Steel vs. Reinforced Concrete
 

I work for a real estate developer, but I am not familiar with construction, so I have a few questions regarding the use of steel vs. reinforced concrete when building high rises.

I have seen new office buildings in Orange County being built out of Steel and residential towers being built out of reinforced concrete. Why the difference? Is reinforced concrete a better product for residential? Is there a big cost difference between the two skeletons? Given that California shakes, are seismic concerns an issue for one but not the other after certain heights? Thanks.

Concrete allows better sound isolation than steel. You may notice that there is a demand for residential buildings to be made out of concrete than out of wood (three floor apartment) and steel on the consumer end.

The cost difference between steel frame and RC frame is, on average, small. However, regional differences in labour & material often favour one system over the other.

Cost aside, we generally see faster construction times in steel frames. While the steel frame may rise quickly on site, what is not generally known, is that the procurement, fabrication, and delivery of steel may require 6 to 10 months of lead time. Concrete structures can go up without that excessive lead time and essentially start work on "day 1".

Design wise, steel is definitely lighter (not always a good thing) and more flexible. To help counter this inherent flexibilty, designers often couple the frame with concrete or masonry infill panels and/or "cores". Usually this works out well because most codes require some fire separation at the stairwell(s) and block or solid concrete does the job nicely.

Concrete frames really shine in super-slender structures because they can produce a very stiff system even within a small profile. Carnegie Hall Tower is such an example where the tower is 50' wide in the upper portion. Designers rejected a steel frame only because they could not get it to develop sufficient lateral stiffness. The extra mass of concrete structures also produces better damping, meaning that it adsorbs energy quicker and produces better (generally speaking) dynamic performance. This is advantageous when controlling any movements - whether seismic or wind induced.

One should also recognise the newer composite systems (RC is itself a composite system) whereby conventional structural steel sections are either filled or encased with concrete. The overall performance of these types of frames are generally better than either of the constituent parts could offer individually. To take full advantage of these new hybrids we also ofter use "high performance" steels, concretes, polymers, composites, etc.

Thank you gentlemen. I will be posting more questions in the near future and it would be great to continue learning from your insight.

Another advantange is the thickness of floors. Steel is hierarchical: Decking rests on joists, joists on beams, beams on girders. This can make for a very thick floor. Concrete only requires 8" in which all electrical and mechanical can be run. Over a whole building, a developer may be able to fit another floor in given the height limit.

in addition to the other distinctions already pointed out and speaking from a chicago persepctive, where almost all of our new residential/hotel highrises are RC and almost all of our new commercial offcie highrises are steel frame/concrete core, it is said that steel framing is prefered in office buildings because it is more economical than RC in super long clear span applications, which is a BIG selling point for commerical office space. conversely, super long clear spans are not particularly advantageous in residential/hotel programs as room sizes are typically far smaller and are thus more easily worked into and around a tighter column grid, which is said to economically favor RC.

Yeah, I was going to mention that steel provides for longer spans/bays, which are big selling points of new office space.

Something I found odd, though, is that there is a new high-rise hospital tower going up here in Lansing. It's only 9 stories (140 something feet, and 180 something overall), and they used concrete for all floors but the top floor in which they framed in steel. I found it very odd and was told that the construction company wanted to use steel, but the hospital insisted on concrete and as a compromise the construction company found out how much concrete they could use to get a steel floor out of this. It still didn't make any sense to me how this worked, and it looks odd.

*EDIT* - I just found out why they did the top floor of this hospital tower in steel, and it's because the original 3-story podium completed at the end of 2005, was framed in steel (I'd totally forgot), and the hospital insisted on concrete for the remaining 6 stories, but it was found that the steel podium couldn't handle more than 5 concrete-framed floors, so the top was completed with steel to minimize the street on the podium.

Yes and no. All floor systems are hierarchical as you noted, but they do not normally rest upon one another. Instead we frame them into one another so that T/Steel is a common elevation at each floor. The hierarchy applies more to the load transfer sequence, as in: slab/sheathing - purlin/girt - joist - beam - girder - column - footing.

In both systems (str'l steel and RC) we usually find the best economy when the Span (btw. columns or walls) to Depth (of floor) ratio is ~25, although anything between 20 and 30 is reasonable.

One serious drawback to long span steel floors in particular is their tendency to "bounce" or develop unwanted dynamic characteristics. RC slabs, waffle slabs, and P/T slabs can span as long (or longer) than steel and don't bounce as much (again they have better damping), but it does cost to use these construction techniques.

