Gable Endwall Panel ("Truss") - Design
When a roof is framed with prefabricated wood roof trusses, the gable endwall is typically framed with an endwall panel, supplied by the truss manufacturer, that is commonly (and incorrectly) referred to as a gable endwall "truss".
Design and construction of gable endwall panels is very often grossly deficient, especially for panels with high verticals.
During the past 30 years, the average number of new homes built in the US is well north of one million per year. http://www.nahb.org/generic.aspx?sectionid=819&genericcontentid=554&channelID=311
According to the main truss industry organization (SBCA), "market share" (percent of total roof area) for roof trusses was 65 percent from 1997 to 2001. Percent of houses is less since a house with roof trusses generally has more roof area than a house with rafters. However, is it likely that the number of new houses built with roof trusses during the past 30 years is somewhere between 20 and 40 percent of all new houses, or 6 million to 18 million.
Based on many years experience evaluating design and construction of truss installations, it is reasonable to conclude that at least 30 percent of prefabricated gable endwall "truss" panels do not have adequate design capacity to resist code-specified design wind pressure. Estimating that 50 percent of new houses are built with gable endwalls, the number of houses with defective gable endwall panels is then (at a minimum) somewhere between 900,000 (nine hundred thousand) and 2,700,000 (two million 700 hundred thousand) due to generic defects with the design and construction process.
Major damage generally does not occur due to basic safety factors used for design and the very infrequent occurrence of wind speeds approaching design wind speed. However, severe damage can occur, and has occurred, for very high speed wind speeds, especially during hurricanes.
Even without major damage, leaks can be caused by excessively flexible endwall verticals, especially with an adjacent lower roof.
Primary deficiency is lack of adequate design capacity to resist design wind pressures specified by the building code. Construction defects also occur, independent of design defects. However, deficient design is most often the basic cause of defective gable endwall construction.
Design By Truss Manufacturer
In general, the truss manufacturer does not design the gable endwall panel to resist wind pressures. The truss designer usually includes a note on the truss diagram to explain that the "truss" is not designed to resist wind. Even if design for wind is specified (on building plans), or requested by a design professional during review, the truss manufacturer will likely ignore the specification or resist a request, claiming that "standard" endwall bracing is the responsibility of the building designer.
Architects generally fail to include any details at all, in building design plans, for bracing gable endwall panels. Typically, the architect incorrectly assumes that; (1) Truss manufacturer has designed the gable endwall panel to resist wind, or (2) Truss manufacturer supplies "standard" gable endwall bracing details such that the architect does not have to provide any other details.
All-too-often, the result of such assumptions is that no bracing is installed to brace the gable endwall panel, or grossly inadequate bracing is installed.
SBCA Standard Detail - Deficiencies
Below is a markup of a standard detail published by the Structural Building Components Association (SBCA) for bracing gable endwall panels.
This detail is essentially conceptual only. Complete design must be provided by the building designer.
SBCA was formerly the Wood Truss Council of America (WTCA).
|WTCA Gable End Bracing with comments 022110.pdf (PDF — 791 KB)|
This "standard" detail requires substantial additional design of connections between the various bracing elements, as well as specific design for each endwall panel.
Even considering that the SBCA detail notes the need for final design, the following deficiencies remain;
(1) Lack of note to emphasize that roof sheathing must be nailed to the upper blocking between trusses. This is essential since lateral force must be transferred to the roof sheathing, acting as a diaphragm.
(2) A note specifies that connection at base of the sloped brace must be designed "to resist both pushing and pulling forces". However, for the detail shown, design of a connection to resist outward "pulling" force is difficult at best. At the very least, a practical connection detail should be included.
(3) There is no explanation to clarify whether or not the small steel connection plates at each end of panel verticals have been designed to resist wind forces. For the verticals that are reinforced (not braced), total wind force to be resisted at each end of the vertical is not changed. Since the truss diagram generally notes that the endwall "truss" has not been designed to resist wind, it is reasonable to conclude that the steel connection plates also have not been designed to resist out-of-plane wind force. Even if a valid case could be made that plates provide moment-resistance (as a force couple), these very thin plates can not resist shear force from design wind pressures without severe distortion due to bending. Also, any moment resisted by the plates will tend to twist the chords excessively.
