When given a truss design project, the Truss Designer needs all of the loading parameters before starting work on it. This article will specifically discuss the Thermal Factor (Ct), which is part of the snow load calculations. As with any load parameter, using the wrong value will result in a truss design that is either underloaded or overloaded. An underloaded truss can increase your organization’s liability exposure. An overloaded truss puts your organization at a disadvantage when bidding.
Metal Plate Connected (MPC) Wood Trusses shall be designed per ANSI/TPI 1-2014, the National Design Standard for Metal Plate Connected Wood Trusses, based on provisions in the IBC-21, Section 2303.4.6, and the IRC-21, Section R802.10.2. Per Section 6.2.1 of ANSI/TPI 1-2014, the code-referenced edition of ASCE 7 shall be used in determining the loads.
The American Society of Civil Engineers (ASCE), in partnership with the Structural Engineers Institute (SEI), oversees and publishes ASCE/SEI 7 Minimum Design Loads and Associated Criteria for Buildings and Other Structures, the design standard commonly known as ASCE 7. This standard is referenced in both the IBC and the IRC and thus is a code requirement for load determinations by a Registered Design Professional (RDP). The 2016 edition will be referenced throughout this article.
Thermal Factor Ct
Projects that follow the IBC are required, per Section 1603.1.3 of the IBC-21, to include the seven load parameters used in snow load calculations directly in the Construction Documents, including the Ct factor. When you are working on a residential project following the IRC, you may not find the snow load factors within the Construction Documents because the IRC does not require them to be specified by the Building Designer. But as mentioned above, ANSI/TPI 1 requires using ASCE 7 for the loading.
As shown in Figure 1, ASCE 7-16 Table 7.3-2 applies to “all structures” with only four exceptions: structures kept just above freezing and others with cold ventilated roofs (…), unheated and open-air structures, freezer buildings, and continuously heated greenhouses (…). A residential Truss Designer will not be expecting to work on projects that fall into the last three exceptions, so we will focus only on “all structures” and the first exception. [For table, See PDF or View in Full Issue.]
Regarding the first exception, “Structures kept just above freezing and others with cold, ventilated roofs (...),” do you know if your residential project would fall into this category rather than simply “all structures”? If the project you are working on includes attic insulation along the ceiling plane, then the question is: does the insulated ceiling assembly yield a “cold, ventilated space” beneath the roof surfaces? If the answer is yes, then the exception applies and the Ct factor to use is 1.1.
Design Responsibilities per ANSI/TPI 1 Chapter 2
With respect to responsibilities, the following are the pertinent definitions and language.
Building Designer
The Building Designer is responsible for specifying all loads, including snow loads, in the Construction Documents.
Truss Designer
The Truss Designer is responsible for the preparation of the Truss Design Drawings based on:
“…Truss design criteria and requirements set forth in the Construction Documents or as otherwise set forth in writing by the Building Designer as supplied to the Truss Designer by the Contractor through the Truss manufacturer.”
Regarding loads, ANSI/TPI 1 Section 2.3.5.5 (f) requires the Truss Design Drawings (TDD) to include “(6) Environmental load design criteria (wind speed, snow, rain, seismic, and all applicable factors as required to calculate the Truss loads); and…”
Regarding TDDs that are required to be sealed and signed, ANSI/TPI 1 Section 2.3.5.3 states:
“Where the Legal Requirements mandate a Registered Professional Engineer for buildings, each individual Truss Design Drawing shall bear the seal and signature of the Truss Designer.”
Truss Manufacturer
Per the design standard, the Truss Manufacturer
“…shall obtain the Truss design criteria and requirements from the Construction Documents.” They shall then “ …communicate the Truss design criteria and requirements to the Truss Designer.”
When a Truss Manufacturer decides on any of these load parameters, they may increase their liability exposure.
Discussion and Recommendations
When Ct is not provided in the Construction Documents or is not available in writing from the Building Designer, it may be an opportunity for the Truss Manufacturer to interact with the RDP to clarify the appropriate load factors not provided in the Construction Documents. In our experience, some RDPs that design wood structures are not familiar with the ANSI/TPI 1 provisions, particularly the standard responsibilities defined in Chapter 2.
The snow load parameters, beyond Ground Snow (Pg) load, that are typically needed for today’s truss design software are the Snow Exposure Factor (Ce), Snow Load Importance Factor (Is), and Thermal Factor (Ct). The net result of a Truss Manufacturer reaching out to the RDP for confirmation of a load parameter could be viewed by the RDP as a demonstration of professionalism, truss design knowledge, expertise, and an indication that the trusses would likely be manufactured to the ANSI/TPI 1-2014 Chapter 3 quality requirements.
As suggested in “Building Code Adoptions and Using AI for Ethical Feedback” in the December issue, using the “ADD” acronym is recommended: Advise the client of relevant codes, Discuss with the client, and Document their decisions. Furthermore, the Building Designer is the most suitable party to specify the necessary load parameters for the design and manufacture of engineered wood trusses.
About the Authors
Frank Woeste, P.E., Professor Emeritus, Virginia Tech (VT), conducted wood engineering research and taught wood design courses throughout his 26-year tenure at VT. In cooperation with other faculty and wood industry professionals, he has developed and participated in VT continuing education programs for more than 30 years.
Marvin Strzyzewski, P.E., Senior Design Engineer at Truss Engineering Co. (a MiTek affiliate), Testing and Regulatory Compliance team, is active in developing building codes and design standards that affect the wood truss industry. He speaks at numerous conferences providing CEU training for building officials, architects, engineers, and contractors.