structural concrete vs steel for mid-rise buildings

[jett]

I've noticed a lot of mid-rise buildings in seismic active regions are made of concrete...why is that? don't you want the material to be ductile?

[SFUVancouver]

In Vancouver structures more than four storeys tall cannot be made of wood. For residential mid-rises less than four storeys in height wood is the dominant building material.

That leaves reinforced concrete and steel for mid-rises above four storeys. For the last decade or more concrete has been used almost without exception in Vancouver.

A large part of the reason for this is that Vancouver is very well supplied by concrete vendors and it has a mature building/forming industry, both of which make for predictable costs and timely construction schedules. A further factor is that all new parking in Vancouver must be below grade and this requires a concrete substructure that is continued above grade for simplicity.

Lastly, Vancouver is in a seismically active area with strict structural building codes and reinforced concrete meets this with ease.

To the best of my recollection in Vancouver there are only two new mid-rise steel buildings going up, with the exception of the immense mid-rise height, 1.1 million square foot Vancouver Convention Centre expansion. (image)

[natelox]

Concrete can be ductile with the addition of reinforcing steel. Steel construction is very much a column and beam system. Concrete can be (and is very often) built in this manner, but concrete shear walls offer much better lateral resistance than the moment frames or tension rods typically seen in steel construction. Shearwalls, especially massive (as in weight) ones, dissipate energy more readily than moment frames or tension rods.

Concrete columns are protected from lateral forces by the reinforcing ties. Circular columns have better seismic resistance than square because of the nature of the rebar. Circular columns use a spiral rebar to tie the vertical rebar together. If the concrete cracks or shatters because of an earthquake, the helical rebar ties will keep the broken pieces in place, and the column will retain its ability to resist compression.

Furthermore, there is the added advantage that concrete is always fireproof, and steel needs an applied fireproofing. Since fires typically do as much damage, if not more, than lateral movement during the aftermath of seismic activity, concrete may be more beneficial (Fire is dangerous post-seismic activity because underground water supply lines tend to rupture, rendering fire-hydrants and fire-fighters without water). Fire-rated gypsum board, which typically protects steel, could easily fail during significant lateral movement.

[XSpringfieldGuy]

Originally Posted by natelox

Concrete can be ductile with the addition of reinforcing steel. Steel construction is very much a column and beam system. Concrete can be (and is very often) built in this manner, but concrete shear walls offer much better lateral resistance than the moment frames or tension rods typically seen in steel construction. Shearwalls, especially massive (as in weight) ones, dissipate energy more readily than moment frames or tension rods.

Concrete columns are protected from lateral forces by the reinforcing ties. Circular columns have better seismic resistance than square because of the nature of the rebar. Circular columns use a spiral rebar to tie the vertical rebar together. If the concrete cracks or shatters because of an earthquake, the helical rebar ties will keep the broken pieces in place, and the column will retain its ability to resist compression.

Furthermore, there is the added advantage that concrete is always fireproof, and steel needs an applied fireproofing. Since fires typically do as much damage, if not more, than lateral movement during the aftermath of seismic activity, concrete may be more beneficial (Fire is dangerous post-seismic activity because underground water supply lines tend to rupture, rendering fire-hydrants and fire-fighters without water). Fire-rated gypsum board, which typically protects steel, could easily fail during significant lateral movement.

I agree very much with this. However, the blame on the steel, well, I don't call it steel, its a form of metallurgy. I should have said this before, its not steel, its so many varieties of different elements. ASME, ASTM have a variety of code that can be mixed with polymers, other elements and even concrete that are more withstanding that simple rebar and Concrete. Cost withstanding, probably and issue. I work in the LNG pressure vessels and Nuclear reactors, so its probably more of an issue with us than fabrication of buildings.

