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RE: STAAD-Concrete sectional properties

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I'll take a stab at SOME of your questions:

Firstly, are you designing for gravity, seismic or both?  I would say that
Ieff would be used for gravity analyses only if the member dimensions, or
loading, were such that seviceability problems could be an issue (e.g. long
span with shallow depth, heavy traffic loading etc.)

For seismic, though, I feel that cracked properties are much more
appropriate for analysis IF it is an in depth analysis on a large or
complicated building.  I say this because the cracked properties will result
(in the analysis AND during a major shake) in larger deformations, which
will need to be accomodated by non-ductile members or elements of the
structure - examples would include "gravity" columns supporting large loads
in a building with a perimeter SMRF.  During an earthquake, the perimeter
frames would be expected to incur more cracking more quickly than the
internal non-ductile frames, and hence even if the perimeter frames were
initially much stiffer, they would soften leading to increased deformation
demands on the interior frames.  Not accounting for this through the use of
cracked sections would lead to an underestimation of the possible inelastic
demands on the interior columns.

That said, addressing your questions below:

1.  I don't, but others might (not too helpful, I realize)

2.  I would use cracked sections, IF you are doing seismic design (I assume
you are).  Relating to the example above, if you are assuming that the
shearwalls take almost ALL the load in that direction (E-W), you could be
underestimating the possible demand on the frames in that direction.  One
way to check would be to reduce the wall K by 1/2 (arbitrary but pretty
conservative).  If this doesn't result in a noticeable increase in the frame
demand in that direction, I wouldn't worry about it.  For the N-S direction,
cracked sections would be important only if there were other elements that
could be affected, or if P-delta may become a concern.

3.  No idea, I don't use STAAD.

4.  Others have given additional references for this, but the basic approach
is to assume an applied moment, and figure out what the extent of cracking
would be under that loading. If you are talking about beams and columns, I
would use a moment curvature approach.  For walls that responde primarily in
flexure, same approach.  For squat walls, I would refer to Paulay and
Priestley's book on R/C seismic design - they give some values.

5.  Someone else will have to field this one, I have something due in an
hour!

Regards,

T. Eric Gillham PE

-----Original Message-----
From: John Holliday [mailto:john.holliday(--nospam--at)chemtex.com]
Sent: Friday, October 12, 2001 1:09 PM
To: seaint(--nospam--at)seaint.org
Subject: STAAD-Concrete sectional properties


I am reviewing the STAAD input for a Reinforced Concrete six story
industrial building and have a few questions that I hope someone can answer.
1.  Is it usual practice to use the member properties in ACI 10.11.1?



2.  Section R8.6.1 talks about using gross section properties for all
members in braced frames. My building has two parallel shear walls in the
east-west direction (in combination with moment resisting frame) and only a
moment resisting frame in north-south direction. Can I use gross section
properties?



3.	STAAD PDelta analysis has been performed. The engineer who done this
analysis informed me that the PDelta analysis in STAAD takes into account
(automatically) reduced section properties due to cracking, therefore it is
acceptable to input gross section properties. Is this correct?

4.	I reviewed the UCB on this subject. Section 1633.2.4 states that flexural
and shear properties shall not exceed one half of the gross section
properties unless a rational cracked-section analysis is performed. Could
someone explain to me what a rational cracked-section analysis is.

5.	Section 1910.11.1 of the UCB gives similar member properties as ACI
10.11.1 but seems to contradict the requirements of UCB Section 1633.2.4
i.e. using 50% of gross. Can someone clarify this for me.

Thanks in advance for any help.

John Holliday  P.E.



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