The New Truss Design Standard: Enter to Win A Copy of ANSI/TPI 1-2014 National Design Standard for Metal Plate Connected Wood Truss Construction

If you are like me, then you enjoy this time of the year. Instead of looking back and reviewing the events of the past year, this is the month for looking ahead at the year to come and what’s in store. So what is in store for 2015?

For the truss industry, there is a new truss design standard that was just released the last week of December. Still hot off the press, the ANSI/TPI 1-2014 standard is a revision to the 2007 edition and is referenced in the 2015 International Building Codes.

While the 2015 I-Codes might take some time for some municipalities to adopt, others are gearing up for adoption of the 2015 I-Codes as early as mid-2015. Either way, it is always good to know what is in the latest and greatest code-referenced design standards. So here’s a look at the new ANSI/TPI 1-2014 truss design standard:

The New ANSI/TPI 1-2014 Standard

The New ANSI/TPI 1-2014 Standard

First, here is a brief primer on the TPI 1 standard. The Truss Plate Institute (TPI) published the first truss design criteria in 1960. Many updates to these design criteria followed after that, and in 1995, TPI published its first ANSI-accredited truss design standard, ANSI/TPI 1-1995. Subsequent editions of this American National Standard have included ANSI/TPI 1-2002, ANSI/TPI 1-2007, and now ANSI/TPI 1-2014. All of the TPI standards, including archived copies going all the way back to TPI-60, are available from TPI (www.tpinst.org). Here is a link to the overview of non-editorial changes from ANSI/TPI 1-2007 to ANSI-TPI 1-2014.

While the 2007 edition included many significant revisions to the previous edition, the 2014 standard has relatively few substantive changes to the 2007 edition, which is good news for those who are still trying to catch up. Chapter 2 covers the design responsibilities involved in metal plate connected wood truss construction and looks different at first glance because it has been reorganized. However, the actual “Design Responsibilities” as they were defined in TPI 1-2007 have not changed.

In short, two separate sections in TPI 1-2007, which address design responsibilities in projects that require registered design professionals and projects that do not, have now been combined into one section. The “Truss Design Engineer” is simply referred to as the “Truss Designer” and the “Registered Design Professional for the Building” is simply the “Building Designer.” If the project requires registered design professionals, then the Truss Designer and Building Designer will be registered design professionals. Regardless of whether or not those two parties are registered design professionals, their responsibilities relating to the design and application of metal plate connected wood trusses are the same, so defining those responsibilities once within the TPI standard simplifies things and makes more sense.

Not new to the wood industry, but new to TPI 1-2014, are provisions for Load and Resistance Factor Design (LRFD). AF&PA incorporated LRFD provisions into the 2005 National Design Specification (NDS) for Wood Construction, and the TPI standard has followed suit, using the same basic approach as the NDS.

The section in TPI 1-2014 with the most changes is the section on deflection criteria. The deflection criteria have been revised in the last three editions of the TPI standard. Starting in TPI 1-2002, a requirement was added to consider creep in total deflection calculations. However, specific creep factors were not specified in the standard and were only presented in the Commentary. In the 2007 edition, creep factors were moved into the standard, and the total deflection calculation explicitly specified a component due to creep of no less than 50 or 100 percent of the initial deflection for long-term loads for dry and green (wet service) use, respectively. This was consistent with the 1.5 and 2.0 creep factors specified in the NDS for total deflection calculations for seasoned and unseasoned conditions.

Between the 2007 and 2014 editions, an inconsistency was discovered between the TPI 1 deflection criteria and the deflection limits in the U.S. model building codes. While the intent of the TPI standard was to present the same basic L/xxx deflection limits for Live Load and Total Load as the model building codes, it was discovered that the IBC deflection limits for “DL + LL” were actually intended to address only the creep portion of the dead load deflection plus the immediate live load deflection. So although long-term deflection including proper creep considerations can be an important consideration in the overall design of the building, it is not intended to be used to limit the design of a truss with respect to building-code established limits on vertical deflection.

Excerpt from the ANSI/TPI 1-2014 Commentary

Excerpt from the ANSI/TPI 1-2014 Commentary

To resolve the issue of inconsistent methods used in the building industry to specify deflection limits, the 2014 edition now distinguishes between the following:

• “Deflection due to Live Load Plus Creep Component of Deflection due to Dead Load” for purposes of meeting the IBC deflection limits for DD + LL, which is defined as

ΔCR = Δ LL + (Kcr ‐1) x Δ DL

• “Long-Term Deflection”, which includes the full effect of creep but for which there are no explicit deflection limits specified in TPI

• “Deflection due to Total Load”, which is based on the full load (including both dead load and live load), but includes no explicit creep factors. The deflection due to total load has the same deflection limits as the IBC deflection limits for DD + LL, but this is not a mandatory check in TPI; it only applies to trusses if the Building Designer specifies that such a check due to total load be performed. Further, any consideration for creep in that calculation would also have to be specified by the Building Designer.

