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Re: C &C wind load on parapets - really around 45 psf?

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Sharad,

How much of a change are the higher wind pressures having on your final design? 

You can always consider doing a site specific wind study to allow you to design to a lessor wind pressure.  You would have to get approval from the building official, but I would believe they would accept it, just as they do for a Site specific ground motion study, where you can possibly design for a lessor building code base shear than the standard static code base shear.

This would take some time, and you would have to findout how the wind study is done, using measured wind speeds for that site specific area over a given time period, but you may find in a number of areas that the pressures are decreased below building code calculated pressures.

Michael Cochran SE

-----Original Message-----
From: Sharad T. Patel <sharad(--nospam--at)pbastructural.com>
To: seaint(--nospam--at)seaint.org
Sent: Tue, 13 Jan 2009 9:47 am
Subject: RE: C &C wind load on parapets - really around 45 psf?

Thanks for your input, Conrad.

Regarding the application of C & C loads, I believe they are applicable
to the design of parapet studs in wood framed buildings.  The GCp number
you give below are for MWFRS (ASCE7-05 6.5.12.2.4) used to determine the
total wind load to the roof diaphragm and shear walls.  I agree with
those numbers (total GCp of 2.5), but I do believe that, since there is
a specific section under C & C for parapets (6.5.12.4.3), that C & C
loads apply to the design of parapets.

I wish I was familiar with AS codes for comparison, but I am not.

As to how the C & C loads are used:  we are using them to design the
parapet stud size / spacing / bracing and connection to roof diaphragm.
The roof diaphragm itself need not have the C & C loads carried through
to it.

Sharad T. Patel, SE
Patel Burica & Associates, Inc.

-----Original Message-----
From: Conrad Harrison [mailto:sch.tectonic(--nospam--at)bigpond.com] 
Sent: Monday, January 12, 2009 8:03 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: C &C wind load on parapets - really around 45 psf?

Sharad,

I think it is unreasonable to be applying C&C pressures to structural
primary elements. I believe that is a misinterpretation of the
experimental
data.

>From the top of my head using AS1170.2. Maximum Roof Cp=-0.9 behind
parapet,
windward wall Cp=+0.7. Give Cpn= 1.6 net across parapet.

ASCE7-05 Clause 6.5.12.2.4 GCpn = +1.5, GCpn=-1

AS1170.2 doesn't have C&C but a local pressure factor Kl on areas 'a x
'a
and 'a/2 x a/2' for cladding and immediate supporting members only. Kl=2
maximum for low rise buildings, on 'a/2 x a/2': applied to external
pressure
coefficients only.

If apply then Cpn = 1.6*2 = 3.2 net across parapet wall. But we would
only
apply to small areas on cladding and say supporting girts, but not to
say a
column extending from ground to top of parapet. If there are short
infill
studs the height of the parapet then would apply the higher pressure to
those.

Sharp edges generate high turbulence: eaves, ridge, wall corners and
edges
of walls (top & ends).

Read the commentary to ASCE7-05 those C&C pressures are the extremes,
irrespective of direction of airflow: they do not peak at the same time
across the entire tributary area of an element unless that element is
small.

The importance of which is that: in a grid structure, when one point
experiences the peak, it can share and distribute the load to other
members
not yet experiencing the peak. So my understanding is that C&C like
pressures are to stiffen the cladding and immediate support so that such
structure is strong enough locally to distribute to the larger
structural
elements. And otherwise prevent breach of the fabric by direct action of
the
wind which would increase internal pressure coefficient to that
experienced
on the surface where the hole is formed.

The pressure coefficients given in the codes are mainly obtained from
model
tests on shapes, not buildings. So they provide a guide to expectations
from
different shapes. So for real buildings, obtain conservative estimate by
reviewing all the figures tested. For example airflow over a sawtooth
roof
towards wall.

AS1170.2 basically uses the same pressure coefficients as given in
ASCE7-05
Fig6-6: there are no C&C pressure coefficients in AS1170.2 just the
magnification factors for localised peaks. A diagram like Fig6-11C I
consider to be misleading. Zone 2 on the left hand eaves does not
experience
the GCp along the entire length of the roof at the same time, nor does
the
right hand eaves experience such peak pressure at the same time as the
left
hand eaves.

To design an economical structure, it is necessary to take advantage of
the
nature of the loading, and investigate pattern loading and/or load
sharing
within a grid.

Because as you say the pressures are high, and therefore need to be
accommodated in a reasonable manner.

Feedback on other threads so far is ASCE7-05 is highly prescriptive, and
few
want to think about wind loading, just want a simple single pressure to
apply to projected areas.

The other issue is I also apparently have more reasonable building
officials/engineers reviewing my work, and more willing to accept
rational
application of Australian codes.

It is necessary to have rational interpretation of the code, because as
you
say, as the parapet wall increases in height it moves further from the
roof
and the airflow over the roof is now changed. Thus there will still
likely
be turbulence and suction behind the top edge of the wall, but it is
unlikely to match the pressure coefficient on the roof. And for that
matter
what is the pressure coefficient on the roof under such conditions:
unlikely
to be that given in the code. Just one of the day to day problems of
using
the wind loading code for real buildings.

Wind loading from the code is your best guess, and risky. And wind
tunnel
tests are no use if the data cannot be transformed into useful guidance
for
design.

The load is high, but have to find a justifiable/defendable approach
which
permits reducing the effect. And common view maybe that C&C pressures
have
to be used. But how are they used?



Regards
Conrad Harrison
B.Tech (mfg & mech), MIIE, gradTIEAust
mailto:sch.tectonic(--nospam--at)bigpond.com
Adelaide
South Australia




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