Truss Engineering Tips: Designing Extended TC Bearing Trusses in ANSI/TPI 1-22

What is an extended top chord bearing truss? For those who don’t see these types of trusses on a daily basis, an extended top chord bearing truss is simply that which contains a top chord extension or tail supported by a bearing. This tail can then bear on a wall, beam, another truss, or other types of support from below. These types of trusses can be more common in commercial buildings, or in applications where trusses hang down from a wall, such as in an exposed ceiling space. The design of this extension and the plate at the nearest joint are critical, as these elements transfer the loads from that entire end of the truss to the supporting elements below. [For all images, See PDF or View in Full Issue.]

How are extended top chord bearing trusses designed using the traditional Prescriptive Method existing in ANSI/TPI 1-22 and prior? In previous versions of ANSI/TPI 1, extended top chord truss designs were limited to a handful of allowed truss “details” – each based on an actual, strength-tested configuration. These “details” had specific requirements including gap limitations (no greater than a 0.5” gap was allowed between the bearing and the end vertical web), specific diagonal web angles, lumber species/grades, lumber sizes and stacks, web configurations, plate sizes and positions/orientations, etc. Depending on the truss “detail” used to design a given truss, a prescriptive allowable reaction could not be exceeded at the top chord bearing. These prescriptive limits were laid out in a table in ANSI/TPI 1 (see Table 7.4-1) and called the Prescriptive Method.

What important changes were made to the designing of extended top chord bearing trusses in the latest 2022 ANSI/TPI 1 update? The key update to these types of trusses in the latest edition of ANSI/TPI 1 includes a new, optional alternative to this method based on new testing data. This new method, called the Analytical Method, replaces the strict “details” and table with a series of equations based on a single detail (shown below), which can be adjusted to accommodate other web and chord configurations, plate sizes and positions/orientations, bearing gap sizes, lumber species/grades, etc. As a result, this method can calculate the allowable reaction of any extended top chord truss, regardless of its design, thus allowing users nearly limitless design possibilities. For example, larger bearing gaps can now accommodate fire separations for balloon framed walls, and more.

This set of four equations calculates separate allowable reactions based on the stress versus capacity of:

1. The compression perpendicular-to-grain in the extended top chord lumber above the bearing,
2. The bending in the extended top chord lumber above the bearing,
3. The shear in the extended top chord lumber above the bearing, and
4. The rolling shear of the top chord lumber at the joint nearest the top chord extension.

Each of these allowable reactions are calculated separately, and the overall minimum result is taken to be the maximum allowable reaction of the extended top chord bearing.

As mentioned, the Analytical Method is an optional alternative to the existing Prescriptive Method. Depending on the truss, the new Analytical Method can give comparable, more conservative, or more liberal results versus the Prescriptive Method, but both are considered valid methods.

Note: The Analytical Method has only been added to ANSI/TPI 1 at this time. All Canadian designs must continue to follow methods outlined in TPIC.

How can reaction failures in extended top chord bearing trusses be fixed? Extended top chord bearing trusses can fail for many reasons, including those common to other types of trusses (lumber failures, plating failures, deflection failures, bearing width failures, etc.). However, a type of failure unique to these specific trusses is an exceeding of the allowable reaction limit, as determined by the Prescriptive Method table or Analytical Method equations mentioned previously. This failure can be identified by the Error message shown in Studio. The exact wording depends on which Method is being used, as well as additional settings:

If using the Prescriptive Method, there are very limited options for fixing reaction failures. Typically, the truss must be redesigned using a different design “detail” that has a higher allowable reaction limit than the bearing in question. Using the Analytical Method, however, so many aspects of the truss design come into play when calculating the allowable reaction. This provides designers with a large array of options for resolving failures, but also means that Studio cannot attempt all solutions without specific input from the user. When faced with this Error message, you might be left not knowing where to even start.

Before getting into specifics for any given truss, let’s first take a look at the possible options that might resolve reaction failures in extended top chord bearing trusses using the Analytical method:

