Concrete slabs+columns vs slabs+beams+columns

[antonba]

I live in Seattle and have noticed that reinforced concrete buildings from the 80s have not just slabs and columns like newer buildings but also beams. I am talking about residential buildings and I am sure it's not steel.

Is there any advantage to this? Why was the standard changed? Does the new beam-less standard have better seismic performance?


On this one you can see the beams and columns very well - note that the "bay windows" are attached to the beams and also the balcony railings sit on such beams - they are about as wide as they are tall:

[Jasoncw]

It always amazes me how minimal reinforced concrete structures really are. Most of the time you just need a few inches of slab and some columns and you're good to go. Intuitively it feels like there needs to be more material, but there doesn't.

idk much about the trends in engineering. But beams make the formwork more complicated and reduce the ceiling height (which increases costs), and I think nowadays there are greater pressures and expectations of cost effectiveness. Beams let you have bigger column spacing but you can reduce the need for wide spans through design (especially in residential buildings and hotels where there are a lot of walls where columns can go without interfering).

Architecturally in the 80s there was still the trend, continued from the 70s and 60s, of having aggressively expressed tectonics. During that time architects would have been more likely to want to have the different structural elements (slabs, beams, and columns) put together and expressed very clearly. And you can't express beam constructional logic if there are no beams. So back then there was a lot of beam overkill and a lot of beams sticking out of facades and stuff like that.

There's a good example of that here https://www.flickr.com/photos/iqbalaalam/17051761672/

[mthd]

there are code limitations on different types of structures systems by height. until recently, concrete buildings over a certain height (varies by jurisdiction, was 240' in San Francisco, for example) required a redundant lateral system, which meant frames of beams at the perimeter plus a concrete core. newer codes allow performance based design in which any design that can be proven to resist appropriate loads can be approved, usually with peer review. this has eliminated a lot of mass in building structures, particularly in reinforced concrete.

to achieve the 25-35 foot spans of residential buildings, concrete slabs also require post-tensioning. this has been around for a while, but availability of the specialized experience varies by market. it's become very common on the west coast in concrete buildings, generally yielding a 7" or so slab with columns about 30' out from the core and 20 to 30 feet on center on the other direction. this and the concrete shear walls around the stairs and elevators and other services make up the entire structure. there are no beams.

tectonics or not, very few architects would ever use architectural 'expression' as a justification for a massively heavier structure. in fact, it's usually the opposite - an attempt to justify a lighter but more complex and more expensive structure by making it part of the character of the building.

[ardecila]

^ I think you hit the nail on the head with that one...

More generally, there are one way and two-way slabs in addition to flat plate... for highrise construction, the flat plate is generally preferred for the flexibility it allows in laying out walls, mechanical systems, etc. For more specialized structures like parking garages or warehouses, you may see the simple one-way and two-way designs.

Even more complex configurations are possible, with waffle slabs, drop panels, and/or capitals. These have generally fallen out of favor.

[dchan]

^ It's not simply about flexibility vis a vis flat plate construction.

Flat plate construction is fine for residential and most office buildings because those structures don't require a high live load rating. It's not appropriate for garages and warehouses because those structures require a higher live load rating, and the columns will punch through the slab when the load is too high.

Drop panels and capitals are used to spread the load of the floor over a wider area on top of the columns, as well as to reduce the likelihood of column punch-through.

In general, certain structures and buildings were built more heavily and robustly in the old days because the designers didn't have the engineering theory that we have available nowadays. That is, they overbuilt the structures just to be safe. Today's structures are designed with cost in mind, and all really you need to do is meet the minimum factors of safeties as laid out by the building/engineering codes.

[harryc]

One current example ....

[ardecila]

Originally Posted by dchan

In general, certain structures and buildings were built more heavily and robustly in the old days because the designers didn't have the engineering theory that we have available nowadays. That is, they overbuilt the structures just to be safe. Today's structures are designed with cost in mind, and all really you need to do is meet the minimum factors of safeties as laid out by the building/engineering codes.

It's not just about value-engineering per se. Post-tensioning is relatively new technology but it goes a long way toward making flat plat feasible/cost effective.