(4) A "ceiling diaphragm" is shown, without any notes to emphasize that any such ceiling diaphragm must be specifically designed by the building designer to be effective structurally. In particular, there must be adequate connection between edge of ceiling drywall and top of wall under the endwall panel. Without an effective ceiling diaphragm, wind force distributed to bottom chord bracing will try to push the bottom chords laterally, completely contrary to key conditions for design of roof trusses.
Unless adequate design of an endwall panel can be provided by the truss manufacturer, to resist code-specified wind pressures, the building designer should specify a site-built endwall, with wall studs oriented as for a typical framed wall. Of course adequate details must also be provided, including essential details to brace top of wall below the gable endwall.
If the site-built gable endwall is to be installed on top of a wall below, a a gypsum board (drywall) ceiling diaphragm can be provided. However, adequate details are essential. IRC 2006 includes basic requirements that are often overlooked by architects and code officials.
An alternate approach is to install floor sheathing in the attic space to provide bracing as a floor diaphragm. As for any diaphragm design, necessary details must be provided to ensure wind force can be transferred into side walls (parallel to direction of wind force).
The use of full-height wall studs, from floor to roof line, might be considered to eliminate the need for bracing top of wall below gable endwall. However, for design wind pressure, lateral deflection of such studs (without lateral bracing) may still be enough such that the ceiling will attempt to act as a brace.
Article From Structure Magazine
An article about design of gable endwall panels (termed "frame" in the article) has been published in Structure magazine (August 2007) by two engineers with Qualtim, a consulting firm that provides engineering services for SBCA.
|C-StructuralDesign-Gjinolli_Aug_07.pdf (PDF — 1 MB)|
The article includes several excellent diagrams to illustrate concepts and details.
However, the authors tend to overlook key problems with actual design as performed by practicing architects today.
The authors provide the following explanation about the design process (underline made for emphasis);
The building designer, knowing the intended flow of loads for the entire building, is best qualified to take the resultant loads from the gable end frame and transfer them safely through the structure. Many building designers include gable end bracing details in the construction documents for the project (figure 10) and some prescriptive details (figure 11) are available as well.
To assist the building designer in determining the bracing requirements to transfer lateral loads from the gable end frame into the roof and/or ceiling diaphragm, many truss designers provide standard design tables and details based on typical design assumptions used by building designers.(figure 12)
These details provide a variety of options for restraining and bracing web members in a gable end frame against lateral loads applied perpendicular to the frame. Options vary by design load assumptions, web spacing, and web length, and may include individual web member reinforcement, horizontal bracing, and/or diagonal bracing (Figure 5a and 5b). These standard tables and details do some of the work for the building designer with respect to incorporating the gable end frame into the overall design of the structure, but they do not take the place of a complete and necessary flow of loads analysis by the building designer.
Based on my experience, the claim that "many building designers" include gable endwall panel bracing details in building design plans is misleading at best and grossly incorrect at worst. A large number of architects performing residential design in New Jersey and Pennsylvania not only omit all truss bracing details, but also are not aware that such bracing details are even necessary. If these architects are cognizant of this design requirement at all, they assume (incorrectly) that the truss manufacturer and builder are responsible for bracing design.
The claim that truss designers provide standard details is also misleading. The truss manufacturer generally provides a set of truss diagrams to the builder, along with a placement / layout plan. The manufacturer may or may not also submit a set to the architect, with or without "standard" details for bracing gable endwalls.
For more than a few projects, truss diagrams sealed by the professional engineer (PE), ultimately responsible for truss design, are often submitted to code officials only after construction has started.
Often, the builder then also does not forward truss diagrams to the architect.
In New Jersey, code officials generally do not keep any record, other than the truss diagrams, of the name of responsible truss engineer. On the truss diagrams, the name of the truss engineer is very often not printed and is not always legible from the raised seal. The point is that, if code officials do not keep any record of the truss engineer, they surely are not keeping score to see if the architect performs detailed review of truss design diagrams.
For many projects, even if the builder submits truss diagrams to the architect, only a cursory review is performed, if any. The vast majority of architects today do not know how to perform structural review of truss diagrams, particularly with respect to overall design of the roof framing system. The general attitude of these architects is that they can validly assume that the truss designer and builder are completely responsible not only for the individual truss diagrams, but also for the entire roof truss system, including all bracing.
See "Roof Truss Bracing Design" for more general discussion of deficiencies with overall design of roof truss systems.