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Wind load on elevated structures? 1
- Thread starter JKJohn
- Start date
Bagman2524
Structural
The wind speed calculated in ASCE-7 chapter 6 is a 10 meter height (or below).
- Thread starter
- #4
Ron, Thanks for the wakeup call.
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
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
SeanStructural
Structural
Wind flowing under the house will cause negative pressure under the house and positive pressure inside the house. This will cause the house to pull down toward the ground. The load is downward. Not uplift
BridgeBuilder88
Structural
The wind flowing over the house has a larger distance to travel, therefore less velocity.
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.
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.
SeanStructural
Structural
Air going under the car causes a drag not uplift!!
Bagman2524
Structural
I'm with BridgeBuilder on this one. The wind will cause an uplift which needs to be checked per the overhang section in ASCE-7
BridgeBuilder88
Structural
Not to point this thread towards cars, but like you said it does cause drag, but also uplift. Have you ever watched GT racing? There are plenty of instances... take a look.
"Air going under the car causes a drag not uplift!!"
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?
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?
SeanStructural
Structural
Which code you mean? I am not sure ASCE 7, addresses this at all?
xyuengineer
Structural
I think it depends on the shape of the floor, if the base floor is flat, treat the floor like inverted roof and thus the force is downward, or suction. This is assuming the predominent wind direction is level not vertical. If the structure is located in the hill or slope, it may be possible to subject to vertical wind load.
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1
- #14
You are all correct, but it seems you do not see that the other guy is correct too. This is how all of your correct statements go together.
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.
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.
First, as referenced in thread isn't drag a horizontal force and not vertical so it would not affect uplift. Second, the wind velocity is less at ground level than at roof level, so the downward force would be minimal. Third, please post a picture showing, or information stating, the floor of a house being sucked out while the roof is still in place.
While a downward force on a floor is a possible, with only a 4' space the force on the house is uplift.
While a downward force on a floor is a possible, with only a 4' space the force on the house is uplift.
Now that the house is on stilts, the floor could be considered another lateral "wall" and experience similar (albeit reduced based on proximity to grade) suction forces that the side walls will experience.
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.
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.
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