Examining Roof Truss Connector Importance
Originally published by: Construction Magazine — December 20, 2011
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To most builders, roof truss connectors are a mere side-note of construction. In truth, it’s one of the most important elements of a building, notes Marty Ruch and Steve Brekke, both with USP Structural Connectors, a division of MiTek.
“When you think about it,” says Brekke, USP’s Engineering Operations Manager, the framing members are rarely the failure mechanism, it’s the connection between them. We know that’s the weakest point and unfortunately, from an engineering standpoint, that’s one of the least covered topics when it comes to higher education.”
Engineering schools may ignore it, but not the connector industry.
“You look a USP structural connector and it looks like metal bent with holes in it, but it’s actually a highly engineered product that’s extensively tested to meet strict codes,” says Ruch, USP’s Director of Marketing and Retail Sales. “Below our offices here we have a state of the art test lab where pretty much all day every day we pull wood apart and test connections to see where the failure modes are.”
Because trusses are no longer a DIY project but rather highly engineered and manufactured in a controlled environment, failures are becoming less likely. Additionally high tech design software provides precision designs. In fact, USP’s parent company, MiTek Industries, is a major global player in design software for the components industry. Despite all these advances, Ruch and Brekke do see some common errors being made in the installation of their products that can lead to problems with load capacity in particular. One is the use of nails that are too short for use in today’s joist hangers.
“The most common mistake we hear about in the engineering department is the fact that some people are still using Teko nails where they shouldn’t,” Brekke explains.
It should be noted that although Teko is a brand name, some regions of the country use the term Teko nails generically for joist hanger nails. Once commonly used for that purpose, their inch-and-a-half length doesn’t meet today’s requirements for all joist hangers.
So what is the correct option?
“We’ve got some connectors that are dependent on what we call double shear nailing, or slant nailing to achieve the published load capacity. There are different words for it – but double-shear nailing can increase the load capacity of our hangers so you get a higher capacity using fewer fasteners. You take a considerable load reduction using short nails with this style of hanger,” Brekke says.
Ruch adds: “There’s a direct relationship load performance for a specific connector and the nails that are called out for that connector. The connectors are tested and evaluated with specific nails called out in the fastening schedule. There’s a lot of generic nails for a lot of other purposes and they tend to be softer and therefore they don’t have the shear capacity of the structural nails. So if you use a fastener that isn’t specifically called for that hanger, you can dramatically impact the load capacity of that connector in a negative way.”
At USP, some connectors are installed using WS wood screws. These structural screws are included with the connector. For structural wood screws, you might go to a local dealer for extra supplies, but Ruch cautions, “be sure your screws are structurally rated.”
For connectors installed with nails, the nails are not provided with orders. To determine the correct nail, use the Fastener Schedule provided, or just look at the connector. USP embosses a diagram on their specialized double shear nail hangers.
Another installation problem builders may encounter is a truss that doesn’t fit correctly.
“Every once-in-a-while the end of the truss may be shorter than what was designed for and then it creates a gap from the header to where the heel of the truss starts. That gap can throw off the load bearing of that connector,” Ruch says.
Brekke notes, “Our hardware is truly for rough framing. Sometimes things don’t fit together that great and a ½ inch, or more, gap can make a big difference. One of the biggest problems is that we learn about this after the fact. It gets discovered by a building official during an inspection when all the framing is done. It’s much easier to correct these problems during construction.”
“Jobsites aren’t always perfect environments,” Ruch sums up. He and Brekke encourage builders to “do your calculations” early and often and don’t hesitate to contact your supplier if you need answers.