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RE: 97 ubc section 1630.8.2.1

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Martin,
Excellent explanation - I appreciate and will post it on the
Structuralist.Net discussion forum for future reference by those who need
it.
Dennis S. Wish PE


> -----Original Message-----
> From: Martin W. Johnson [mailto:MWJ(--nospam--at)eqe.com]
> Sent: Friday, March 16, 2001 9:03 AM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: 97 ubc section 1630.8.2.1
>
>
>
>
> I suggest you design the beam and supporting columns for omega
> (2.8) times the
> forces from the wall, times 1.4 if your forces have already been
> reduced to ASD
> levels..  Remember that the shear strength of the wall that you
> are using is an
> ASD value.  If you look at the plywood shear wall test data that
> was developed
> at UCI using shear panels with height/length of about 1, the
> actual ultimate
> strength tended to be about 2.5 to 3 times the tabulated
> allowable shear values.
> The code provision is intended  to ensure that the supporting
> beams, trusses,
> columns, etc., do not fail due to downward forces caused by this
> overstrength.
> Although the code applied force will be large,  the code allows you to use
> special inrease factors when you calculate the strengths of the supporting
> members.
>
> The SDM example suggest that the hold-downs for the shear wall
> might also need
> to be designed using this increased force.  I personally do not
> agree with this.
> While the irregularity condition can be detrimental to the
> elements below the
> wall, the shear wall itself is not different in essence that any
> other shear
> wall in the building, so should we single out this one wall to
> require stronger
> hold-downs?  The concern is for the downward force, not the upward one.
> Presumably, if the ultimate tested sthrength of a shear panel can
> be as much as
> 2.5 to 3 times greater than the ASD shear, while the maximum
> strength of the
> hold downs may only be about 1.7 times greater than the permitted design
> strength, then the hold-downs that we use in walls may often be a
> "weak link"
> that develop yielding.  If we start increasing the strength of the uplift
> connection, then the possible downward force at the toe of the
> wall is able to
> correspondingly increase, and it is the compression force that the code is
> concerned with.
>
> Incidentally, the UCI test data also includes some data for shear
> panels with
> height/length of 2.  The ultimate strength of these panels tends
> to be about 30
> percent less than for the more square panels.  So why don't our
> codes make any
> distinction between allowable shear values for shear walls based
> on h/L?  It
> seems that the factor of safety is varying.
>
> regards,
> Martin
>
>
>