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RE: IBC 2000 Wind Design

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Regarding Brian's response to item 1: You must also consider D + Lr
(no 0.75 factor) and D + W (again, no 0.75 factor).  You cannot add an
additional small transient load to justify a large reduction in the
major load.  IBC Sec. 1605.1 indicates that "effects from one or more
transient loads not acting shall be investigated."

You asked, "Is there a reason that design per IBC 2000 is more
conservative that past codes?"  Setting aside the contractor's
standard "rationale" of "we've always done it that way," there are two
ways to rationalize the 1/3 stress increase: strain rate effects and
reduced likelihood of occurrence.  Fluctuating wind loads, although
dynamic, change too slowly to produce strain rate effects in typical
construction materials (with the exception of wood, which already has
a load duration factor).  The reduced likelihood of occurrence for
multiple transient loads is the basis of the 0.75 (= 1/(4/3)) factor
that is permitted; because the rationale is limited to multiple
transients, application of the factor has the same limitation.  [A
third reason to arbitrarily increase strengths when dealing with wind
loads would be to produce the effect of designing for more frequently
occurring wind loads (return period of less than 50 years).  If that
is the real reason, the loads, and not the capacities, should be
changed.]

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Michael Valley, P.E., S.E.                   E-mail: mtv(--nospam--at)skilling.com
Skilling Ward Magnusson Barkshire Inc.              Tel:(206)292-1200
1301 Fifth Ave, #3200,  Seattle  WA 98101-2699      Fax:        -1201

-----Original Message-----
From: Brian K. Smith [mailto:smithegr(--nospam--at)bellsouth.net]
Sent: Wednesday, October 03, 2001 6:59 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: IBC 2000 Wind Design


1.	If you combine your out of plane wind loads with the roof live
loads you
can reduce the two transient loads by 0.75, with no increase in
allowable
stresses.  This is consistent with ASCE 7-98.

2.	The two columns are for positive (pressure) and negative (suction)
forces.


Brian K. Smith, P.E.
Bossier City, Louisiana

-----Original Message-----
From: Sherman, William [mailto:ShermanWC(--nospam--at)cdm.com]
Sent: Tuesday, October 02, 2001 6:16 PM
To: SeaInt Listserver (E-mail)
Subject: IBC 2000 Wind Design


In trying to apply wind design per IBC 2000, several questions have
come up:

1. IBC Section 1605.3.1.1 states that increases in allowable stresses
for
materials may not be used for allowable stress design with the load
combinations of Section 1605.3.1. A 0.75 reduction factor is allowed
only
when two or more transient loads are combined. As I interpret this,
when
designing a CMU wall for out of plane wind load, one can no longer
allow a
1/3 increase in allowable stress - is this correct? (Section 2107.1
references ACI 530 for masonry design but deletes Section 2.1.1.1 of
ACI
530, thus deleting the increase in allowable stress allowed by that
code.)
Is there a reason that design per IBC 2000 is more conservative that
past
codes?

2. IBC Table 1609.6.2.1(2) lists two load values within each column
and row
as tabulated. Where does the code define what these two values
represent?
One is positive and one is negative - I believe that they are windward
and
leeward wind pressures, but I don't see where the code explains what
the two
values represent.

3. In this code (and in ASCE 7), it appears that the leeward (suction)
pressure is higher than the windward pressure. Under previous UBC
codes, the
windward pressures were higher than the leeward pressures. Why does
wind act
differently under the newer codes?


William C. Sherman, PE
Camp Dresser & McKee, Inc.
Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com


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