3000 psi concrete requiremen

huangyhg

3000 psi concrete requirement
i have a small residential metal building foundation (30 x 42) on uneven topo with some exterior foundation walls having up to four feet of concrete exposed to the weather.  the owner received a comment back from the local jurisdiction (king county) that 3000 psi concrete is required in this area of exposed concrete.  this will throw the concrete into the inspected range since it is over 2500 psi.  the requirement seems ridiculous to me.
does anyone know of a specific code reference in the ibc requiring this 3000 figure in this circumstance?  if not, then i may be dealing with a local upgrade, or an over exuberant plans examiner trying to be an engineer.  
yea... like that never happens.   
mike mccann
mmc engineering
ibc references aci 318 and aci 318 requires a minimum of 2,500 psi.  see aci section 5.1.1.
the 3,000 psi requirement could possibly be a local amendment but you'd have to check with that jurisdictions website or call the bldg department to see where that comes from.
also, according to the ibc, a building official has the right to increase the minimum requirements of the ibc (they can't diminish them).   
mike,
see 2003 ibc section 1904 and table 1904.2.2(2).  king county wa may be in a moderate or severe exposure region.  by this table it would require 3,000 psi concrete because it is a vertical surface exposed to the weather.
  
ralph
structures consulting
northeast usa
in our area, you will not even buy or get 2500 psi concrete, unless it is specifically specified in writing. some lightweight concrete fire protection may be exempted from the requirement, but it may still be over the minimal 3000 psi limit.
this is not a code provision, but a general industry standard to protect the supplier from future nuisance claims later. since there is just such a small cost differential, there is really no problem.
you have a hard time getting concrete here for an external slab unless it is 4000 psi and air entrained unless the person on site signs and it responsible. - life is too short and precious for "penny-pinching" on materials when labor is the real cost.
localities have some latitude on codes and acceptance, but not every supplier is willing to accept the consequences.
i agree with concretemasonry - 3,000 psi mix is the minimum i use for any structure, reinforced or non-reinforced. 2,500 is only used for curbs, gutters and sidewalks and this is required by local code.

i will not argue here, but note that simpson only requires 2500 psi for most of their holddowns, but provide tables for strengths in excess of that figure too.  
i see the table referred to here and it looks like 3000 psi is required as it is in a 40 psf snow load area.  moreover, and more importantly, the owner is refrigerating the slab with cooling coils and insulating under the slab of which i was not aware.  
i also see that he is installing 40 psi foam under the spread footings too.  i don't like it, but if the 40 psi figure is the compressive strength of the foam, i guess it is ok.
thanks guys.   
mike mccann
mmc engineering
concretemasonry:
in your area, minnesota, i can completely understand that.  however, our winter weather is much milder here and 2500 psi is regularly specifleied and available, or at least it has been in the past.  
from the looks of the ibc table referred to above though, it looks like i will have to change my thinking and specifying on the subject, at least pertaining to commercial.  i am going to check the irc to see if the same table applies.
mike mccann
mmc engineering
mike -
the code is only the least restrictive standard that you can use and be legal, but it is not always correct or proper.
in our area, there is little external control (especially with extremely price conscious contractors) that the industry has some concrete major producers have set a higher standard because the ultimate use of the material is not known. some suppliers will not unload without a signed waiver if it is apparent it is for exposed purposes. - it eliminates nuisance claims and suits where the timing and placement could really be the culprits. there will always be the low priced producers because they think they have some cost saving secrets.
the difference in the per yard material cost is minimal and not significant when compared to the labor and potential liability for the supplier and contractor. the price people just go to the small low bidders and try to get performance. at times, i feel like we are in a jungle surrounded out of area bidders that is sitting on good aggregate deposits.
i am not in the ready-mix business, but have been involved for over 40 years (starting as summer concrete inspector on state projects)and marveled at the differences across the country and in other countries. - now al franken will have to be "minnesota nice" if that is possible after his 25 year hiatus.

just wondering if we are becoming "code" engineers or are we "thinking" engineers?  do we "follow" the code even if it isn't an absolute necessity from a design point of view?  (i'm thinking about the recent post of hitting bedrock "within" the frost depth required.) 3000 psi (18mpa) is okay by me - but when i do leveling concrete (blinding concrete/mud mat) i wouldn't specify 3000 psi nor even 2500 psi concrete - yet if the code requires . . .   
mike,
i think that with the old 97 ubc that we use to spec. 2500 psi concrete to not require special inspection on residential projects.
now, section 1704.4 exceptions allows light framed construction with out special inspection. but, i don't see anything in the 2006 ibc that required 3000 psi concrete.
maybe the building official is just adding his own specs to the code. you might ask him where he has found something, in the code, that required 3000 psi concrete.
i suspect, that when push comes to shove, that the concrete probably has a minimum compressive strength greater than 3000 psi.
the reason a lot of codes require 3,000 psi and a lot of engineers, suppliers and contractors use it for structures is that it has been shown that over the years, it provides a good level of performance. not just because some code official published it. and by the way, most codes are written and edited by practicing engineers. if there are special circumstances, this minimum requirement or local preference should be evaluated to see if something stronger might be required.