C &C wind load on parapets - really around 45 psf?
Sharad T. Patel
My office has recently studied the components and cladding (C & C) wind loads on parapets in order to design the parapets on four story wood framed buildings. I would like to present our conclusions and request everyone’s input as to whether you believe we have interpreted the code correctly. I would also like to know, if our analysis is correct: Do you all believe that the C & C wind loads required by ASCE for use on parapets are reasonable?
Here is our analysis:
Using 6.5.12.4.4, the code requires GCp factors as follows (for h less than 60’, not near corners, assuming impervious finish on the parapet, and same structure (stud) takes load on both sides of parapet):
For Load Case A (windward parapet):
Windward side of the windward parapet: per Figure 6-11A, about 1.0 (zone 4), with a 10% reduction allowed if the roof is flat.
Leeward side of the windward parapet: per Figure 6-11B, about 1.8 (zone 2, roof element, it is our understanding that the negative roof wind pressure is to be applied to the back of the parapet)
Total GCp factor: 2.7 (assuming flat roof)
Basic wind pressure, qp is 17 psf (85 mph, h=50’, exposure C).
Wind load on parapet: 46 psf.
This is greater than a floor live load. Does anyone really think that we need to design parapets for this much wind load?
If you assume exposure B, which is reasonable almost anywhere in southern California, the load would be 34 psf.
To us, it seems that the GCp factor of 1.8 required on the leeward side of the windward parapet is very high. If there was a wind storm and I was on the roof, this is where I would want to be. And I don’t think that I would experience any suction if I was huddled at the corner where the roof meets the parapet. And how about the case of a very tall parapet – say 10’ or so. Does the GCp of 1.8 apply to the entire height of this parapet?
For load Case B, which is essentially the leeward parapet, the code requires GCp factors of 1.0 on the back of the parapet, and 1.1 on the front of the parapet, for a total of 2.1 ASCE-7 seems to be saying that, since there is no roof on the leeward side of the leeward parapet, the GCp is 1.1 instead of 1.8.
All of your input will be appreciated!
Thanks,
Sharad T. Patel, SE
Patel Burica & Associates, Inc.
Conrad Harrison
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.te...@bigpond.com
Adelaide
South Australia
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Conrad Harrison
Another point which may help.
The MWFRS pressure coefficients are determined from the maximum coefficient
which occurs in a 45 degree sector either side of the wind direction
considered: transverse, longitudinal. These give an approximation of the
distribution of wind loading experienced by building as the wind blows in a
given direction. The C&C ignore direction.
ASCE7-05 Fig6-6 indicates that the side wall pressure coefficient is
Cp=-0.7, whilst AS1170.2 indicates the pressure is stepped (-0.65,
-0.5,-0.3,-0.2) in similar manner to that on roof for longitudinal. Note we
have one extra step of 3h.
As move away from windward edge the wall pressure coefficient and roof
coefficient for longitudinal loading are thus decreasing. Thus a point along
the length of the building is reached where the transverse loading is
controlling. Also the windward edge, end frames only have half the load
width of typical frames: so higher pressures there tend not to be critical.
Also for longitudinal loading the parapet has suction on both faces,
therefore not a critical condition: the transverse loading is.
Pressure coefficients for transfer loading of roofs are typically lower than
the longitudinal pressure coefficients, until reach a point along length of
roof where longitudinal have stepped lower. So in terms of AS1170.2 we could
be applying the local pressure factor to lower pressures (Use ASCE7 Fig6-6
for Cp). For example Cp=2*-0.9 at windward edge longitudinal loading versus
Cp=2*-0.6 leeward face of transverse loading, versus say Cp=2*-0.4 on
windward face for transverse loading (assuming 20 degree roof). But leeward
has suction on both faces of parapet so not critical in terms of maximum
load: just important with respect to location of compression flange. Taking
Cp=+0.8 for windward wall (AS1170.2 has +0.7), then get Cpn=2*(0.8+0.4) =
2.4, smaller than the extreme Cpn=3.2 previously estimated.
Ok! To ASCE7-05 C&C pressures apply irrespective of direction. However what
ever the prescriptive requirements in our codes, the wind pressures are
still distributed across the surfaces. And ASCE7 commentary does indicate
permitted to use any rational methods in the literate: so seek texts and
research papers which identify the distribution.
Good luck.
Sharad T. Patel
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.
ml...@aol.com
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
Conrad Harrison
Your approach seems reasonable. Also using AS1170.2, kl=1.25 for positive
pressures on windward wall, so my estimate of C&C drops lower Cp=
1.25*0.8+2*-0.4 = 1.8.
However I am aware that whilst AS1170.2 and ASCE7-05 are basically based on
research more than 30 years old, the C&C pressures in ASCE7-05 are
relatively current research and do suggest that AS1170.2 may be under
estimating cladding pressures. But then Holmes in wind loading of structures
does indicate that Cp on a roof surface can get as high as Cp=-20, and that
is not in either code. Some complex statistics are used to account for the
duration of a spike in loading and its magnitude and frequency of
occurrence.
The C&C pressures are spikes in pressures on small areas, but the spikes do
not occur over the entire surface at once. Small vortices attach themselves
to the surface travel along, detach and get swept away in the main flow of
the fluid. So whilst the individual stud has to be assessed for the extreme
loading, that load can be shared by other studs if there is a grid which
attaches them together. For example the noggins and plates connect studs,
battens connect rafters, joists connect bearers. Displacement of one leads
to displacement of others.
In Australia our timber framing code AS1684, and domestic metal framing code
AS3623 (now replaced by NASH specification), all contain formula for load
sharing between members. Though from memory they mostly relate to live
loading requirements. For example timber rafters are considered to share
load, if the fascia board has a minimum stiffness. In effect the load is
reduced for the individual rafter.
