How do concrete and steel compare in the construction of large commercial buildings? Office Buildings, Hotels, Sports Complexes, and other buildings have the purpose of bringing people inside. The goal is to keep occupants comfortable and safe, while keeping the cost associated with the construction and maintenance of the building low. Both materials have advantages and disadvantages. However, when it comes right to it, the differences between the two materials balance themselves out. There is no truly better building material.
The first aspect that must be analyzed when choosing a structural material is the strength of the material. Until recently steel had a far greater strength to weight ratio. This resulted in taller and lighter buildings. However this is no longer the case. According to the Portland Cement Association (2007):
In the 1950s, 5000 psi (34 MPa) was considered high strength; by 1990, two high-rise buildings were constructed in Seattle using concrete with strengths of up to 19,000 psi (131 MPa). Ultra-high-strength concrete is now manufactured with strengths in excess of 21,750 psi (150 MPa) (Precast).
These new ultra-high-strength concrete mixtures allow a building to be built with concrete and have its structural members be of similar dimensions as a steel building. The concrete building will still have more mass, but not by a great amount as in the past. Elasticity adds to the overall strength of the material. The Modulus of Elasticity (the amount of stress-strain were a material still acts elastically) for concrete is resulting in a range of roughly 3,500,000 psi to 9,400,000 psi while steel has a Modulus of Elasticity of 27,000,000 psi (Spiegel, 2003, p. 8). In this respect steel is a clear winner, and it is for this reason that structural concrete has steel reinforcing embedded within the concrete.
Figure 1 Stress-Strain Curve for Concrete. Note. From Spiegel, L., P.E. & Limbrunner, G., P.E. (2003). Reinforced Concrete Design, 5th ED. Upper Saddle River: Prentice Hall, Pearson Education Inc.
Figure 2 Stress-Strain Curve for Steel. Note. From Spiegel, L., P.E. & Limbrunner, G., P.E. (2002). Applied Structural Steel Design, 4th ED. Upper Saddle River: Prentice Hall, Pearson Education Inc.
Another factor that must be considered when choosing a structural material is its ability to resist seismic activity. Ductility has been found to be an important structural attribute. “Ductility is the ability of a structure to remain standing even after parts of it have been stressed beyond design allowable stresses. Deformation of parts will serve to transfer loads to the other less heavily loaded areas” (Spiegel, 2002, p.7). The mass of the structural members has also been thought to help to resist the forces induced by seismic activity. The Cement Association of Canada states (2006), “Lateral stiffness, or resistance to horizontal movement, make concrete the product of choice when constructing in areas where high winds, hurricanes, tornadoes or seismic conditions are considerations” (Structural Integrity: Ideal for Strict Specifications). This has been the general consideration when designing structures for high lateral stresses. Ongoing research, especially after recent earthquakes in both California and Japan, has lead engineers to look towards ductility more. From the Portland Cement Association (2007):
Contrary to popular belief, a structure’s likelihood of surviving an earthquake depends more on how well the structure is engineered than on what type of material is used to build it. During a severe earthquake that struck Kobe, Japan, on January 17, 1995, concrete buildings and steel buildings in the downtown area of the city shared comparable fates: just 4.9 % of concrete buildings and 5.3 % of steel buildings collapsed (Resisting Earthquakes).
These numbers show that the material is not as important as previously believed, and proves that either material can be just as strong when engineered properly.Aesthetics can also be an important factor when choosing a building material. The owner does not want an ugly building. Newer technologies now allow architects to use steel and concrete not only for structural strength, but for aesthetic appeal as well. The British Constructional Steelwork Association, in there Merits Booklet (2002) calls steel aesthetically pleasing because it has clean lines, is long spanning and has many architectural possibilities (What the Client Wants: Prestige). Recent advances in fire protection have also allowed steel structural members to be left exposed. Concrete buildings have not been thought to be having the same capabilities. According to the Cement Association of Canada (2006), The aesthetic appeal of concrete is infinite; concrete can be colored to any specification, and the textures that can be applied are endless. They go on to state, “Concrete texture can resemble smooth, high-polished granite or gutsy, exposed aggregates with a rugged feel. Other possibilities include tumbled cobblestone, brick, cultured limestone, slate, flagstone or river rock” (Aesthetic appeal). Two well-known examples of this are the Guggenhiem Museums. The Guggenhiem in New York City, designed by Frank Lloyd Write, is a concrete building, while the Guggenhiem in Bilbao, Spain, designed by Frank Gehry, is a structural steel building.
