From: "T. Eric Gillham PE" <gk2(--nospam--at)kuentos.guam.net>
Date: Mon, 15 May 2000 07:52:12 +1000
>If you follow the code, you are allowed to assume?
>yield curvature as Ec/(Length of wall). This amounts to assumed maximum
>concrete strain (0.003) divided by wall length. (Sorry - can't write the
>greek letters). Two questions come from this:
>1) Is the point to find the concrete or steel yield point?
>2a) If it is steel, shouldn't the curvature be (Es+Ec)/(Length of Wall)
>2b) If it is concrete, shouldn't the curvature be Ec/C`u?
For a wall undergoing inelastic action, the following will be true:
ult curv = yield curv + plastic curv
Therefore, if the wall experiences minimal curv at yield, it must have
significant plastic curv capacity in order to produce the required ult curv.
As I see it, the code is giving you an easy out by letting you
CONSERVATIVELY compute the yield curv by using lw instead of c'u. Since
lw>c'u (unless you have a HUGE axial load, in which case the code doesn't
allow you to use the wall anyway), then by using lw you will decrease the
calculated yield curv and therefore you will need to deal with a higher
plastic curv demand in order for the wall to have adequate ult curv
Sooooo, I believe the answer is that using lw gives the designer an easy,
albeit very conservative, estimate of the yield curv.
>My thought is this, if the wall is pushed into the plastic region we
>should be providing boundary confinement. The elastic region would end
>at one of the two states listed in 2a or 2b. The limit state listed by
>the UBC is non conservative if my assumptions listed in 2a and 2b are
Regarding your assumptions above, the code is IMO interested in the expected
maximum conc compressive strain, and that is the reason for the BE
requirements. Steel strain is assumed to not be a problem, and in my
experience it rarely is. However, for reasons stated above, I don't see
that the UBC allowance of the use of lw in lieu of c'u as being
T. Eric R. Gillham PE
PO Box 3207 Agana Guam 96932
Ph: (671) 477-9224
Fax: (671) 477-3456