• A.  Switching to the Prescriptive Method – This could either increase or decrease the reaction allowed at the bearing, but since both methods are acceptable, it can’t hurt to try! This can be done be either right clicking on the EnvData button, or navigating to the EnvData settings, and turning off the “Use TPI analytical method for Extended TC.” This option can only be used if the truss meets the design criteria of the Prescriptive Method. If it does not, Studio will either give an error, or grey out the save button on the given dialog box where it sees invalid input (such as for bearing gaps > 0.5”).
• B.  Using stronger top chord lumber species and/or grades – Strengthening the material used in the extension can greatly improve reaction limits.
• C.  Increasing extended top chord lumber width – Add a stack to the extended top chord lumber if using wx2 oriented lumber or increase the extended top chord lumber size if using 2xw oriented lumber. Alternatively, you can add a partial slider rather than adding a stack to the entire panel as long as the interior portion is 12” or longer from the outside edge of the web(s).
• D.  Decreasing end vertical web lumber width – Remove a stack from the end vertical web lumber if using wx2 oriented lumber or decrease the end vertical web lumber size if using 2xw oriented lumber. If already using the smallest size, try removing the end vertical for an “open” end design. This may seem counterintuitive because you are removing lumber that should be providing more support. However, extending the XECB length (shown in the prior diagram as Figure 7.3-9) by adding more web lumber at the top chord joint can actually worsen existing moment/bending issues. This solution is unique to the Analytical Method, as the opposite is true for the Prescriptive Method.
• E.  Decreasing diagonal web lumber width – Remove a stack from the diagonal web lumber if using wx2 oriented lumber or decrease the diagonal web lumber size if using 2xw oriented lumber. If already using the smallest size, try flipping the diagonal web so that it ends at the bottom chord rather than the extended top chord joint. As mentioned in option D, this can be counterintuitive and gives opposite results in the Prescriptive Method.
• F.  Changing the orientation of the diagonal web member or adjusting its cut at the top chord panel joint to reduce web scarf length – Rotating the diagonal web member by shortening the first web panel can reduce the length of the web scarf cut at the top chord joint. This change, or similar manual web cut changes at the top chord joint, can affect reaction limits by shortening that XECB length shown in the diagram.
• G.  Decreasing the gap between the bearing and the webs – Increasing the bearing width to end closer to the webs or moving the webs closer to the bearing by reducing the input gap can help reduce moment/bending in the extension.
• H.  Enlarging or repositioning the plate – Larger plates or plates that cover larger portions of the top chord (especially closer to the bearing) can help provide top chord lumber strength.
• I.  Rotating the plate – The angle of an extended top chord bearing joint plate relative to either the top chord or diagonal web can impact the equations differently. Try switching between these two automatic orientations (i.e., plate parallel to the top chord versus parallel to the diagonal web) using the “For TC Bearing with no End Vertical, Plate parallel to diagonal web” and “For TC Bearing with End Vertical, Plate parallel to diagonal web” EnvData settings, or try rotating the plate to any other angle using Plate Properties or the Move Plate tool. Keep in mind that the best solutions try to maximize the plate area on the top chord, especially nearest (or over) the bearing. Please note that the truss needs to be reanalyzed after each attempt to update the results. Make sure to turn on “Hold plate” prior to rerunning to ensure that your plate changes stick.
• J.  Adding truss plies or changing the lumber orientation – Increasing the out-of-plane depth of your truss at the bearing can help spread out the loads across the supporting wall, thus allowing for higher reaction limits at the bearing. The best way to do this is by increasing the number of plies of your truss (which improves all of the reaction equations), but switching from 2xw to wx2 oriented lumber can have the same effect on compression perp. calculations. In contrast, the other three equations improve with a change from wx2 to 2xw oriented lumber instead, due to the vertical increase in top chord lumber size.
• K.  Reducing load – If possible, reducing loading or increasing the number of trusses in the roof or floor system to reduce spacing can help get the bearing reaction to be within allowable limits.
• L.  Disallowing wane in the extended top chord – Lumber defects, such as wane (reduced lumber cross section due to the presence or sloughing off of bark when lumber comes from the outer edges of the log), can reduce the strength of the top chord lumber as well as reduce the effectiveness of the extended top chord joint plate, especially if located on or near the top chord extension. One of the equations includes a factored reduction in the lumber strength (0.67) if allowing for wane in the extension. If this is the equation that controls the failure, removing that allowance for wane can improve allowable reactions. This setting can be turned on or off via the “Allow wane in the extended top chord portion of top chord bearing trusses” EnvData setting. Disallowing wane in the extended top chord will add a note to the truss design drawing, and will require an inspection of the extension to ensure the absence of wane. Plant personnel must be made aware of this requirement so that the proper lumber can be selected to avoid wane in the extension.
• M.  Use net/reduced reactions – For extended top chord bearing trusses, ANSI/TPI 1 allows a reduction in load located directly above the bearing when designing the extension. This is because that load is considered to be supported by the wall or supporting member rather than the truss extension itself. Users can utilize this reduction by turning on the “Allow reduced TC bearing reactions” EnvData setting, leading to fewer failures.

Any of these options have the potential to help resolve bearing reaction failures, or it may take a combination of these to get a particular truss to pass. So, how can you tell what changes might have the most impact? First, analyze the truss and navigate to the “Eng Calcs” audit tab. If this is not visible, you’ll first need to activate it using either the “Audit Engineering Calculations” EnvData setting, or using the “Generate All Audits” setting in the EnvData right click menu. Once in this audit tab, scroll down to the “ANSI/TPI 1-2022 TC Bearing Analytical Method reaction calculations” section. As a reminder, there are four equations that result in an allowable reaction value (R_). The lowest value is the one that controls the design.

Once you’ve identified the controlling (lowest) reaction equation, the following table outlines which of the above steps can improve that value. If more than one equation has a lower value than the actual bearing reaction, multiple steps may need to be taken.

In the example truss used in this article, rolling shear was the controlling reaction equation, so options C and H were attempted by the user simultaneously by adding a stack to the top chord and increasing the top chord joint plate size. The truss now passes the Analytical Method bearing reaction limit, as shown below. A passing extended top chord bearing design in Studio displays a Warning message with the allowable reaction stated. This is informational only, and should not be confused with the Error message of a failing design.

Try as many solutions as you’d like, and as always, we’re here to help! Contact your Simpson representative with any questions and we’d be happy to try to find a passing design with you.

Copyright © 2025 Simpson Strong-Tie Company Inc. All Rights Reserved