1. Feb 4, 2013

### Physics_Kid

this falls into civil engineering, but as we all know everything boils back to physics. and, i couldnt find any good civil forums to join.

so, i have a basic Q but need help understanding some things.

1) i am building patio cover approx 50ft long x 14ft off the house. it has a pitch around 6 degrees.
2) i need to submit design loads. wind load is my last piece. i need to calc at 90mph. but at this low angle and low elevation (~8ft) its more like a airfoil, a very flat airfoil. what airfoil equations to use? i initially though i would calc max downforce or uplift by considering the area as a vertical plane using the UBC '97 equation force=A*Ce*Cq*Qs, but this may yield way more force than i need to account for.

any ideas here?

2. Feb 4, 2013

How far in depth do you want to go? You could write a program to calculate the net up/down force that results from flow over geometry that you input. This method basically calculates the pressure coefficient at each geometry point and integrates it over the surface to determine a net force value. This is how I've done it before and would give much better results than assuming it's an airfoil.

I'd be more concerned about the force exerted on it by the air due to the surface area of the cover.

3. Feb 4, 2013

### Physics_Kid

the UBC equation is the force on the complete area @ 90MPH, but UBC is for vertical walls. i dont think my small flat cover will see wind vectors that are completely vertical on either top or bottom side (although a 90mph downward micro-burst is possible but not likely).

its a open cover so i dont expect to see large pressure diffs between top and bottom areas. the wind load #'s are never likely to be seen, but town code says it must be included in the design, etc.

4. Feb 4, 2013

Why not just assume it's possible and state that it's a very conservative estimate? That's what I would do.

5. Feb 5, 2013

### Physics_Kid

i am not 100% that the std UBC calc for a vertical wall yields greater force than say a 90mph wind gust where the air moves rapidly down the roof and over the cover. this would fall into airfoil arena, no?

i'll ask my town folks what they expect for a wind calc.

6. Feb 5, 2013

### 256bits

You do have a national building code, no?
I believe structures in the USA have to withstand a 90mph wind.
You could design it as an open roof or as an overhang with the guidlines from the code, but it takes some time to figure out the formulas and designations.
Ask your townfolk what is the latest edition applicable to your structure, and there might be a copy in the library for you to peruse.
The code bases a design on downforce and uplift.

Last edited by a moderator: May 6, 2017
7. Feb 5, 2013

### Physics_Kid

yes, downforce and uplift is what we design to, but those are vector summations. an uplift is most likely due to air pressure under the cover (like a strong ground wind coming low and pressurizing the air under the open cover, would need a lot of this to get significant pressure diff), and air pressure due to the cover acting like an airfoil.

the other std force i have is UBC calculation for basic plate that is subject to a wind at 90 degrees.

pic of how i might see a force from acting like an airfoil.
http://img221.imageshack.us/img221/8441/67599143.png