Thus studs in parapet are probably attached by a plate at the top. So when
one stud experiences the spike in pressure the others don't and can help
resist total displacement of the highly loaded stud. The problem is ASCE7-05
doesn't mention the footprint or area of influence of the vortex causing the
spike: therefore don't know how many studs are influenced at once. So appear
to be stuck applying high load to individual stud with no load sharing.
Gautam Manandhar
List members:
I have a curved beam (horizontal). The center line of the beam layout is horizontally offset from the supporting post by about 10+ % of the span. The arch wants a channel section and will not allow a tube section. Can anyone suggest a quick method to check for flexure and torsion. DL is 480plf , LL = 800 plf. The project in under construction and the detail is due first thing to-morrow morning.
Gautam
.
Mark Johnson
| Can you model it in a FEA program like RISA or SAP? Break it into small pieces if necessary. HTH MJ --- On Tue, 1/13/09, Gautam Manandhar <Gautam_M...@ci.richmond.ca.us> wrote: |
Acharya, Suresh
From: Gautam Manandhar [mailto:Gautam_M...@ci.richmond.ca.us]
Sent: Tuesday, January 13, 2009 4:35 PM
To: sea...@seaint.org
Subject: RE: curved beams
Padmanabhan Rajendran
| The only reference that quickly comes to my mind is AISC's "Torsion Analysis of Steel Members". Rajendran |
--- On Wed, 1/14/09, Gautam Manandhar <Gautam_M...@ci.richmond.ca.us> wrote: |
From: Gautam Manandhar <Gautam_M...@ci.richmond.ca.us> |
Padmanabhan Rajendran
| Guatam, Can you design a tube section to carry all the loads but weld a channel on the outside face (visible face) to keep the arch happy? |
Rajendran --- On Wed, 1/14/09, Gautam Manandhar <Gautam_M...@ci.richmond.ca.us> wrote: |
From: Gautam Manandhar <Gautam_M...@ci.richmond.ca.us> |
Harold Sprague
Regards, Harold Sprague
Date: Wed, 14 Jan 2009 11:37:38 -0800
From: praje...@ymail.com
Subject: RE: curved beams
To: sea...@seaint.org
| The only reference that quickly comes to my mind is AISC's "Torsion Analysis of Steel Members". Rajendran --- On Wed, 1/14/09, Gautam Manandhar <Gautam_M...@ci.richmond.ca.us> wrote: From: Gautam Manandhar <Gautam_M...@ci.richmond.ca.us> |
Windows Live™ Hotmail®: Chat. Store. Share. Do more with mail. See how it works.
Jimenez, Jorge A.
We can ask to Stephen Hawking for an antimatter universe with other gravitational laws ideal for some architects, or help them to realize the limitations of our one.
Jorge Jimenez, PE
From: Harold Sprague
[mailto:sprag...@hotmail.com]
Sent: Wednesday, January 14, 2009
4:57 PM
To: sea...@seaint.org
Subject: RE: curved beams
There is another relatively
painless method contained in Design of Welded Structures by Blodgett.
Regards, Harold Sprague
Garner, Robert
Did anyone watch L.A. Ink where Kat tatoos an Architect?
David Topete
David Topete, SE
Rhkr...@aol.com
Interviewer (pig): "What are you doing, Rat?"
FLR: "I am Frank Lloyd Rat, Master Architect, and this is a scale model of my latest masterpiece, an office building made of nothing but rotating cheese."
Engineer: "This could be really hard to build."
FLR: "My engineer is quite lame."
Ralph Hueston Kratz, S.E.
Richmond CA USA
Jorge Jimenez, PE
Sent: Wednesday, January 14, 2009 4:57 PM
To: sea...@seaint.org
Subject: RE: curved beams
There is another relatively painless method contained in Design of Welded Structures by Blodgett.
Regards, Harold Sprague
From: praje...@ymail.com
Subject: RE: curved beams
To: sea...@seaint.org
The only reference that quickly comes to my mind is AISC's "Torsion Analysis of Steel Members".
Rajendran
--- On Wed, 1/14/09, Gautam Manandhar <Gautam_M...@ci.richmond.ca.us> wrote:
From: Gautam Manandhar <Gautam_M...@ci.richmond.ca.us>
Subject: RE: curved beams
To: sea...@seaint.org
Date: Wednesday, January 14, 2009, 12:35 AM
List members:
I have a curved beam (horizontal). The center line of the beam layout is horizontally offset from the supporting post by about 10+ % of the span. The arch wants a channel section and will not allow a tube section. Can anyone suggest a quick method to check for flexure and torsion. DL is 480plf , LL = 800 plf. The project in under construction and the detail is due first thing to-morrow morning.
Gautam
.
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Daniel Popp
From: "Rhkr...@aol.com" <Rhkr...@aol.com>
To: jaji...@csagroup.com; sea...@seaint.org
Sent: Wednesday, January 14, 2009 12:20:48 PM
Subject: Re: curved beams
Garner, Robert
First the tatoo artist couldn't figure out what the architect had drawn for his tatoo. It made absolutely no sense. When she figured it out, she realized it was not possible to do it in line work. So after figuring out what the Archie wanted, she completely redrew it for practicality, then went to work. But the Architect was afraid to let just anyone know he had a tat, so he had her stop it short just above his short-sleeve line. So you have this fantastic convoluted shape that just seems to stop with loose ends just above his sleeve line - imagine a skyscraper elevation cut off at about mid-level of the first floor with the columns dangling in space. That's his tattoo.
David Topete
David Topete, SE
Jn...@aol.com
Johnson & Nielsen Associates
Palm Springs, CA