Multi-Story Pattern Live w/ Lateral
Multi-Story Pattern Live w/ Lateral
(OP)
In multi-story buildings, we of course design concrete beams for skipped live loads for gravity combination. Now, looking at the overall building under seismic or wind, I know high-rise engineers that routinely ignore pattern live loadings in combination with lateral.
I can't find anywhere in ACI or IBC that allows you to ignore skipping the live load on the same floor level or disregarding patterning it on or alternate levels in combination with lateral. I don't think that ETABS or RAM will even combine skipped live with lateral which may be part of the reason they are ignoring it as well.
Do you think this is being overly conservative if I combine the effects of pattern live loads with lateral for a typical office building?






RE: Multi-Story Pattern Live w/ Lateral
RE: Multi-Story Pattern Live w/ Lateral
There are some levels with storage and mech room live (100 psf) that other levels do not have. I don't know.
What if it were a multi-level parking deck where there were a much greater chance of pattern loading during seismic? Still overkill?
RE: Multi-Story Pattern Live w/ Lateral
I think it would be quite rediculous to try to checkerboard live loadings all over the building while simultaniously performing a lateral analysis especially considering the live load being designed for is probably 4 times what will every be there and the seismic load being checked is being determined based on a largely arbitrary R factor.
RE: Multi-Story Pattern Live w/ Lateral
It does seem that patterning a live load that is already high to meet a certain probability of exceedance, with a load pattern that is unlikely to actually occur, is overkill.
Without intending to take the thread off topic, why would you consider the inelastic reponse modification factor to be arbitrary, Willis?
RE: Multi-Story Pattern Live w/ Lateral
RE: Multi-Story Pattern Live w/ Lateral
One immediate problem I see with a catmatic reading of the pattern loading code language is practicality.
I'm not sure it's even possible to pattern load the structure to ensure the absolute worst load on every element, from a roof beam moment connection down to each anchor rod. That might be a thousand load combinations for all I know.
Also, how do second-order effects come into all this? If I put the live load everywhere, that would cause second-order effects to be larger than if I leave off some LL.
In 3D, do I need to leave off live load strategically to make the most severe centroid location, simultaneously considering pattern live load, FOR every little element of the structure?!?
LOL, it's a good question, but I think this subject is best left "not thought about" any deeper than folks usually do. :)
RE: Multi-Story Pattern Live w/ Lateral
"For determining column, wall, and beam moments and shears caused by gravity loads, the code permits the use of a model limited to the beams in the level considered and the columns above and below that level."
Reading through ACI R8.9, I remember my concrete professor saying you can take the gravity moments from a model that considers the columns directly below and above the floor being analyzed as fixed and then add these gravity moments to the moments found from a separate full lateral model that is full height or approximated by portal method etc..
271828:
If I used this allowed simplified gravity model, then I can automatically disregard a full height live load pattern analysis (with or without lateral included) as we have all seen in structural analysis books to get column moments. I would only be left with the decision to combine the worst skipped moments from the gravity model at any one level with the overall lateral moments. Which I agree would still be an overkill in itself.
RE: Multi-Story Pattern Live w/ Lateral
And I'd like to take this opportunity to vent on our revered academians out there who write the codes. This is a prime example of how individual code provisions, when combined, create a exhausting complexity that most engineers must resolve to either ignore, or fake, in their designs.
As 271828 mentions above: "That might be a thousand load combinations..." to meet the code. This is correct.
To meet the technical intent of the code we'd have some sort of patterned load combination on every bay, odd/even bays, adjacent bays, in both directions for each floor. And technically, there are patterned combinations for each story (odd/even/adjacent stories) that would have to combine with the odd/even/adjacent bays.
My gripe is with the apparent ignorance of the code writers in understanding that their silence on this issue within the code is appalling.
RE: Multi-Story Pattern Live w/ Lateral
Forgetting about the lateral part and only thinking about dead+live:
For concrete buildings, I don't think you would have to check story to story pattern loads since you can use a single floor model per ACI 8.9. But for steel buildings where there are moment frames all over the place, I think you would have to check all patterns on the same floor and floor to floor patterns since there is no single floor model assumption allowed, correct?
RE: Multi-Story Pattern Live w/ Lateral
RE: Multi-Story Pattern Live w/ Lateral
1. Base plate moments and axial loads. Rememeber that some skip loading patterns will have bigger moments, but smaller axial loads. Haven't started talking about shears yet...
2. Positive moment on every beam.
3. Negative moment on every beam.
4. Unbalanced moment at every beam-column connection (panel zone check).
5. Every beam shear, both ends.
6. Column moments.
I'm sure there are others.
I'd be willing to bet $2 (LOL) that it would take hundreds if not thousands of LCs to ensure that each of those was maximized individually. It would really be interesting to start drawing influence lines of such a frame and see how the LCs start piling up.
The floor-by-floor approach seems more reasonable to me.
RE: Multi-Story Pattern Live w/ Lateral
RE: Multi-Story Pattern Live w/ Lateral
RE: Multi-Story Pattern Live w/ Lateral
If I had to analyze a 5-story, 4x4 bay all moment frame steel building, I honestly would not know what to do about all this pattern checking. I was kind of hoping someone on here has done this before and could tell us what simplifying assumptions they used. (I assume we are only talking about gravity only now and ignoring the skipped live combined with lateral stuff.)
RE: Multi-Story Pattern Live w/ Lateral
haynewp - yep - I agree with the ACI "fix the far end of the columns" provision so that technically floor-to-floor pattern loading for concrete structures can probably be ignored....but again, if you're analyzing a full moment frame for a multi-story building, I get so tempted to consider it due to its extreme influence on column moments.
RE: Multi-Story Pattern Live w/ Lateral
RE: Multi-Story Pattern Live w/ Lateral