Comcast Building in Philadelphia, PA
This is the fifth in the Engineer’s Corner series of articles in ESCSI Lightweight Design eNews that address issues related to concrete made using expanded shale, clay, or slate (ESCS) lightweight aggregate that has been manufactured using the rotary kiln process at temperatures around 2000 deg. F. When the aggregate particles reach these high temperatures, the material softens, and bubbles are formed within the particles as gases are released from within. These bubbles become permanent pores in the aggregate when it cools and hardens. The pores in the vitrified ceramic aggregate particles give lightweight aggregate its reduced density and other unique properties.
The first three articles in the Engineer’s Corner series addressed common myths or misconceptions about lightweight aggregate or lightweight concrete. The fourth article in the series introduced a second direction for the series by presenting a topic for which structural lightweight concrete provides significant design advantages: the common and efficient use of structural lightweight concrete on composite metal decks for floors in buildings.
This fifth article follows up on the fourth by considering the cost of using lightweight concrete for floor slabs in buildings by addressing the following myth or misconception about lightweight concrete in buildings:
Myth — Lightweight concrete can’t provide an economical solution for floors slabs in buildings because the cost of lightweight concrete is greater than normal weight concrete.
While it is true that lightweight concrete costs more on a per cubic foot basis than normal weight concrete, that does not mean that it cannot provide a more economical solution for a multi-story or high-rise building when the cost of the entire building is considered, including framing, foundations, excavation, and even transportation of concrete.
A second myth or misconception that will be addressed more fully in a later article in this series needs to be addressed briefly here:
Myth — Lightweight concrete can’t be successfully pumped to the upper floors of a high-rise building.
This question must be addressed here, or else the rest of the discussion may be dismissed by some readers. Properly proportioned lightweight concrete has been pumped successfully to the top of high-rise buildings by using appropriate mix designs and equipment. For now, please see the information, project gallery, and ESCSI publication “Go With The Flow” which can all be accessed at the following webpage.
Examples of buildings where lightweight concrete has been successfully pumped to the top of the structure include the 1100 ft tall Wilshire Grand Center in Los Angeles, CA; the 1070 ft Salesforce Tower in San Francisco, CA; the 973 ft tall Comcast Building in Philadelphia, PA; and the 742 ft tall Lake Street Building in Chicago, IL, all of which are discussed on the webpage mentioned above.
But now I’ll move on to the myth to be dispelled in this article by looking at cost comparisons for several buildings:
Enough detail is provided on the final example in the referenced report that one could take the information, as I have, and put it into a spreadsheet to allow you to make your own comparisons.
This 50-story building was designed and constructed entirely of lightweight concrete. It was a masterful design by the great engineer Fazlur Khan with Skidmore, Owings & Merrill in Chicago, IL. When completed in 1971, it was the world’s tallest reinforced concrete building. In an outstanding article published in ACI Symposium Volume SP 29 (1), he discussed design issues that he encountered related to lightweight concrete during the design process and how they were all successfully addressed. A companion article on the testing and quality control of the lightweight concrete used for the building is also published in SP 29 (2). A third interesting article on the building is available on a Princeton University website (3).
While a cost comparison is not provided in his article on the design of the building, he makes some excellent points related to the cost of the structure. As Khan (1) described, foundations for high-rise buildings in Houston must be floating mats since clay extends to a depth of more than 2000 ft. The maximum excavation depth in the city had been established by prior experience to be about 60 ft, which limited the building height for a normal weight concrete structure to 35 stories (since the weight of the structure had to be approximately equal to the weight of soil removed for the excavation). However, a taller building was desired to improve the economics of the project for the investors. The only way to achieve an increased structure height was to reduce the mass of the structure by using lightweight concrete. Even the mat foundation, which was 8 ft 3 in. thick, was constructed of lightweight concrete to reduce the required excavation depth. Khan states the following on page 2 of the article (1):
The decision to build the world’s tallest concrete building, therefore, was made on only one basic structural condition—that the structural system must produce the most economical total building. This led to the final conclusion that lightweight concrete must be used for the construction of the entire building including the heavy mat foundation.
Khan’s preliminary analysis indicated that a 52-story building constructed using high-strength lightweight concrete with a unit weight of 115 lb/ft3 could be built on the Houston site for the same cost as a 35-story building constructed using normal weight concrete. Therefore, this was the concept that was used (although other references accessed on the internet all state that the building is 50 stories). One Shell Plaza is a striking example of how the benefits of lightweight concrete can be used in some situations to obtain a more economical solution when it is considered as an important factor in the optimization of the entire project.
While only a few details of the actual design of the Bank of America Building in Atlanta, GA, are available, a simple comparison of the concrete cost for the floors in the building was developed based on the total floor area and the thickness of concrete floors on metal deck. A major factor in the selection of lightweight concrete for the floors in this building was that required fire rating determined the thickness of the floor system. For a lightweight concrete floor, the concrete can be thinner for the same fire rating, so the weight of floor slabs in the building is reduced not only because of the reduced concrete density but also because of the reduced thickness.
Even when only a simple evaluation of the cost of the floor concrete is considered, it is demonstrated in the table below that the total cost of the concrete floors would be reduced by 24% for a total savings of over $1,000,000 for this building. The table below also shows that the total weight of the floors in the building was reduced by nearly 33% when lightweight concrete was used. Therefore, additional cost reductions would also be achieved when the savings from all sources were considered, such as the reduced material required for the framing system and foundations because of the reduced structure weight.
