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Dual system combined concrete shear wall/concrete SMRF

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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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