Sway frame column design moments 1

[tkday]

Per ACI 318-05 Chapter 10.13, if I understand correctly, you allowed to use one of 3 methods to determine your Moments due to sway. If I'm using formula 10-18, what moment am I multiplying the factor by? Will this be my moment produced by eccentric loading?
If this is not the case then what is the difference between the two moments determined in eq.'s 10-15 and 10-16 which should give you your total max moments due to sidesway. They both are defined as factored end moments but ACI-318-05 does not clearly define the mathematical difference, or just reader error (as usual).
Please help

 

[JAE]

For each load combination separately:

[blue]1. You analyze your structure and get separate non-sway moments in your columns (M1 and M2 at each end). This is usually the gravity loads applied to the structure with their load combinations applied per each load combination.

2. Then you analyze it using sway loads (lateral wind, seismic, etc.) with applicable load factors for each combination, and get your sway moments, M1s and M2s.

3. You then calculate the design moment [δ]M1s from Equation 10-18 using M1s = Ms and then calculate [δ]M2s from the same equation using M2s = Ms...again, for each load combination.

4. Your total design moment is then calculated by equations 10-15 and 10-16 for each load combination.[/blue]

You have to do this for all required load combinations and in each lateral direction.

So if you have 14 required load combinations, you will have 14 x (2 directions) = 28 different [δ]'s to calculate.

 

[tkday]

Thank you JAE,
so this brings up another question. For this particular situation I am designing a Caisson. The client has provided a moment and axial load (worst case, both at top) for a round caisson. They claim that earth movement is quite frequent and substantial, thus large moment at the upper end. I've been told to assume pinned at the bottom(no moment) and fixed at the top, where it meets slab. However, it sounds as though sway is typically associated with lateral forces such as wind & seismic. Would I consider this a sway frame, or is my whole approach faulty? These moments are appearantly attributed to seismic earth pressures. I want to design to restrain movement.

Thanks again

 

[JAE]

Well, technically, ACI 318 doesn't apply to drilled piers (see chapter 1).

But under a seismic event, if your pier is providing lateral stability to a structural frame, it may be that you still jump into 318 to design it.

If you have liquefaction potential in the soils during a seismic event, then you can't count on the soil at all to provide lateral stability to the structure - you would just have oatmeal around your piers. If that's the case, then the moment they give you should reflect the moment that occurs without any help from the soil.

If you don't have liquefaction, then ultimately the soil would provide lateral restraint in your structure and the moment at the top of the pier could be such that it can go plastic (i.e. you wouldn't have to count on the column-slab stiffness to provide lateral restraint) and you could possibly consider this as not a sway frame.

My guess is that you would consider it as a sway frame to be safe. (conservative assumption).

 

[tkday]

Thank you thank you. This clears things up. If I can pick you brain once more: it is my understanding that slenderness is partially accounted for in your kL/r. Is the other end of the story included in the reduced EI, and other factors?

 

[JAE]

Not sure what you are asking here. Please describe what you mean by "other end of the story".

What you've described is a drilled pier (column) supporting a mat slab or other type of slab system that are interlocked such that moment can be transfered between pier and slab system. The bottom of the pier is pinned (free to rotate at its end with zero moment).

 

[tkday]

Well I'm treating this as a pinned at bottom fixed at top column which gives me a k=2, and therefore a kl/r of 60 for a 30' column. This puts me above the kl/r=22 cut off for slenderness.I'm curious how slenderness is accounted for in my design, besides my k. Or is this where it's covered? I guess my real question is, if my column were not slender and also resistant to sidesway, am I excluded from 10.11 altogether. O.k. I checked commentary and it seems I answered my own question. Thanks much