Wind load on elevated structures?
Wind load on elevated structures?
(OP)
Which ASCE Section applies on houses that are only a few feet above the ground?
When was the last time you drove down the highway without seeing a commercial truck hauling goods?
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Wind load on elevated structures?
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RE: Wind load on elevated structures?
RE: Wind load on elevated structures?
RE: Wind load on elevated structures?
My project is a beach front house. It will be elevated 4 feet above the ground. The site is flat.
The question is what is the effect of elevating the structure 4 feet on wind load? And where is that covered in ASCE 7
RE: Wind load on elevated structures?
RE: Wind load on elevated structures?
RE: Wind load on elevated structures?
Bernoulli's equation tells us that the pressure above the house will be less than below... which causes uplift.
The same is the case for airplane wings, the top of the wing is longer than the bottom, creating uplift, that's how planes fly...
Not to mention automobiles... The top of the car is obviously longer than the bottom, which causes uplift due to air velocity, so the closer to the ground the car is, the less air can go under the car resulting in less uplift.
I'm not a physicist.. but I'm pretty sure that's how it works.
RE: Wind load on elevated structures?
RE: Wind load on elevated structures?
RE: Wind load on elevated structures?
http://www.youtube.com/watch?v=Krq5IRZGbio
RE: Wind load on elevated structures?
check out F1 cars ... their underbodies are carefully designed to maximise downforce from airflow under the car's body.
the point is the aerodynamics are way too confused to say upforce or downforce will be generated ... ground effects, local trees, landscaping, etc all have an effect. i think the code provided a conservative solution and as an alternative you could spend a tonne of money modelling and analyzing your specific design to prove that in no case is uplift generated ... but it'd be Way easier to work with the code, no?
RE: Wind load on elevated structures?
RE: Wind load on elevated structures?
RE: Wind load on elevated structures?
We need to see this as two examples. First, the wing and resulting uplift. Second, the principals of low and high pressure with regard to a shell structure.
The home with a pitched roof will act like a wing and uplift will be generated on the overall building because it is a longer distance over the roof than under the floor and therefore the wind must travel at a higher velocity over the roof and produces a low pressure above the house. A lower pressure than what is found below the house. Now, whether this uplift will overcome the reduced dead load of the project is what you need to calculate and resist with appropriately located and sized hold-downs.
Now, the house is also similar to a shell structure like a tank in the sense that air flowing past the structure's exterior will make lower pressure than is inside the home. Therefore the pressure inside the home will push out on all the walls (and in this raised case, the floor too). So, this definitely causes there to be a force that will try to rip the floor off the bottom of the house, but I would not consider it a down-force. To be conservative in your design you should neglect this force and only consider the uplift for your hold-down design. However, you should consider this force acting alone when designing the components that will keep the floor attached to the rest of the house because this outward pressure (also known as suction) is a real force that needs to be accounted for.
Seeing as this is a beach house, it is likely that ocean winds will not be obstructed by anything and winds are likely to be frequent and substantial. Definitely take as many conservative approaches as you can in this design.
Note that in the automobile examples given that the shape of the car causes uplift in general because the top is longer from front to back than the bottom (due to distance differences in a curve and straight line). To overcome this uplift affect in high speed vehicles the addition of a "spoiler" or airfoil at the rear, and in some cases the front, of the car. The airfoil is a wing turned upside down to counteract the uplift forces without adding much to drag. Drag is certainly there, but reduced due to its aerodynamic shape.
RE: Wind load on elevated structures?
While a downward force on a floor is a possible, with only a 4' space the force on the house is uplift.
RE: Wind load on elevated structures?
Does that convey a reasonable way of thinking of a simplified model or am I way off?
So, I agree, uplift is the overall result on the structure.
Down force on the floor is not very strong.
No, the floor won't be ripped off the bottom of the structure.
I am just stating that the force on the floor will likely be suction just like the lateral walls and the roof. But when compared to the roof, the roof has a higher suction force and so the overall result will be uplift for the entire structure.
RE: Wind load on elevated structures?
By the Australian wind code the positive pressure coefficient for the windward wall of an elevated building increases from 0.7 to 0.8.