Truss Plate Design and Bending Moment
 Although truss plates have been around for nearly 60 years, the behaviour of truss plates is not fully understood. Engineers in the truss industry are always working to better understand and predict the performance of truss joints. The effect of bending moment on truss plates is one aspect that has been researched and tested extensively in the past ten years.
 

So what exactly do we mean by moment? Moment is the stress caused by a bending action, or by a force acting about a pivot point. Moment creates stress in a truss joint as the joint tries to resist turning or rotation.

 
Historically, truss joints were often considered to be “pin” joints, meaning that the joint acted like it was connected with a single pin, like a bolt or nail. In a pin joint, the ends of the truss members are free to rotate in response to bending stress. Thus, the pin joint must only resist axial tension and compression forces. Consider the following diagram of a pin joint. We have two members connected with a single bolt or nail. If we apply a load to the end of the member as shown, the member will rotate because the pin joint offers no resistance to rotation. The moment stress in this joint is zero.

 

 

 

 

 

 

Now consider the same joint, but this time we have a truss plate making the connection. When we apply the load, the member will bend, but not rotate at the joint because the truss plate prevents rotation in the joint. Moment stress is present in this joint.
 
 

 

 

 

 
 
 
 
 
 
 
 
 
 
Prior to the 2002 edition of the TPI truss design standard, there was no standard method of incorporating moment stress into the design of the plate. In the 2002 edition, a procedure for designing mid-panel splice joints for moment was introduced. This procedure was based on peer-reviewed research that including testing of actual splice joints. Under this new procedure, both the tooth grip capacity and the steel tension capacity of the joint were analyzed for the combined stresses caused by moment and axial (tension or compression) force.

 

For the 2007 edition of the TPI design standard, the moment design procedure was modified (based on additional testing and research) and now more joint types are designed for moment stress. In addition to mid-panel splices, TPI 2007 requires that moment be considered for panel point splices, pitch breaks, and corner joints.

 

            If you’re running truss designs using TPI 2007 (which is required by the new IBC/IRC 2009 building code), you may find that some plate sizes have increased as result of the moment check. However, you can alleviate some plating issues by using a more complete plate inventory (namely the MT18HS 18-gage high strength plate). Also, changing the chord geometry or chord cutting (for example, running the vertical through the chord, or changing the plumb cut pitch break to one at the angle bisector) may result in a smaller plate. On some corner joints, changing the size of the end vertical may help.

 
- Employee Spotlight Article Author
 Phil O'Regan

 

Phil O’Regan is a licensed engineer who started at Robbins Engineering more than 11 years ago after completing a Master’s degree in wood structures engineering. Phil began as the Research & Development engineer, testing full-scale trusses, investigating truss joint behavior, and improving truss analysis methods. Some of his work has been published in peer-reviewed journals, and his work on truss plate design for moment has been incorporated into the ANSI/TPI truss design standard.

 

- Employee Spotlight - Mike Magid

 

Mike has been working in the truss industry for twenty eight years, the last twelve for Robbins.

His industry experience includes twelve years as a truss engineer with Truswal Systems and four years with Truswood, a component manufacturer located in Raleigh, NC.

Seven months after being hired by Robbins, Mike obtained his PE license and soon became Robbins

Chief Engineer.  Along with his engineering duties, Mike has been working with the programming staff in updating the Online Plus software to the latest Building Codes and TPI standards.

Mike graduated from Lawrence Technological University, a private college in Southfield, Michigan, with

a degree in Architecture and Construction Engineering. 

He resides in Lutz, Florida with his wife Linda, and three children, Hilary, Joel and Anna.
 

- Employee Spotlight - Joaquin Velez
 

Originally from Puerto Rico, Joaquin resides in Tampa.  He is a graduate of the University of South Florida with a BS degree in Civil Engineering.  In 2008, he obtained his Professional Engineering license from the state of Florida.  Joaquin has been working in the truss industry for eleven years, all of them with Robbins Engineering, Inc.  He is a great asset to the engineering department, especially when it comes to tackling complex and challenging engineering repairs. Joaquin also handles all the hinge plate designs received from customers.

 
 
6904 Parke East Blvd Tampa Florida 33610
Phone: 813-972-1135  Fax: 813-971-6117  Email: TPA-Information@mii.com