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Re: ANSI/TIA-222-G-2005

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Daryl:

I don't know if it will help or not, but below is the commentary for the
section on the "Directionality Factor".

"C6.5.4.4 Wind Directionality Factor. The existing wind load
factor 1.3 in ASCE 7-95 included a wind directionality factor
of 0.85 [Refs. C6-10 and C6-11]. This factor accounts for two
effects: (1) The reduced probability of maximum winds coming
from any given direction and (2) the reduced probability of
the maximum pressure coefficient occurring for any given wind
direction. The wind directionality factor (identified as Kd in the
standard) has been hidden in previous editions of the standard and
has generated renewed interest in establishing the design values
for wind forces determined by using the standard. Accordingly,
the Task Committee onWind Loads, working with the Task Committee
on Load Combinations, has decided to separate the wind
directionality factor from the load factor and include its effect in
the equation for velocity pressure. This has been done by developing
a new factor, Kd , that is tabulated in Table 6-4 for different
structure types. As new research becomes available, this factor
can be directly modified without changing the wind load factor.
Values for the factor were established from references in the literature
and collective committee judgment. It is noted that the kd
value for round chimneys, tanks, and similar structures is given as
0.95 in recognition of the fact that the wind load resistance maynot
be exactly the same in all directions as implied by a value of 1.0.
A value of 0.85 might be more appropriate if a triangular trussed
frame is shrouded in a round cover. A value of 1.0 might be more
appropriate for a round chimney having a lateral load resistance
equal in all directions. The designer is cautioned by the footnote to
Table 6-4 and the statement in Section 6.5.4.4, where reference
is made to the fact that this factor is only to be used in conjunction
with the load combination factors specified in Sections 2.3
and 2.4."

HTH,

Scott
Adrian, MI


On Tue, 27 Jun 2006, Daryl Richardson wrote:

> Fellow engineers,
>
>         Perhaps one of you could educate me regarding the "Directionality Factor" indicated by Ed in the posting below.  I'm interested in the basics behind this factor rather than anything code specific for the obvious reason that I don't live in a location governed by the code.
>
>         I've always had the opinion that the design level wind loading resulted from high speed winds surrounding a low pressure storm centre and, hence, could come from any direction (unless, of course, you are located in a mountain valley, in which case it could only come from up or down the valley.
>
> Thanks for your indulgence.
>
> Regards,
>
> H. Daryl Richardson
>   ----- Original Message -----
>   From: Ed.Haninger(--nospam--at)Fluor.com
>   To: seaint(--nospam--at)seaint.org
>   Cc: seaint(--nospam--at)seaint.org
>   Sent: Tuesday, June 27, 2006 12:13 PM
>   Subject: Re: ANSI/TIA-222-G-2005
>
>
>
>   One of the big differences is the "Directionality Factor".  See ASCE 7-05 Section 6.5.3 (1) for the start of the procedure.  The higher wind load factor is partially compensated by the addition of the Directionality Factor.
>
>   Ed Haninger
>   Fluor
>
>
>
>        Drew Morris <dmorris(--nospam--at)bbfm.com>
>         06/27/2006 10:52 AM
>         Please respond to seaint  To seaint(--nospam--at)seaint.org
>               cc
>               Subject Re: ANSI/TIA-222-G-2005
>
>
>
>
>
>
>
>
>   Both ASCE 7 -02 and 7-05 have 1.6W in the basic load combination using strength design.  The 1.3W used to be in the older alternative basic load combination (ASD) in the 2003 and older IBC.
>
>   G. Bunkers wrote:
>   The lastest version of the code for towers - TIA-222-G-2005 has went to
>   Limit State design and shows the following load combinations:
>
>   1. 1.2D + 1.0 Dg + 1.6 Wo
>   2. 0.9 D + 1.0 Dg + 1.6 Wo
>   3. 1.2 D + 1.0 Dg + 1.0 Di + 1.0 Wi + 1.0 Ti
>   4. 1.2 D + 1.0 Dg + 1.0 E
>   5. 0.9 D + 1.0 Dg + 1.0 E
>
>   My question is why the 1.6 factor for wind.  ANSI / ASCE
>   states  1.3 .
>
>   Gary Bunkers P.E.
>
>
>
>
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