Braced Frame Spread Footings 1

[marinaman]

I hope by asking this question, it may lend clarity to all:

Braced frame footing design is not clearly defined anywhere that I can find. I have looked in the AISC design guides, in ASCE 7, in numerous foundation design books and guides that I have, on the internet, and within this forum. I can never find a clearly defined procedure for the design of braced frame shallow spread footings. That said, I'm going to pose a scenario here, and I'd like to hear feedback as to how you guys go about the design.

Lets say you have a chevron braced frame. Column bases are pinned. The gravity loads have been developed. The lateral forces have been developed using Equivalent Lat Force. The braced frame has been analyzed via RISA 2D. The braced frame column reaction, on one of the columns, is as follows:

Dead Load: 300 kips (150 kips are composite floors and steel framing)
Live Load: 280 kips
Seismic Vertical Load: +/- 200 kips
Seismic Horz Load: +/- 60 kips

ASCE 7: 12.8.5, says that the structure must be designed for overturning.

That said, I'd like to hear how you guys design your footing size (mass) for uplift.

Are you taking ASCE 2.4.1 load combo "8" and simply setting 0.6D = 0.7E (uplift)......and whatever the 0.6D is short, you make up for via mass of the footing?
Are you taking ASCE 2.4.1 load combo "8", figuring the reduced .6 dead, taking the 0.7 earthquake uplift and multiplying by 1.5 (factor of safety), subtracting the reduced dead from the 1.5 earthquake uplift, and then the "net" is the footing mass needed?
Or some other method?

I keep hearing all kinds of thoughts on this. I would like to have a definitive answer on determining footing mass v/s seismic uplift at braced frame columns.

 

[Once20036]

I`m in the first camp, and include top bars in these footings so that the mass being used to resist the uplift forces is fully reinforced.

In your second option, where is the FS=1.5 coming from? The only spot that i`ve seen that is the retaining wall section of the code, which doesn't require the 0.6D combination.
I think 0.6D & a FS=1.5 is duplicating two methods to achieve the same goal.

 

[phamENG]

The first one. Footing self weight is dead load, and it gets worked into the load combination the same as any other dead load. No need to introduce an additional safety factor.

 

[driftLimiter]

I use the 0.6D + 0.7 E LC.

To check against 1.5 FOS for overturning, I get the factored OTM and the Factored RM. The RM > 1.5*OTM.

As far as the actual resistance you have your Dead loads on the the frame. The mass of the footing. Mass of any soil on top of the footing (or a cone if you want to push it). Also include some mass tributary of other foundations or slab.

 

[marinaman]

driftlimiter's comment is where my confusion is coming from.

The 1.5 Factor of Safety v/s overturning is a common comment among the more experienced guys in the office, but the younger guys tell me that its 0.6D = 0.7E and that the 0.6 is the old 1.5 Factor of Safety.

I think when using the 0.6D = 0.7E for uplift, there's no need to check anything using the 1.5 FOS anywhere....but our more experienced guys say yes, use the 1.5 also.

Confused.

 

[driftLimiter]

Yea I wrote my response and after realized the confusing part. Seems like 1.5x for seismic overturning of frames isn't a code reference.
I checked the last one that I did and I used 0.6D+0.7E with a minimum FOS against overturning of 1.1.

This was based on the 1.1 FOS requirement for retaining walls under earthquake overturning. I realize that this isn't a code requirement for braced frames.
TBH I wanted a lot more resistance because I was trying to develop the brace tension, but everywhere you look into this in the code it seems like it is pushing you to not worry so much about it.

The code allows you to reduce seismic overturning effects at the foundation, allows you to not used omega or capacity limited loads there as well.

All of my mentors always check overturning against the FOS 1.5 and 1.1 even with load combinations maybe this is overkill now that you have us looking.

 

[JAE]

The 0.6 factor on DL is essentially your 1.5 safety factor. (1/0.6 x 0.9 = 1.5)
If you include your footing mass in the DL then the 0.6 applies to the overall DL.
If your footing mass is not in the DL, then a separate 0.6 factor on it is warranted.

The 0.7 factor on E is simply bringing the ultimate E value down to a "service" level to use with service DL in checking overturning.