Site Class C vs. D 1

[crlev]

Does anyone know if there is historical data or information that compares the building costs of building an office tower (approx. 250 feet, 25 stories) on a seismic Site Class C vs. a Site Class D. I am trying to work up a quick number to compare the two. The overall building size and construction has yet to be determined. I was hoping for a rough idea, percentage based or unit price savings etc.. Thanks for any input.

 

[Grizzman]

Site class D is the default you use when you do not know. I'm sure site class C will result in lower seismic loads, which may allow you to classify in a lesser seismic design category. The lesser SDC's will allow for less stringent lateral loads constraints and are typically less expensive. If it won't allow you to reclassify the SDC, the difference in expensive will only be marginal. If you reclassify the SDC, but don't modify your design to use a more efficient lateral system, you differences in expense will only be marginal.
that being said, most geotech reports will not reclassify a project to a lesser site class unless you specifically ask them to. I don't think they mind, but most buildings are designed before the soil tests are completed (and as such are designed for site class"D")so changing to a more favorable site class does not generally do anything. and leaving it will be conservative.
Sorry it's not a cookbook answer.
G

 

[crlev]

Thanks for the response Grizzman. The geotech is willing to reclassify the site as site class C if more excavation is done to get the building closer to the bedrock. In addition, the building must be isolated from surrounding soils. I am trying to compare the costs of this against designing in site class D. I have ran a few quick numbers and it doesn't look like the SDC will change. What does this mean? Isn't the design the same if the SDC is the same for both sites? (meaning costs would be the same) Where would my savings be?

 

[Grizzman]

certain resisting systems are not allowed in higher SDC's. for example, "ordinary reinforced masonry shear walls" are not permitted in SDC D, but are permitted in SDC C. thus you'd have to go with special reinforced masonry shear walls, which only provide a marginal increase in 'R' values for more stringent reinforcing requirements.
Obviously you're not going to have masonry shear walls on a 25 story building, but I think the example clarifies what I'm saying.

 

[prsconsultant]

You need to find out if your seismic design category changes by going from a site class D to C. If site class D puts you into a SDC "D" vs SDC "C" for a site class C, it is worth taking the extra steps to fall in a site class C. You would be spending a lot of money in seismic detailing if you fall in SDC "D".