Building Construction Question

Discussion in 'Community Discussion' started by barkomatic, Apr 12, 2011.

  1. barkomatic macrumors 68040

    Joined:
    Aug 8, 2008
    Location:
    Manhattan
    #1
    I don't know if there are any structural engineers on this board but I thought I'd ask opinions about something that has peaked my curiosity.

    Across the street from me, there is multi-level building being constructed and currently its on about the 15th floor. Nothing unusual about that in NYC except that every building that size that I've ever seen built has had a steel frame--a structure of I-beams that are riveted together around and upon which the walls and floors are installed.

    This building has no steel frame. What they been doing instead is using steel rebar and pouring concrete over it. Rinse and repeat for every floor. I've honestly never seen such a tall building being constructed this way and it doesn't strike me as terribly stable. I guess we aren't going to get any earthquakes in NYC but still. :)
     
  2. Illuminated macrumors 6502a

    Illuminated

    Joined:
    Sep 25, 2008
    Location:
    Denver
    #2
    I'm not an engineer of any sort, but I do know that this is called reinforced concrete. Been around since around the 1800's...

    ...It's as safe, if not safer than the building you described with the steel I-beams and what not...
     
  3. bgd macrumors regular

    Joined:
    Aug 30, 2005
    Location:
    SG
    #3
    I noticed in London that commercial buildings (office buildings, etc) were built using steel framing. Residential blocks, on the other hand, where generally built with reinforced concrete.

    Did ask an engineer friend about that and he couldn't give a reason.

    Here in Singapore it all seems to be reinforced concrete.
     
  4. robbieduncan Moderator emeritus

    robbieduncan

    Joined:
    Jul 24, 2002
    Location:
    London
    #4
    Most of the office blocks at Canary Wharf don't have steel frames. They are built using the core with cantilevers system (frequently with multiple cores allowing for redundant fire escapes). The cores are poured re-enforced concrete.
     
  5. SuperCachetes macrumors 6502a

    SuperCachetes

    Joined:
    Nov 28, 2010
    Location:
    Away from you
    #5
    I'm not an engineer but an architect.

    A lot of issues come in to play when deciding whether to go concrete or steel, but the short answer is that reinforced concrete structures are quite stable and good for VERY tall buildings, as in 1,000 feet high and more. For all practical purposes, either system can be made sufficiently resistant to seismic events in equal measure.

    The longer answers of why one over the other have to do with building code and economics. Any building that tall these days has to be a certain construction type, where the building structure itself (what holds the building up) has to be resistant to fire. Concrete is inherently resistant to fire, at least in terms of what code requires. Steel can be made resistant, but via applied fireproofing, which is messy and expensive. You very rarely see this fireproofing as a person moving through a building because it's ugly, and not especially immune to wear-and-tear - so it's usually covered up. But if you see something that looks like rough, spattered concrete or blown-in insulation coating the underside of a floor or on steel framing, that's what that is.

    So the economics of all the triggerwork required to form, mix, and pour concrete a few hundred feet in the air has to be weighed against fabricating steel (usually a long lead-time item), staging it on the site, erecting it, and then fireproofing it in place. The intended use of the completed building, the contractor's expertise, market conditions, the regional preferences, the owner's insurance requirements, and the site itself all can impact the decision. It's hard to say, based on your post, what might've influenced the choice of concrete across the street from you - but rest assured, it's perfectly normal. :)
     
  6. barkomatic thread starter macrumors 68040

    Joined:
    Aug 8, 2008
    Location:
    Manhattan
    #6
    This is very insightful, thank you. It just seemed like the reinforced walls and floors of the new building are very thin. It has the look of a structure that would fall over pretty easily. The older, masonry building next door looks very solid in comparison -- but I guess that wall thickness might not necessarily be an indicator of strength if the thicker wall isn't reinforced.
     
  7. blow45 macrumors 68000

    Joined:
    Jan 18, 2011
    #7
    the architects post was spot on, I am an engineer btw.

    concrete is reinforced because it has very poor tensile stress properties and very good compression ones, hence the steel. I wasn't aware that it wasn't used as much in the states because in continental Europe, and much less in the UK, it's standard practice, and they are very robust buildings, esp. for places with earthquakes. As for the wall thickness I am not sure what you are referring too, walls don't have to be thick, columns and slabs have to be thick structurally, but thickness is not the only measure as the quality of the concrete, and to a greater extent the quantity of the reinforcement plays a big role. You will also notice that parts of steel are left uncovered as they build the concrete up and they might be protruding from the concrete because they have to tie in and fasten with the steel on the next floor, etc. Concrete (which is a ratio of cement and aggregate) has to be exposed to water while it sets, that's the activator, and it sets in pretty much the same time frames, but there are other types of faster setting concrete for more critical applications where you want it to set faster, and of course there are all sorts of specialty admixtures according to the elements it will be exposed, the locale, etc. Maintenance requires that you inspect for cracks and above all that you inspect that the reinforcement is not exposed to rust, or is internally rusting because chemicals penetrated it, saline water is esp. detrimental for example. Masonry has to be thicker because it's not reinforced and it's not in one piece, they just pile it up.
     
  8. skunk macrumors G4

    skunk

    Joined:
    Jun 29, 2002
    Location:
    Republic of Ukistan
    #8
    Concrete frames have always been more popular in the UK than in the US, for some reason, probably to do with the fireproofing aspect as noted above.
     
  9. blow45 macrumors 68000

    Joined:
    Jan 18, 2011
    #9
    is that the reason? I thought it was mainly because the supplies of steel were ample but I could be wrong. What I do know is that wrt continental Europe the UK has much more of a tradition in steel frames.
     

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