Metal v. Concrete: Metal's Prespective
Originally published by: Metal Construction News — May 2, 2012
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How does metal stand up against concrete?
Tilt-up concrete construction is a popular method of construction that rivals metal construction. According to the Tilt-up Concrete Association, at least 10,000 buildings are constructed annually that enclose more than 650 million square feet. Tilt-up concrete construction is not new; it has been in use for well over a century.
Nationwide, more than 15 percent of all industrial buildings are tilt-up, ranging in size from 5,000 to more than 1.5 million square feet. A tilt-up building's walls are created horizontally, similar to a 19th century barn-raising. Workers lay out wood forms on the project's floor slab and set up grids of rebar inside each form. They then pour concrete into the form and when they dry (or cure), remove the forms to leave large slabs of concrete panels. The panels are then lifted with a large crane or "tilted up" into position around the building's slab, and held upright by critical connections to a relatively lightweight roof system. An experienced tilt-up crew can erect as many as 30 concrete panels in a single day.
Consider the following factors when competing with concrete tilt-up for building construction.
Building size matters
The building's size is a factor. For projects less than 50,000 square feet, steel is generally the least expensive alternative. For a building this size, the fixed costs of a tilt-up construction project (like the rental of a large crane) make it more expensive than steel, even though concrete is usually a less-expensive raw material. However, as projects become larger than 50,000 square feet, the lower price of concrete starts to offset tilt-up construction's fixed costs and this method becomes cost-competitive with a metal building. "For very large structures where the walls are a lesser percent of the cost, then tilt-up might be better," says Larry Stevens, general manager at Rapidset Buildings, Colorado Springs, Colo. "To compare the cost, the issue is scale."
With buildings requiring longer spans and bays, a pre-engineered metal building is the right choice. Metal's spanning capacity is better than concrete's. This is important for offices because it increases floor space flexibility. Metal's customizable column locations and depths offer adaptability to most floor plans, an important factor in remodeling. The greater number of support columns needed in concrete construction limits the moving of walls and partitions, and it reduces the amount of useable square footage.
One perceived drawback to long-span steel floors is their tendency to "bounce" or develop unwanted dynamic characteristics. Because concrete tilt-up has better damping, it doesn't bounce as much. However, builders can limit the "bounce" in steel floors by limiting deflection using beams with a higher moment of inertia (i.e. deeper length) that will limit deflection. Steel members require less complicated foundation systems. This gains importance when soil stability is poor.
Cantilevering with concrete is expensive, demanding form set-up and carpentry to hold the concrete until it cures. Cantilevering with steel joists means only specifying an extended end or a full-depth cantilever direct from the factory. Steel buildings also permit more glass on the building's exterior than concrete tilt-up. This reduces a building's overall energy consumption.
Also, "there are many systems available for insulating steel structures," says Craig Madsen, owner of J & M Steel Solutions, Lehi, Utah. "The cost per R-value is much less to insulate than concrete tilt-up."
"Steel can exceed the design capabilities of tilt-up in some instances," says Wayne Stellmacher, president of Keller Inc., Kaukauna, Wis. "I believe the benefits of steel buildings have become much more widely communicated and are now commonplace for many commercial constructions. The possibilities are endless in steel construction."
Steel is 100 percent recyclable and "greener" than concrete. "Old metal buildings can be demolished and recycled, making the mining of raw materials less burdensome and having an overall manufacturing process that has less impact on the environment," says Madsen. In the United States, today's wide-flange metal beams or columns are produced from approximately 95 percent scrap material. Steel's recyclability allows contractors to take advantage of LEED points. Building owners can work toward LEED certification by incorporating energy saving strategies that affect the complete cycle of a building from best construction practices to occupation, through the end of the building's useful life. Strategies include choosing environmentally sensitive building designs, as well as materials and other assorted criteria, each of which can earn points toward certification.
Time and cost savings
Steel is procured as a complete form and can be utilized in a building immediately, improving erection speed. Metal buildings can be erected in cold climates; however, concrete requires hoarding and heating when the temperature drops. Concrete tilt-up also requires mixing of different materials in exact quantities and qualities, a very time-consuming process.
