Wind load uplift on Foundation question

[SYLK]

I am designing an open mono-slope structure and have the MWFRS & C&C wind pressures calculated.

I took the wind pressures from C&C and multiply the zone areas with their respective area pressures, adding the zones downward forces and zones uplift forces individually, I obtained the total downward force and uplift force for the entire building.

I have 6 columns in 2 x 3 configuration. That means the two central columns would carried twice the load (by tributary area) of that by each of the other 4 columns. I am designing an isolated footing to support each column loads.

Here are my questions. 1.) Why is the uplift load combination 0.6+W ?
My confusion is: Dead Load is downward force and the uplift force is upward force, and adding the 0.6D to W would increase the upward force: say, my wind uplift force is 3000 lb and my superstructure DL is 1000, that would mean 600+3000 = 3600lb. Should we subtract 600 from 3000?

2.) My 3000 lb uplift force is from C&C and is an LRFD load. That need to be changed to 3000 x 0.6= 1800 to ASD's, correct? Then 0.6D + W is = 600 +1800 = 2400 lb - that is the uplift force for the foundation design?

 

[GEE8B]

Dead load is always downward (direction of gravity). Wind load can be upward or downward (suction/pressure). Wind forces away from the surface are (-). Wind forces towards the surface are (+). So the “W” in 0.6D+W can be negative or positive. Pressure and suction values are typically per ASCE7, and it is important that you stay consistent with your signage.

Note too that the load combination in question is at strength level (i.e., LRFD). The “corresponding” service level (ASD) combination is 0.6D+0.6W.

Again, it depends on direction of the wind resultant. If it is upward, then dead load opposes it. If it downward, then it is cumulative with dead load. Dead load will never increase a net uplift, but a wind load can increase a cumulative downforce.

 

[phamENG]

GEE8B, good summary of wind load directions (have schools stopped teaching vectors all of a sudden!?), but I disagree on the load combination.

0.6D+W is an ASD combination from ASCE 7-05 and earlier. It changed to 0.6D+0.6W with 7-10 when wind load calculation shifted to strength design levels.

Similarly LRFD went from 0.9D+1.6W to 0.9D+1.0W.

Don't forget there's also a D+0.6W (or D+W for older codes) to use when wind and dead load are acting in the same direction.

 

[GEE8B]

And your disagreement is warranted.

Mistake is mine RE: the LRFD form of the LC. 0.9D+1.0W was the intended reference. Whiffed badly on that in my haste. Thanks for the correction, Pham.

 

[Aesur]

@SYLK - I believe I saw in another post that you didn't have a copy of ASCE, you really need to get a full copy of this; your company should have it, if they don't, make them purchase it. Is there a reason you are working on a structural design without a mentor? I don't mean to sound rude, but the posts you have been creating and the questions you are asking have all been for the same structure and very similar, and it is my opinion that you need a proper mentor.

In addition to what GEE8B and phamENG said above, you should also consider MWFRS for foundation design, for open structures, sometimes the MWFRS loading is actually higher than the C&C loading based on trib to the column/footing.

 

[SYLK]

thanks @ Aesur, I didn't work on structural projects and therefore won't be able to get a mentor. This is just a one time project I wanted to build at my backyard. When you first posted the C&C data you ran for me I had a hard time understanding those as I didn't have ASCE 7-16 chapter 30. A friend of mine explained to me and help me with the data I needed. I am great to see professionals here wanted to help and make sure we did the right thing.