Long-span composite decks could offer designers new flooring options
August 8, 2002
Unshored, deep steel decking that spans 20 feet—it's a good concept on paper, but can it serve customers in the real world? At least one engineer is convinced it can, and he's taking his message to the masses.
When it comes to unshored steel floors in their buildings, structural design engineers are used to thinking about the world in certain lengths—typically 8 or 10 feet, maybe up to 15 ft., but rarely 20 ft., especially with steel.
But that is just what Virginia Tech (www.vt.edu) Professor Sam Easterling is proposing that structural designers consider when putting together future projects.
The idea is to use a steel deck that is deeper, heavier, and longer than normal floor deck while using fewer structural members to support it. Easterling used two different prototypes for his study, both comprising unshored slabs that were 20 ft. long. Total depth of the first slab was 8.5 in. (a 6-in.-deep deck rib covered by 2.5 in. of concrete); the second used a 4.5-in.-deep deck with 2.5 in. of concrete for a total depth of 7 in. (see Figure 1).
While the heavier, deeper deck is more expensive, a project using the material could more than make up for that cost by having fewer supporting (or secondary) structural members to fabricate, erect, and fireproof, Easterling's studies show.
"The reason for increasing the span of the deck is to cut down on the number of secondary steel framing members," Easterling said. "While the individual members there may be deeper or heavier, there would be fewer structural steel pieces and fewer pieces to fabricate, erect, and fireproof. Overall, the thought was that there would be a net savings."
That's the way it works out on paper, anyway. The professor's work looks so good on paper, in fact, that it earned him the American Institute of Steel Construction's (www.aisc.org) 2002 T.R. Higgins Award. Easterling is to give half a dozen Higgins lectures on the project nationwide this year, which gives him the opportunity to evangelize design engineers on his vision.
He already has a believer in Walter Schultz, a structural research engineer at Nucor's Vulcraft facility (www.vulcraft.com) in Norfolk, Neb. Schultz has done research work with Easterling on other projects and has bounced around a few ideas with him on this one.
"I suspect there will be a growing market for it," said Schultz, who noted that the lower fireproofing and erection costs represent big advantages for the idea. And while he's only done analysis on the idea so far, he does plan to manufacture some specimens in the future, he added.
The two composite slabs used in the study had different depths and support configurations, but both provided evidence that long-span deck could work in the field.
"Actual manufacturing is not the issue," Easterling said. "If there's a market, you can bet manufacturers will produce it. Right now there's just no demand. Manufacturers are not interested in leading the charge and having new tooling made just to push a research project."
Problem is, designers aren't going to be too keen to forsake tried-and-true materials such as precast concrete for steel in their commercial building projects, especially if they have no physical proof that this concept works economically.
The answer would seem to be getting someone to take a chance. But that costs money, which a lot of people aren't exactly flush with right now.
And then there's the little issue of finding the stuff. Several companies make deeper-deck products akin to Easterling's concept, and many buildings in the U.K. have employed Slimdek® and Slimflor® decking systems manufactured by Corus' Precision Metal Forming group (www.pmf-corus.co.uk). The SD225 flooring used in the project, which has ribs almost 9 in. deep, can span more than 21 ft. unpropped, depending on the slab's weight. The system uses rolled, asymmetric steel beams to create the floor's platform. It is then laid over with concrete.
The Daresbury Park office complex in Runcorn, a city in western England about 190 miles northwest of London, is a prime example of deep steel decking in action.
"SD225 has already been used in three office buildings in this complex, and I am particularly pleased with the way it allows for the integration of M&E [mechanical and electrical] services under the floor," Senior Project Engineer Christopher Woodhead (formerly of the Shepherd Gilmour firm in Manchester) was quoted as saying on the Corus Web site.
Matthew Barlow, who currently works for Shepherd Gilmour and is familiar with the Daresbury Park project, said his firm stopped using Slimdek when Corus raised prices but did consider it a good choice then.
"It was working out to be the most economic [option] at the time," Barlow said.
The good thing about projects such as the Daresbury Park complex is that "you can point to other related work," Easterling said. "This is not simply some crazy guy in Virginia saying this is a good idea. That's been kind of a fortunate thing."
But here in the states, it's just a wee bit difficult to get experience with material that for all intents and purposes does not exist.
"Nobody has any [experience with it] because nobody makes it," said Kurt Swensson, owner of KSI Structural Engineers in Atlanta, whose bread-and-butter jobs are research and development laboratories. Swensson said he needs a deep slab that spans 20 ft. and is deep enough to house the intricate ventilation, electrical, and other mechanical systems used by his customers.
"I haven't used them because the infrastructure is not there yet, meaning the metal deck industry isn't supplying the type of profiles we need for the applications we need," Swensson said. "The metal deck manufacturers aren't going to make it without a demand for it. But there's not going to be a demand because nobody is going to use it."
"The way you break the cycle is to find a very particular project where this is the only system that will work," Swensson added. "I haven't found that yet."
And one isn't likely to pop up in the near future, according to Nucor's Schultz.
"It's on the horizon, I guess you could say," Schultz said.
Right now people in the Southeast trust concrete because it meets their key needs—large clearances, good floor-to-floor heights for mechanical systems, and more-than-adequate weight capacity, according to Swensson.
"You start talkin' to owners and architects, and before you even design it, they don't want to do it," Swensson said of steel designs. "It's not even a cost issue; they don't have a system they can look at and say, 'This works.'"
At least one steel decking company executive disagrees, saying that cost isn't just a big issue with building designers—it's the only issue.
"Unfortunately in this country, as in many other countries, economics are what's key," said David Landis, Vice President of Epic Metals (www.epicmetals.com), Rankin, Pa. "That's the reality."
Landis said that deck as deep as the one proposed by Easterling requires too much steel to cover the required floor area to be economically attractive. If the less costly alternative looks and works the same, designers and builders are going to opt for it, he said.
"If you can buy a $20,000 car that does the same thing that an $80,000 car does, and if they both look the same, which would you buy?" he said.
Another fly in the ointment is what Swensson perceives as structural steel manufacturers' unwillingness to innovate.
"Nobody is going to fund any research or underwrite any prototypes to do anything with it [deep steel decking]," Swensson predicted. "The industry wants researchers to show that it’s a 100 percent economically viable system before they put any money into it, and you can't do that.
"I'd rather design out of steel than concrete—it takes me less time," he confided. "[But] I look at concrete and what concrete has done: They've got new systems. The concrete systems [people] are being innovative in solving building problems, and the steel guys aren't. And if they're losing market share, that's why."
Just wait—people will come around to the concept in time, Schultz said.
"Things change slowly in this business," Schultz said. "I'm optimistic."