Recommendations for live load value of existing residential highriser 2

[Iasonasx]

I was asked to check if the columns of a 30-story condominium building are properly designed to today's standards to carry the loads they are supposed to. I can use the ASCE 7-22 for the live load reduction etc, but I am not sure if I should just stick to a 40 psf load for Live load value that is the minimum we recommend. I never design for minimum even though I know that statistically that is already actually very conservative. Sure we have 100 psf for corridors, but even with that, in a plan that looks more like a labyrinth with corridors going around and units in really odd forms, I wonder if I could get a general idea of what is a good average value to consider. With design specifications for f'c of 3ksi, I am skeptical about how conservative I should be. Any suggestions?

 

[mtu1972]

My career started just over 50 years ago in Michigan. I worked on commercial buildings, but nothing over six stories tall. 3 ksi concrete was the standard back then. As I had learned LFRD (now USD) my boss checked all of my early calcs as to how they compared to his WSD results. Part way through my first year, he was OK with the new analysis method. I also think the load factors were 1.4D + 1.7L back then.

Those buildings included hotels, hospitals, apartment buildings, offices, and schools. We used 40 psf for residential areas and 100 psf for corridors and stairs. The hospitals had some other loads for mechanical equipment, etc. The schools may have had their own code required live loads.

Calcs were done by hand, and frames were analyzed with 2D moment distribution methods.

gjc

 

[lexpatrie]

It's still an exit, it's how you get to the stairs from the private and public rooms. There's a fire-rating involved, after all, that's part of the picture. The reason they word it that way is for all the transfer corridors in taller buildings where the egress stairs aren't in the same plan location all the way up the building, for example. I may be pounding a square peg through a round hole, but that's my explanation, taller buildings have those mechanical levels and you usually have horizontal transfer corridors there that are the egress path.

I don't have design experience that far back, as I wasn't a practicing engineer in utero and prior.

As to load combinations, it's not that simple, though. Unless you are dealing with a newer structure, because ACI 318-1963 had 1.5D + 1.8L and different reductions for strength (Not sure they even called them phi factors that far back), so there's been like three different load combinations and the reductions also changed, 1.5D + 1.8L, 1.4D + 1.7L, and 1.2D + 1.6L. Source: eng-tips thread

If you are dealing with the South Florida Building Code, somewhere along there they had tables for allowable live load reduction that were percentages based on number of supported floors that didn't consider tributary area. Obviously at some point tributary area became a thing, but I don't know when.

I haven't found the Miami-Dade (and previously Dade) county amendments, but Broward county has their amendments available if you know the right search terms. The amendments are at least potentially similar. If you want to be really literal, Dade County and Miami-Dade (after the name change) didn't actually adopt the most recent ACI for about twenty years, so potentially somebody could design under 1963 all the way into the early eighties. You can argue responsible designers wouldn't do that, but then again, you're presuming the building you're looking at had a "responsible designer", and there's no evidence of that, a priori.

What would be useful to everyone is some kind of semi-comprehensive runthrough of the codes for both wind and live/dead/rain loads "back in the day" up to current. But I've never seen one, and/or whoever has it considers it their IP and doesn't want to publish it. One would think the Practice Periodical on Design and Construction would be interested in such an article. I can't, I don't have the base SBC, SFBC, and BOCA codes from then.

Regards,
Brian