ASD Footing with an LRFD Structure

[ElliottJames]

Older PE here. I admit I'm rusty and I'm doing my best to get up to current methods and standards.

Project is a self-supported lattice tower with a simple gravity mass footing. Tower mfr. shared some sample calcs with me. Calcs show LRFD basis for wind force and sizing the lattice members (1.2DL and 1.6LL). I'm fine with that. They show a tower moment capacity of 20,000 ft-lbs at the design wind speed. They then take the 20,000 ft-lbs and reduce it to 12,500 ft-lbs, along with a presumptive 1500 psf soil bearing strength, and check overturning moment "with ASD."

Why not take the wind force and the tower as moment arm getting the overturning moment with simple statics, thus using DL+LL with the 1500 psf soil strength. If the tower members are sized with 1.2DL+1.6LL, then the mfr's calcs are suggesting (1.2DL+1.6LL)/1.6 = overturning moment. That confounds me. Which approach is more reasonable?

 

[Enhineyero]

The tower mfr.'s calculation is the usual approach for structures designed using LRFD.

I believe the 20,000 ft-lbs moment is mostly due to the wind load (which is at a the design level, factored load). Now the foundation will be sized using 1500psf soil bearing capacity, which is in a service level, therefore the 20,000 ft-lbs needs to be reduced into a service level "ASD" load. Otherwise, the footing size would be excessive.

Then check the over-turning using the total gravity loads of the tower and foundation against the moment caused by the wind. (all loads in service level)

 

[dik]

You may request the 'breakdown' of the loading into DL, LL and WL... when I do a load takedown, I usually have the various components and divide them by the load factor to get the service load to design the foundations.

Dik

 

[bpstruct]

I too would request a breakdown. If you have the calcs, you might find what you need buried in there somewhere. But I wouldn't try to back in to it. We waste too much time doing that sort of thing....at least I know I do.

 

[ElliottJames]

Thank all of you very much for your responses. Enhineyero your explanation was clear and helpful -- and you're right, using your suggested method keeps the footing size reasonable.

 

[RFreund]

I would think you would have a "compression case" and a "uplift case".

For Compression:
LRFD = 1.2DL + 1.6WL
Equivalent ASD per mentioned in the OP: 1.2/1.6DL + 1.6/1.6WL = 0.75DL + 1.0WL
ASD = 1.0DL + 1.0WL
Equivalent method is slightly unconservative.

Fore uplift:
LRFD = 0.9DL + 1.6WL
Equivalent ASD per mentioned in the OP: 0.9/1.6DL + 1.6/1.6WL = 0.5625DL + 1.0WL
ASD = 0.6DL + 1.0WL
Equivalent method is slightly conservative for uplift.

EIT

http://www.HowToEngineer.com

 

[msquared48]

I talked with Peter at TNXTower about three or four months ago on this, and he told me that sometime in the future, they were going to provide the working reactions as well as the factored reactions in the printout. Doesn't help much now, but to approximate, I just divide the current reaction by 1.5 and run with it for the foundation reactions. Depending on the relationship of dead to live loads, 1.4 could even be more appropriate...

In actuality, depending on what load case controls the various components of the reactions, that adjustment factor unfortunately will vary. It would be nice to know and it's just a tweek in the computer program...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com