On Site at SIPA - Fall/Winter 2004 (Plain Text Version)
In this issue:
Strict new wind codes could provide SIP builders with a new competitive edge.
by Ted Cushman
The unprecedented series of storms that struck Florida this summer has focused new attention on the problem of building homes to withstand high winds. But even before the recent storms, wind-resistant design was a hot topic in coastal areas near the Atlantic Ocean and Gulf of Mexico. From Texas to Maine, authorities in most of the coastal states have started to enforce the tough new wind-resistant design and construction details required by the updated International Building Code (IBC) and International Residential Code (IRC). The strict new codes are a new headache for builders near the coast, but they could prove a boon to the SIP industry, and to builders who use SIPs.
SIP builders often have to provide design-engineering data on their panels and connectors, while a traditional acceptance of conventional construction methods commonly lets stick builders be "grandfathered in." Along the coastline where design wind speeds exceed 110 miles per hour, however, the free ride for stick construction is over -- every permit application, regardless of materials and methods, will need to be accompanied by valid design engineering that shows how the building will resist storm wind forces. The change puts stick construction and SIP construction on a level playing field -- now everyone has to show numbers.
And when it comes to numbers, the new coastal requirements may even give SIP builders an advantage — if they take the time to get familiar with the new rules. SIPs have already demonstrated equal or superior structural strength when compared to conventionally stick-framed walls. Depending on the situation, that edge in axial strength and racking resistance can give SIP walls a load-resisting capacity equal to or better than even an engineered shearwall element, built by extra-close nailing of plywood or OSB to wall studs and plates. In many cases, SIPs can achieve these high design strengths without added fasteners or straps -- and therefore without added material and labor cost.
Architect Bill Chaleff designs and builds with SIPs on the east end of Long Island, where design wind speeds are set at 120 miles per hour. "I've already gotten permits for two houses, one in Easthampton and one in Southhampton, under the new building code here, by showing my calculations to the building department to demonstrate that I can meet the required design loads," say Chaleff. "And the amazing thing is, I didn't have to change anything about the way I fasten the panels together. I'm using the same fasteners at the same spacing that I have always specified for our designs."
It adds $15,000 to the cost of a big custom house -- and that means the cost curves for SIP and for stick construction have crossed. We can build for the same cost as stick-built -- and of course we have much better energy performance."
There's a question, too, says Chaleff, about whether stick-built structures with all the extra nailing and connectors required by code actually have the strength in practice that they have on paper. "The builder's association here is concerned about what all those additional nails are doing to the wood," he says. "When you put that many nails into a stud, it often splits the wood severely, and that may be compromising the strength of those connections."
Experience from the recent Florida storms may shed light on that question. So far, reports indicate that homes built to the new standards adopted since Hurricane Andrew have generally performed far better than homes built under previous codes. This would tend to support the idea that new nailing and fastening requirements have been effective. Also, testing of stick-built wall and roof assemblies under simulated seismic loading at facilities like the new Simpson Strong-Tie test lab in California, and destructive testing of building assemblies by Clemson University researchers on the East Coast, have provided a solid basis for concluding that stick frames with added nailing and fastening are much stronger than without.
However, the experience of some SIP houses that went through the recent Florida storms adds weight to the SIP industry's argument that SIP construction provides superior strength against storm-wind loading. Three houses using a specialized wall SIP that has a cement-board skin, and with metal-and- EPS roof panels, took a direct hit from Hurricane Charley as it made landfall in Port Charlotte, Florida. The three emerged with barely a scratch, although a flying palm tree did puncture one roof panel.
Equally dramatic was the experience of the "Not So Big Show House," a demonstration home being built using Insulspan SIPs as a new-technology showcase home for the 2005 International Builders Show. Builder Cameron Bradford, the general contractor for the show home, says, "In mid-construction, we have actually been hit by three hurricanes: Charley, Frances, and Jeanne. Frances brought 105-mph measured winds at the airport. And our house survived without any damage at all."
Remarkably, the SIP structure held together even though its openings were not protected. "We didn't have the windows or doors in yet," says Bradford, "and the wind came right inside the building. The house wasn't officially designed for that, because the design assumes storm-rated windows."
When wind enters a building, internal pressure and external suction create a combined load on the structural connections — it's a scenario that often leads to total collapse of the building. But the Orlando house, with SIP exterior walls and a flat SIP ceiling on the second floor supporting a truss roof, is "like a plywood bomb shelter," says Bradford. "It was not damaged at all."
Bradford's company had several nearby homes under construction using more conventional methods. "We had one house where the second floor was stick framed, above a first floor of reinforced concrete masonry units -- stacked block with the steel and grout in the cores. The stick-built second-floor walls were sheathed and braced; that all ended up on the ground. At another house, the cement block walls had been laid, but not yet grouted. Those walls were knocked right down."
The Orlando show house is Bradford's first experience with SIPs, but after one house, he's a convert. "The biggest advantage is the speed of construction," he says. "It took us just four days to get the house closed in -- first floor walls, second floor deck and walls, and second floor ceiling panels. And that's including the learning curve, doing it for the first time." A "very professional" site rep from Insulspan provided excellent guidance, says Bradford.
An online slideshow of the Orlando House under construction is available at http://www.aswebuild.com/Bradford. A stick-built pool house that is part of the same project allows for comparison of methods -- the connector straps applied to all studs are clearly visible in the stick-framed exterior walls of the pool house. "The reinforcement for the SIP structure was a lot simpler to understand and to install," says Bradford. "We used Simpson hold-downs at every panel joint into the slab foundation — that was definitely overkill. Then we had flat straps spanning the first floor frame, just at the panel joints." The connectors for the stick portion were considerably more time-consuming, Bradford says. "In a stick frame, you're installing two connectors for every stud, and six or eight nails per connector," he explains. "That is a lot of nailing. The SIP walls were much, much faster."
For SIP builders as well as SIP suppliers, the challenge will be education. To gain the most advantage from the structural strength of a strong component, it has to be used effectively, and its capabilities have to be effectively communicated to the building department. The new requirements in coastal areas give SIP builders an opportunity -- get there first with the best solution. But to do it, they'll have to understand the wind-resistant design process.
Wind-resistant design involves two different, but related, aspects. The code has requirements both for structural elements (known as the "Main Wind Force Resisting System," or MWFRS) and for exterior finish elements (called the "Components and Cladding," or C&C). Wind-resistant design starts with the MWFRS; once that's established, the C&C elements also have to be specified in accordance with code.
With their proven racking resistance and axial load-bearing strength, SIPs can readily do the heavy work in the MWFRS, acting not just as shearwalls but also as horizontal diaphragm elements like the ceiling of the Orlando House. But the "unit performance" of the SIP structure will also depend on how wall and diaphragm elements are connected -- so builders and manufacturers will need to be ready to document the strength of fasteners, and to illustrate the performance of joints and corners with examples of compliant assemblies.
They'll also need to be ready with details for anchoring SIP shearwall elements into the foundation -- a critical element of bringing lateral loads and uplift loads down into the ground.
And when that's all understood, there is still the issue of components and cladding. Builders will have to be clear on how windows, trim, roofing, porches, and other elements on the building exterior get firmly fastened to the SIP walls and roof. In general, that's no different for SIP houses than for stick-built homes, but clear details and easily understandable specs will be a big advantage when it's time to apply for a building permit.
Equipped with knowledge about the structural and finish requirements of the new code, SIP builders near the coast are now looking forward to a whole new game on a level playing field. After all, as thousands of visitors to the Orlando concept houses will see for themselves: This is a system that has already stood up to the storm.