Image 1 Photo of Guggenhiem Museum in New York. Retrieved August 10, 2007 from http://www.greatbuildings.com/cgi-bin/gbi.cgi/Guggenheim_Museum.html/cid_cr1037_b.html
Image 2 Photo of Guggenhiem Museum in Bilboa, Spain. Retrieved August 10, 2007 from http://www.greatbuildings.com/cgi-bin/gbi.cgi/Guggenheim_Bilbao.html/cid_1028276211_Bilbao_017.html
Many factors will be considered when choosing a structural material. The one factor that will likely be the most important to the owner will be the economics of the project. There are many dynamics that lend to the overall economics of building construction. The first factor is the initial cost of the project. The availability of materials will be a major factor in the cost of materials. Another factor is the availability of skilled labor. These two factors together help to determine the overall initial cost of construction.
More important is the sustainability of the finished building. Both materials can be readily recycled or made from recycled material. According to the Cement Association of Canada (2006):
Concrete is an inert material that is easily recyclable. Old concrete that has reached the end of its service life can be reused as aggregate for new concrete mixtures. The addition of industrial by-products such as fly ash, silica fume and blast furnace slag make concrete less permeable while incorporating materials that would otherwise be deposited in landfill sites (Recycling).
Structural steel is an environmentaly friendly material as well. The Steel Recycling Institute states that structural steel used in this country is 95% recycled material (2005, p.1).Another important factor of sustainability is the energy efficiency of a building. The Cement Association of Canada says (2006), “The mass of a concrete structure makes it a significant thermal reservoir with the ability to store large amounts of energy” (Environmental Responsible: Energy Efficiency). They go on to state (2006), “By storing and releasing the energy needed for heating or cooling, concrete delivers year-round energy benefits” (Environmental Responsible: Energy Efficiency). A study that was conducted at the Ryerson University in Toronto, Canada seems to refute these claims. According to Mark Gorgolewski, P.E. (2007), “In any one location, the two buildings perform almost identically, which suggests that the steel-framed office building has sufficient thermal mass to generate the same benefits in energy use as the concrete building” (p.1). Another aspect of thermal mass is the mass enhanced R-value of the exterior walls. According to Alex Wilson in an article in Environmental Building News (1998), “[the thermal mass of exterior walls is only effective in areas where] outdoor temperatures cycle above and below indoor temperatures within a 24-hour period” (When is mass enhanced R-value effective?).
Figure 3 Predicted Energy-Use Comparison Chart. Note. From Gorgolewski, M, Ph.D. (2007, January). Framing systems and thermal mass [ Electronic Version]. Modern Steel Construction. 45-46. Retrieved July 26, 2007, from http://www.aisc.org/Template.cfm?Section=Technical_Answers&template=/ContentManagement/ContentDisplay.cfm&ContentID=33021
There are many decisions that must be made when deciding on a building material. Strength, aesthetics and sustainability are but a few factors that are used to determine the better choice. There is no clear winner when it comes to building materials. Concrete and steel compare very well to each other. The decision as to which material to use is dependant on many factors. Availability of the material and skilled workers, environmental considerations and the overall aesthetic look of the building are but a few of the considerations made when designing a building. The buildings use will also have to be used as bases for material selection. In the end there is no clear winner between steel and concrete a building material.
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