In recognition of the increased creep in trusses compared to solid sawn beams, the creep factors have been increased to 2.0 and 3.0 for dry and green (wet service) use, respectively. For purposes of deflection checks in accordance with the IBC, these factors reduce to 1.0 and 2.0, respectively, since the equation for “Deflection due to Live Load Plus Creep Component of Deflection due to Dead Load” uses KCR-1 rather than KCR as the factor on the immediate deflection due to dead load.

What does this all mean? For the majority of truss applications (e.g., dry-service), the effect of switching from TPI 1-2007 to TPI 1-2014 will be a change in creep factor from 1.5 to 1.0, unless additional requirements are specified by the Building Designer. Those additional requirements may include a limit on long-term deflection or a check for total load deflection (subject to the TPI deflection limits), including any considerations for creep.

A complete listing of the changes in TPI 1-2014 and more discussion about these changes are available in the TPI 1-2014 Commentary.

Now is your chance to win a copy of the ANSI/TPI 1-2014 standard for your own design library! Simply post a truss-related question, comment or idea for a future truss-related blog topic, and we will enter you into a drawing during the week of Jan 15-22. One winner will be picked at random. We look forward to hearing from you!

19 thoughts on “The New Truss Design Standard: Enter to Win A Copy of ANSI/TPI 1-2014 National Design Standard for Metal Plate Connected Wood Truss Construction

  1. Two Questions:
    Who should specify permanent bottom chord bracing in the absence of a gyp ceiling?

    ANSI/TP1 allows for the truss designer to pick any PS-20 compliant species for truss chords, with no restriction on specific gravity. How can the building designer conservatively and economically design the hurricane ties for use in high wind areas of the country?

    • The Building Designer is responsible for the permanent bracing, including permanent bottom chord bracing. The truss design drawing will specify the maximum on-center spacing for the bottom chord
      lateral restraint that is required (in the absence of a directly applied rigid ceiling) based on the load conditions for which the truss has been designed. For example, the bottom chord bracing
      requirements might be specified on the truss design drawing as “Sheathed or Purlins at 6-3-0”. The Building Designer is responsible for specifying the size and material of the continuous lateral
      restraint (purlins) that are to be spaced no greater than 6’-3” o.c., the
      connections of the lateral restraint to the trusses, and the means of bracing the lateral restraint against lateral movement, such as by adequate diagonal bracing. The Building Designer is the Owner of the Building or the person that contracts with the Owner for the
      design of the building and/or who is responsible for the preparation of the Construction Documents. An excellent resource for information pertaining to permanent bracing is the Building Component Safety Information (BCSI), “Guide to Good Practice for Handling, Installing, Restraining & Bracing of Metal Plate Wood Trusses.” Section BCSI-B3 (also available as a stand-alone Summary Sheet) provides guidelines for individual chord and web member permanent restraint/bracing, including minimum recommendations for bracing materials, connections, and diagonal bracing intervals. This document is available from SBCA; for more information, visit http://www.sbcindustry.com/content/1/building-component-safety-information-bcsi.

    • ANSI/TPI 1 establishes minimum requirements for the design and
      construction of metal-plate-connected wood Trusses. That doesn’t mean that a Building Designer can’t require above-minimum requirements for his/her project, such as more restrictive deflection criteria, higher than code-minimum loading requirements, and even minimum allowable truss chord material. For example, if the Building Designer wants to utilize the higher design values for hurricane ties and girder tiedowns that are often associated with Southern Pine lumber (compared to SPF, for example), he/she can specify SP truss chord material, or perhaps a minimum specific gravity for the truss chord material. Of course, that may also require the wall framing to be SP or other high-density material, depending on what the truss is being tied down to. Another, and perhaps the best, means of
      achieving economical roof system designs is by having a good collaboration between the Building Designer and the truss supplier/Component Manufacturer. Whereas the Component Manufacturer (CM) knows the most economical lumber selections for the truss based on their inventory and/or material flow through their plant, the Building Designer knows what the most economical material would be for the overall building system. Therefore, the most economical designs can be achieved by collaboration during the planning stages, so that the strengths of both parties can be utilized. The good Component Manufacturer understands that an efficient and cost effective roof framing system means that there might need to be a little more cost built into the truss, whether to reduce the amount of roof system bracing required, or to allow for more economical connections throughout the structure.

  2. The biggest truss related issue we usually have are related to damage of trusses, both in the field during construction or after occupancy (such as after a fire). A good blog topic might be how to design a fix for broken or partially burned members; or possibly when is it best to replace a truss as opposed to fixing it.

