High uplift on a braced frame

[UJS]

Hello. I am currently designing a platform for a client who does not want any footing. The column is connected to a base plate and grouted around the plate. I have been getting an uplift of around 24 Kips, while designing with Risa 3D, which does not seem logical. I went with dynamic analysis for seismic, which is giving me such high uplift. I also tried with ELF method and although the uplift is lower, it is still around 16 kips. Is there a way to reduce the uplift to around 9-10 kips?

 

[driftLimiter]

No footing for a braced frame eh? Curious where the 9-10 kips is going since thats what your asking for.

Anyhow, there is one place you can look to reduce seismic uplift demand on foundations....
Review ASCE7-22 section 12.13.4 (this section exists in older versions of the code as well).

Still curious how a braced frame works with no foundation. What about the downward loading and stability of it? 10 kips is an awful lot of slab to rely on.

 

[UJS]

That's what clients want. They do have a concrete floor (about 7" thick) and the posts are supposed to land on top of it, connected to a base plate. The loading is just DL and LL for platforms, staircases and some equipment loads.

 

[driftLimiter]

Okay so thats what they want. But you have your analysis saying i need this much upward and downward reaction capacity. The question is can you achieve adequate performance with the 7" slab ?

 

[lexpatrie]

With a legit footing.

Just because somebody "wants" something doesn't make you obligated to provide it. Life-Safety is paramount.

 

[phamENG]

Sounds like that client needs an education. Either give them that education, or decline the job. You can't safely do what is being proposed.

 

[JLNJ]

How did you get so much uplift on just a platform? Something doesn't seem right. Please give us a little more info.

 

[JoshPlumSE]

Okay so thats what they want. But you have your analysis saying i need this much upward and downward reaction capacity. The question is can you achieve adequate performance with the 7" slab ?

I had a client once (when I was a junior engineer) who had lots of experience with their slab and had restrictions on what we could do when attaching to it based on the previous work that they'd done.... Because they knew EXACTLY what the slab could take. Granted, this was a nuclear facility.

Some comments about the dynamic analysis in RISA (or any other program):
1) When you do a Response Spectra analysis, this is a statistical combination of multiple modal results. For this reason, it loses it's signage. Think the simple SRSS combination method where you have + 10 kip (uplift) for one mode and -5 (downward) load for another mode. SRSS result = sqrt(10^2 + -5^2) = +11.2

2) If you have multiple RSA load cases at the same time, you need to be cautious about how you combine them. Don't just add them together. Like if you have the same numbers +10k reaction for an ELX and +5k rection for an ELY response spectra, then you'd get 15 kips rather then the 11.2 you'd get if you used an SRSS of the two earthquake results.

3) Alternatively, you could use the 100% + 30% combination method which would give you 11.15 kips. This method is merely an approximation of a more statistical method of combination.

 

[strucbells]

I don't know why you would do an RSA for a platform. A simple ELF model seems sufficient.

Can you give us an idea of the size and construction of the platform? Is seismic or wind dominating your lateral?

Usually for miscellaneous industrial platforms (think bar grating, handrail and framing, steel or aluminum construction) that I have worked on the dead load and seismic weight is pretty small so seismic loads are relatively small. Usually wind (if this is exterior) isn't too bad either because they don't have a large profile.

Doesn't seem like that big of a deal to sawcut out a section of slab, dig out a little footing and pour some concrete, but I don't know your client's facility or limitations.

Also, depending on the relative size of your platform and your connection details, your stair stringers may also be providing bracing to your platform.

 

[KootK]

Is there a way to reduce the uplift to around 9-10 kips?

If it's spatially acceptable, You might use two adjacent braced frames to increase the lever arm of the entire braced frame assembly.

If this would get you to a load that you'd be willing to resist with the SOG, it may well be cheaper than foundation work.

 

[UJS]

Thank you everyone for your inputs. I found out that I was using a very minimum response modification factor. When I went back and used the correct one, I ended up with 5.7 Kips of uplift. Is that somewhat reasonable?

 

[strucbells]

How would we have any idea with no information about the dimensions or construction of the platform or the loads it is subject to?

1 kip of uplift or 100 kips of uplift could be "somewhat reasonable" based on the info you have provided to us.

 

[structSU10]

do a simple hand calc to validate the RISA model, that is how you know if your result is reasonable or not. Then have a senior engineer at your company check your work.