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Proposals for "Z" values for different soil condit

 
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PostPosted: Fri Oct 15, 2004 10:59 am    Post subject: Proposals for "Z" values for different soil condit Reply with quote

From: Prof. A R Chandrasekaran
To: All Interested

1. Background:

Prior to 2002 version, an indirect provision in the form of BETA factor for
different soil-foundation systems were specified to increase the design
forces. The Spectra shape did not distinguish the type of soil. In 2002
version, soil effects have been included in the normalised shape of design
spectra but the "Z" values were not goverened by soil types.
It is certain that microzonation studies would suggest changes to both "Z"
values as well as shape of spectra depending on soil types. However, so far,
reports of such studies in India do not give the conclusions and
recommendations in such explicit form and is given in an implied form which
cannot be interpreted easily by design engineers.

Engineers need information only in the form of Normalised Shape of Design
Spectra and the multiplication factor (Z) to convert it as design spectra
for the site [including soil effects] for different values of damping.

A recent paper by Prof. Vincent Lee which appeared in ISET Journal Vol. 39,
No.4, Dec 2002 [actuaaly this journal was issued by ISET only in Oct 2004
!!], suggests Horizontal and Vertical Spectra for two types of Soil
conditions and is given for different MM intensities. Since IS:1893 zoning
is supposed to be based on MMI, the conclusions may be of interest to us. In
particular, it is seen [Fig. 8, p229 of the paper - this Fig has been
scanned and sent to administrator SEFI as I cannot include any attachment
with this message] that the soil effects depends on the period of the
structure. For systems with period less than about one second, soil values
are more than that of rock whereas for periods greater than about one
second, rock values are more than that of soil.

2. New Proposals for Discussion and Implementation eventually by CED 39 of
BIS [IS:1893]

A.        "Z" Values

ZONE         STIFF SOIL       MEDIUM SOIL        WEAK SOIL
V                 0.27                    0.36
0.45
IV                0.18                    0.24
0.30
III                0.12                    0.16
0.20
II                  0.075                  0.10
0.125

Notes:
1.The values in medium soil are same as that in IS:1893:2002
The values in stiff soil is reduced by 25% and values in soft soils
increased by 25%
2. I propose stiff soil to have shear wave velocity [Vs] of 1200 m/sec or
above
Medium soil to have 700 m/sec and weak soil to have 200 m/sec
3. for the purpose of design these values of Vs be evaluated at the lowest
level of the foundation,
that is at the base of Raft or Spread or Column footing and bottom tip of
bearing piles.
In general, I expect that even for weak soil at surface, at the founding
level, Vs would be atleast 400 m/sec
4. The code should reflect change in "Z" with respect to soil as it appears
to be a fact.

B. Shape of Normalised Spectra

I am not making any proposals at this stage. However, it appears that the
present 2002 version causing increase throughout the period range [beyond a
nominal short period range] has to change. Lee's suggestion for a turnover
at about one second could be a starting point for debate.

PS:
If the administrator is not able to post the Fig. 8 of Prof. Lee's paper, I
can send it as email attachment to those who request me.

Posted via Email
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