uplift calculations for open detached carport 2

[davidecates]

I am a registered Civil PE but work mostly in roadway and storm design. I have a friend who started building a detached open carport. The dimensions are as follows:
20' x 24' and he's using 6 pressure treated 6x6s 12' tall for the columns, roof trusses, OSB plywood roof sheathing, shingles, double 2x8 beams and 4x4 lateral bracing for the columns (with 1' of overhang).
He started construction by digging 30" holes and compacted native soil around them (no concrete). The building inspector came and said he needed an engineer's letter stating that the structure wouldn't blow away.
I have calculated the entire weight of the structure to be 5442 lbs which equates to 907 lbs per column which converts to 4,317 psf downward force on each column (907/[(5.5/12) x (5.5/12)]). My question is how do I calculate the upward force to see if he needs to pour concrete around his columns? Thank you in advance.

 

[davidecates]

I forgot to add that the roof is a gable

 

[msquared48]

You really need to contact a local structural engineer who knows how to used the local wind provisions of the current ASCE7. Just based on a minimum 10 psf uplift, I see a net uplift force on each column without looking at the code provisions, which could make it much worse, especially if you are in a high wind area.

Mike McCann, PE, SE (WA)

 

[JeanLucPicard]

David,

The inspector is right.

You must have some US codes to calculate wind actions, case of uplift or the "explosion" in order to calculate what you're looking for. It should contain winds velocity for a certain region, density, tubroblence factors and so on... I'm from Europe, so I use Eurocode EN1991-1-4 for wind actions together with the national annex. If you're using some kind of structural software like Autodesk ROBOT, by feeding the software with correct codes and structural details, the software itself can calculate the wind actions. If now, I bet there is some US standalone software that could easily calculate what you need.

I dug up my latest design, and have calculated a maximum characteristic uplift W=48,56 kN on one column. This calculation was done on timber hall 30m×30m×11m, location: Croatia, also, it was not multiplied by the safety factors.


I hope this helps!

Live long and prosper!

 

[SteelPE]

A friend who asks you to help him out with this letter is a friend you don't need. You need to tell your friend that you can't provide him with a letter and maybe point him in towards someone who can. Someone who is familiar with calculating wind uplift, more than likely a structural engineer.

 

[Joel900]

Using ASCE7-10 C&C method for open pitched roof and assuming 120 MPH wind speed, exposure C, Kzt = 1, Kd = 0.85, Gust factor of 0.85, and a roof mean height of 15 feet, 4/12 slope, and clear wind flow ... Being that the tributary area of a post is greater than 4*a^2, I calculate the ultimate uplift pressure to be 22 psf.

Using 0.6D+0.6W, the net uplift for a column is roughly 512# which equates to needing about 6 cubic feet of concrete to satisfy equilibrium with 0.6D+0.6W.

 

[hokie66]

And it is not just uplift. There will be lateral load, and the posts may not be able to handle it as cantilevers.

 

[JStephen]

Be prepared for the "But Jim Bob build one just like this, and it's been there for 40 years, you must be doing something wrong!"

Realistically, there are probably a LOT of carports in use that don't meet any building standard, but then again, if they get blown away, nobody's too worried about it.

 

[hokie66]

Nobody, except the neighbour where the roof lands.

 

[Ron]

Agree with hokie66. OP said the roof is gabled....picks up a lot of lateral load in two directions plus oblique.