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Wind Loads

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Andrew,

I typically will refer to ASCE Transactions, 1961, Part IV, and the paper, 
"Final Report of ASCE Committee on Wind," for unusual structures (and usual 
structures) such as you describe.

The paper has several pages of diagrams of structures and the wind forces on 
various parts from wind impinging from various directions.  These diagrams 
were from wind tunnel tests for the Swiss building code.  Canopy structures, 
such as pitched or inverted gull wing, with and without trains/other 
obstructions adjacent or under are included.  These diagrams had been 
included in early editions (1st and 2nd) of the AITC manual.

If I have a roofed building without walls constructed adjacent to the wall of 
a larger building, I can see no difference between that and a 
canopy/overhang.  The wind can get in and under the roof and can only get out 
by going out the sides.

A number of years ago, I was called out to look at an hanger that had 
suffered wind damage.  The building had one end open and the other three 
sides sheathed and was located at the airport where the National Weather 
Service had its weather station.  IIRC, the open end was about 100' wide and 
had 3 or 4 20'-25' bays perpendicular to the open end.  The roof sheathing 
was ripped off the bay adjacent to the open end and part of the next bay, the 
first two frames had bottom flange buckling between compression flange 
bracing, and 3/8" (or was it 5/8", I'll have to look at the photos) plate in 
the frame's panel zone had failed in tension.  The loss of roof sheathing at 
the open end indicated that the wind got in and could only get out at the 
same location it got in, and that the combination of internal pressure and 
external suction destroyed the roof and the first two frames.

NWS records indicated that the maximum wind speed in that storm was in 60's, 
well below the 75 mph fastest mile design wind that was then in effect.

HTH

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Andrew Kester wrote:

. > 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

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