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FW: Dual system combined concrete shear wall/concrete SMRF

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Tom:

I have given this particular subject a lot of thought in the past, and
arrived at the conclusion that an R/C "dual system" composed of a SMRF
infilled with a wall will act like a wall, and WILL NOT exhibit dual system
behavior as I  see it.  By dual system behavior, I mean cantilever wall /
frame interaction.

One way to look at it is to first imagine a 10 story building with ONLY R/C
walls, no other elements providing transfer of gravity loads to the
foundation, nor any other elements providing lateral stiffness.  Assuming
also that the walls have a sufficiently heavy foundation, they will act like
cantilevered walls - very simple.

Now imagine that you start arbitrarily thickening the ends of the walls to
form BE's.  The walls become flanged sections, but they still act like
cantilevered walls.

Now go ahead and start thickening the areas just below the slabs at each
level to form bond beams.  No special reinforcement in the "beams", they are
just thickened potions of the wall.  As I see it, the behavior of the wall
will not have changed by virtue of the beams, since the entire element is
still just a wall that happens to have some arbitrarily thickened portions.

Finally, if ductile reinforcment is placed in the beams, again it will not
alter the behavior of the element, and it is still a cantilevered wall.

My point is that the act of placing a SMRF integrally with a wall results in
a wall, and the wall will yield at the base (assuming a heavy foundation),
and dual system behavior will not manifest, only cantilevered wall response.
This assumes that the wall has been designed to exhibit a ductile, flexural
response mechanism (i.e. no diagonal tension, crushing or sliding shear
failure precedes flexural yield), in which case the frames will never get to
act.

In order for a dual system to exist, the frames and the walls must be able
to exhibit their own particular behavior, which will occur when the frames
and walls exist in parallel, not series.

My $.02.

T. Eric Gillham PE

-----Original Message-----
From: Tom Higgins [mailto:thiggins(--nospam--at)grpmack.com]
Sent: Thursday, January 24, 2002 11:25 AM
To: 'seaint(--nospam--at)seaint.org'
Cc: mhettum(--nospam--at)grpmack.com
Subject: Dual system combined concrete shear wall/concrete SMRF


Colleagues,

Please consider and comment on the following:

In Table 16-N, the 1997 UBC permits unlimited height in Seismic Zones 3 & 4
for structural systems of the type Dual System/Shear Walls/Concrete with
SMRF.  The most favorable R factor, 8.5, is granted, on a par with the most
ductile steel systems such as Dual System/Steel EBF/With Steel SMRF and pure
steel SMRF.

Now consider the case of a concrete SMRF and shear wall occupying the same
bay.  First let's review some commentary from the 1999 SEAOC Blue Book.
Section 104.6.5 gives the definition of a dual system and is the same as the
UBC.  The commentary says, "The special moment-resisting frame acting
independently shall be designed to resist not less than 25 percent of the
total required lateral force, including torsion effects.  Columns of the
frame system may also function as the boundary elements of shear walls."
The commentary goes on to say that a combined column/boundary element must
be designed for forces associated with the shear wall and forces associated
with the frame.  This seems obvious.  Now, what if the shear wall and frame
are actually combined in a single integral unit in the same bay?  Also, what
if the entire seismic force resisting system is done this way?

Detour for a minute to think about steel dual systems.  I have the
impression that engineers are not hesitating to combine some types of steel
braced frame and steel SMRFs in the same bays.  Intuitively it seems that
satisfactory results are likely.  Imagine a steel SMRF combined with a
special concentric braced frame.  Suppose that the braced frame is of the
two-story-X or zipper-column type so failure of the beams due to unbalanced
vertical brace forces is precluded.  In a strong seismic event, the system
first responds to load as a stiff braced frame.  After a considerable
struggle, with lots of dissipated energy and plastic deformation, the braces
effectively fail and cease to resist significant forces.  The sites of the
plastic deformation in the braces are separated from the key members of the
SMRF.  In fact, specific locations in the gusset plates have been designed
to form plastic hinges.  The less stiff SMRF therefore remains, probably
essentially undamaged, to provide reserve resistance at higher levels of
drift.  A life-safety level of performance is achieved, as the building is
still standing, though severely damaged.  This is a description of pretty
good behavior in a major seismic event.  The key point is that having the
steel braced frame in the same bay as the steel SMRF probably doesn't impair
the ability of the SMRF to function as desired.

Now, back to concrete.  If a concrete SMRF is equipped with concrete shear
panels, forming an integrated shear wall/SMRF assembly in a single bay, a
desirable two-stage response to heavy seismic forces, analogous to that
described above, is conceivable if the shear panel is designed to fail
first, shedding load to an unimpaired frame.  However, if the construction
is entirely conventional, consisting only of concrete and rebar, isn't it
likely that shear cracking in the shear panel will extend across the
boundary element/SMRF columns and promote local integrity problems at an
early stage of loading?  When the shear panel is destroyed, what are the
chances the boundary element/column will not have suffered a level of damage
that will render it incapable of delivering effective reserve resistance?
Where is the traffic cop who says to the cracks "don't go here, or, if you
must, limit your width to X?"  All I can think of is very high reinforcement
levels in the boundary elements and low levels in the shear panels, combined
with limited shear panel thickness.  I still sense trouble.  This doesn't
seem like a system with predictable behavior.

On the other hand,  I see real potential in composite boundary elements.  A
boundary element with a heavy wide-flange core would be pretty tolerant of
severe local distress in surrounding concrete.  Better yet, if you ignore
connection problems, would be a cylindrical steel shell with a concrete
core.  With a boundary element of that type, the shear panel could break
down with the variety and unpredictability typical of concrete and not be
expected to cause local damage to the column/boundary element so severe that
it would be rendered useless.

A side note:  I'm haven't been too worried about the SMRF beams, perhaps
because P-delta isn't an issue, an perhaps because I feel the surrounding
floor system helps insure their integrity.

So, have we any opinions about the concept of a concrete SMRF and shear wall
occupying the same bay?  Is it legitimate?  Is it widely practiced?  Can the
possible weaknesses touched on here be reliably addressed?  Is there any
literature?

Tom

Thomas B. Higgins, P.E., S.E.

Group Mackenzie
0690 S.W. Bancroft Street
Portland, OR 97201
Phone (503) 224-9560
Fax (503) 228-1285
<http://www.groupmackenzie.com>
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