Article No: 267

2009-12-22 10:24:12
Spectacular Spans
By: Larry Storer


Photo by Hal Nelson
 
A decade ago, all residential concrete floor systems with spans up to 20 feet were done using techniques borrowed from commercial construction, and the experienced crews and heavy equipment made second-story concrete floors very expensive. The same was true for smaller multi-family residences such as apartment complexes and condominiums.

Then in the late 1990s, the technology caught up to the need, and short span concrete floor decks were economically feasible and could be done with fewer workers with less experience using less heavy equipment.

Industry data indicates that concrete floors were used in less than 5 percent of the single-family homes in the early 1990s, but 10 years later the use had jumped to more than 16 percent. Those numbers have continued to grow as people apply to the floor deck and roof decisions the same logic that brought them to build concrete walls in the first place: strength and durability; sustainability with its environmental and green benefits; fire, insect and noise resistance; and lower insurance rates.

Systems are now engineered to provide adequate strength for floor decks with clear spans possible up to more than 50 feet. Common floor systems suited to homes and small buildings include: Composite steel joist, suspended concrete floor with pre-galvanized steel joists, cold-formed steel, rigid stay-in-place foam forms, block and joist system, concrete on steel decks, precast planks, concrete on fiber glass joints, precast hollow-core planks and autoclaved aerated concrete panels.
   
AN ICF IN ARIZONA

When Hal Nelson, owner of Insulated Concrete Co. (halnelson_ccaz@msn.com), built a two-story 6,500-square-foot insulating concrete form (ICF) house in Payson, Ariz., the owner went with Metwood Building Solutions (metwood.com) cold-formed steel floor system both inside and out.

“This summer and retirement home, which sits at about 5,000-foot elevation, was built into a sloping hillside with great views, and the owner wanted three decks and wanted to include radiant heating,” Nelson said. “The entire house was built with the Metwood post and beam system for the basic structure, so the decision to do the decks that way was simple.

“We love the Metwood concrete floors and decks concept and sell it whenever we can,” he said, explaining that having Metwood systems in the portfolio of solutions puts his company in  more competitive position in making the sale. Nelson said it wasn’t hard to sell this owner, a Bechtel Corp. engineer, on the Metwood system because it made sense to him.

The walls of the Payson house are insulating concrete forms (ICFs) from Arxx Corp., although Nelson said he has installed Metwood floor systems with Quad-Lock and Poly-Steel, as well as other ICF manufacturers. “Metwood works so well with all ICFs.”

Two small decks are cantilevered off the second floor to the west, about 15 feet above the ground. The south deck, which is 20-foot by 15-foot and captures a priceless view, is partially roofed, and the Metwood beams and posts carried up to that roof structure.

“Basically we used their TruSPAN metal beams and posts and a metal decking and had variations in the deck profile that ranged from 2 inches to 4 inches with rebar suspended in the valleys of that profile.” The decks were finished off with rod-iron railings around them.

“It gets reasonably cold here, so concrete for the house and the decks has an advantage over wood.The owner was so sold on radiant heating that some consideration early-on was given to putting it on the decks as well as throughout the house. Unfortunately, they get a little snow up there. A snowstorm may dump a few inches that will take a few days to melt-off, so the radiant heating on the decks was dropped.

Nelson said he used Metwood’s ThroughSPAN metal beams for the channel so that utilities could be routed through the middle of the beams, and also used the FloorSpan system. “Metwood light-gauge steel beams are reinforced with rebar, making them so much stronger than wood beams. We were also able to run longer spans without having to have vertical supports.”

Metwood Vice President Al Smith said cold-formed steel has been the workhorse of commercial and industrial construction, but Metwood has taken the benefits of that technology into residential and light commercial buildings.

“The TruSpan and ThroughSpan beams all come from what we created call the SpanTechnologies family, and our TruSpan, ThroughSpan, FloorSpan and DeckSpan products are what we sell nationwide for residential and light commercial buildings because the smaller, lightweight beams have the ability to span further with less depth.”

FloorSpan systems provide a radiant heat-ready floor for a concrete pourover, and all components are light enough to install by hand, can be trimmed on the job and fastened into place with self-drilling screws.

Kipton Tewksbury, a contractor in Arlington, Mass., wrote in an article in the Journal of Light Construction about retro-fitting a structural ridge project using Metwood TruSpan products, that the Metwood beams are stronger, lighter, more compact and easier to install than comparable laminated veneer lumber (LVL) or steel I-beams.

“The manufacturer’s load calculations showed that, in some cases, a 71⁄4-inch-deep TruSpan beam could bear the same weight as a 16-inch-deep LVL,” Tewksbury  stated. “As for weight, our ridge beam weighed about 22 pounds per lineal foot, compared with 50 pounds per lineal foot for a steel I-beam and even more for an LVL. And the fact that you can buy it with 2-by nailers on the top and bottom edges meant we could use normal wood connectors for making attachments.”

The Metwood’s light-gauge cold-formed steel construction TruSpan beam is a hollow rectangular beam made from 14-gauge cold-formed steel. Its cross section consists of two C-sections that have been welded together, with steel reinforcing bars at each corner.

Tewksbury said that by beefing up the internal rebar (increasing from a No. 4 to a No. 9 rebar, for example), Metwood can increase a beam’s carrying capacity without making it any bigger. TruSpan can be custom made or can be cut with power tools onsite and screwed into place with self-tapping screws eliminating the need for any welding. Using three or four ply chambered TruSpan beams, spans of up to 50 feet can be achieved with less depth and having a higher strength-to-weight ratio than wood.

Metwood’s Smith said that the ThroughSpan beam differs from a TruSpan beam in that it has openings in the beam that allow utilities to pass through the beam for routing general utilities and the ability to hide utilities that would hang beneath the joist. The openings range from a 5 ½-inch on a 9 ¼-inch height beam to a 9 ¼-inch opening on a 14-inch height beam.

DeckSpan provides a deck surface for concrete pourover that will accommodate any type of handrail and any kind of aesthetic floor treatment, from all decorative concrete designs and colors to wood or ceramic tile.

He said residential builders are beginning to discover the benefits of cold-formed steel for residential and light commercial buildings.

“Especially attractive is that the concrete floor system products can achieve higher load capacities than wood and can achieve longer spans with fewer vertical supports and require less space,” Smith said. “In addition, the products can be easily handled on an ICF site, and all trimming can be done with a reciprocating saw or circular saw with a metal cutting blade, therefore no welding or cutting torches are required.”