The National Design Standard for Metal Plate Connected Wood Truss Construction, ANSI/TPI 1-2022 has been updated to require a design load check that accounts for a Person Loading of a roof truss chord without structural wood sheathing. As discussed in the ANSI/TPI 1 Commentary, this loading requirement is “intended to represent the occurrence of a person on the bottom Chord of a truss,” not the roof man-load in ASCE 7.
The Person Loading requirement language from ANSI/TPI 1-2022 is given in Section 6.2.2.5:
“6.2.2.5 Person Loading
In addition to any load cases required for the applicable end use, each roof Truss spanning 18 feet (549 cm) or more shall be designed for a single concentrated load at the mid-point of any Chord panel that is not sheathed with structural wood sheathing. This concentrated load shall be 300 lbs. (1335 N) and shall be applied concurrently with dead load. It shall not be applied concurrently with any other load. The load duration factor for this load case shall be 1.6 or less.”
Consensus Process for Updating ANSI Standards
ANSI has specific requirements to create or update a standard. The committee must be balanced, meaning certain percentages are required to be Producers, Users, and General Interest members. Specifically, no one interest group can outnumber the other two interest groups. Revisions to an ANSI design standard is an evolutionary process that involves consensus of a committee, with public input, that deems the change is needed for various justifiable reasons, such as product performance and safety.
The Person Loading revision is an example of the evolutionary nature of ANSI design standards. In the mid-1990s, I used the Purdue PPSA2 program to analyze a 4/12 W-Truss, 2 ft. on-center with a maximum span of 30-5-0 using 2x4 No.2 Southern Pine chords. A man-load, which at that time was 250 lbs., was placed at the center of one of the bottom chord panels in addition to the truss design dead load. The analysis revealed that the 10-ft. lower chord panel was overstressed.
As a member of the WTCA Engineering Committee, I suggested a prescriptive rule to limit the maximum span of a 2x4 bottom chord panel to 8-ft. as a safety concern for a person that might stand on a truss chord panel. However, a consensus on this proposal at that time was not reached by the committee.
My “story” is an example of how the consensus process works and at times may be slow. While serving on the 2022 TPI 1 revision committee, I was pleasantly surprised when a Person Loading was proposed. As the following example will demonstrate, a 10-ft. chord panel that is not sheathed with structural wood sheathing can produce an overload when subjected to Person Loading.
Person Loading Stress Analysis Example
The ANSI/TPI 1 revision that includes a Person Loading offers a good example of a member subjected to combined bending and tension stresses (ANSI/TPI 1-2022, Eq.E7.3-10 and 11).
W-Truss and Load Analysis Assumptions:
1. W-Truss, 4/12, 2-ft. on-center
2. 2x4 No.2 Southern Pine, Top and Bottom Chords
3. Span = 30-ft.
4. Loading, 20-10-10 (completed structure)
5. 300-lb. concentrated load mid-panel
6. Load Duration Factor (LDF) = 1.6
The first lower chord panel was analyzed with a 300-lb. concentrated load applied mid-span as depicted in Figure 1 [for figure, See PDF or View in Full Issue].
Under the design dead load and the Person Loading, the panel axial load (T) was 1,715 lbs. and the mid-panel bending moment at the 300-lb. load was 7,990 in.-lb. The equation for calculating the strength of a member subject to bending and tension stress (Eq. E7.3-10) is given by
ft/F′t + fb/F*b ≤ 1
where
ft is the design tension stress in the member,
F′t is the adjusted allowable tensile strength,
fb is the design bending stress in the member, and
F*b is the adjusted allowable bending strength.
For the example problem,
ft = T/A = 1,715 lbs./(1.5 in. × 3.5in.) = 327 psi ,
fb = M/S2×4 = 7,990 in.-lb./3.06 in.3 = 2,611 psi .
For a solid-sawn member in which structural wood sheathing is not attached, the repetitive member increase, Cr, equals 1.10 for both bending and tension [ANSI/TPI 1-2022, 6.4.2.1(b)]:
F′t = Ft × LDF × Cr = 675 psi × 1.6 × 1.10 = 1,188 psi ,
F*b = Fb × LDF × Cr = 1,100 psi × 1.6 × 1.10 = 1,936 psi .
Substituting the number into the combined stress equation, the result is
ft/F′t + fb/F*b ≤ 1
327/1,188 + 2,611/1,936 =
0.28 + 1.35 = 1.63 > 1 .
For the truss first panel, the combined stress index indicated a 2x4 No.2 was overstressed by 63% and inadequate with respect to strength.
The equation for calculating the lateral stability of an unbraced member (Eq. E7.3-11) subject to bending and tension stress is given by
fb – ft ≤ F′b
where F′b is calculated by multiplying F*b by a stability factor, CL, that is always less than 1.0. The purpose of the stability check is to guard against the “twisting” potential of a member when the compression edge of the lumber member is not fully braced. For this case, it’s not necessary to calculate CL as the 2x4 No.2 member was already overstressed. Thanks to modern wood truss industry software, the requisite truss design checks are a matter of selecting the proper “buttons.”
Why Person Loading Matters
ANSI/TPI 1 Person Loading involves important safety concerns. As such, I encourage component manufacturers (CMs) to educate truss technicians on the importance of implementing the Person Loading check as required by ANSI/TPI 1-2022 before the standard is referenced in the applicable building code. The use of a revised and updated design standard is generally accepted by registered design professionals and regulatory officials, plan reviewers, and other officials.
Historically, some component manufacturers have limited panel lengths based on experience, performance, and good judgment. The ANSI/TPI 1-2022 update provides a consistent method to check panel length for a Person Loading and eliminates “rules-of-thumb” or “prescriptive rules” for limiting panel spans. The Person Loading provision applies to all chord grades, sizes, species, specie groups, and Multi-Species and Country Grade-marked lumber (as per the 2018 National Design National Design Specification® (NDS®) for Wood Construction Supplement).
As the only wood truss design standard referenced in the U. S. building code evolves, it is encouraging to see the truss industry stakeholders work together to reach an ANSI consensus on a wide range of updates and changes that strengthened the standard with respect to truss engineering, quality, performance, and construction and public safety.
Frank Woeste, P.E., is Professor Emeritus, Virginia Tech and a wood construction consultant. Frank, along with his colleagues, has developed and presented continuing education programs for more than 30 years. He is a member of the ANSI/TPI 1 Project Committee for revision of the truss design standard and the AWC Wood Standards Design Committee, and serves as the ALSC Consumer Representative (Alternate). Comments are welcome and can be sent to fwoeste@vt.edu.