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wind loads II

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Scott - The wind load mitigation bill died in congress in committees,
whatever that means. According to .

Thank you everyone for your views and insight on wind loads. The distinction
I would like to make is that I am talking about wind LOADS, not design of
lateral systems and such to resist them. That part is relatively
straightforward. You do not have all these problems with ductility,
resononance, natural frequency, etc. like with seismic. My problem is it
seems that we know plenty about seismic LOADS (I could be way off, not my
area of expertise), but we still do not have a complete picture of wind
LOADS. We all seem to have some problems determing which wind loads to use
for a particular structure, but once we have that, the structural analysis
part is pretty straightforward because they are treated as static loads in
simple PSF terms.

I think this is easily solved with more money and wind tunnel testing on
different types of structures. Maybe we have lots to learn about both, but
still it seems the focus of most research and funds is on seismic despite
the annual $40B in damage from wind loads. OF course, the damage due to wind
loads could have a lot less to do with engineering knowledge and more to do
with poor codes, poor residential construction, etc.

Now, for are particular case we are dealing with now, it is a highly
scrutinized job for the DOT, so we want to cover all of our bases. This is
also a very unique structure, unlike the other canopies we have dealt with,
it is not attached or part of another building. It does have a building at
one end, but the effects are negligible. Just so to be clear, this is a
canopy over a toll plaza on a toll road. It is massive in that it is over
100' long and 10' deep at the fascias and parapets. It has metal deck on the
roof and ceiling. The ceiling is about 19ft off the ground.

This is what we came up with, after analyzing it several different ways.
FBC= Florida Building Code, load tables developed from ASCE I believe.

Specific loads for our structure, so you can see the difference:

ASCE 7-98
Open Buildings
Roof Uplift= 23.5psf, No lateral loads given

ASCE 7-98
Sign Loads (to be used for lateral loads to fascia, etc.)
P lat=38.1psf

ASCE 7-98
Partially Enclosed
Roof uplift= 43.6psf
P lat= 35.5psf

Roof Overhang
Int zone = 44psf
Ext zone= 56.2psf

Lateral Loads (both surfaces, entire structure)
IZ= 22.2

OK, lot to digest there. So if we use overhang loads, these are at least 2x
that of the open building loads. They do closely match the partially
enclosed loads from ASCE.

Lateral loads of ASCE sign loads are close to that of partially enclosed
lateral loads.

Our conclusion is to use the Partially Enclosed ASCE loads as a basis, but
slightly increased when compared to the other loads. This way all cases have
been considered and this has to be the most conservative method of modeling
this structure.

P roof= 45psf
P lat= 38.5psf

Now if you read Scott's response, I agree with him that the way to model a
canopy attached to a structure, like at a hotel entrance, shopping plaza or
grocery store, etc., is to treat it like a MWFRS overhang (or C C if that is
what you are designing). I understand this large increase in uplift due to
two things:
-direct uplift as the wind passes over the top surface of the overhang
-pressure from beneath, on the ceiling or soffit of the overhang, due to the
wind hitting the surface of the building and being trapped underneath the
overhang/canopy, and having no where to go but up. This trapped pressure to
me is much like a partially enclosed building, probably why the uplift loads
of overhangs and partially enclosed buildings are so close.

So my conclusion:

If a canopy is attached to a wall or building, then use overhang loads. This
building or structure will impede the flow of the wind causing a build up of
pressure, much like in a partially enclosed building.

A free standing canopy, like at a gas station or a park pavilion, would be
modeled using Open Building loads, with lateral wind loads taken from Signs
(as Tripp suggested I believe).

But since our structure seems of extreme importance (failure could block a
major thoroughfare in an evacuation or return), has such a deep fascia, and
is unlike the above mentioned structures, we are using higher , more
conservative loads as I gave above.

Any further comments, criticisms, or opinions are gladly welcomed.


Andrew D. Kester, EI

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