Steel buildings take an average of one-third less time to erect than other types of construction. According to The Metal Initiative, a program of the Metal Construction Association, "Typically metal wall panel systems offer lower installed costs than that of tilt-up and comparable to synthetic stucco or EIFS, because they can often be installed faster than other wall materials, resulting in savings in construction costs."
The Steel Joist Institute reports that designers of a 26-story office building and parking garage in Cincinnati saved $2.1 million in foundation and column costs for a 10-level garage because of steel's lighter weight, as compared to an all-concrete structure. A significant consideration in selecting the steel system was the construction time reduction, 56 weeks versus an estimated 61 weeks for concrete.
Also, the Institute cites a steel joist system used in a Buffalo, N.Y, apartment complex demonstrating the design flexibility and cost savings of steel joist construction as compared with traditional concrete construction. These buildings combine first-floor commercial space with three floors of housing units, including ground-level parking beneath the living quarters. After investigating alternatives, the designer found a combination of steel frame, steel joists, concrete floors with metal decks, vertical K-bracing and moment-resisting frames to be the most economical. The Institute reports the design saved $300,000.
An update to an independently produced cost comparison study for commercial buildings proves that structural steel framing solutions remain faster to build and more cost effective than reinforced concrete alternatives. The study, conducted by a team including Davis Langdon, Arup and MACE, considers two typical modern commercial developments.
A range of steel-, composite- and concrete-based frame solutions for both buildings were fully designed and priced. First carried out in 1993, the study is the established industry standard. The building design and construction methods are regularly reviewed and updated to reflect modern practice.
The study shows that while the cost of a frame and floor is a relatively small part of the total cost of a development, the selection of a steel frame reduces timescales and has a beneficial effect on other major variable cost items such as foundations, cladding and services. This leads to significant cost savings for the overall building project.
Cement, used to make concrete, can experience a shortage in material availability. The shortage of concrete's availability due to shipping rates and imported materials can drive up costs and limit supplies, despite growing demand. "Construction and material costs of metal buildings are substantially less than those of concrete tilt-up," Stellmacher says.
Natural disaster durability
Metal buildings have better strength and flexibility to protect in natural disasters like earthquakes, tsunamis and hurricanes. Stronger, more flexible steel buildings can withstand much of the pressure that topples traditional structures, and save lives in the process.
Steel is ideal for seismic zones, reports Larry Williams from the Steel Framing Alliance, since it can bend and absorb energy from earthquakes. Also, steel's ductility and strength make it an effective blast-resistant structural material.
A collapsed concrete structure is often the cause of fatalities and injuries. Under high winds, a lightweight, tilt-up building roof can become compromised, adversely affecting its connection
to its concrete panels. The Joplin, Mo., Home Depot, a concrete tilt-up building, has gained attention because of the 200-mph tornado winds that impacted it in May 2011. After its roof was lifted off, its concrete panels fell in huge sections like dominoes. Seven people were killed in the front of the store when the 100,000-pound walls collapsed on them. Twenty-eight people in the back of the store survived when those walls collapsed outward. Only two of the slab walls in the Home Depot remained standing.
But, concrete can resist fire for extended lengths of time better than steel. Because of this, concrete structures are less likely to collapse in the event of a fire. Steel will both soften and melt when exposed to high temperatures for longer time periods. But builders can improve safety by using spray-on fireproofing spray to strengthen the steel.
"For many types of disasters we can easily design and engineer what is needed to keep the occupants safe," says Stevens. "The strength and durability of a pre-engineered steel building, if designed correctly, can easily exceed your customers' expectations."
The best of both
Often, depending on the design needs, buildings are built with both concrete and steel for the best possibilities. It is not uncommon for buildings to incorporate concrete tilt-up in their lower portion and steel in the upper. "I think that pre-engineered steel can complement tilt-up structures," says Stevens.
"A hybrid type building using steel column and roof system is a great way to go. A full tilt-up with open web joist is very technically difficult to build. It takes experience to form, pour and tilt the panels especially if they have openings for doors or windows. But with a pre-engineered metal building combined with a partial tilt-up system, even though the expertise is still needed to lift the panels, the panels have a weld point and together they complement each other.