You can limit the 'bounce' by limiting deflection. Using a beam with a higer moment of inertia (read: deeper) will limit deflection. A deeper beam is also taking up more head room. Anyways, you're right: steel is framed into one another. But you still have your decking resting/welded onto beams which are much deeper than a concrete slab. Plus concrete carries all the services. If we're talking economy, a waffle slab is definetly not. It would take a very long time to form it.

You can get pre-formed waffle forms, but they are less and less common these days. Using pre/post-tensioning concrete is usually a bit more common.

Deflection is independent fundamental frequency which is a function of both EI/L and basic mass (m) of the system. The "comfort" of a given floor is thus derived by not how far it deflects, but rather how fast it deflects (i.e. acceleration). CSA S16.1 (Appendix G or I perhaps) has a good discussion on long span floors and determining if it may have unwanted vibration characteristics. S6-00 has delta versus frequecy curves for vehicular structures.

Another reason can be fire code issues, depending upon your code and type of use a RC building will not need additional fire protection that a steel framed building would need. For typical business occupancy the protecting of a steel structure would not be a prohibitive cost, but if you are doing something like research labs or some manufacturing some of the fire protection coatings can get expensive and be compromised by faulty installation or damaged and have to be redone to get an occupancy permit.

It really depends on what the function of the building is and what the requirements are to determine the best way to go.

nevermind

Here in earthquake country, there's another issue discussed here: http://www.fema.gov/news/newsrelease.fema?id=6202

Money quote:

Obviously, steel frame construction was not abandoned and I have watched new office highrises that were steel-framed go up in San Francisco in the late 90's. But it seemed obvious to me that the robustness of the framing had been greatly increased over what I had seen in buildings prior to 1991 and that has to have increased the cost relative to RC.

It is a difficult comparison to make. In any earthquake resistant design, you want a measure of ductility - which is not really the same as strength. I think what was been seen in these structures was that the buildings did sway (meaning they achieved their desired ductility), but high stresses occured at rigid joints (because the differential rotations became large). One way to counteract that has been the development of "strong column/weak beam" design. In this approach, a weak point is purposefully placed in the beam near the column. As the building racks and stresses build up, the "weak point" goes plastic and prevents additional damage to the joint.

Engineers have always idealised a structure developing "plastic joints" which would essentially act like a hinge. It may sound dangerous, but the beam still remains connected to the column, only it can rotate considerable amounts without damaging the connection. What engineers couldn't do so well, was predict with certainty where these joints would form. So now, the strong column/weak beam design allows engineers to essentially place a "fuse" in their structures (just like an electrical circuit) so it "blows" when it becomes overloaded.

I can't recall if this point was noted previously in the thread, but probably the single largest drawback to RC construction (from a seismic point of view) is its mass. Force = Mass x Acceleration. The more mass a building has, the more force it develops under seismic ground motion (and hence acceleration). However, they are generally stiffer too, so in the end displacements and rotations are smaller and the level of stress/strain discussed above is not achieved.

I'm sure you are familiar with "performance-based design" addressing this issue and perhaps even the current controversy over it in San Francisco and, I assume, other places. If not, see http://sanfrancisco.bizjournals.com/...26/focus2.html .

Money quote:

Yes codes are an interesting animal, esp. in the US. Most countries only have one (building code), although even here in Canada there are a few provincial variations on the National Bldg. Code. In the US, there is the UBC/IBC, BOCA, SBC, and numerous state and municipal variations.

I can't say I recognise the term "performance based design", but in general the US codes are moving toward Limit State Design approach (also called LRFD) in which various "limits" of failure are defined (strength, buckling, fatigue, service). Every structure then has to have all it's parts stay inside all of those predefined envelopes.

What becomes difficult with any code, is the assignment of factors and combinations. Also with seismic analysis, there are many ways to analyse a structure: modal, spectral, etc. I would like to see more of the "guts" from a performance based approach to compare and contrast.

A related article, good read...

Concrete on the Rise in City Building Projects

BY JOAN VOLLERO - Special to the Sun
March 29, 2007
URL: http://www.nysun.com/article/51455

Everyone who sees the new world headquarters of Barry Diller's IAC is likely to see something different. Architect Frank Gehry's first structure on Manhattan's West Side Highway could be said to resemble billowing sails or a pleated skirt, but the real sleight is concealed: It is concrete, not steel, that makes up the building's backbone.

The reinforced concrete slabs and tilted concrete columns were not part of the first blueprints. "It was originally designed by Gehry in steel," the executive director of the New York Concrete Promotion Council, Carmine Attanasio, said. The group charged with boosting concrete's presence in the city points to the IAC building as just one of several "conversions," the industry's term for buildings conceived in steel but that wind up with concrete in the finished product. Mr. Gehry changed his plans after an engineer, Vincent DeSimone of DeSimone Consulting Engineers, suggested he use concrete, Mr. Attanasio said.

