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RE: Wind load design for Photovoltaic panel installations

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Very well said, Conrad.

With your permission, I would like to submit your post to my peers at
the San Diego Development Services Department.

Sincerely,

Robert Garner, S.E.

-----Original Message-----
From: Conrad Harrison [mailto:sch.tectonic(--nospam--at)bigpond.com] 
Sent: Wednesday, August 13, 2008 11:55 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Wind load design for Photovoltaic panel installations

Dennis,

Jordan's approach may be the more difficult to sell to plan checkers,
but it
is potentially the more correct approach.

In Wind loading of Structures, by John Holmes, the starting point for
the
whole subject is with aerodynamics and fluid flow. He also provides some
guidance for design of roof mounted solar hot water systems, and
antennae.

Using AS1170.2 we have drag coefficients for various structural
sections,
and also frictional drag requirements over cladding. But still solar
panels
and other roof mounted structures are outside the scope of the code: but
then so are most real things.

Using the code some judgement needs to be made regarding the
interference of
airflow over the roof and the obstruction of the solar panel. Free
monoslope
roofs give some guidance, but gap between panel and roof would have some
form of venturi effect. Then there will be turbulence where the airflow
hits
and leaves the solar panel obstructing airflow over the roof surface.
The
direction of the wind pressure could be uncertain.

Consider another example, a skillion roof lean-to against the wall to a
higher level gable roof. Does the wall experience wind ward positive
pressure, or more a continuation of the roof suction? What height above
the
skillion roof experiences suction, and when does the interference
between
the wall and roof cancel out, and the windward wall pressure dominate? 

Using the wind loading code is highly dependent on making judgement
calls of
worst case conditions for real world buildings which seldom have the
shapes
used in the wind tunnel testing.

It is also a matter of how easy it is for you to apply your own design
factor to magnify design loads to allow for uncertainties, and how well
the
results are then accepted by those who have to pay for the
implementation,
and how practical it is to implement.

It is outside the scope of the code, it can therefore only be a
judgement
call: unless some manufacturer willing to do wind tunnel testing, in
which
case they will have the test results and no other manufacturer will.

Here we don't have the same liability issues as you have in the US, so
here
all parties are generally more willing to take the risk and make and
accept
judgements. If risks are explained then losses are also more acceptable.
Insisting on great expense to strengthen something, then experiencing
loads
greater than design load and still loosing is good cause for owners to
be
unhappy. A balance needs to be made between the expense now, and the
cost of
replacement. The hazard to life, is not addressed by the magnitude of
the
load, but by the mode of failure and circumstances. When a severe
weather
warning is issued then expect hazard to life: not the least of which is
trees being uprooted and crashing through house roofs or bringing cables
and
utility poles down. Direct action of wind on the roof is minor hazard to
life: unless in hurricane or tornado territory.

Also it is not the 50 year mean return period wind speed need to worry
about, but the ultimate limit state wind speed with a mean return period
of
500 years: and roughly 5% probability of being exceeded for a 25 year
life
expectancy for a building: this is the wind which will start to tear the
building apart. Combining this with the 5 percentile resistance, the
probability of the load exceeded and the system below strength is very
low:
more likely to be over strength and under loaded if everything worked
out
right. None the less failure is inevitable. The question is will it
happen
in our life times, will the buildings last long enough that we can move
forward, how often are we hampered from moving forward because of need
to
rebuild: both as a society and as an individual?

My point is that generally things are built before regulations are
imposed.
Regulations are imposed because of perceived hazard or a few failures,
and
initial regulations tend to be prescriptions based on what is known to
be
working versus what is known to be failing.

So when there is no existing specific guidance, any rational
justification
is better than none. Engineering doesn't make it safe; it simply reduces
the
risk of failure compared to having no engineering. With no engineering
best
guess of the risk of failure is 50%: either it will or it won't. With
engineering hope to do better than that.

So one option is to start with the resistance of what the installers
prefer
to do, then work that back to the maximum pressure coefficient which can
be
applied, then compare that against the code. If lucky will get a
coefficient
greater than anything in the code.

Using AS1170.2 I can go one step further, and maintain risk at 5% and
adjust
life expectancy of the structure, on condition do not drop design wind
speed
below 30m/s. (we vary site wind speed with height, then calculate
pressure
compare ASCE7 which varies pressure with height)

I think some solar hot water systems only have a life of about 10 years,
so
why design the support structure for a longer life, if it needs to be
modified for new systems? And who knows how long a solar cell remains
operational before pollution damages it. Of course the support structure
may
be there for as long as the house, with solar panels replaced
occasionally.

Further if solar panels are to be mounted on roof tops, then the
manufacturers should consider designing the panels and support structure
to
suit the potential reserve capacity in the variety of existing roofs
rather
than imposing wide scale assessment of existing dwellings. That is the
panel
manufacturers should do the wind loading research and publish the
recommendations for support structure.

Conventional light timber framing represents the minimum resistance
available, if consider that is over stressed in the first instance then,
zero reserve resistance for additional loading. Therefore need an
independent support structure reaching to roof level if that is
impractical,
unsightly and unacceptable: then the alternative is to accept lower
design
load. A lower design load reduces life expectancy if risk level is
maintained.

And by all accounts from most members of the list conventional timber
framing in the US is considered over stressed when checked against the
structural codes. Therefore if resistance calculated by codes is
considered
accurate, then the design load needs to be lower for it to be
acceptable,
which once again for a given risk means the life expectancy of
conventional
framing is lower than engineered construction. That lower life
expectancy is
acceptable to the community and authors of the codes; otherwise
conventional
framing would be rejected by the regulators.

All structures designed to a code of practice are at risk of failure, it
is
unhelpful to think in terms of providing safety: we are dealing with
uncertainty; there may be no safety at all. Also lawyers don't care what
codes say, but what the designer should have been expected to know.

So sometimes it is necessary to take the difficult path and educate the
building officials, or assist them by getting calculations independently
reviewed before submitting for checking. Another perspective is if the
building officials ask the questions and cause the problems for owners
and
builders alike, then they better be capable of understanding the
solution.
The person in the building department, who posed the problem, usually
can,
and is also likely to accept rational judgement when the codes have no
specific requirements. Also when all the problems and practicalities are
pointed out, building departments are amenable, and introduce some
guidelines for acceptable practice, so as not to cripple local business
and
the building products they are supplying.
  
Whilst most of the work I do comes from manufacturer/builders I don't
much
like such businesses which rely on external consultants. I believe they
should employ engineers on staff and properly design and engineer their
products, and otherwise properly assess individual projects and provide
proper documentation for building approval. Rather than wait until the
building officials issue them with 4 pages of questions, then turn up at
some consultant's office crying for help: and a time constraint to avoid
paying the approval fees again. If the product is appropriately designed
then the manufacturer would be one step in front of the regulators.

Given that the owners and contractors will want a 10 minute solution and
low
fee, and supplying at maybe a rate of one or more per day.

Really need to assess how quickly can get a conservative answer, and
what
risks you are exposed to.

The simple answer is to try pushing some questions back, to both the
manufacturers and the building departments. See what practical
compromise
can be reached.

(Oh! the probability stuff is in the commentary to ASCE7, and also the
book
by Holmes it is also in AS1170.2 but the probability models are for
Australian conditions.)



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



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