[urbanlife]

plus another big factor is china, the cost of steel is so high right now, that it makes much more sense to build with concrete.

actually, funny that wood was mentioned. A while back to stimulate development in the city, developers are allowed to use wood in their buildings, but the buildings can only be 5 stories high. The exception to that rule, is if the first floor is a concrete base the total building can then be 6 stories high. Since that went through, Portland has been getting alot more 6 story buildings.


oh and structural concrete does have a type of steel in it, thus making it very strong to tension and compression, thus if used correctly is fine in seismic areas. Actually the worse thing you can build in seismic areas is unit masonry construction, like brick buildings. A strong earthquake can tear a unreinforced brick building apart.

[Startvcr]

1. What dramatically affects the cost of a highrise (as it increases in height)

Understand the "CORE" is the critical component with respect specific materials (reinforced concrete) and it configuration. Is there a reasonable optimun height before cost jump? Say 30 floor or is aspect ratio adequate

2. Floor plates (are they typically around 10,000 sqft) plus Core Area.

3. Rule of thumbs: $/ft2 for plates or $/m3 for Cores (at 50,000psi +)

[Kelvin]

Cost increases w.r.t. largely because of the cycle time involved (e.g. it takes longer to move materials up) and requires effort (e.g. use of cranes, etc). Overall productivity decreases thus cost rises. At some point the cost to build higher is outweighed by the economic return on the property, so that's where they decide to build no further (theoretically). Obviously in some famous cases, decisions to build higher were made solely to underwrite the developer's egos.

2. Floor plates sizes vary widely.

3. Building unit costs are almost always reduced to the cost per unit (gross) floor area and include all the entirety of the property (incl. land). Unit costs also vary widely depending on materials used, labor (union/nonunion), local experience, availability of material, etc.

[Coldrsx]

1. after floor 5, your tower has the infrastructure to support 6 floors or ~20 floors...regardless of how high you want to go. Generally speaking 20-40 floors is most efficient but design can range so vastly that height only factors in so much. Remember that height generates far more money per floor as you go up, but yes it does cost more.

2. point towers such as ones in vancouver tend to have smaller floor plates of 6000 to 8000sqft whereas large buildings in chicago/nyc can have 10-13,000sqft plates. If i were to be general here, i would say 8000 - 10,000 gross.

3. costs for building are determined largely by where you are in the country, access to labor and materials, and design specs. Wood framed units are generally about 25-30% less than concrete to build.

[mhays]

One factor I don't see here is that concrete sways and vibrates less. That's why laboratories are nearly always concrete.

[Trantor]

Brazil has since forever built highrises in reinforced concrete. We never built steel framework.

[speedy1979]

Originally Posted by Trantor

Brazil has since forever built highrises in reinforced concrete. We never built steel framework.

Is that because of the damp tropical climate (concrete encases the steel reducing the likelihood of corrosion) or is that due to labor cost?

[Trantor]

Originally Posted by speedy1979

Is that because of the damp tropical climate (concrete encases the steel reducing the likelihood of corrosion) or is that due to labor cost?

well, i have no idea. You will have to ask a real brazilian architect or engineer

this beauty here, the Banespa building (1947), is reinforced concrete, and was the tallest reinforced concrete building in the world when it was finished, as well as the tallest highrise in the world outside USA (or so I heard )


picture courtesy of Arrakeen´s travelogue
http://www.arrakeen.ch/brasil6/brasil2006.html

[vanhattan]

Thanks for the photo. It is a very handsome building. Sort of reminds me of a modified version of the Empire State Building.

[mthd]

Originally Posted by speedy1979

Is that because of the damp tropical climate (concrete encases the steel reducing the likelihood of corrosion) or is that due to labor cost?

labor cost - in countries where the formwork can be built cheaply, concrete becomes more attractive than steel which has a very high material cost and also a high design and detailing cost.

in countries where the labor involved in constructing formwork is very expensive, the material cost of steel becomes lower in proportion and the time savings and flexibility of steel buildings become more attractive.

residential towers are also much much more likely to be concrete because they don't have the extensive mechanical systems of office towers, so they don't need a large ceiling space. this means the ability to use the underside of the concrete slab as the ceiling saves a lot of finishing costs, and the structure is inherently fireproof. concrete columns are also harder to design as the floor to floor height increases, which tends to push office buildings towards steel in many countries.