A comprehensive cost comparison for a 5-story building in Salt Lake City, Utah (5), which includes evaluation of the building framing system and foundations, was developed in 2017 for the Utelite Corporation, one of the member companies of ESCSI. The 90 x 150 ft building with 30 x 30 ft bays was designed for seismic lateral loads as well as conventional live and dead loads. The building design used a steel frame with floors comprised of were concrete on a metal deck while the roof was just a metal deck. Complete details of the assumptions, inputs, and detailed material quantities can be found in the report (5). An updated version of the report is now available (6), which uses the original design with current costs.
The report summarizes data for building designs with the following basic parameter: a design 5.25 in. thick lightweight concrete (LWC, 110 pcf) floors with a 2-hour fire rating; and three designs for normal weight concrete (NWC, 145 pcf) floors with fire ratings of 0, 1, and 2 hours with corresponding floor thicknesses of 5, 5.5, and 6.5 in., respectively.
Component and total costs for materials and labor for the two designs with a 2-hour fire rating (5.25 in. LWC floors and 6.5 in NWC floors) are summarized in the table below. The differences between costs for the building designs with lightweight and normal weight concrete floors are also presented, as well as the ratio of the difference in cost divided by the cost of the normal weight concrete floor design (differences and ratios for quantities where the cost for the lightweight concrete design is greater are negative). Current estimates for material and labor costs used in the study (6) are given in the second table.
From this comparison of results for the 2-hour fire rating designs, it can be seen that the cost of the lightweight concrete was greater than the normal weight concrete by 7.5%. It can also be noted that more shear studs were required for the building with the lightweight concrete deck resulting in an increased cost in that area. However, even with the increases in components of material costs, the overall cost of the building with lightweight concrete floors was 9.2% less than the overall cost of the building with normal weight concrete floors because of significant savings in other areas. The source of the greatest cost savings was the footings where the material cost of footings in the building with lightweight concrete floors was reduced by 27% compared to the design with normal weight concrete floors. This reduction, as well as the 10.5% reduction in steel costs for the frame, are the result of the large reduction in dead load due to the compounding effects of reduced concrete density and also a thinner floor. The report states that the total floor weight was 144,450 and 176,850 lbs for the lightweight and normal weight concrete designs, respectively, which is a reduction of 18.3%. It should be noted that the reduction in floor weight also reduced the seismic mass and base shear by 23% and 21%, respectively, which contributed to the additional savings in foundation costs.
This comprehensive cost comparison is an excellent example of the potential savings that can be achieved when lightweight concrete is used in a structure, even though the cost of the material itself is greater than the cost of normal weight concrete. This clearly demonstrates the fact that the economics of using lightweight concrete cannot be assessed solely on the cost of the material itself, or even the comparative cost of the floors in a building, but should be examined based on the complete design to get a full understanding of the benefits of using lightweight concrete because significant savings can be achieved in the frame and foundations resulting from reduced dead load.
The One Shell Plaza Building in Houston, Texas, is an excellent example of how the overall building concept was significantly improved when lightweight concrete was used. In this case, a 50-story lightweight concrete building, which included lightweight concrete in the building frame and foundations, could be constructed for the same cost as a 35-story normal weight concrete building. However, no details of the cost comparison are presented in the cited articles.
The second cost comparison, which was based on an evaluation of the cost of floor slabs only in the Bank of America Building in Atlanta, Georgia, provides a limited insight into the potential cost savings for buildings when lightweight concrete is used.
The comprehensive example of the 5-story commercial building in Salt Lake City, Utah, provides a much more complete understanding of the potential savings when using lightweight concrete for floors in a multi-story commercial building. In this case, if only the cost of the floors had been considered, the designer would have assumed that using lightweight concrete was not an economical solution. However, when the complete design was compared, it was found that the use of lightweight concrete provided savings in the cost of the building frame and foundations that were more than enough to overcome the additional cost for the lightweight concrete. The updated report for this example (6) provides a detailed approach to developing a cost comparison between lightweight and normal weight concrete designs.
The first two comparisons were made with dated cost data; the third used current data. However, all costs used were estimates. The costs of lightweight and normal weight concrete will also vary in different parts of the country. Therefore, these comparisons should only be used as an indication of the potential for savings when lightweight concrete is used in both high-rise and low- to mid-rise buildings. Readers are encouraged to make their own comprehensive cost comparisons for projects by considering the overall effect of using lightweight concrete on the total project, including the frame and foundations. Concrete costs used in such comparisons should be obtained from local concrete suppliers so that local market conditions will be accurately represented.
While preparing this article, current costs were obtained from several markets in the Southeast. It appears that in major cities, the cost of normal weight concrete is about $145/CY, while the cost for lightweight concrete is $175 to 180/CY. These differences are subject to change with time and location since many factors affect material prices. Again, readers are encouraged to check local sources for current pricing when looking at comparative designs.
These three different examples clearly demonstrate that lightweight concrete can provide a more economical solution to a building. However, it is also clear that the comparison must be based on a complete design of the structure and foundations because significant savings may be achieved in these elements that can offset the increased cost of lightweight concrete. It should also be noted that these evaluations are limited, and some additional savings may be realized in other areas, even as seemingly minor as the decreased number of trucks required to deliver the concrete due to the decreased volume of concrete in the structure, which reduces costs and greenhouse gasses. In some locations, such as New York City, the use of lightweight concrete allows mixer trucks to deliver full loads because of the load restrictions on city streets, further improving the overall benefit of using lightweight concrete for a structure. For a comparison that will reveal the true savings potential for the use of lightweight concrete, as many factors as possible should be considered.