Another such "conversion," 505 Fifth Ave., peers over Bryant Park from across 42nd Street.

Ever since the terrorist attacks of September 11, 2001, concrete's presence in New York has been on the rise. Due to concrete's fireproofing qualities, it is especially being used in elevators, stairways, and other means of exit. It is also being used to insulate steel beams. "One of the advantages of concrete is it is an inert material. It doesn't combust," an engineer who is a member of the NYCPC, Michael Mota, said. "It can withstand two to three hours of fire without really a lot of damage."

Constructed after September 11, the U.S. Mission to the United Nations at 45th Street and First Avenue is one New York building that opted for concrete from the start. Architects Gwathmey Siegel purposely constructed thick exterior walls using reinforced concrete, because the material can withstand car bombs and chemical or even biological terrorist attacks.

Speed is another factor in the decision to use concrete. "You can build structures, especially residential structures, faster, and occupy them sooner" with concrete, Mr. Mota said. The industry standard is to build at the rate of a floor every two days. "That's something that's unprecedented outside of New York City," he said. "Usually, if you go to other parts of the country, concrete construction is on a five-day cycle."

The Solaire and Tribeca Green are residential towers that rapidly rose in Lower Manhattan. Made of reinforced concrete plates, both received official LEED status from the U.S. Green Building Council. While the Tribeca Green offers such features as "rainwater recycling for landscaping and maintenance" (perhaps to offset the karmic cost of 24-hour valet parking and a "pram room"), the building's concrete superstructure was also brought up to green standards. In the concrete mix, about 50% of the cement was replaced by slag, which significantly reduces carbon dioxide emission and requires less energy to produce, according to the Slag Cement Association.

It seems that not even Mother Nature herself can slow concrete's forward march. Because concrete is sensitive to temperature, it can crack if exposed to excessive heat or cold after being poured. However, construction demands have transformed it into a year-round industry, with a few exceptions for the coldest days of the year.

In fact, the only thing threatening to freeze the concrete industry is concern over supply. Two years ago, a shortage of portland cement, an ingredient in concrete, led to a concrete shortage in New York City. This shortage coincided with an unprecedented building boom in China, which is where a lot of cement was being diverted. Tishman Construction Corp. was initially concerned about finding enough concrete to build the reinforced core of 7 WTC (the building's superstructure is steel), but concrete producers and suppliers rose to the task.

Since then, producers have had a chance to catch up and are better equipped to meet China and India's growing appetite for building materials. America is also boosting its supply by importing about 20% of the cement it consumes yearly. "Right now, we're consuming everything we're producing. There is no surplus," Mr. Mota said. "Whatever is being produced is being consumed."

The New York Building Congress's 2006–08 construction outlook shows there is reason for concern over building material supply and the means of carting it. Its biannual report lists the rising costs of commodities such as concrete and steel and the high price of fuel as causes for concern. There are also questions about the number of qualified contractors able to execute multiple overlapping construction schedules. "Lower Manhattan over the next several years has more construction planned than maybe has ever occurred in a square mile of a city," the president of the construction and real estate trade group, Richard Anderson, said.

Despite of the daunting number of stadiums, skyscrapers, condominiums, and transit hubs slated for the coming years, the concrete industry is optimistic.

"There may be here or there … " Mr. Mota said, stopping himself, "I don't want to use the word ‘shortages,' because that alarms people, but I think all in all we'll be able to meet the demand."

in cold climates steel frame is often preferred because it can be erected in winter, where the hoarding and heating costs for concrete can be prohibitive.

concrete is often being chosen today in 'green' buildings, because it has the ability to act as a thermal mass....helps shave the peaks mechanical loads.

There was a great deal of discussion about building with steel after the collapse of the World Trade Center. In a catastrophic fire with intense heat the steel can bend and lose its strength. The coating that they put on steel beams during construction is not enough protection. A hi-rise fire is very difficult to deal with no matter what materials are used. The article above suggests that concrete surrounding steel or just reinforced concrete will withstand much more than just steel. I realize that steel might be more economical but from a safety perspective I wonder why that isn't more of an issue.

Reinforced Concrete v structural steel
 

hey mate(s)

I have to write a report for uni and so far your insights have helped a lot.

I'm a first year student so my breadth of knowledge isnt nearly as deep as yours.

I'm trying to gather information which compares reinforced concrete to structural steel

Your knowledge would be greatly appreciated

Kind Regards,

charlie