huangyhg

wind load on an open structure
i am currently looking at an open structure for a project. the problem i am having is with the h/l ratio. per asce 7-05, figure 6-18b, the minimum h/l ratio is 0.25. my structure has an h/l ratio of approximately 0.16.
i looked through the asce 7, and did not find an alternative way to determine the wind loads for the building.
i am considering just ignoring the ratio and proceeding with the analysis, but i was wondering if anyone knew of a "legal" alternate way of doing this analysis, or any other suggestions on this matter.
thanks
that's a good one. the values are based on wind tunnel testing, so there's really no way to quantify what the effect would be.
for a very squat building, there might be some addtional pressures from the wind entering and leaving the open sides. it would be very conservative to assume the ends are solid and add those forces to the figure 18b forces if that doesn't greatly change your design.
well...i should have been a little clearer on my first post. my structure is a prefab metal building which will serve as an overhead cover for fish hatcheries. all sides are open, the roof has a 4/12 pitch, a 152' span, and a 13' eave height. basically this results in a h/l ratio of about 0.16.
what is frustrating is the asce 7-05 gives no parameters of what to do when you don't meet the minimum or maximum h/l.
i'd use the same parameters for the upward/downward roof forces and add in the horizontal force as if the sides were solid. i don't think the roof forces will increase, but the wind trying to squeeze past that 13 ft eave might create a horizontal force.
as far as asce 7 not having any parameters for your case, you've got to admit it's somewhat unusual.
depending on how you finish the ceiling, open walls could create some interesting effects on vertical pressure. the national building code of canada (nbc) has a number of coefficients listed for a variety of roof shapes.
i do not know what "h" is in your code, but if h/l is about 0.16, i am guessing that "h" is either the height of the sloping roof or the vertical distance from the ground to the middle of the slope.
if you are interested, i could send you some material from the nbc.
best regards,
ba
what are the dimensions of your building.
for all intensive purposes, this could be considered an agricultural building, therefore, no "finishes" to really speak of.
the 'h' dimension, per asce 7, is the mean roof height...the average of the dimension between the eave height and the top of the peak.
the plan dimensions of the building are 152'x 354'. the eave height is 13' with a gable roof slope of 4/12.
i'm sorry rwpe, i just can't help it. the phrase is "for all intents and purposes."
in the nbc, "h" is called "reference height" and is defined the same way, i.e. mid-height of sloping roof. the only way it enters the calculation is its effect on the exposure factor, ce. if the building is in open terrain, ce is defined as (h/10)^0.2 where h is in meters.
in your case, h = 13 + 152/12 = 25.7' or 7.82m, giving a value of ce of 0.952, close enough to 1.0.
hope that helps.
best regards,
ba
i suggest you treat this structure as a hybrid.
use 16-8b for 26' at the eaves and 6-11d for the middle 100 ft.
the rational behind this approach is that the greatest turbulence occurs at the eaves.
just to be safe, i would check using 16-8b for the whole roof and compare it with the above approach to see which produces the greatest horizontal and vertical loads and go with the highest load.
it gets even more complicated if you have a back wall.
just be glad you didn't have to design this with asce 7-02.
figure 16-8b didn't exist.

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