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FW: Rebar in members resisting earthquake force

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It is a bit chancey commenting on a structure that I have not examined the
plans for, but I will give it a shot (speaking in general terms only).

As I understand things, the required ratio of Fu/Fy>1.25 relates to the
basic assumption taken by code writers that structures perform well in EQ's
by virtue of 3 things:  1) overstrength of the structure means that it
actually has a higher capacity than we are accounting for and 2) ductility
requirements for members allow them to undergo inelastic action (hinging)
while remaining stable, and finally, 3) damping and period elongation
effects will reduce the expected demand in the structure as the EQ proceeds
to crack and soften the members.

What this all means is that initially, at least, the demand on the structure
is much higher than its capacity (looking at it as a design), but in the
real world, the structure is stronger so demand/capacity is reduced a bit,
then, as the EQ actually occurs and damages the structure, the demand is
reduced (period and damping effects) to the point where demand=capacity,
assuming that the members are constructed such that undesirable behavior
(diag tension failure, crushing of core concrete etc.) in critical members
is avoided.

Although I believe this to be a simplistic explanation which leaves a lot
out, it suffices, and if you are interested, you can read SEAOC's version of
this in the 1990 Bluebook, Appendix 1E2a-Rw starting on page 146-C
(Commentary section), with the most pertinent (IMO) figure on page 149-C.

Anyway, it would appear that the 1.25 ratio is meant to insure the minimum
amount of expected strain hardening to make this whole thing work.

If you have less than 1.25, in your case 1.17, does this mean that it won't
work?  IMO, not necessarily.  If you are lacking on the strength end,
because of insufficient SH, then you can make it up on the ductility end.

Rather than go the old (and outdated) force based route, try the
displacement path.  Calculate your expected displacements of the structure,
find out the required rotations in your members, then figure out how much of
that will have to be through inelastic action, and finally check if your
members as designed can handle that much plastic action.  If so, then you
are fine.  If not....

In the end, though, I would be very much suprised if this DOES result in a
problem, provided that there are no other (major) deficiencies.

On another note, depending on your foundation setup, you probably ARE going
to get inelastic rotations at the column/wall to foundation interface.  This
is likely to be your location for maximum moment, and unless you have
designed the system to have overstrength at the base, forcing the yield
mechanism up into higher members in the building, I would say that this is
exactly where you will get the larger inelastic rotation demands.  So, I
would definitely check it.

Hope this helps.

T. Eric R. Gillham PE
PO Box 3207 Agana Guam 96932
Ph: (671) 477-9224
Fax: (671) 477-3456
Pgr: 720-8891
eric(--nospam--at)gk2guam.com <mailto:eric(--nospam--at)gk2guam.com>


-----Original Message-----
From: Soojin Hur [mailto:sjhur(--nospam--at)hec.co.kr]
Sent: Wednesday, March 22, 2000 6:11 PM
To: SEAOC
Subject: Rebar in members resisting earthquake force


I designed a R.C building in Zone 4, which building is dual structure
(moment resisting frame + shear wall) with reinforcement of ASTM A615
Grade 60.

According to 21.2.5 in ACI 318, there are two additional requirement for
reinforcement resisting earthquake-induced force, one of which is as
follows:
  '(b) actual ultimate tensile strength / actual tensile yield strength
> 1.25'

However, construction team at site have already finished concrete work
for foundation and part of column and shear wall by reinforcement with
the ratio = 1.17.

With this condition, how can I approach to check the acceptability of
the existing structure? Is the following approach reasonable?

- Since the ratio is related with inelastic rotation capacity, and
plastic hinge is not likely to occur at joints between foundation and
column, column and wall, so 'No Problem'.
- Since the ratio is also related with response modification factor (R),
check the existing structure with reduced R-value. If this is correct,
how to quantify it?

Any input from anybody would be appreciated.

Thanks in advance.

Soojin Hur
HDEC, Korea
e-mail : sjhur(--nospam--at)hec.co.kr