd qtr.
鈥淪eismic design and steel connection detailing鈥? 2003 nascc proceedings
鈥淎 commentary on seismic provisions for special concentrically braced frames and special truss moment frames鈥? 2003 aisc proceedings
also typeiv is in okc, wind speed = 90 , this matches the work our firm did to compare r=3 vs. r=5 and r=6. it was no contest, every aspect (connection material quantity, weld volume, bolt count, etc.) of the connection cost increased when a higher r value was used.
but isn't a roof diaphragm also a part of the "system"? i would think so. even if the inelastic brace force is much higher, the least amount of load delivered into the brace would be the deck tearing away. it is the weak link in the chain of the load path.
the metal deck does not fail inelastically (as far as i understand it) and thus it would apply...not the inelastic brace force or a force from a higher order analysis that would not consider the deck.
what do you think?
jae- the concept of using "the maximum force, indicated by analysis, that can be transferred to the brace by the system" only applies to scbf, r=6 (ocbf must be designed for yield of the brace).
as unreasonable as it sounds for sdc b in okc, all of the literature available, including contacts to aisc's solution center, is very specific that the phrase which provides the exception in scbf's requires a very complicated non-linear analysis. it all follows out of the definition of scbf per seismic prov section 13.1 (see the reference reading from my prevous response)
our firm has been down this road in another part of the usa that has adopted ibc 2k. unless sdc is d or worse use r=3. aisc even says so when one attends their seminar "practical steel design".
mrstohler,
you refer to the aisc seismic 2002. this and supplement no. 2 is not referenced in ibc 2000.
for the ibc 2000, only the 1997 aisc seismic provisions and the supplement no. 1 applies. so your statement "the maximum force, indicated by analysis, that can be transferred to the brace by the system" only applies to scbf" is true for only the 1997 aisc seismic with supplement no. 2 applied. supplement no. 2 may not have been adopted by the specific city or governing authority as most all editions of the ibc 2000 did not include supplement no. 2.
so in some cases, technically, an engineer is faced with the quandry of which parts or editions of the aisc seismic spec to use - the legally adopted code (which may not include supplement no. 2) or the "latest" specifications which would include supplement no. 2.
this is sort of a nit-picking point in a way, but legally and technically, the provisions you refer to may not apply. so typeiv would have to check to see what's correct for the jurisidiction of his/her project.
i will check out the aisc website and see if i can find some of your references on this per supplement no. 2. if supplement no. 2 applies, then what you describe makes perfect sense.
i don't understand how it allows you to limit the connection to the max force that can be transferred by the system and not the brace yield force. if the brace doesn't yield, then the frame cannot perform inelastically and the r=(higher than 3) doesn't seem justified.
i would think that the max force the system has to deliver would have to be at least equal to the force:
1) enough to yield the brace
2) force from r=3
whichever is less.....i am just thinking out loud here

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