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Re: Wind Loads

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Andrew,

Last year I completed the design of two canopy structures for a bus terminal project on the West Coast and in a desert location.  One was a typical gas station structure over the refueling islands and the other was a large parking lot shade structure (90 ft by 120 ft).  We agonized over all the same issues as you have.  The criteria we came up with was rather simple and we feel conservative enough.  This was a UBC design however we came up with our own force/pressure coefficients.  We used the following UBC Cq values (similar to ASCE Cf):

0.70 up
0.30 down
1.30 horizontal

We took the worse cases for total uplift and downward pressures to design the trusses which were spaced every 24 feet.  The shade structure has tall multilevel moment frames down the center so for overturning we took the maximum uplift on one side, the maximum down pressure on the other, and full horizontal on the projected area.

Thomas Hunt, S.E.
ABS Consulting




"Andrew D. Kester" <andrew(--nospam--at)baeonline.com>

02/20/2003 05:54 AM
Please respond to seaint

       
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        Subject:        Wind Loads



This is an age old questions in our office, and I have asked professors and
lecturers and many others this question, with no definitive response.

According to FL Building Code 2001 / IBC/ ASCE  (you pick), how would you
model a canopy structure? What we have always done is model it as an "open
building", then check loads according to ASCE. Then we have compared those
results with overhang loads (when the canopy is attached directly to a
building) according to the FBC. Well, FBC overhang loads always govern, and
seem plenty high and thus conservative. It usually does not matter too much
because canopies are a small component, so if we are overdesigning a bit it
does not make much of a difference in cost. It seems justifiable to use an
overhang load when the canopy is directly connected to the main structure
because the way the wind is hitting the structure and loading the canopy,
with pressure building up underneath and suction forces on the top, should
be like that of an overhang. This type of structure, at least in FL (it
rains a lot) and most places come think of it, are outside almost every
building entrance, especially long ones outside of shopping plazas, grocery
stores, hospital and hotel entrances, and in FL, there are lots of these at
schools connecting outside classrooms. In residences, these are carports and
porches.

PROBLEM- how should you model a freestanding canopy, that is either far away
from the main structure, or not connected at all? For example, I am thinking
first of gas station canopies because of their prevalance. But our specific
issue is that of a toll plaza canopy. This has a depth of 10' and a length
of over 120'. It stands around 20' off of the ground. It is completely
seperate from any structure. Unlike the shallow depth of a gas station type
canopy, with little issue with lateral loading, this has 10' of fascia that
is exposed to lateral loads. Should you model this as a MWFRS, as an
enclosed building? It has a ceiling, walls, and roof, but it is 20ft off the
ground. It is not pressurized inside the canopy. Or as a partially enclosed
or open building? We thought of even using AASHTO sign loads or ASCE sign
loads, but then again, this is not really a sign at all. Component and
cladding loading is a seperate issue.

Any insight or "what I do"'s would be more then welcome. Basically, my
answer from "wind experts" is that they are doing testing, and more testing
needs to be done, and they need to add this to the building codes or ref
docs like ASCE. I see this as a major gap in the code as most buildings have
this type of structure, it is extremely prevalent and common. Also, if you
look at historical storm damage, these types of structures seem to be the
first or only part of the structure to fail or sustain the most damage.
These lightweight structures can also become flying debris. So I see them as
a very succeptible part of a structure in wind loading. This should be in
the codes specifically if you ask me.


Thanks,

Andrew Kester, EI
Longwood, FL