  3. What is the best way to prevent interior drywall damage from truss uplift and still adequately brace your interior walls to the truss system? Opinions vary greatly in my experience.

    • The truss industry has a document that addresses this exact question. The document is called “Partition Separation Prevention and Solutions (How to Minimize Callbacks Due to Gypsum Cracking at the Wall/Ceiling Interface)”, and is available from SBCA. A non-printable PDF of this document can be viewed at http://www.sbcindustry.com/ttbpartsep. In short, the best way to prevent drywall cracking and still provide lateral support at the top of the partition wall is with the use of a flexible connection between the
      truss and partition, and by properly floating the drywall joints. Although this can be done with blocking (as shown in some of the details in the SBCA document), those of us at Simpson Strong-Tie think that the easiest way of accomplishing this is with a slotted clip such as our STC roof truss clips. These clips get installed to the top of the partition wall and then get nailed to the truss bottom chord through the center of a slot that allows for vertical movement, while still providing lateral support at the top of the wall. Our DS drywall clips can be used in conjunction with the STC clips to secure the drywall to the wall. Then, to allow the drywall ceiling to float, it should not be fastened to the bottom chord within 16” from the wall. When all of this is done, it enables there to be movement between the truss and the wall, with either one able to move up or down independently of the other, without causing cracking in the drywall at the wall/ceiling interface.

  4. Like Luke, I would like to see some details for attaching partition walls to the underside of trusses. There are many details out there. I would like to see what works best and maybe which details to avoid.

    • As mentioned in the answer to Luke’s question, details for attaching partition walls to the underside of trusses can be found in “Partition Separation Prevention and Solutions (How to Minimize Callbacks Due to Gypsum Cracking at the Wall/Ceiling Interface),” available from SBCA. The details shown in that document have proven effective in minimizing cracking at the wall/ceiling interface. A non-printable PDF of this document can be viewed at http://www.sbcindustry.com/ttbpartsep. A detail that should be avoided is toe-nailing the truss to the partition!

    • ANSI/TPI 1 does not publish lumber design values, but rather states that the design values must come from an accreditation body that complies with PS-20. Further, the Commentary states that the
      design values for typical lumber used in metal plate connected wood trusses are established by the Board of Review of the American Lumber Standards Committee (ALSC), as it is the only accreditation body recognized in accordance with PS-20. Therefore, when the new ALSC-approved Southern Pine design values went into effect, they were used in truss designs as of the ALSC implementation date (June 1, 2013). So although the revision of Southern Pine design values isn’t reflected explicitly in the truss design standard, it is addressed by the requirement for values to come from an accreditation body that complies with PS-20, which is ALSC.

    • Congratulations Donald! You have won a copy of the ANSI/TPI 1-2014 standard for your own design library! We will email you the details.

  5. I’d like to see some discussion of vertical up and down movement of the support walls due to moisture variations under the support wall footings, with specific attention to differential movements between exterior and interior walls and the effect of extremely dry air adjacent to interior support walls during winter months when the building is being heated.

  6. I have seen in Hawaii X-strapping of the trusses along the full length of the residential home structure with timber 2×4 purlins @ 24″ oc running perpendicular to the trusses without a plywood sheathing diaphragm. Corrugated metal roofing is then attached to the purlins. What type of strapping is available by Simpson for this type of installation?

    • The change in deflection criteria in ANSI/TPI 1-2014 does not necessarily affect the cambering of trusses, since camber is a separate consideration that can be addressed independently of the deflection criteria. Per ANSI/TPI 1-2014 Section 2.3.2.4(g), the amount of camber required for any given truss should be specified in the Construction Documents by the Building Designer. How much camber should be used is a judgment call that depends on a lot of different factors, and is best based on experience. So any amount of camber that has proven successful in the past for a particular application can still be used for trusses designed to ANSI/TPI 1-2014. Note that the ANSI/TPI 1 Commentary still mentions 1.5 times the dead load deflection as an amount of camber that might be specified to counter the deflection due to dead load and prevent ponding in parallel chord roof trusses.

  7. I would like to learn more about when the TC26 should be used for trusses with horizontal movement, and why trusses can’t be designed with both bearings pinned with rafter ties installed per your building code. Being able to run trusses with both bearings pinned and then using some kind of rafter tie system would make larger scissor type trusses much more economical to build and could give a truss manufacturer a leg up when competing against conventional framing.

  8. I Completed a Job A while Back where information on Chemical reactions to Plates were Scarce. I can remember the option was Using Stainless Steel plates or Coating the standard Galvenized type with a Vapor Barrier to minimize Cost Difference. But also the Plate Grip Difference analogy/analysis between the 2 types of plates ,When using Fire Retardant Lumber /Chemical storage. I realized the difference in plate size required to plate same truss joints. Can you elaborate

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