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UBC Wall Boundary Element Detailing Requirements`

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If anyone has any thoughts on this matter please post a response:

I need to know the basis for the UBC's (and SEAOC Bluebook's) requirements
concerning hoops in an R/C walls boundary elements.

Specifically, why is the ratio of length to width of the hoops limited to
3:1?  ACI 318 does not contain this requirement (which was started in
UBC94), and neither the 1990 nor 1996 Bluebooks address this specific

I guess I fail to see the reasoning behind the requirement if taken in
conjunction with the requirement that a hoop leg or cross tie must be
present at least every 12" (since such a leg would provide a point of
confinement every 12").

Furthermore, for a 24" wall (my case), accounting for cover this would
limit my hoops to 54".  The 54" long legs of the hoop are not, in my
opinion, going to offer a whole lot in the way of confinement pressure
(perpendicular to the 54" direction) since it is much too long.  Therefore,
my question is: why limit it to 54" if the cross ties are going to do all
the confinement work anyway?  If it were 60" or even 84" it would not
reduce the confinement effect because it already has none.

Also, I am a bit curious about two requirements which seem to conflict (to
me, anyway).

>From UBC97 (or UBC94):

1921. "The ratio of the length to the width of the hoops shall not
exceed 3.  All adjacent hoops shall be overlapping."

1921. "Horizontal reinforcement shall not be lap spliced within a
the boundary zone."

First the UBC limits the length of the hoops, forcing a designer to overlap
them.  Then it states that lap splicing is not allowed.  Does this mean
that boundary element steel cannot be used for horizontal reinforcement
(for shear resistance)?  It seems irrational to me to require both boundary
element hoop steel (which can be very heavy in terms of area), and then
require that additional horizontal steel be put in to resist shear.

Any and all help on these questions would be greatly appreciated.

T. Eric Gillham PE