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US-04: Bottom-chord bracing for metal plate-connected wood trusses used in light-commercial applications

Report ID: 985

Published: CROSS-US Newsletter 2 - September 2020

Report Overview

Concerns regarding metal plate-connected wood trusses in light-commercial construction when subject to net uplift.

Report Content

<p>A reporter is concerned about metal-plate connected wood trusses in light-commercial construction &ndash; specifically the use of these trusses with no ceiling (exposed trusses) or a suspended ceiling grid.&nbsp; These light-framed roofs will typically experience a net uplift even under moderate wind loading, which will cause the bottom chord to go into compression to resist the net uplift. &nbsp;These bottom-chord compression members almost-always require bracing to achieve adequate strength &ndash; similar to columns. &nbsp;Generally speaking, on light-commercial jobs, the Building Designer will defer to the Contractor, Truss Manufacturer, and Truss Designer<sup>1</sup> to provide temporary and permanent bracing, and the Truss Manufacturer/Designer will only provide the typical details &ndash; such as in BCSI-B3<sup>2</sup>, if at all. &nbsp;Truss Manufacturers/Designers should pay attention to the architectural drawings and the reflected ceiling plan to see if additional bracing is needed to resist net uplift loads. &nbsp;The Building Designer must coordinate bracing for these components, as it is part of the structural load path. &nbsp;Unfortunately, the scope of services on most light-commercial projects are lacking requirements for parties to coordinate between disciplines, which leaves many of these structures susceptible to wind-uplift related failure.</p>

Comments

The typical governing standard for this construction is ANSI/TPI 13 as referenced by the applicable edition of the International Building Code (IBC).  Wind uplift pressures are prescribed by ASCE 7 and the IBC in the United States.

A failure from the conditions noted is usually related to a lack of coordination and confusion about the responsibilities of the various entities, particularly the Building Designer, the Contractor, the Truss Manufacturer, and the Truss Designer.

The Building Designer is ultimately responsible for the design of the overall building structure.  Design can be delegated but overall responsibility cannot.  The issue reported here is not one of building codes or material selection, but rather of the failure of the Building Designer to review the delegated design work, and to reasonably ensure that their design intent has been achieved in the field.

Local building officials generally rely on the Building Designer to provide a complete structural load path.  Some jurisdictions may not check and/or peer review light commercial construction and rely on a design presented in submitted engineering drawings to provide a minimum level of structural performance during a wind event.

ANSI/TPI 1 Para. 2.3.3.1 allows methods of restraint/bracing of trusses and their members to be accomplished by standard industry details, substitution with reinforcement, or project-specific design.  This breadth of approach allows for specific determination of such bracing by the Structural Engineer of Record (SEOR) or Truss Design Engineer, requiring care in communication amongst parties involved in the bracing design and installation.

ANSI/TPI 1 Para. 2.3.2.4 contains a detailed list of information required of the Construction Documents for the truss design and the supply of related Structural Elements, including specification of all loads on the trusses.  Para. 2.3.4 requires the Contractor to provide all relevant documents to the Truss Manufacturer.  This information must be, in turn, transmitted to the Truss Designer.  Overestimation of deadload can be unconservative in determination of net uplift, so the Building Designer and the Truss Designer must be clear in communicating and understanding what deadloads are applied in various locations.  (For example, some portions of a building may have ceilings, and some may not.)  In these cases, provision of a complete set of design drawings (not just structural) to the Truss Designer is helpful.

ANSI/TP 1 Para. 2.3.1.6.2 requires Special Inspection, including of restraint and bracing, but only for clear spans of 60 ft (18.3 m) or more.  For lessor spans, which would include significant light-commercial construction that is the subject of this report, field observations by the Building Designer are especially important.

Limited scopes of services mentioned by the Reporter, while unfortunate, cannot be an excuse by the parties involved.  The best designs, details, etc. are of little value if coordination and follow through do not take place at the critical times during the project life.

This report is about light-commercial design. This also applies to residential or any project without sufficient bottom-chord bracing.

Additional references4,5,6 are included below.

1. Titles of responsible parties mentioned herein follow Ref. 3.  Note that on most projects the Building Designer is the Registered Design Professional or Structural Engineer of Record.
2. BCSI is Building Component Safety Information, which is published by the Structural Building Components Association.
3. ANSI/TPI 1 – 2014, National Design Standard for Metal Plate Connected Wood Truss Construction.
4. Josh Bartlett, “Wood Truss Bracing, whose Job is it Anyway?", STRUCTURE, March 2005 (https://www.structuremag.org/?p=6905).
5. The Truss Plate Institute publishes standards and guidelines on this subject:
Metal Plate Connected Wood Truss Handbook, Wood Truss Council of America.


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