Quote:
Originally Posted by Pedestrian
As a general rule of thumb, in San Francisco office towers are structural steel-framed with concrete elevator cores, of course, but residential buildings are nearly all reinforced concrete like the Millennium tower. I believe it's because the flexibility and swaying that's unavoidable in steel buildings during "minor" tremors would unnerve residents in buildings full of them.
|
That's the case for most residential condo buildings here in NYC as well. Steel structures are designed for commercial office buildings, and reinforced concrete is designed for residential buildings.
Why is this? My main hypothesis is that it comes down to two factors:
1. Minimizing floor-to-floor height while maximizing ceiling height.
2. MEP services
3. Fireproofing concerns
First, we must understand the difference between steel structures and reinforced concrete.
Steel structures rely on the large girder beams resting on columns as the main support. Resting on top of these girder beams are joists that span the girder beams. Then a reinforced concrete floor (or more usually, a concrete over metal deck floor) rests on top of the joists and the girders. The result is that thick steel girder beams will run across the ceiling in regular intervals. For aesthetic purposes, these structural elements are usually covered up by drop ceiling panels, and the "nominal" ceiling height would be measured from the floor to under the thickest steel girders:
Whereas, for reinforced concrete buildings, the underslab joists are much thinner and built monolithically with the floor above using a single pour of concrete. Therefore, reinforced concrete structures can achieve a higher "nominal" ceiling height while minimizing actual floor-to-floor height.
The next reason (based on my hypothesis) is MEP services. Commercial offices need substantial MEP services (HVAC, electrical wiring, data lines, fire sprinklers, etc.), and the space above the drop ceilings can be used to hide these mechanical elements. Residential buildings have fewer MEP needs, so they have no need for a cavernous empty space above drop ceilings. Therefore, reinforced concrete structures are better for residential needs.
Finally, fireproofing. Steel is not fireproof (as we all have been aware for almost 20 years), and needs to be fireproofed. Steel fireproofing is fairly flimsy and weak, and often needs to be inspected and re-applied as-needed. That may be feasible in office buildings, where tenant turnover and facility renovations are fairly regular. But in residential buildings, condo owners would likely balk at regular intrusions into their space. Therefore, it's best to have a naturally fireproof structural material that does not require periodic maintenance, and reinforced concrete fits the bill (that is, as long as there are no water problems that may damage the reinforcement over time).