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FW: Seismic design of flatplates

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Einar:

In your original post, you stated that the expected 500 year return
effective peak ground acceleration is .15g for your structure.  Unless you
are talking about the REDUCED or DESIGN (Rw or equivalent reduction factor
>1) effective peak ground acceleration being .15g, this sounds pretty low.
If indeed it is this low, then the structure is probably in the equivalent
of Zone 2A or less wrt the 1994 UBC (as a reference only).  In this case,
1.5% of interstory drift would be too high.

If, on the other hand, you are using .15g as a DESIGN (Rw=6 lets say)
acceleration, then 1.5% of interstory drift is not that far off.  Remember
that the design of flat connections is done with the assumption that minimal
ductility is present, therefore you need to be designing for maximum
expected deformations, which will be significantly higher than those which
you will get from an elastic analysis based on uncracked sections and a
reduced DESIGN base shear.

In any case, you can make an educated guess about the expected interstory
drift by converting your acceleration spectra into a deformation spectra
(for the 1994 UBC, a=ZICg, d=(w^2)*a where w=frequency, a=pseudo
acceleration, d=displacement).  Use a reasonable estimate of the cracked
period of your structure to obtain an expected displacement at the height of
the resultant for your structure (2/3 of H would be reasonable IMO, assuming
predominant first mode response for a shear wall structure, ASSUMING your
shearwalls extend to the foundation without a soft or weak first story).
Once you have an estimate of the expected displacement, distribute this over
the height of the structure (weighting it accordingly for hinge formation)
to get your expected max interstory drift.  This should let you know how
close 1.5% is.

Hope this helps.

T. Eric R. Gillham PE
PO Box 3207 Agana Guam 96932
Ph: (671) 477-9224
Fax: (671) 477-3456
Pgr: 720-8891
eric(--nospam--at)gk2guam.com <mailto:eric(--nospam--at)gk2guam.com>

-----Original Message-----
From: Einar Einarsson [mailto:einar(--nospam--at)istak.is]
Sent: Tuesday, December 19, 2000 3:18 AM
To: 'seaint(--nospam--at)seaint.org'
Subject: RE: Seismic design of flatplates


I was hooping to hear what is a common practice in situatons like I have
with shearwalls to take the horisontal earthquake force and laterial
interstory drift ratio of 0.15%.
I am aware of recent articles in ACI Structural Journals on this subject by
Megally and Ghali. They state that a slab-colmn connection must prossess the
ability to undergo a specified minimum laterial interstory drift ratio of
1.5%, which is 10 times what I calculate in my situation. Vu must not exeed
0.4 C Vc, where C is strength-reduction factor. This limit on Vu may be
difficult to satisfy without the use of drop panels or use of stud shear
reinforcement according to M. and G..  I assume they must be talking about
the use of flatplate on columns without shearwalls to resist earthquake
force.

Einar Einarsson, CE

-----Original Message-----
From: Eric Lehmkuhl [mailto:eric_lehmkuhl(--nospam--at)kpff.com]
Sent: 15. desember 2000 23:56
To: seaint(--nospam--at)seaint.org
Subject: RE: Seismic design of flatplates


I would recommend studrail reinforcement for both strength and ductility.
There have been several articles on this in recent ACI Structural Journals.

Eric Lehmkuhl,  S.E.
KPFF Consulting Engineers, San Diego
(619) 521 8500 phone
(619) 521 8591 fax
eric_lehmkuhl(--nospam--at)kpff.com

-----Original Message-----
From: Einar Einarsson [mailto:einar(--nospam--at)istak.is]
Sent: Thursday, December 14, 2000 6:39 AM
To: 'seaint(--nospam--at)seaint.org'
Subject: Seismic design of flatplates


Hello.

I would like to hear from you.
 I am designing 7 story high concrete building.  It has shearwalls to resist
the  wind and earthquake loads and flatplates on columns to resist vertical
loads. My consern is about punching of the flatplates because of eccentric
punching during earthquake.  The expected effectiv ground peak acceleration
is 0.15g at the building site with 500 years return.
Do you have any recommendations.  Should I reduce the nominal shear strenght
of the concrete and provide shear stirrups ?
Are there any recommendations from Europe i.e. ceb/fip.

Einar Einarsson, CE.