SDC of site category F structure

[ramihabchi]

Hi
In ASCE for site class F we cannot determine Fa and Fv from given tables as it refers that a soil response analysis must be done.in this case what are the values that should be given for the engineer as result from the soil analysis and how the Seismic design category is determined?I mean should the soil analysis give Fa and Fv for the engineer along with surface ground acceleration time history?or he should calculate them to know SDs and therefore SDC of the structure?
Thank you.

 

[ramihabchi]

I mean how can we determine seismic design category of class F site?

 

[HTURKAK]

Refer to Table 20.3-1 Site Classification

A. Hard rock
B. Rock
C. Very dense soil and soft rock

.....

F. Soils requiring site response analysis
in accordance with Section 21.1 . Soils requiring site-specific evaluations

and 21.1 SITE RESPONSE ANALYSIS . So , Class F site is, Soils requiring site-specific evaluations by an experienced geotechnical group.

and pls look to 11.4.8 Site-Specific Ground Motion Procedures.

excerpt of the 11.4.8

(A site response analysis shall be performed in accordance with Section 21.1 for structures on Site Class F sites, unless
exempted in accordance with Section 20.3.1. A ground motion hazard analysis shall be performed in accordance with
Section 21.2 for the following:
1. seismically isolated structures and structures with damping systems on sites with S1 greater than or equal to 0.6,
2. structures on Site Class E sites with Ss greater than or equal to 1.0, and.
3. structures on Site Class D and E sites with S1 greater than or equal to 0.2.
EXCEPTION: A ground motion hazard analysis is not required for structures other than seismically isolated structures
and structures with damping systems where:
1. Structures on Site Class E sites with SS greater than or equal to 1.0, provided the site coefficient Fa is taken as equal to
that of Site Class C.
2. Structures on Site Class D sites with S1 greater than or equal to 0.2, provided the value of the seismic response
coefficient Cs is determined by Eq. (12.8-2) for values of
T ≤ 1.5Ts and taken as equal to 1.5 times the value
computed in accordance with either Eq. (12.8-3) for TL ≥
T > 1.5Ts or Eq. (12.8-4) for T > TL.
3. Structures on Site Class E sites with S1 greater than or equal to 0.2, provided that T is less than or equal to Ts and the
equivalent static force procedure is used for design....)

 

[ramihabchi]

No I meant that for each SDC the code refers to a clause to design structural element accordingly.if soil type is F geotechnical must specifty SDC or some parameters that indicate which clause is the most adequate.for example special moment frame/ordinary moment frame..etc.am I right?

 

[HTURKAK]

O.K. The steps that you are expected to follow ;

- Define risk category of the bldg . from Table 1.5-1 ( Risk Category of Buildings and Other Structures...). You may find the more detailed description of Risk Category at IBC .. Table 1604.5.

- Define the Importance factor from Table 1.5-2 acc. to Risk Category of Buildings and Structures.

- Assign the SEISMIC DESIGN CATEGORY acc. to section 11.6 . Pls look TABLE 11.6-1 Seismic Design Category Based on Short-Period
Response Acceleration Parameter ( Sds) and TABLE 11.6-2 Seismic Design Category Based on 1-s Period Response Acceleration Parameter (Sd1) ..

- Select the SFR acc. to Table 12.2-1 Design Coefficients and Factors for Seismic Force-Resisting Systems. Notice that you may select the SFR acc. to limitations and may select SMRF instead of OMF or IMF .. But inverse is not true.